HomeMy WebLinkAboutGPA 09-01; ROBERTSON RANCH PA 22; PRELIMINARY STORMWATER MANAGEMENT PLAN; 2009-01-08I
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PRELIMINARY STORM WATER MANAGEMENT PLAN
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TABLE OF CONTENTS
ITEM PAGE
COVERSHEET ....................................................................................... 1
TABLE OF CONTENTS............................................................................ ii
1.0 IN"TRODUCTION.......................................................................... 1
1.1 PURPOSE.................................... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 LOCATION....................................................................... 1
2.0 PROJECT REVIEW & PERMITTIN"G PROCESS.................................... 2
2.1 DETERMINE APPLICABLE STORM WATER BMP
REQUIREMENTS............................................................... 2
2.2 PRIORITY PROJECT PERMANENT STORM WATER BMP
REQUIREMENTS............................................................... 2
3.0 PREPARE AND SUBMIT APPROPRIATE PLANS................................. 2
3.1 REQUIRED BMP ELEMENTS................................................ 2
4.0 PERMANENT BEST MANAGEMENT PRACTICES SELECTION............. 2
4.1 IDENTIFY POLL UT ANTS FORM THE PROJECT AREA............ 3
4.1.1 ANTICIPATED POLLUTANTS OF CONCERN............... 3
4.1.2 POTENTIAL POLL UT ANTS OF CONCERN................... 3
4.2 IDENTIFY POLLUTANTS OF CONCERN IN RECEIVING
WATERS .......................................................................... 4
4.2.1 IDENTIFY THE HYDROLOGIC UNIT CONTRIBUTION... 4
4.2.2 IDENTIFY 303(d) IMPAIRMENTS IN THE RECEIVING
WATERS ............................................................... 4
4.3 BENEFICIAL USES OF RECEIVING WATERS.......................... 4
4.3.1 BIOL-PRESERVATION OF BIOLOGICAL HABITATS
OF SPECIAL SIGNIFICANCE..................................... 4
4.3.2 RECl -CONTACT RECREATION............................... 4
4.3.3 REC2-NON-CONTACT RECREATION........................ 4
4.3.4 EST-ESTUARINEHABITAT .................................... 4
4.3.5 MAR-MARIN"EHABITAT ....................................... 4
4.3.6 WILD -WILDLIFE HABITAT.................................... 4
4.3.7 RARE-RARE, THREATENED, OR ENDANGERED
SPECIES............................................................... 5
4.3.8 MIGR-MIGRATION OF AQUATIC ORGANISMS.......... 5
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5.0
6.0
TABLE OF CONTENTS (CONTINUED)
ITEM PAGE
4.4 IDENTIFY CONDITIONS OF CONCERN................................. 5
ESTABLISH PERMANENT STORM WATER BEST MANAGEMENT
PRACTICES................................................................................ 5
5.1 SITE DESIGN BMPS......................................................... .. 5
5.1.1 MAINTAIN PRE-DEVELOPMENT RAINFALL RUNOFF
CHARACTERISTICS................................................ 6
5.1.2 MINIMIZE IMPERVIOUS FOOTPRINT........................ 6
5.1.3 CONSERVENATURALAREAS................................. 6
5.1.4 MINIMIZE DIRECTLY CONNECTED IMPERVIOUS
AREAS .................................................................. 6
5.1.5 MAXIMIZE CANOPY INTERCEPTION AND WATER
CONSERVATION CONSISTENT WITH THE CARLSBAD
LANDSCAPE MANUAL............................................ 6
5.1.6 CONVEY RUNOFF SAFELY FROM TOPS OF SLOPES..... 6
5.1.7 VEGETATE SLOPES WITH NATURAL OR DROUGHT
TOLERANT VEGETATION....................................... 7
5.1.8 STABILIZE PERMANENT CHANNEL CROSSINGS...... 7
5.1.9 INSTALL ENERGY DISSIPATERS 7
5.2 SOURCE CONTROL BMPS 7
5.2.1 DESIGN OUTDOOR MATERIAL STORAGE AREAS TO
REDUCE POLLUTION INTRODUCTION...................... 7
5.2.2 DESIGN TRASH STORAGE ARES TO REDUCE
POLLUTION INTRODUCTION................................... 8
5.2.3 USE EFFICIENT IRRIGATION SYSTEMS AND
LANDSCAPE DESIGN.............................................. 8
5.2.4 PROVIDE STORM WATER CONVEYANCE SYSTEM
STENCILING AND SIGN AGE.................................... 8
5.3 INDIVIDUAL PRIORITY PROJECT CATEGORIES..................... 9
5.4 TREATMENTCONTROLBMPS............................................ 9
5.4.1 TREATMENT CONTROL BMP DESIGN STANDARDS..... 9
5.4.2 TREATMENT CONTROL BMP SELECTION............... 9
5.4.3 POLLUTANTS OF CONCERN.................................... 10
5.4.4 STRUCTURAL TREATMENT CONTROL BMP
SELECTION........................................................... 10
5.4.5 TREATMENT CONTROL BMP INFORMATION............. 11
5.4.6 STRUCTURAL TREATMENT LIMITED EXCLUSIONS.... 11
5.5 PERMANENT BMPS APPLICABLE TO THE PROJECT SITE........ 11
CONSTRUCTION STORM WATER BMPS.. ... . .. . ... . ..... .. ... . .. . ... .. . ... . ..... 12
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PRELIMINARY STORM WATER MANAGEMENT PLAN
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TABLE OF CONTENTS (CONTINUED)
ITEM PAGE
7.0 IMPLEMENTATION & MAINTENANCE REQUIREMENTS.................. 12
7.1 OPERATION AND MAINTENANCE PLAN.............................. 12
7.2 ACCESS EASEMENT/AGREEMENT....................................... 12
8.0 APPENDICES.............................................................................. 13
APPENDIX A: VICINITY MAP
APPENDIX B: STORM WATER STANDARDS QUESTIONNAIRE
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TABLE OF CONTENTS (CONTINUED)
ITEM
APPENDIX C: CITY OF CARLSBAD STANDARDS EXCERPTS
PAGE
• TABLE 1: STANDARD DEVELOPMENT PROJECT & PRIORITY PROJECT STORM
WATER BMP REQUIREMENTS MATRIX
• TABLE 2: ANTICIPATED AND POTENTIAL POLLUTANTS GENERATED BY
LAND USE TYPE
• TABLE 3: NUMERIC SIZING TREATMENT STANDARDS
• TABLE 4: STRUCTURAL TREATMENT CONTROL BMP SELECTION MATRIX
• APPENDIX B: DRAFT ENVIRONMENTALLY SENSITIVE AREAS WITHIN THE
CITY OF CARLSBAD MAP
APPENDIX D: PRELIMINARY HYDROLOGY STUDY
• 'PRELIMINARY DRAINAGE STUDY FOR ROBERTSON RANCH PA22' PREPARED
BY O'DAY CONSULTANTS, INC DATED DEC. 18, 2008
APPENDIX E: BENEFICIAL USES OF RECEIVING WATERS
APPENDIX F: 2006 CW A SECTION 303(d) LIST OF WATER QUALITY LIMITED
SEGMENTS
APPENDIX G: SOURCE CONTROL BMP FACT SHEETS
• CITY OF CARLSBAD GS-16: REFUSE BIN ENCLOSURES
• CASQA SD-10: SITE DESIGN AND LANDSCAPE PLANNING
• CASQA SD-11: ROOF RUNOFF CONTROLS
• CASQA SD-12: EFFICIENT IRRIGATION
• CASQA SD-13: STROM DRAIN STENCILING
• CASQA SD-32: TRASH ENCLOSURES
APPENDIX H: TREATMENT CONTROL BMP FACT SHEETS
• CASQATC-30VEGETATEDSWALE
• CASQA MP-52: DRAIN INSERTS
• CASQA TC-60: MULTIPLE SYSTEM FACT SHEET
• CASQA SD-20: PERVIOUS PAVEMENTS
• CASQA TC-32: BIORETENTION
• SECTION 6: LONG-TERM MAINTENANCE OF BMPS
APPENDIX I: APPLICABLE MANUFACTURER'S BMP INFORMATION
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TABLE OF CONTENTS (CONTINUED)
ITEM
APPENDIX J: MAP EXHIBITS
• BIORETENTION SIZING CALCULATIONS
PAGE
• PA 22 PRELIMINARY STORM WATER MANAGEMENT PLAN BMP EXHIBIT
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1.0 INTRODUCTION
Federal, state and local agencies have established goals and objectives for storm water
quality in the region. The proposed project is a priority project as defined in Order No.
2007-01 by the San Diego Region of the California Water Quality Control Board. As a
result, the project is subject to SUSMP requirements. In addition, prior to the start of
construction activities, the project will comply with all federal, state and local permits
including the Stormwater Management Plan (SWMP) required under the County of San
Diego Watershed Protection, Stormwater Management, and Discharge Control Ordinance
(WPO) (section 67.871), the City of Carlsbad's Standard Urban Storm Water Mitigation
Plan, and the National Pollution Discharge Elimination System (NPDES) from the
Regional Water Quality Control Board (RWQCB).
The purpose of this SWMP is to address the water quality impacts from the proposed
improvements as shown on the Tentative Map. This report will provide guidelines in
developing and implementing post construction Best Management Practices (BMPs) for
storm water quality.
Planning Area 22 (PA 22), located south of Cannon Road, will contain approximately 5.5
acres of office commercial development and RV storage. The SWMP for the entire
Robertson Ranch East Village project includes the PA 22 site for the temporary RV
parking. This SWMP details the BMP' s specific for this Tentative Map. See Appendix D
for Preliminary Hydrology Study and Appendix J for Preliminary Storm Water
Management Plan Exhibit.
1.1 PURPOSE
This Storm Water Management Plan has been written to the standards set forth in
the City of Carlsbad Engineering Standards, Volume 4: Storm Water Standards
Manual (BMPs) (2008 Edition). This report will reference these standards as
'City Standards.'
1.2 LOCATION
The project is located according to the vicinity map found in Appendix A.
2.0 PROJECT REVIEW & PERMITTING PROCESS
In order to complete the Project Review & Permitting Process section of the City
Standards the Storm Water Standards Questionnaire has been completed. For reference,
this document has been included in Appendix B: Storm Water Standards Questionnaire.
2.1 DETERMINE APPLICABLE STORM WATER BMP
REQUIREMENTS
Based on the completed Storm Water Standards Questionnaire completed for the
project (see Appendix B: Storm Water Requirements Applicability Checklist) the
following requirements apply to the project:
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2.2 PRIORITY PROJECT PERMANENT STORM WATER BMP
REQUIREMENTS
Based on Section 1: New Development of the Storm Water Standards
Questionnaire, the project meets priority requirements.
Since the project is subject to priority project standards the design should
incorporate all applicable requirements identified in Sections 2.3.3 'Establish
Permanent Storm Water Best Management Practices' per the City Standards.
This should include Low Impact Development (LID) site design and source
control BMPs; BMPs applicable to individual priority projects; and treatment
control BMPs. Further guidance on these requirements can be found on page 10
of the City Standards.
3.0 PREPARE AND SUBMIT APPROPRIATE PLANS
Based on the general categories of storm water requirements that are applicable to the
project as described in section 2, the following categories from Table 1: Standard
Development Project & Priority Project Storm Water BMP Requirements Matrix found
on page 6 of the City Standards apply:
4.0
3.1 REQUIRED BMP ELEMENTS
The following are required BMP elements for the project and one or more
applicable BMPs must be utilized:
• Site Design BMPs
• Source Control BMPs
• Applicable to Individual Priority Projects
o Surface Parking Areas
• Treatment Control BMPs
PERMANENT BEST MANAGEMENT PRACTICES SELECTION
This section will identify what permanent best management practices will apply to
project:
4.1 IDENTIFY POLLUTANTS FROM THE PROJECT AREA
Based on the general pollutant categories and project categories as described in
Table 2: Anticipated and Potential Pollutants Generated by Land Use Type found
on page 8 of the City Standards, the project has the following pollutants:
4.1.1 ANTICIPATED POLLUTANTS OF CONCERN
The following are anticipated pollutants of concern for the project:
• Heavy Metal
• Organic Compounds (petroleum hydrocarbons)
• Trash & Debris
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• Oil & Grease
4.1.2 POTENTIAL POLLUTANTS OF CONCERN
The following are potential pollutants of concern for the project:
• Sediment
• Nutrients ( due to landscaping)
• Oxygen Demanding Substances ( due to landscaping)
• Pesticides ( due to landscaping)
4.2 IDENTIFY POLLUTANTS OF CONCERN IN RECEIVING
WATERS
This section will identify the pollutants or concern, if any, in the receiving waters
of the project proposed drainage pattern.
4.3
4.2.1 IDENTIFY THE HYDROLOGIC UNIT CONTRIBUTION
The project is located in the Los Monos Hydrologic Subarea (904.31) of
the Agua Hedionda Watershed in the Carlsbad Hydrologic Unit in the San
Diego Region.
4.2.2 IDENTIFY 303(d) IMPAIRMENTS IN THE RECEIVING
WATERS
In the 2006 CWA Section 303(d) List of Water Quality Limited Segments,
Agua Hendionda Creek as an impaired water body. Portions of Carlsbad
where construction sites have the potential to discharge into a tributary of
a 303(d) or directly into a 303(d) water body or sites located within 200
feet of an ESA require additional BMP implementation.
BENEFICIAL USES OF RECEIVING WATERS
The beneficial uses for the hydrologic unit are included in Appendix E and a
summary of the applicable definitions are listed below. This information comes
from the Water Quality Control Plan for the San Diego Basin.
4.3.1 BIOL -PRESERVATION OF BIOLOGICAL HABITATS OF
SPECIAL SIGNIFICANCE
This beneficial use includes uses of water that support designated areas or
habitats, such as established refuges, parks, sanctuaries, ecological
reserves, or Areas of Special Biological Significance (ASBS), where the
preservation or enhancement of natural resources requires special
protection.
4.3.2 REC-1 CONTACT RECREATION
This beneficial use includes uses of water for recreational activities
involving body contact with water, where ingestion of water is reasonably
possible. These uses include, but are not limited to, swimming, wading,
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water-skiing, skin and SCUBA diving, surfing, white water activities,
fishing, or use of natural hot springs.
4.3.3 REC-2-NON-CONTACT RECREATION
This beneficial use includes the uses of water for recreational activities
involving proximity to water, but not normally involving body contact
with water, where ingestion of water is reasonably possible. These
include, but are not limited to, picnicking, sunbathing, hiking, camping,
boating, tide pool and marine life study, hunting, sightseeing, or aesthetic
enjoyment in conjunction with the above activities.
4.3.4 EST-ESTUARINE HABITAT
This beneficial use includes the uses of water that support estuarine
ecosystems including, but not limited to, preservation or enhancement of
estuarine habitats, vegetation, fish, or wildlife (e.g., estuarine mammals,
waterfowl, shorebirds).
4.3.5 MAR -MARINE HABITAT
This beneficial use includes uses of water that support marine ecosystems
including, but not limited to, preservation or enhancement or marine
habitats, vegetation such as kelp, fish, shellfish, or wildlife (e.g., marine
mammals, shorebirds).
4.3.6 WILD -WILDLIFE HABITAT
This beneficial use includes uses of water that support terrestrial
ecosystems including but not limited to, preservation and enhancement of
terrestrial habitats, vegetation, wildlife, (e.g., mammals, birds, reptiles,
amphibians, invertebrates), or wildlife water food and sources.
4.3.7 RARE-RARE, THREATENED, OR ENDANGERED
SPECIES
This beneficial use includes uses of water that support habitats necessary,
at least in part, for the survival and successful maintenance of plant or
animal species established under state or federal law as rare, threatened or
endangered.
4.3.8 MIGR -MIGRATION OF AQUATIC ORGANISMS
This beneficial use includes uses of water that support habitats necessary
for migration, acclimatization between fresh and salt water, or other
temporary activities by aquatic organisms, such as anadromous fish.
4.4 IDENTIFY CONDITIONS OF CONCERN
In order to determine if the project proposed drainage patterns will affect the
downstream conditions, a copy of the project's Preliminary Drainage Study is
including in Appendix D: Site Drainage Study. This report is title 'Preliminary
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5.1.3 CONSERVE NATURAL AREAS
This Site Design BMP entails concentrating or clustering development on
the least environmentally sensitive portions of a site while leaving the
remaining land in a natural, undisturbed condition and incorporates the use
of natural drainage systems to the maximum extent practicable
5.1.4 MINIMIZE DIRECTLY CONNECTED IMPERVIOUS
AREAS
This Site Design BMP entails minimizing directly connect impervious
areas where landscaping is proposed and attempt to direct runoff from
impervious surfaces such as sidewalks, parking lots, walkways, and patios
to landscaping areas.
5.1.5 MAXIMIZE CANOPY INTERCEPTION AND WATER
CONSERVATION CONSISTENT WITH THE CARLSBAD
LANDSCAPE MANUAL
This Site Design BMP entails maximizing canopy interception and water
conservation consistent with the Carlsbad Landscape Manual to preserve
existing native trees and shrubs, to plant additional native or drought
tolerant trees, and to plant large shrubs in place of non-drought tolerant
exotic species. The project will utilize this Site Design BMP by having
the landscaping designer utilize the applicable City of Carlsbad Landscape
Manual and any other applicable City of Carlsbad Standards.
5.1.6 CONVEY RUNOFF SAFELY FROM TOPS OF SLOPES
This Site Design BMP entails conveying runoff safely from the tops of
slopes and in channels. The project will utilize this Site Design BMP as
much as possible. There are tops and bottoms of slopes on the project site.
Each top and bottom of slope will be designed so that runoff will safely be
conveyed away from them.
5.1.7 VEGETATE SLOPES WITH NATURAL OR DROUGHT
TOLERANT VEGETATION
This Site Design BMP entails the vegetation of slopes with native or
drought tolerant vegetation where practically consistent with the Carlsbad
Landscape Manual. The project will utilize this Site Design BMP by
having the landscaping designer utilize the applicable City of Carlsbad
Landscape Manual and any other applicable City of Carlsbad Standards.
5.1.8 STABILIZE PERMANENT CHANNEL CROSSINGS
This site Design BMP entails the stabilization of permanent channel
crossings. This Site Design BMP will not be applicable to the project due
to no permanent channel crossing being present on the existing or
proposed site designs.
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5.2
5.1.9 INSTALL ENERGY DISSIPATERS
This Site Design BMP entails installing energy dissipaters at the outlets of
new storm drains, culverts, conduits, or channels that enter unlined
channels. This is to be done in accordance with the applicable standards
and specifications to minimize erosion. Energy dissipaters shall be
installed in such a fashion as to minimize impacts to the receiving waters.
The project will utilize this Site Design BMP as needed to protect the
proposed and existing storm drain inlets, culverts, conduits, and channels.
SOURCE CONTROL BMPS
Source Control BMPs are comprised of the following BMPS:
• Design Outdoor Material Storage Areas to Reduce Pollution
Introduction
• Design Trash Storage Areas to Reduce Pollution Introduction
• Use Efficient Irrigation Systems and Landscape Design
• Provide Storm Water Conveyance System Stenciling and Signage
5.2.1 DESIGN OUTDOOR MATERIAL STORAGE AREAS TO
REDUCE POLLUTION INTRODUCTION
This Source Control BMP entails placing any and all potentially hazardous
materials that have a potential to contaminate urban runoff in storage areas
on site that are enclosed in structures such as, but not limited to, cabinets,
sheds, or other similar structures that prevent and contain with rain, runoff,
or spillage. In addition, secondary structures such as berms, dikes, or
curbs will be utilized out side of the storage structure to further prevent
contamination. The storage areas shall be paved with a sufficiently
impervious material to contain leaks and spills, and shall have a roof or
awning to minimize direct contact with precipitation within the secondary
containment area. Because the project does not proposed any material
storage areas this Source Control BMP will not be utilized.
5.2.2 DESIGN TRASH STORAGE AREAS TO REDUCE
POLLUTION INTRODUCTION
This Source Control BMP entails designing trash storage areas to reduce
pollution introduction. Trash Storage Areas shall be paved with an
impervious surface, designed not to allow runoff from adjoining areas,
screened or walled to prevent off-site transportation of trash, and contain
attached lids on all trash containers that protects them from precipitation.
Alternatively, the trash enclosure can contain a roof or awning to
minimize direct contact with precipitation. The project will utilize this
Source Control BMP by designing and building the trash storage areas
according to the City of Carlsbad Standard Drawing GS-16 and in
accordance with CASQA SD-32: Trash Enclosures. These areas will be
paved with an impervious surface, graded to drain away from the
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enclosure, and screened and walled to prevent off-site transport of trash.
Trash containers will contain attached lids that exclude rain to minimize
direct precipitation. A copy of each of these is provided in Appendix G.
5.2.3 USE EFFICIENT IRRIGATION SYSTEMS AND
LANDSCAPE DESIGN
This Source Control BMP entails employing rain shutoff devices to
prevent irrigation during precipitation and this requires all landscaping
aspects to be designed per the Carlsbad Landscape Manual. The project
will utilize this Source Control BMP by having the landscaping designer
utilize the applicable City of Carlsbad Landscape Manual and any other
applicable City of Carlsbad Standards. In addition, site irrigation will also
be designed in accordance with CASQA SD-10: Site Design and
Landscape Planning. A copy of this has been provided in Appendix G.
5.2.4 PROVIDE STORM WATER CONVEYANCE SYSTEM
STENCILING AND SIGNAGE
This Source Control BMP entails providing storm drain conveyance
system stenciling and signage. This shall be done by providing concrete
stamping, porcelain tile, insert permanent marking or approved equivalent
as approved by the City of Carlsbad, of all storm drain conveyance system
inlets and catch basins within the project area with prohibitive language
(i.e. "No Dumping-I Live Downstream') satisfactory to the City
Engineer. In addition, signs shall be posted and prohibitive language
and/or graphical icons, which prohibit illegal dumping at public access
points along channels and creeks within the project area, trailheads, and
parks shall be used. The project will utilize this Source Control BMP by
utilizing CASQA SD-13: Storm Drain Stenciling. A copy of this is
provided in Appendix G.
5.3 INDIVIDUAL PRIORITY PROJECT CATEGORIES
Where identified in Table 1 of the City Standards, the following requirements
shall be incorporated into priority projects: Surface Parking Areas. Surface
parking areas ( covered and uncovered) where landscaping is proposed shall
incorporate landscape areas into the drainage design. Parking that is in excess of
the project's minimum requirements ( overflow parking) may be constructed with
permeable paving subject to the City Engineer's approval. The project will utilize
this Individual Priority Project Category BMP by incorporating the proposed
landscaping areas in the drainage pattern as much as feasibly possible.
5.4 TREATMENT CONTROL BMPS
Where identified in Table I of the City Standards, and after site design and source
control BMPs have been incorporated into the project design, treatment control
BMPs may then be utilized.
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5.4.1 TREATMENT CONTROL BMP DESIGN STANDARDS
Treatment Control BMPs shall be designed to infiltrate, filter, and/or treat
runoff from the project footprint per Table 3: Numeric Sizing Treatment
Standards. A copy of Table 3 is provided in Appendix C. There are four
guidelines that need to be followed for Treatment Control BMPs:
• All Structural Treatment Control BMPs shall infiltrate, filter,
and/or treat the required runoff volume or flow prior to
discharging to any receiving water body supporting beneficial
uses.
• Multiple post construction Structural Treatment Control BMPs
for a single priority project shall collectively be designed to
comply with the numeric sizing treatment standards.
• Shared BMPs shall be operational prior to the use of any
dependent development or phase of development. The shared
BMPs shall only be required to treat the dependent
developments or phases of development that are in use.
• Interim storm water BMPs that provide equivalent or greater
treatment than is required may be implemented by a dependant
development until each shared BMP is operational. If interim
BMPs are selected, the BMPs shall remain in use until
permanent BMPs are operational.
5.4.2 TREATMENT CONTROL BMP SELECTION
All projects classified as Priority Projects in the City of Carlsbad shall
select a single or combination of treatment control BMPs from the
categories listed in Table 4: Structural Treatment Control BMPs Selection
Matrix. A copy of Table 4 is provided in Appendix C. This selection
matrix will determine the most efficient removal BMP for the pollutants of
concern from the project site. The most efficient device or combination of
devices shall be utilized to maximize pollutant removal.
5.4.3 POLLUTANTS OF CONCERN
Based on the above findings for the proposed site usage the project has the
following pollutants of concern:
• Sediment (Anticipated)
• Nutrients (Potential by use)
• Heavy Metals (Anticipated)
• Organic Compounds (Anticipated)
• Trash & Debris (Anticipated)
• Oxygen Demanding Substances (Potential by use)
• Oil & Grease (Anticipated)
• Pesticides (Potential by use)
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The other pollutants of concern have not been accounted for because the
project landscaping will be designed and installed per the City of Carlsbad
Landscape Manual and will incorporate efficient irrigation, pest resistant
species, and incorporate drought resistant native species of planting.
5.4.4 STRUCTURAL TREATMENT CONTROL BMP
SELECTION
Based on the pollutants of concern present from the project site and the
removal efficiencies listed in Table 4: Structural Treatment Control BMP
Selection Matrix, the Structural Treatment Control BMP with the most
efficient removal efficiencies for the project are as follows (listed most to
least efficient):
• Vegetated Swale
• Bioretention (see Appendix J for sizing calculations)
• Filter Inserts
• Pervious Pavement
Based on the above mentioned removal efficiencies and limited space on
site, the project shall incorporate a combination of biofilters and drainage
inserts on site. Higher removal efficient structural treatment control
devices could not be used on site due to the limited space to accommodate
their proper design and implementation.
5.4.5 TREATMENT CONTROL BMP INFORMATION
Based on the above mentioned removal efficiencies, the project shall
incorporate Suntree Technologies Inlet Basket System ( or approved
equivalent) and Bioclean Environmental Services, Inc ( or approved
equivalent) products in the proposed locations shown on the Preliminary
Storm Water Management Plan Exhibit in Appendix J.
5.4.6 STRUCTURAL TREATMENT LIMITED EXCLUSIONS
No Structural Treatment Limited Exclusions apply to this project as
defined in the City Standards.
5.5 PERMANENT BMPS APPLICABLE TO THE PROJECT SITE
Based on the above findings, the following permanent BMPs will be established
for the project:
Site Design BMPs:
• Maintain Pre-Development Rainfall Runoff Characteristics
o Minimize Impervious Footprint
o Minimize Directly Connected Impervious Areas
o Maximize Canopy Interception and Water Conservation
Consistent with the Carlsbad Landscape Manual
• Protect Slopes and Channels
o Convey Runoff Safely From Tops of Slopes
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PRELIMINARY STORM WATER MANAGEMENT PLAN
ROBERTSON RANCH, P.A. 22
o Vegetate Slopes with Natural or Drought Tolerant Vegetation
• Vegetated Swales (LID)
Source Control BMPs:
• Design Trash Storage Areas to Reduce Pollution Introduction
• Use Efficient Irrigation Systems and Landscape Design
• Provide Storm Water Conveyance System Stenciling and Signage
Applicable to Individual Priority Projects
• Surface Parking Areas
Treatment Control BMPs:
• Suntree Technologies Catch Basin Insert
• Bioclean Environmental Services, Inc Bio-Sorb
6.0 CONSTRUCTION STORM WATER BMPS
Based on the size of the project site the City of Carlsbad will require that a separate
document be prepared. If the project site is less then one acre then the project will
require a Water Pollution Control Plan. If the project site is one acre of more then a
Storm Water Pollution Prevention Plan will be required. Since the project has a disturbed
area more than one acre a SWPPP will be required. Construction BMPs and any
applicable design standards can be found in this separate document.
7.0 IMPLEMENTATION & MAINTENANCE REQUIREMENTS
Once the City of Carlsbad approves all the project BMPs, the applicants and the City
Project Managers must ensure proper implementation of the selected BMPs. In order to
accomplish effective implementation and maintenance the City of Carlsbad may require
that some sort of Permanent BMP Maintenance Agreement be entered into. This will be
at the discretion of the City of Carlsbad Staff. In the event that a Permanent BMP
Maintenance Agreement is required by City Staff, the following items will need to be
addressed:
7.1 OPERATION AND MAINTENANCE PLAN
The applicant shall include and Operation & Maintenance Plan (O&M), prepared
satisfactory to the City, with the approved maintenance agreement, which
describes the designated responsible party to mange the storm water BMPs,
employee's training program and duties, operating schedule, maintenance
frequency, routine service schedule, specific maintenance activities (including
maintenance of storm water conveyance system stamps), copies of resource
agency permits, and any other necessary activities. At a minimum, maintenance
agreements shall require the applicant to provide inspection and servicing·of all
permanent treatment BMPs on an annual basis. The project proponent or City-
approved maintenance entity shall complete and maintain O&M forms to
document all maintenance requirements. Parties responsible for the O&M plan
shall retain records for at least 5 years. These documents shall be made available
to the City inspector upon request at any time. In addition, CASQA Section 6:
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PRELIMINARY STORM WATER MANAGEMENT PLAN
ROBERTSON RANCH, P.A. 22
Long-term Maintenance of BMPs shall be utilized as guide. A copy of this is
provided in Appendix H.
7.2 ACCESS EASEMENT/AGREEMENT
If a permanent BMP requires access for maintenance, as part of the O&M plan,
the applicant shall execute and record an access easement or agreement that shall
be binding on the under lying land throughout the life of the project in favor of the
party responsible for maintenance, until such time that the permanent treatment
BMP requiring access for maintenance is removed or replaced. The City shall
approve any changes to the permanent BMPs, O&M plans, or access agreements.
The agreement shall include a provision that gives the City the right, but not the
obligation to perform the maintenance. The party responsible for BMP
maintenance will pay the City for any and all costs uncured by the City for
maintaining any BMPs. The agreement will provide a cost recovery provision in
favor of the City satisfactory to the City Attorney.
8.0 APPENDICES:
The following Appendices are included:
Appendix A: Vicinity Map
Appendix B: Storm Water Standards Questionnaire
Appendix C: City of Carlsbad Standards Excerpts
• Table 1: Standard Development Project & Priority Project Storm
Water BMP Requirements Matrix
• Table 2: Anticipated and Potential Pollutants Generated by Land Use
Type
• Table 3: Numeric Sizing Treatment Standards
• Table 4: Structural Treatment Control BMP Selection Matrix
• Appendix B: Draft Environmentally Sensitive Areas Within the City
of Carlsbad Map
Appendix D: Site Drainage Study
• Preliminary Drainage Study for PA 22 by O'Day Consultants, Inc date
December 18, 2008.
Appendix E: Beneficial Uses of Receiving Waters
Appendix F: 2006 CW A Section 303( d) List of Water Quality Limited Segments
Appendix G: Source Control BMP Fact Sheets
Carlsbad Standard Drawing GS-16: Refuse Bin Enclosures
CASQA SD-10: Site Design and Landscape Planning
CASQA SD-11: Roof Runoff Controls
CASQA SD-12: Efficient Irrigation
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PREL~MINARY STORM WATER MANAGEMENT PLAN
ROBERTSON RANCH, P.A. 22
CASQA SD-13: Strom Drain Stenciling
CASQA SD-32: Trash Enclosures
Appendix H: Treatment Control BMP Fact Sheets
CASQA MP-52: Drain Inserts
CASQA TC-30: Vegetated Swales
CASQA TC-60: Multiple System Fact Sheet
CASQA SD-20: Pervious Pavement
CASQA TC-32: Bioretention
Section 6: Long-term Maintenance of BMPs
Appendix I: Applicable Manufacturer's BMP Information
Appendix J: Map Exhibits
• PA 22 Preliminary Storm Water Management Plan BMPs Exhibit
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I SECTION 1
NEW DEVELOPMENT
PRIORITY PROJECT TYPE YES NO Does you project meet one or more of the following criteria:
1. Home subdivision of 100 units or more. v Includes SFD, MFD, Condominium and Apartments
2. Residential development of 10 units or more. v Includes SFD, MFD, Condominium and Apartments
3. Commercial and industrial development greater than 100,000 square feet including parking areas.
Any development on private land that is not for heavy industrial or residential uses. Example: Hospitals,
Hotels, Recreational Facilities, Shopping Malls, etc. ✓
4. Heavy Industrial I Industry greater than 1 acre (NEED SIC CODES FOR PERMIT BUSINESS TYPES) v SIC codes 5013, 5014, 5541, 7532-7534, and 7536-7539
5. Automotive repair shop. V SIC codes 5013, 5014, 5541, 7532-7534, and 7536-7539
6. A New Restaurant where the land area of development is 5,000 square feet or more including parking ✓ areas.
SIC code 5812
7. Hillside development ✓ (1) greater than 5,000 square feet of impervious surface area and (2) development will grade on any
natural slope that is 25% or Qreater
8. Environmentally Sensitive Area (ESA). ✓ Impervious surface of 2,500 square feet or more located within, "directly adjacent"2 to (within 200 feet),
or "discharQinQ directly to"3 receivinQ water within the ESA 1
9. Parking lot. / Area of 5,000 square feet or more, or with 15 or more parking spaces, and potentially exposed to urban
runoff
10. Retail Gasoline Outlets -serving more than 100 vehicles per da'i. ✓ Serving more than 100 vehicles per day and greater than 5,000 square feet
11. Streets, roads, driveways, highways, and freeways. v Project would create a new paved surface that is 5,000 square feet or greater.
12. Coastal Development Zone. v Within 200 feet of the Pacific Ocean and (1) creates more than 2500 square feet of impermeable
surface or (2) increases impermeable surface on property by more than 10%. .
1 Environmentally Sensitive Areas include but are not limited to all Clean Water Act Section 303(d) impaired water bodies;
areas designated as Areas of Special Biological Significance by the State Water Resources Control Board (Water Quality
Control Plan for the San Diego Basin (1994) and amendments); water bodies designated with the RARE beneficial use by
the State Water Resources Control Board (Water Quality Control Plan for the San Diego Basin (1994) and amendments);
areas designated as preserves or their equivalent under the Multi Species Conservation Program within the Cities and Count
of San Diego; and any other equivalent environmentally sensitive areas which have been identified by the Copermittees.
2 "Directly adjacent" means situated within 200 feet of the environmentally sensitive area.
3 "Discharging directly to" means outflow from a drainage conveyance system that is composed entirely of flows from the
subject development or redevelopment site, and not commingled with flow from adjacent lands.
Section 1 Results:
If you answered YES to ANY of the questions above you have a PRIORITY project and PRIORITY project requirements DO
apply. A Storm Water Management Plan, prepared in accordance with City Storm Water Standards, must be submitted at
time of application. Please check the "MEETS PRIORITY REQUIREMENTS" box in Section 3.
If you answered NO to ALL of the questions above, then you are a NON-PRIORITY project and STANDARD requirements
apply. Please check the "DOES NOT MEET PRIORITY Requirements" box in Section 3.
SWMP Rev 6/4/08
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/ SECTION 2
SIGNIFICANT REDEVELOPMENT: YES NO
1. Is the project redeveloping an existing priority project type? (Priority projects
are defined in Section 1)
If you answered YES, please proceed to question 2.
If you answered NO, then you ARE NOT a significant redevelopment and you ARE NOT subject to
PRIORITY project requirements, only STANDARD requirements. Please check the "DOES NOT MEET
PRIORITY Requirements" box in Section 3 below.
2. Is the project solely limited to one of the following:
a. Trenching and resurfacing associated with utility work?
b. Resurfacinq and reconfiqurinq existinq surface parkinq lots?
C. New sidewalk construction, pedestrian ramps, or bike lane on public
and/or private existinq roads?
d. Replacement of existing damaged pavement?
If you answered NO to ALL of the questions, then proceed to Question 3.
If you answered YES to ONE OR MORE of the questions then you ARE NOT a significant redevelopment
and you ARE NOT subject to PRIORITY project requirements, only STANDARD requirements. Please
check
the "DOES NOT MEET PRIORITY Requirements" box in Section 3 below.
3. Will the development create, replace, or add at least 5,000 square feet of
impervious surfaces on an existing development or, be located within 200
feet of the Pacific Ocean and (1 )create more than 2500 square feet of
impermeable surface or (2) increases impermeable surface on property by
more than 10%?
If you answered YES, you ARE a significant redevelopment, and you ARE subject to PRIORITY project
requirements. Please check the "MEETS PRIORITY REQUIREMENTS" box in Section 3 below.
If you answered NO, you ARE NOT a significant redevelopment, and you ARE NOT subject to
PRIORITY project requirements, only STANDARD requirements. Please check the "DOES NOT MEET
PRIORITY Requirements" box in Section 3 below.
I SECTION 3
Questionnaire Results:
✓ MY PROJECT MEETS PRIORITY REQUIREMENTS, MUST COMPLY WITH PRIORITY
PROJECT STANDARDS AND MUST PREPARE A STORM WATER MANAGEMENT PLAN FOR
SUBMITTAL AT TIME OF APPLICATION.
□ MY PROJECT DOES NOT MEET PRIORITY REQUIREMENTS AND MUST ONLY COMPLY
WITH STANDARD STORM WATER REQUIREMENTS.
Applicant Information and Signature Box This Box for City Use Only
Address: Assessors Parcel Number(s):
.CANNON 'g.D. /68-0StJ-S5 4-JG8-3tf,()-03
Applicant Name: , Applicant Title:
City Concurrence: I YES I
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By.
Date:
Applicant Signature: Date: Project ID:
NO
SWMP Rev 6/4/08
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PRELIMINARY STORM WATER MANAGEMENT PLAN
ROBERTSON RANCH, PA 22
APPENDIX C: CITY OF CARLSBAD STANDARDS EXCERPTS
Table 1: Standard Development Project & Priority Project Storm Water BMP
Requirements Matrix
BMPs Applicable to Individual
Priority Proiect Catef(ories (3)
<;;; Q)
8 "' ci/l "' "' "' ~ "' "' ;;,-, ~ "' oJ) "' "' oJ) ~ "" "' "" c Q) -~ ·a Q) g -< ..c: :> oJ) "' ·c: ~ "' "' u
"' ..c: "' Q) ~ "' Q ~ u "' 'O Q) "' 8 "' 'O "' .i "' u "' & ~ § 0 "' § ~ ~ p.. 1Z •.p ~ ...:I c c ...
~ Q) ~ Q) s 0 Q) c Q) 0 u 'O :'9 bJ) u oJ) Site Source "' 'i'.l c :.2 .9' ~ 'tl i ·;;; Treatment :> "' c ·«i ::, ~ 0 Q) ·c: ~:;;a ~ rE' ::, &: Design Control p.. . ;J Q > 0 Cl'.l Control
BMPs(l) BMPs(2) oJ ..0 p.. u 'O ,; ...... bii ..d ·-...:., BMPs(4)
Standard Projects R R 0 0 0 0 0 0 0 0 0 0 0
Priority Projects:
Detached
Residential R R R R R s
Development
Attached
Residential R R R s
Development
Commercial
Development
> 100,000 ft2
R R R R R R s
Automotive R R R R R R R s Repair
Restaurants R R R R s
Hillside
Development R R R R s
<5, 000 ft2
Parking Lots R R R(5) s
Streets,
Highways, & R R s
Freeways
R = Required; select one or more applicable and appropriate BMPs from the applicable steps in Section III.2.a-d, or
equivalent as identified in Appendix C.
0 = Optional or may be required by City Staff. As appropriate, applicants are encouraged to incorporate treatment
control BMPs and BMPs applicable to individual priority project categories into the project design. City staff may
require one or more of these BMPs, where applicable.
S = Select one or more applicable and appropriate treatment control BMPs from Appendix C.
(1) Refer to Section III.2.A.
(2) Refer to Section III.2.B.
(3)
(4)
(5)
Priority Project categories must apply specific storm water BMP requirements, where applicable. Priority projects
are subject to the requirements of all priority project categories that apply.
Refer to Section III.2.D.
Applies if the paved area totals >5,000 square feet or with> 15 parkign spaces and is potentially exposed to urban
run-off.
Source: City of Carlsbad Public Works Department Standard Urban Storm Water Mitigation
Plan Storm Water Standards (2008, page 6)
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PRELIMINARY STORM WATER MANAGEMENT PLAN
ROBERTSON RANCH, PA 22
APPENDIX C: CITY OF CARLSBAD STANDARDS EXCERPTS
Table 2: Anticipated and Potential Pollutants Generated by Land Use Type
General Pollutant Catel(ories
Project
Categories Sediments Nutrients Heavy Organic Trash Oxygen Oil Bacteria Pesticides
Metals Compounds & Demanding & &
Debris Substances Grease Viruses
Detached
Residential X X X X X X
Development
Attached
Residential X X X p(l) p(2) pCI)
Development
Commercial
Development
> 100,000 ft2
pCll p(l) p(2) X p(5) X p(3)
Automotive X xC4)(5J X X
Repair
Restaurants X X X X
Hillside
Development X X X X X
<5, 000 ft2
Parking Lots p(l) p(l) X p(l) X
Streets,
Highways, & X p(l)X X X(4) X p(5) X
Freeways
Notes:
X = Anticipated
P = Potential
(1) A potential pollutant if landscaping exists on-site.
(2) A potential pollutant if the project includes uncovered parking areas.
(3) A potential pollutant is land use involves food or animal waste areas.
(4) Including petroleum hydrocarbons.
(5) Including solvents.
.. Source: City of Carlsbad Public Works Department Standard Urban Storm Water M1t1gation
Plan Storm Water Standards (2008, page 8)
X
X
p(5)
X
pCll
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PRELIMINARY STORM WATER MANAGEMENT PLAN
ROBERTSON RANCH, PA 22
APPENDIX C: CITY OF CARLSBAD STANDARDS EXCERPTS
Table 3: Numeric Sizing Treatment Standards
Volume
1. Volume-based BMPs shall be designed to mitigate (infiltrate, filter, or treat) the volume of run-off
produced from a 24-Hour 85th percentile storm event, as determined from Isopluvial maps contained in
the County of San Diego Hydrology Manual.
OR
Flow
2. Flow-based BMPs shall be designed to mitigate (infiltrate, filter, or treat) the maximum flow rate of
run-off produced from a rainfall intensity of 0.2 inch of rainfall per hour for each hour of a storm
event.
Source: City of Carlsbad Public Works Department Standard Urban Storm Water
Mitigation Plan Storm Water Standards (2008, page 14)
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PRELIMINARY STORM WATER MANAGEMENT PLAN
ROBERTSON RANCH, PA 22
APPENDIX C: CITY OF CARLSBAD STANDARDS EXCERPTS
Table 4: Structural Treatment Control BMP Selection Matrix
Treatment Control BMP Catef(ories
Pollutant
Of Biofilters Detention Infiltration Wet Ponds Drainage
Concern Basins Basins (Il or Inserts
Wetlands
Sediment M H H H L
Nutrient L M M M L
Heavy M M M H L
Metals
Organic u u u u L
Compounds
Trash & L H u u M
Debris
Oxygen L M M M L
Demanding
Substances
Bacteria u u H u L
Oil & M M u u L
Grease
Pesticides u u u u L
Notes:
(1) Including trenches and porous pavement.
(2) Also known as hydrodynamic devices and baffle boxes
L: Low removal efficiency
M: Medium removal efficiency
H: High removal efficiency
U: Unknown removal efficiency
Filtration Hydrodynamic
Separator
Systems <2l
H M
M L
H L
M L
H M
M L
M L
H L
u L
Sources: Guidance Specifying Management Measures for Sources of Nonpoint Pollution in Coastal Waters ( 1993 ),
National Stormwater Best Management Practices Database (2001), and Guide for BMP Selection in Urban
Development Areas (2001 ).
Source: City of Carlsbad Public Works Department Standard Urban Storm Water Mitigation
Plan Storm Water Standards (2008, page 15)
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PRELIMINARY STORM WATER MANAGEMENT PLAN
ROBERTSON RANCH, PA 22
APPENDIX D: DRAINAGE STUDY
Please see attached.
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PRELIMINARY STORM WATER MANAGEMENT PLAN
ROBERTSON RANCH, PA 22
APPENDIX E: BENEFICIAL USES OF COASTAL WATERS
Please see attached.
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PRELIMINARY STORM WATER MANAGEMENT PLAN
ROBERTSON RANCH, PA 22
APPENDIX F: 2006 CWA SECTION 303(d) LIST OF WATER QUALITY
LIMITED SEDIMENT
Please see attached.
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PRELIMINARY STORM WATER MANAGEMENT PLAN
ROBERTSON RANCH, PA 22
APPENDIX G: SOURCE CONTROL BMP FACT SHEETS
Please see attached.
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SD-10 Site Design & Landscape Planning
Designing New Installations
Begin the development of a plan for the landscape unit with attention to the following general
principles:
■ Formulate the plan on the basis of clearly articulated community goals. Carefully identify
conflicts and choices between retaining and protecting desired resources and community
growth.
■ Map and assess land suitability for urban uses. Include the following landscape features in
the assessment: wooded land, open unwooded land, steep slopes, erosion-prone soils,
foundation suitability, soil suitability for waste disposal, aquifers, aquifer recharge areas,
wetlands, floodplains, surface waters, agricultural lands, and various categories of urban
land use. When appropriate, the assessment can highlight outstanding local or regional
resources that the community determines should be protected ( e.g., a scenic area,
recreational area, threatened species habitat, farmland, fish run). Mapping and assessment
should recognize not only these resomces but also additional areas needed for their
sustenance.
Project plan designs should conserve natural areas to the extent possible, maximize natural
water storage and infiltration oppo1tunities, and protect slopes and channels.
Conserve Natural Areas during Landscape Planning
If applicable, the following items are required and must be implemented in the site layout
dming the subdivision design and approval process, consistent with applicable General Plan and
Local Area Plan policies:
• Cluster development on least-sensitive portions of a site while leaving the remaining land in
a natural undisturbed condition.
■ Limit clearing and grading of native vegetation at a site to the minimum amount needed to
build lots, allow access, and provide fire protection.
■ Maximize trees and other vegetation at each site by planting additional vegetation, clustering
tree areas, and promoting the use of native and/ or drought tolerant plants.
■ Promote natural vegetation by using parking lot islands and other landscaped areas.
■ Preserve riparian areas and wetlands.
Ma,Yimize Natural Water Storage and Infiltration Opportunities Within the Landscape Unit
■ Promote the conservation of forest cover. Building on land that is already deforested affects
basin hydrology to a lesser extent than converting forested land. Loss of forest cover reduces
interception storage, detention in the organic forest floor layer, and water losses by
evapotranspiration, resulting in large peak runoff increases and either their negative effects
or the expense of countering them with structural solutions.
■ Maintain natural storage reservoirs and drainage corridors, including depressions, areas of
permeable soils, swales, and intermittent streams. Develop and implement policies and
2 of4 California Stormwater BMP Handbook
New Development and Redevelopment
www.cabmphandbooks.com
January 2003
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Site Design & Landscape Planning SD-10
regulations to discourage the cleating, filling, and channelization of these features. Utilize
them in drainage networks in preference to pipes, culverts, and engineered ditches.
■ Evaluating infiltration opportunities by referring to the stormwater management manual for
the jurisdiction and pay pa1ticular attention to the selection criteria for avoiding
groundwater contamination, poor soils, and hych·ogeological conditions that cause these
facilities to fail. If necessary, locate developments ·with large amounts of impervious
smfaces or a potential to produce relatively contaminated runoff away from groundwater
recharge areas.
Protection of Slopes and Channels during Landscape Design
■ Convey runoff safely from the tops of slopes.
■ Avoid disturbing steep or unstable slopes.
• Avoid disturbing natural channels.
■ Stabilize disturbed slopes as quickly as possible.
■ Vegetate slopes with native or drought tolerant vegetation.
■ Control and treat flows in landscaping and/ or other controls prior to reaching existing
natural drainage systems.
■ Stabilize temporary and permanent channel crossings as quickly as possible, and ensure that
increases in run-off velocity and frequency caused by the project do not erode the channel.
■ Install energy dissipaters, such as riprap, at the outlets of new storm drains, culve1ts,
conduits, or channels that enter unlined channels in accordance with applicable
specifications to minimize erosion. Energy dissipaters shall be installed in such a way as to
minimize impacts to receiving waters.
■ Line on-site conveyance channels where appropriate, to reduce erosion caused by increased
flow velocity due to increases in tributary impervious area. The first choice for linings
should be grass or some other vegetative surface, since these materials not only reduce
runoff velocities, but also provide water quality benefits from filtration and infiltration. If
velocities in the channel are high enough to erode grass or other vegetative linings, riprap,
concrete, soil cement, or geo-grid stabilization are other alternatives.
■ Consider other design principles that are comparable and equally effective.
Redeveloping Existing Installations
Various jurisdictional stormwater management mid mitigation plans (SUSMP, WQMP, etc.)
define "redevelopment" in terms of amounts of additional impervious area, increases in gross
floor area and/or exterior construction, and land disturbing activities with structural or
impervious smfaces. The definition of" redevelopment" must be consulted to determine
whether or not the requirements for new development apply to areas intended for
redevelopment. If the definition applies, the steps outlined under "designing new installations"
above should be followed.
Janua1y 2003 California Stormwater BMP Handbook
New Development and Redevelopment
www.cabmphandbooks.com
3 of 4
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SD-10 Site Design & Landscape Planning
Redevelopment may present significant opportunity to add features which had not previously
been implemented. Examples include incorporation of depressions, areas of permeable soils,
and swales in newly redeveloped areas. While some site constraints may exist due to the status
of already existing infrash·ucture, opportunities should not be missed to maximize infiltration,
slow runoff, reduce impervious areas, disconnect directly connected impervious areas.
Other Resources
A Manual for the Standard Urban Stonuwater Mitigation Plan (SUSMP), Los Angeles County
Department of Public Works, May 2002.
Stormwater Management Manual for Western Washington, Washington State Department of
Ecology, August 2001.
Model Standard Urban Storm Water Mitigation Plan (SUSMP) for San Diego County, Port of
San Diego, and Cities in San Diego County, February 14, 2002.
Model Water Quality Management Plan (WQMP) for County of Orange, Orange County Flood
Control District, and the Incorporated Cities of Orange County, Draft Feb111ary 2003.
Ventura Countywide Technical Guidance Manual for Stormwater Quality Control Measures,
July 2002.
4 of4 California Stormwater BMP Handbook
New Development and Redevelopment
www.cabmphandbooks.com
January 2003
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SD-11 Roof Runoff Controls
barrels needed is a function of the rooftop area. Some low impact developers recommend that
every house have at least 2 rain barrels, with a minimum storage capacity of 1000 liters. Roof
barrels serve several purposes including mitigating the first flush from the roof which has a high
volume, amount of contaminants, and thermal load. Several types of rain barrels are
commercially available. Consideration must be given to selecting rain barrels that are vector
proof and childproof. In addition, some barrels are designed with a bypass valve that filters out
grit and other contaminants and routes overflow to a soak-away pit or rain garden.
If the cistern has an operable valve, the valve can be closed to store storm water for irrigation or
infiltration between storms. This system requires continual monitoring by the resident or
grounds crews, but provides greater flexibility in water storage and metering. If a cistern is
provided with an operable valve and water is stored inside for long periods, the cistern must be
covered to prevent mosquitoes from breeding.
A cistern system with a permanently open outlet can also provide for metering stormwater
runoff. If the cistern outlet is significantly smaller than the size of the downspout inlet (say ¼ to
½ inch diameter), runoff will build up inside the cistern during storms, and will empty out
slowly after peak intensities subside. This is a feasible way to mitigate the peak flow increases
caused by rooftop impervious land coverage, especially for the frequent, small storms.
Dry wells and Infiltration Trenches
Roof downspouts can be directed to dry wells or infiltration trenches. A dry well is constructed
by excavating a hole in the ground and filling it with an open graded aggregate, and allowing the
water to fill the dry well and infiltrate after the storm event. An underground connection from
the downspout conveys water into the dry well, allowing it to be stored in the voids. To
minimize sedimentation from lateral soil movement, the sides and top of the stone storage
matrix can be wrapped in a permeable filter fabric, though the bottom may remain open. A
perforated observation pipe can be inserted vertically into the dry well to allow for inspection
and maintenance.
In practice, dry wells receiving runoff from single roof downspouts have been successful over
long periods because they contain very little sediment. They must be sized according to the
amount of rooftop runoff received, but are typically 4 to 5 feet square, and 2 to 3 feet deep, with
a minimum of 1-foot soil cover over the top (maximum depth of 10 feet).
To protect the foundation, dry wells must be set away from the building at least 10 feet. They
must be installed in solids that accommodate infiltration. In poorly drained soils, dry wells have
very limited feasibility.
Infiltration trenches function in a similar manner and would be particularly effective for larger
roof areas. An infiltration trench is a long, narrow, rock-filled trench with no outlet that receives
stormwater runoff. These are described under Treatment Controls.
Pop-up Drainage Emitter
Roof downspouts can be directed to an underground pipe that daylights some distance from the
building foundation, releasing the roof runoff through a pop-up emitter. Similar to a pop-up
irrigation head, the emitter only opens when there is flow from the roof. The emitter remains
flush to the ground during dry periods, for ease of lawn or landscape maintenance.
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Roof Runoff Controls SD-11
Foundation Planting
Landscape planting can be provided around the base to allow increased opportunities for
stormwater infiltration and protect the soil from erosion caused by concentrated sheet flow
coming off the roof. Foundation plantings can reduce the physical impact of water on the soil
and provide a subsurface matrix of roots that encourage infiltration. These plantings must be
sturdy enough to tolerate the heavy runoff sheet flows, and periodic soil saturation.
Redeveloping Existing Installations
Various jurisdictional stormwater management and mitigation plans (SUSMP, WQMP, etc.)
define "redevelopment" in terms of amounts of additional impervious area, increases in gross
floor area and/ or exterior construction, and land disturbing activities with structural or
impervious surfaces. The definition of" redevelopment" must be consulted to determine
whether or not the requirements for new development apply to areas intended for
redevelopment. If the definition applies, the steps outlined under "designing new installations"
above should be followed.
Supplemental Information
Examples
■ City of Ottawa's Water Links Surface -Water Quality Protection Program
■ City of Toronto Downspout Disconnection Program
■ City of Boston, MA, Rain Barrel Demonstration Program
Other Resources
Hager, Marty Catherine, Stormwater, "Low-Impact Development", January/February 2003.
www.stormh2o.com
Low Impact Urban Design Tools, Low Impact Development Design Center, Beltsville, MD.
v-.rv,,rw.lid-stormwater.net
Start at the Source, Bay Area Stormwater Management Agencies Association, 1999 Edition
January 2003 California Stormwater BMP Handbook
New Development and Redevelopment
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SD-12 Efficient Irrigation
■ Design timing and application methods of irrigation water to minimize the runoff of excess
irrigation water into the storm water drainage system.
■ Group plants with similar water requirements in order to reduce excess irrigation runoff and
promote surface filtration. Choose plants with low irrigation requirements (for example,
native or drought tolerant species). Consider design features such as:
Using mulches (such as wood chips or bar) in planter areas without ground cover to
minimize sediment in runoff
Installing appropriate plant materials for the location, in accordance with amount of
sunlight and climate, and use native plant materials where possible and/ or as
recommended by the landscape architect
Leaving a vegetative barrier along the property boundary and interior watercourses, to
act as a pollutant filter, where appropriate and feasible
Choosing plants that minimize or eliminate the use of fertilizer or pesticides to sustain
growth
■ Employ other comparable, equally effective methods to reduce irrigation water runoff.
Redeveloping Existing Installations
Various jurisdictional stormwater management and mitigation plans (SUSMP, WQMP, etc.)
define "redevelopment" in terms of amounts of additional impervious area, increases in gross
floor area and/ or exterior construction, and land disturbing activities with structural or
impervious surfaces. The definition of " redevelopment" must be consulted to determine
whether or not the requirements for new development apply to areas intended for
redevelopment. If the definition applies, the steps outlined under "designing new installations"
above should be followed.
Other Resources
A Manual for the Standard Urban Stormwater Mitigation Plan (SUSMP), Los Angeles County
Department of Public Works, May 2002.
Model-Standard Urban Storm Water Mitigation Plan (SUSMP) for San Diego County, Port of
San Diego, and Cities in San Diego County, February 14, 2002.
Model Water Quality Management Plan (WQMP) for County of Orange, Orange County Flood
Control District, and the Incorporated Cities of Orange County, Draft February 2003.
Ventura Countywide Technical Guidance Manual for Stormwater Quality Control Measures,
July 2002.
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SD-13 Storm Drain Signage
DRAINS TO OCEAN" and/or other graphical icons to discourage illegal dumping.
■ Post signs with prohibitive language and/or graphical icons, which prohibit illegal dumping
at public access points along channels and creeks within the project area.
Note -Some local agencies have approved specific signage and/or storm drain message placards
for use. Consult local agency storm water staff to determine specific requirements for placard
types and methods of application.
Redeveloping Existing Installations
Various jurisdictional stormwater management and mitigation plans (SUSMP, WQMP, etc.)
define "redevelopment" in terms of amounts of additional impervious area, increases in gross
floor area and/or exterior construction, and land disturbing activities with structural or
impervious surfaces. If the project meets the definition of "redevelopment", then the
requirements stated under " designing new installations" above should be included in all project
design plans.
Additional Information
Maintenance Considerations
■ Legibility of markers and signs should be maintained. If required by the agency with
jurisdiction over the project, the owner/operator or homeowner's association should enter
into a maintenance agreement with the agency or record a deed restriction upon the
property title to maintai~ the legibility of placards or signs.
Placement
■ Signage on top of curbs tends to weather and fade.
■ Signage on face of curbs tends to be worn by contact with vehicle tires and sweeper brooms.
Supplemental Information
Examples
■ Most MS4 programs have storm drain signage programs. Some MS4 programs will provide
stencils, or arrange for volunteers to stencil storm drains as part of their outreach program.
Other Resources
A Manual for the Standard Urban Stormwater Mitigation Plan (SUSMP), Los Angeles County
Department of Public Works, May 2002.
Model Standard Urban Storm Water Mitigation Plan (SUSMP) for San Diego County, Port of
San Diego, and Cities in San Diego County, February 14, 2002.
Model Water Quality Management Plan (WQMP) for County of Orange, Orange County Flood
Control District, and the Incorporated Cities of Orange County, Draft February 2003.
Ventura Countywide Technical Guidance Manual for Stormwater Quality Control Measures,
July 2002.
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SD-32 Trash Storage Areas
■ Use lined bins or dumpsters to reduce leaking ofliquid waste.
■ Provide roofs, awnings, or attached lids on all trash containers to minimize direct
precipitation and prevent rainfall from entering containers.
■ Pave trash storage areas with an impervious surface to mitigate spills.
■ Do not locate storm drains in immediate vicinity of the trash storage area.
■ Post signs on all dumpsters informing users that hazardous materials are not to be disposed
of therein.
Redeveloping Existing Installations
Various jurisdictional stormwater management and mitigation plans (SUSMP, WQMP, etc.)
define "redevelopment" in terms of amounts of additional impervious area, increases in gross
floor area and/ or exterior construction, and land disturbing activities with structural or
impervious surfaces. The definition of " redevelopment" must be consulted to determine
whether or not the requirements for new development apply to areas intended for
redevelopment. If the definition applies, the steps outlined under "designing new installations"
above should be followed.
Additional Information
Maintenance Considerations
The integrity of structural elements that are subject to damage (i.e., screens, covers, and signs)
must be maintained by the owner/ operator. Maintenance agreements between the local agency
and the owner/ operator may be required. Some agencies will require maintenance deed
restrictions to be recorded of the property title. If required by the local agency, maintenance
agreements or deed restrictions must be executed by the owner/ operator before improvement
plans are approved.
Other Resources
A Manual for the Standard Urban Stormwater Mitigation Plan (SUSMP), Los Angeles County
Department of Public Works, May 2002.
Model Standard Urban Storm Water Mitigation Plan (SUSMP) for San Diego County, Port of
San Diego, and Cities in San Diego County, February 14, 2002.
Model Water Quality Management Plan (WQMP) for County of Orange, Orange County Flood
Control District, and the Incorporated Cities of Orange County, Draft February 2003.
Ventura Countywide Technical Guidance Manual for Stormwater Quality Control Measures,
July 2002.
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PRELIMINARY STORM WATER MANAGEMENT PLAN
ROBERTSON RANCH, PA 22
APPENDIX H: TREATMENT CONTROL BMP FACT SHEETS
Please see attached.
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MP-52 Drain Inserts
one box; that is, the setting area and filtration through media occurs in the same box. One
manufacturer has a double-box. Stormwater enters the first box where setting occurs. The
stormwater flows into the second box where the filter media is located. Some products consist
of one or more trays or mesh grates. The trays can hold different types of media. Filtration
media vary with the manufacturer: types include polypropylene, porous polymer, treated
cellulose, and activated carbon.
Construction/Inspection Considerations
Be certain that installation is done in a manner that makes certain that the stormwater enters
the unit and does not leak around the perimeter. Leakage between the frame of the insert and
the frame of the drain inlet can easily occur with vertical (drop) inlets.
Performance
Few products have performance data collected under field conditions.
Siting Criteria
It is recommended that inserts be used only for retrofit situations or as pretreatment where
other treatment BMPs presented in this section area used.
Additional Design Guidelines
Follow guidelines provided by indivip.ual manufacturers.
Maintenance I Likely require frequent maintenance, on the order of several times per year.
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Cost
■ The initial cost of individual inserts ranges from less than $100 to about $2,000. The cost of
using multiple units in curb inlet drains varies with the size of the inlet.
■ The low cost of inserts may tend to favor the use of these systems over other, more effective
treatment BMPs. However, the low cost of each unit may be offset by the number of units
that are required, more frequent maintenance, and the shorter structural life (and therefore
replacement).
References and Sources of Additional Information
Hrachovec, R., and G. Minton, 2001, Field testing of a sock-type catch basin insert, Planet CPR,
Seattle, Washington
Interagency Catch Basin Insert Committee, Evaluation of Commercially-Available Catch Basin
Inserts for the Treatment of Stormwater Runoff from Developed Sites, 1995
Larry Walker Associates, June 1998, NDMP Inlet/In-Line Control Measure Study Report
Manufacturers literature
Santa Monica (City), Santa Monica Bay Municipal Stormwater/Urban Runoff Project -
Evaluation of Potential Catch basin Retrofits, Woodward Clyde, September 24, 1998
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Drain Inserts MP-52
Woodward Clyde, June 11, 1996, Parking Lot Monitoring Report, Santa Clara Valley Nonpoint
Source Pollution Control Program.
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TC-30 Vegetated Swale
■ Roadside ditches should be regarded as significant potential swalefbuffer strip sites and
should be utilized for this purpose whenever possible.
Limitations
■ Can be difficult to avoid channelization.
■ May not be appropriate for industrial sites or locations ·where spills may occur
■ Grassed swales cannot treat a very large drainage area. Large areas may be divided and
treated using multiple swales.
■ A thick vegetative cover is needed for these practices to function properly.
■ They are impractical in areas with steep topography.
■ TI1ey are not effective and may even erode when flow velocities are high, if the grass cover is
not properly maintained.
■ In some places, their use is restricted by law: many local municipalities require curb and
gutter systems in residential areas.
■ Swales are mores susceptible to failure if not properly maintained than other treatment
BMPs.
Design and Sizing Guidelines
■ Flow rate based design determined by local requirements or sized so that 85% of the annual
runoff volume is discharged at less than the design rainfall intensity.
■ Swale should be designed so that the water level does not exceed 2/ 3rds the height of the
grass or 4 inches, which ever is less, at the design treatment rate.
■ Longitudinal slopes should not exceed 2.5%
■ Trapezoidal channels are nonnally recommended but other configurations, such as
parabolic, can also provide substantial water quality improvement and may be easier to mow
than designs with sharp breaks in slope.
■ Swales constructed in cut are preferred, or in fill areas that are far enough from an adjacent
slope to minimize the potential for gopher damage. Do not use side slopes constructed of
fill, which are prone to structural damage by gophers and other burrowing animals.
■ A diverse selection of low growing, plants that thrive under the specific site, climatic, and
watering conditions should be specified. Vegetation whose growing season corresponds to
the wet season are preferred. Drought tolerant vegetation should be considered especially
for swales that are not prut of a regularly irrigated landscaped area.
■ TI1e width of the swale should be determined using Manning's Equation using a value of
0.25 for Manning's n.
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Vegetated Swale TC-30
ConstJ-uction/Inspection Considerations
■ Include directions in the specifications for use of appropriate fertilizer and soil amendments
based on soil properties determined through testing and compared to the needs of the
vegetation requirements.
• Install swales at the time of the year when there is a reasonable chance of successful
establishment without irrigation; however, it is recognized that rainfall in a given year may
not be sufficient and temporary irrigation may be used.
• If sod tiles must be used, they should be placed so that there are no gaps between the tiles;
stagger the ends of the tiles to prevent the formation of channels along the swale or strip.
■ Use a roller on the sod to ensure that no air pockets form between the sod and the soil.
■ vVh.ere seeds are used, erosion controls will be necessary to protect seeds for at least 75 days
after the first rainfall of the season.
Performance
TI1e literature suggests that vegetated swales represent a practical and potentially effective
technique for controlling urban runoff quality. W11ile limited quantitative performance data
exists for vegetated swales, it is known that check dams, slight slopes, permeable soils, dense
grass cover, increased contact time, and small storm events all contribute to successful pollutant
removal by the swale system. Factors decreasing the effectiveness of swales include compacted
soils, short runoff contact time, large storm events, frozen ground, short grass heights, steep
slopes, and high runoff velocities and discharge rates.
Conventional vegetated swale designs have achieved mixed results in removing particulate
pollutants. A study perlormed by the Nationwide Urban Runoff Program (NURP) monitored
three grass swales in the Washington, D.C., area and found no significant improvement in urban
runoff quality for the pollutants analyzed. However, the weak perlormance of these swales was
attributed to the high flow velocities in the swales, soil compaction, steep slopes, and short grass
height.
Another project in Durham, NC, monitored the performance of a carefully designed artificial
swale that received runoff from a commercial parking lot. TI1e project tracked 11 storms and
concluded that pa1ticulate concentrations of heavy metals (Cu, Pb, Zn, and Cd) were reduced by
approximately 50 percent. However, the swale proved largely ineffective for removing soluble
nutrients.
TI1e effectiveness of vegetated swales can be enhanced by adding check dams at approximately
17 meter (50 foot) increments along their length (See Figure 1). TI1ese dams maximize the
retention time within the swale, decrease flow velocities, and promote particulate settling.
Finally, the incorporation of vegetated filter strips parallel to the top of the channel banks can
help to treat sheet flows entering the swale.
Only 9 studies 11ave been conducted on all grassed channels designed for water quality (Table 1).
TI1e data suggest relatively high removal rates for some pollutants, but negative removals for
some bacteria, and fair performance for phosphorus.
January 2003 California Stormwater BMP Handbook
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TC-30 Vegetated Swale
!Table 1 Grassed swale pollutant removal efficiency data
Rem.oval Efficiencies{% Removal)
Study TSS TP 'IN NOs Metals Bacteria Type
Caltrans 2002 77 8 67 66 83-90 -33 dryswales
Goldberg 1993 67.8 4-5 -31-4 42-62 -100 grassed channel
Seattle Metro and Washington 60 45 --25 2-16 -25 grassed clumnel !Department of Ecology 1992
Seattle Metro and Washington 83 29 --25 46-73 -25 grassed channel Department of Ecology, 1992
Wang et al., 1981 Bo ---70-80 -dryswale
Dorman et al., 1989 98 18 -45 37-81 -dryswale
Harper, 1988 87 83 84 80 88-90 -dryswale
!Kercher et al., 1983 99 99 99 99 99 -dryswale
Harper, 1988. 81 17 40 52 37-69 -wetswale
Koon,1995 67 39 -9 -35to6 -·wetswale
While it is difficult to distinguish between different designs based on the small amount of
available data, grassed channels generally have poorer removal rates than wet and dry swales,
although some swales appear to expo1t soluble phosphorus (Harper, 1988; Koon, 1995). It is not
clear why swales export bacteria. One explanation is that bacteria thrive in the warm swale
soils.
Siting Criteria
The suitability of a swale at a site will depend on land use, size of the area serviced, soil type,
slope, imperviousness of the contributing watershed, and dimensions and slope of the swale
system (Schueler et al., 1992). In general, swales can be used to serve areas ofless than 10 acres,
·with slopes no greater than 5 %. Use of natural topographic lows is encouraged and natural
drainage courses should be regarded as significant local resources to be kept in use (Young et al.,
1996).
Selection Criteria (NCTCOG, :1.993)
• Comparable pe1formance to wet basins
■ Limited to treating a few acres
■ Availability of water during d1y periods to maintain vegetation
■ Sufficient available land area
Research in the Austin area indicates that vegetated controls are effective at removing pollutants
even when dormant. Therefore, irrigation is not required to maintain growth during dry
periods, but may be necessary only to prevent the vegetation from dying.
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Vegetated Swale TC-30
The topography of the site should permit the design of a channel with appropriate slope and
cross-sectional area. Site topography may also dictate a need for additional structural controls.
Recommendations for longitudinal slopes range betvveen 2 and 6 percent. Flatter slopes can be
used, if sufficient to provide adequate conveyance. Steep slopes increase flow velocity, decrease
detention time, and may require energy dissipating and grade check. Steep slopes also can be
managed using a series of check dams to terrace the swale and reduce the slope to within
acceptable limits. The use of check dams with swales also promotes infiltration.
Additional Design Guidelines
Most of the design guidelines adopted for swale design specify a minimum hydraulic residence
time of 9 minutes. This criterion is based on the results of a single study conducted in Seattle,
Washington (Seattle Metro and 1,Vashington Department of Ecology, 1992), and is not well
supported. i\.nalysis of the data collected in that study indicates that pollutant removal at a
residence time of 5 minutes was not significantly different, although there is more variability in
that data. Therefore, additional research in the design criteria for swales is needed. Substantial
pollutant removal has also been observed for vegetated controls designed solely for conveyance
(Barrett et al, 1998); consequently, some flexibility in the design is warranted.
Many design guidelines recommend that grass be frequently mowed to maintain dense coverage
near the ground surface. Recent research (Colwell et al., 2000) has shown mowing frequency or
grass height has little or no effect on pollutant removal.
Summary of Design Recommendations
1) The swale should have a length that provides a minimum hydraulic residence time of
at least 10 minutes. The maximum bottom widtl1 should not exceed 10 feet unless a
dividing berm is provided. TI1e depth of flow should not exceed 2/3rds the height of
the grass at the peak of the water quality design storm intensity. The channel slope
should not exceed 2.5%.
2)
3)
4)
5)
6)
A design grass height of 6 inches is reconm1ended.
Regardless of the recommended detention time, tl1e swale should be not less than
100 feet in length.
The width of the swale should be determined using Manning's Equation, at the peak
of the design storm, using a Manning's n of 0.25.
The swale can be sized as both a treatment facility for the design storm and as a
conveyance system to pass tl1e peak hydraulic flows of the 100-year storm if it is
located "on-line." The side slopes should be no steeper tlian 3:1 (H:V).
Roadside ditches should be regarded as significant potential swalejbuffer strip sites
and should be utilized for this purpose whenever possible. If flow is to be introduced
through curb cuts, place pavement slightly above the elevation of the vegetated areas.
Curb cuts should be at least 12 inches wide to prevent clogging.
Swales must be vegetated in order to provide adequate treatment of runoff. It is
important to maximize water contact with vegetation and tl1e soil surface. For
general purposes, select fine, close-grmving, water-resistant grasses. If possible,
divert runoff ( other than necessary irrigation) during the period of vegetation
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TC-30 Vegetated Swale
establishment. vVhere runoff diversion is not possible, cover graded and seeded
areas with suitable erosion control materials.
Maintenance
TI1e useful life of a vegetated swale system is directly prop01tional to its maintenance frequency.
If properly designed and regularly maintained, vegetated swales can last indefinitely. The
maintenance objectives for vegetated swale systems include keeping up the hydraulic and
removal efficiency of the channel and maintaining a dense, healthy grass cover.
Maintenance activities should include periodic mowing (with grass never cut sho1ter than the
design flow depth), weed control, watering during drought conditions, reseeding of bare areas,
and clearing of debris and blockages. Cuttings should be removed from the channel and
disposed in a local composting facility. Accumulated sediment should also be removed
manually to avoid concentrated flows in the swale. The application of fertilizers and pesticides
should be minimal.
Another aspect of a good maintenance plan is repairing damaged areas within a channel. For
example, if the channel develops ruts or holes, it should be repaired utilizing a suitable soil that
is properly tamped and seeded. TI1e grass cover should be thick; if it is not, reseed as necessary.
Any standing water removed during the maintenance operation must be disposed to a sanitary
sewer at an approved discharge location. Residuals (e.g., silt, grass cuttings) must be disposed
in accordance with local or State requirements. Maintenance of grassed swales mostly involves
maintenance of the grass or wetland plant cover. Typical maintenance activities are
summarized below:
■ Inspect swales at least twice annually for erosion, damage to vegetation, and sediment and
deblis accumulation preferably at the end of the wet season to schedule summer
maintenance and before major fall runoff to be sure the swale is ready for w'inter. However,
additional inspection after periods of heavy runoff is desirable. The swale should be checked
for debris and litter, and areas of sediment accumulation.
■ Grass height and mowing frequency may not have a large impact on pollutant removal.
Consequently, mowing may only be necessary once or twice a year for safety or aesthetics or
to suppress weeds and woody vegetation.
■ Trash tends to accumulate in swale areas, paiticularly along highways. The need for litter
removal is determined through periodic inspection, but litter should always be removed
prior to mowing.
■ Sediment accumulating near culverts and in channels should be removed when it builds up
to 75 mm (3 in.) at any spot, or covers vegetation.
■ Regularly inspect swales for pools of standing water. Swales can become a nuisance due to
mosquito breeding in standing water if obstructions develop ( e.g. debris accumulation,
invasive vegetation) and/ or if proper drainage slopes are not implemented and maintained.
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Vegetated Swale TC-30
Cost
Construction Cost
Little data is available to estimate the difference in cost between various swale designs. One
study (SWRPC, 1991) estimated the construction cost of grassed channels at approximately
$0.25 per ft2• Titls price does not include design costs or contingencies. Brown and Schueler
(1997) estimate these costs at approximately 32 percent of construction costs for most
stormwater management practices. For swales, however, these costs would probably be
significantly higher since the construction costs are so low compared witl1 oilier practices. A
more realistic estimate would be a total cost of approximately $0.50 per ft2, whlcl1 compares
favorably with other stormwater management practices.
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-------------------TC-30
Table 2 Swale Cost Estimate (SEWRPC, 1991)
Unit Cost
Component Unit Extent Low Moderate High Low
Mobilization I Swale '1 $101 $274 $441 $107
Demobil!zation-Light
Site Preparatioo
Clearingb ................ Al:.re 0.5 $2,200 $3,800 $5,400 $1.100 Grubbing' .............. Al:.re 0.25 $3,800 $5,200 $8,600 $950 Ganer.al
Excavatiorfl ............ Yd3 372 $2.10 $3.70 $5.30 $781
Level and Till" ........ Yd2 1,210 ¢0.20 $0.35 ¢0.50 $242
Sites Development
Salvaged Topsoil
Seed, and Mulch1 .. Yd2 1,210 ¢0.40 $1.00 $1.60 $4!!4
Sod9 ...................... Yd2 1,210 $1.20 $2.40 $3.60 $1,452
Subtotal .. -.. -.. $5,118
Contingencies Swale 1 25% 25% 25% $1,279
Total -· ------$8,395
Source. (SEWRPC, 1991)
Noh~: Mobili:zati on/demobilization mfers to the organ izaliai and planning involved in 1'.Btablish ing a vegetative swale.
g Swale has a bottom width of 1.0 foot, a top v.1dth of 10 feet with 1:3 side slopes, and a 1,000-foot length,
b Area cleared= {top width+ 10 feet) x swale length.
e Area grubbed:::: (top width x swale length).
dVolume excavated = (0.67 x top width x swale depth) x swale length {parabolic cross-section).
"'Area tilled= (top width + S(swale depth'.:) x S\vale length (parabolic cross-section}.
3(top width)
r Area seeded= area cleared x 0.5.
9 Area rodded= area cleared x 0,5.
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Vegetated Swale
Total Cost
Moderate H[gh
$274 $441
$1,900 $2,700
$1,300 $1,650
$1,376 $1,972
$424 $605
$1,210 $1,936
$2,904 $4,356
$9,38& $13,660
$2,347 $3,415
$11,735 $17,075
January 2003
- - - - -- - - - ---------Vegetated Swale
Table 3 Estimated.Maintenance Costs (SEWRPC. 1991)
Component Unit Cost
Lawn Mowing $0.8511,000 ft2/ mowing
General Lawn Gare $9.0011,000fWyear
Swale Debris and Utt8r $0.10 / linear foot f year
Removal
Grass ResMding with $0.'30/yd2
Mulch and Fertilizar
Program Administration and $0.15 / linear fool I year,
Swale Inspection plus $251 inspection
Total ..
-~~---·--.
January 2003
Swale Size
(Depth and Top Width)
1.5 Foot Depth1 One-3-Foot Depth. 3ftFoot
Foot Bottom Width, Bottom Wldth1 21~Foot
10•FootTop Width Top Width
$0.14 /linoorfoot $0 .21 / linear foot
$0.18 / linearfoot $0.28 / linear foot
$0.10 / linearfoot $0.10 I linear foot
$0.01 /linoorfoot $0.01 /liMarfoot
$0.15 llinearfoot $0.15 lllnearfoot
$0.58 / linear toot $ 0.76 / linear foot
California Stormwater BMP Handbook
New Development and Redevelopment
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TC-30
Comment
lawn maintenance area=(top
wldlh + 1Qf8:)t)x length. Mow
eight 1imes par year
lawn maintenanm area = (top
wldih + 10 fest) x le ng1h
-
Ama reveget!.11:ad tquals 1 %
of lawn maintenance area per
yoor
Inspect four times per yee r
-
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TC-30 Vegetated Swale
J.Uaintenance Cost
Caltrans (2002) estimated the expected annual maintenance cost for a swale -with a tributary
area of approximately 2 ha at approximately $2,700. Since almost all maintenance consists of
mowing, the cost is fundamentally a function of the mmving frequency. Unit costs developed by
SEWRPC are shown in Table 3. In many cases vegetated channels would be used to convey
runoff and would require periodic mo"ving as well, so there may be little additional cost for the
water quality component. Since essentially all the activities are related to vegetation
management, no special training is required for maintenance personnel.
References and Sources of Additional Information
Barrett, Michael E., Walsh, Patrick M., Malina, Joseph F., Jr., Charbeneau, Randall J, 1998,
"Pe1formance of vegetative controls for treating highway nm off," ASCE Journal of
Environmental Engineering, Vol. 124, No. 11, pp. 1121-1128.
Brown, W., and T. Schueler. 1997. The Economics of Storm water BMPs in the Mid-Atlantic
Region. Prepared for the Chesapeake Research Conso1tium, Edgewater, MD, by the Center for
"\Vatershed Protection, Ellicott City, MD.
Center for Watershed Protection (CWP). 1996. Design of Stormwater Filtering Systems.
Prepared for the Chesapeake Research Consortium, Solomons, MD, and USEPA Region V,
Chicago, IL, by the Center for Watershed Protection, Ellicott City, MD.
Colwell, Shanti R, Horner, Richard R, and Booth, Derek B., 2000. Characterization of
Performance Predictors and Evaluation of Mowing Practices in Biofiltration Swales. Report
to King County Land And Water Resources Division and others by Center for Urban \'\Tater
Resources Management, Department of Civil and Environmental Engineering, University of
Washington, Seattle, WA
Dorman, M.E., J. Hartigan, RF. Steg, and T. Quasebmth. 1989. Retention, Detention and
Overland Flow for Pollutant Removal From Highway Stormwater Runoff. Vol. 1. FHWA/RD
89/202. Federal Highway Administration, Washington, DC.
Goldberg. 1993. Dayton Avenue Swale Biofiltration Study. Seattle Engineering Department,
Seattle, WA.
Harper, H. 1988. Effects ofStormwater Management Systems on Groundwater Quality.
Prepared for Florida Department of Environmental Regulation, Tallahassee, FL, by
Environmental Research and Design, Inc., Orlando, FL.
Kercher, W.C., J.C. Landon, and R Massarelli. 1983. Grassy swales prove cost-effective for
water pollution controL Public Works, 16: 53-55.
Koon, J. 1995. Evaluation of Water Quality Ponds and Swales in the Issaquah/East Lake
Sammamish Basins. King County Surface Water Management, Seattle, WA, and Washington
Department of Ecology, Olympia, WA.
Metzger, M. E., D. F. Messer, C. L. Beitia, C. M. Myers, and V. L. Kramer. 2002. The Dark Side
Of Stormwater Runoff Management: Disease Vectors Associated With Structural BMPs.
Stormwater 3(2): 24-39.Oakland, P.H. 1983. An evaluation of stormwater pollutant removal
10 of 13 California Stormwater BMP Handbook
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Vegetated Swale TC-30
through grassed swale treatment. In Proceedings of the International Symposium of Urban
Hydrology, Hydraulics and Sediment Control, Lexington, KY. pp. 173-182.
Occoquan Watershed Monitoring Laboratory. 1983. Final Report: Metropolitan Washington
Urban Runoff Project. Prepared for the Metropolitan Washington Council of Governments,
Washington, DC, by the Occoquan Watershed Monitoring Laboratory, Manassas, VA.
Pitt, R., and J. McLean. 1986. Toronto Area ·watershed Management Strategy Study: Humber
River Pilot vVatershed Project. Ontario Minist1y of Environment, Toronto, ON.
Schueler, T. 1997. Comparative Pollutant Removal Capability of Urban BMPs: A reanalysis.
Watershed Protection Techniques 2(2):379-383.
Seattle Metro and Washington Depaitment of Ecology. 1992. Biofiltration Swale Performance:
Recommendations and Design Considerations. Publication No. 657. ·water Pollution Control
Department, Seattle, WA
Southeastern Wisconsin Regional Planning Commission (SWRPC). 1991. Costs of Urban
Nonpoint Source Water Pollution Control Measures. Technical repo1t no. 31. Southeastern
Wisconsin Regional Planning Co1mnission, ·waukesha, WI.
U.S. EPA, 1999, Stormwater Fact Sheet: Vegetated Sw·ales, Report# 832-F-99-006
http://www.epa.gov/owm/mtb/veg:swale.pdf, Office of Water, Washington DC.
Wang, T., D. Spyiidakis, B. Mar, and R Horner. 1981. Transport, Deposition and Control of
Heavy Metals in Highway Runoff. FHWA-WA-RD-39-10. University ofv\Tashington,
Department of Civil Engineering, Seattle, V1,T A.
Washington State Department of Transportation, 1995, Highway Runoff Manual, Washington
State Department of Transportation, Olympia, Washington.
Welborn, C., and J. Veenhuis. 1987. Effects ofRunoff Controls on the Quantity and Quality of
Urban Runoff in Two Locations in Austin, TX. USGS Water Resources Investigations Rep01t
No. 87-4004. U.S. Geological Survey, Reston, VA.
Yousef, Y., M. Wanielista, H. Harper, D. Pearce, and R. Tolbe1t. 1985. Best Management
Practices: Removal of Highway Contaminants By Roadside Swales. University of Central
Florida and Florida Department of Transp01tation, Orlando, FL.
Yu, S., S. Barnes, andV. Gerde.1993. Testing ofBestManagementPracticesforControlling
Highway Runoff. FHW A/VA-93-Rl.6. Virginia Transportation Research Council,
Charlottesville, VA
Infonnation Resources
Maryland Department of the Environment (MDE). 2000. Maryland Stormwater Design
Manual. ·www.mde.state.md.us/environment/wma/storrnwatermanual. Accessed May 22,
2001.
Reeves, E. 1994. Performance and Condition of Bioftlters in the Pacific N01thwest. Watershed
Protection Techniques 1(3):117-119.
January 2003 California Stormwater BMP Handbook
New Development and Redevelopment
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TC-30 Vegetated Swale
Seattle Metro and V\Tashington Department of Ecology. 1992. Biofiltration Swale Pe1formance.
Recommendations and Design Considerations. Publication No. 657. Seattle Metro and
Washington Deparhnent of Ecology, Olympia, WA
USEPA 1993. Guidance Specifi.Jing Management lvleasures for Sources ofNonpoint Pollution in
Coastal Waters. EPA-840-B-92-002. U.S. Environmental Protection Agency, Office of Water.
Washington, DC.
Watershed Management Institute (WMI). 1997. Operation, JY[aintenance, and Management of
Stormwater Management Systems. Prepared for U.S. Environmental Protection Agency, Office
of Water. Washington, DC, by the Watershed Management Institute, Ingleside, MD.
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TC-60 Multiple System Fact Sheet
• When addressing multiple constituents th.rough multiple BMPs, one BMP may optimize
removal of a particular constituent, while another BMP optimizes removal of a different
constituent or set of constituents. Therefore, selecting the right combination of BMPs can
be very constructive in collectively removing multiple constituents.
Siting Criteria
Refer to individual treatment control BMP fact sheets.
Additional Design Guidelines
■ When using two or more BMPs in series, it may be possible to reduce the size of BMPs.
■ Existing pretreatment requirements may be able to be avoided when using some BMP
combinations.
Maintenance
Refer to individual treatment control BMP fact sheets.
Cost
Refer to individual treatment control BMP fact sheets.
Resources and Sources of Additional Information
Refer to individual treatment control BMP fact sheets.
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SD-20 Pervious Pavements
■ Permeable pavement can become clogged if improperly installed or maintained. However,
this is countered by the ease with which small areas of paving can be cleaned or replaced
when blocked or dam.aged.
■ Their application should be limited to highways ,,vith low traffic volumes, axle loads and
speeds (less than 30 mph limit), car parking areas and other lightly trafficked or non-
trafficked areas. Permeable smfaces are currently not considered suitable for adoptable
roads due to the risks associated with failure on high speed roads, the safety implications of
ponding, and disruption arising from reconstruction.
■ When using un-lined, infiltration systems, there is some risk of contaminating groundwater,
depending on soil conditions and aquifer susceptibility. However, this risk is likely to be
small because the areas drained tend to have inherently low pollutant loadings.
■ The use of penneable pavement is restricted to gentle slopes.
■ Porous block paving has a higher risk of abrasion and damage than solid blocks.
Design Considerations
Designing New Installations
If the grades, subsoils, drainage characteristics, and groundwater conditions are suitable,
permeable paving may be substituted for conventional pavement on parking areas, cul de sacs
and other areas with light traffic. Slopes should be flat or very gentle. Scottish experience has
shuwn that permeable paving systems can be installed in a ,-vide range of ground conditions, and
the flow attenuation pe1formance is excellent even when the systems are lined.
The suitability of a pervious system at a particular pavement site will, however, depend on the
loading criteria required of the pavement.
Where the system is to be used for infiltrating drainage waters into the ground, the vulnerability
oflocal groundwater sources to pollution from the site should be low, and the seasonal high
water table should be at least 4 feet below the surface.
Ideally, the pervious surface should be horizontal in order to intercept local rainfall at source.
On sloping sites, pervious smfaces may be terraced to accommodate differences in levels.
Design Guidelines
The design of each layer of the pavement must be determined by the likely traffic loadings and
their required operational life. To provide satisfactory peif ormance, the following criteria
should be considered:
■ The subgrade should be able to sustain traffic loading ·without excessive deformation.
■ The granular capping and sub-base layers should give sufficient load-bearing to provide au
adequate construction platform and base for the overlying pavement layers.
■ The pavement materials should not crack of suffer excessive rutting under the influence of
traffic. This is controlled by the horizontal tensile stress at the base of these layers.
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Pervious Pavements SD-20
TI1ere is no current structural design method specifically for pervious pavements. Allowances
should be considered the following factors in the design and specification of materials:
• Pervious pavements use materials with high permeability and void space. All the current UK
pavement design methods are based on the use of conventional materials that are dense and
relatively impermeable. The stiffness of the materials must therefore be assessed.
■ Water is present within the construction and can soften and weaken materials, and this must
be allowed for.
■ Existing design methods assume full fiiction between layers. Any geotextiles or
geomembranes must be carefully specified to minimize loss of friction between layers.
■ Porous asphalt loses adhesion and becomes brittle as air passes through the voids. Its
durability is therefore lower than conventional materials.
The single sized grading of materials used means that care should be taken to ensure that loss of
finer particles between unbound layers does not occur.
Positioning a geotextile near the surface of the pervious consh·uction should enable pollutants to
be trapped and retained close to the smface of the construction. This has both advantages and
disadvantages. The main disadvantage is that the filte1ing of sediments and their associated
pollutants at this level may hamper percolation of waters and can eventually lead to smface
ponc1ing. One advantage is that even if eventual maintenance is required to reinstate
infiltration, only a limited amount of the construction needs to be disturbed, since the sub-base
below the geotextile is protected. In addition, the pollutant concentration at a high level in the
structure allows for its release over time. It is slowly transported in the storm.water to lower
levels where chemical and biological processes may be operating to retain or degrade pollutants.
The design should ensure that sufficient void space exists for the storage of sediments to limit
the period between remedial works.
■ Pervious pavements require a single size grading to give open voids. The choice of materials
is therefore a compromise between stiffness, permeability and storage capacity.
• Because the sub-base and capping \vill be in contact with water for a large pait of the time,
the strength and durability of the aggregate particles when saturated and subjected to
wetting and drying should be assessed.
■ A uniformly graded single size material cannot be compacted and is liable to move when
construction traffic passes over it. This effect can be reduced by the use of angular crushed
rock material ·with a high smface friction.
In pollution control terms, these layers represent the site oflong term chemical and biological
pollutant retention and degradation processes. TI1e construction materials should be selected,
in addition to their structural strength properties, for their ability to sustain such processes. In
general, this means that materials should create neutral or slightly alkaline conditions and they
should provide favorable sites for colonization by microbial populations.
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SD-20 Pervious Pavements
Construction/Inspection Considerations
■ Permeable surfaces can be laid without cross-falls or longitudinal gradients.
■ The blocks should be lain level
■ They should not be used for storage of site materials, unless the surface is well protected
from deposition of silt and other spillages.
■ The pavement should be constructed in a single operation, as one of the last items to be
built, on a development site. Landscape development should be completed before pavement
construction to avoid contamination by silt or soil from this source.
■ Surfaces draining to the pavement should be stabilized before construction of the pavement.
■ Inappropriate construction equipment should be kept away from the pavement to prevent
damage to the surface, sub-base or sub-grade.
Maintenance Requirements
The maintenance requirements of a pervious surface should be reviewed at the time of design
and should be clearly specified. Maintenance is required to prevent clogging of the pervious
surface. The factors to be considered when defining maintenance requirements must include:
■ Typeofuse
■ Ownership
■ Level of trafficking
■ The local environment and any conhi.buting catchments
Studies in the UK have shmvn satisfacto1y operation of porous pavement systems without
maintenance for over 10 years and recent work by Imbe et al. at 9th ICUD, Portland, 2002
describes systems operating for over 20 years without maintenance. However, performance
under such regimes could not be guaranteed, Table 1 shows typical recommended maintenance
regimes:
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Pervious Pavements SD-20
Table 1 Typical Recommended Maintenance Regimes
Activity Schedule
• Minimize use of salt or grit for de-icing
• Keep landscaped areas well maintained Ongoing
• Prevent soil being washed onto pavement
• Vacuum clean smface using commercially available sweeping
machines at the following times:
-End of ·winter (April) 2/3xperyear
-Mid-summer (July/ August)
-After Auhmm leaf-fall (November)
• Inspect outlets Annual
• If routine cleaning does not restore infiltration rates, then
reconstruction of part of the whole of a pervious surface may be
required.
• The surface area affected by hydraulic failure should be lifted for
inspection of the internal materials to identify the location and As needed (infrequent) eJ...'tent of the blockage. Maximum 15-20 years
• Smface materials should be lifted and replaced after brush
cleaning. Geote:i,..'til.es may need complete replacement.
• Sub-smface layers may need cleaning and replacing .
• Removed silts may need to be disposed of as controlled waste .
Pe1111eable pavements are up to 25 % cheaper ( or at least no more expensive than the traditional
forms of pavement construction), when all construction and drainage costs are taken into
account. (Accepting that the porous asphalt itself is a more expensive surfacing, the extra cost of
which is offset by the savings in underground pipework etc.) (Niemczynowicz, et al., 1987)
Table 1 gives US cost estimates for capital and maintenance costs of porous pavements
(Landphair et al., 2000)
Redeveloping Existing Installations
Various jurisdictional stormwater management and mitigation plans (SUSMP, WQMP, etc.)
define "redevelopment" in terms of amounts of additional impervious area, increases in gross
floor area and/ or exterior construction, and land disturbing activities with structural or
impervious surfaces. The definition of" redevelopment" must be consulted to determine
whether or not the requirements for new development apply to areas intended for
redevelopment. If the definition applies, the steps outlined under "designing new installations"
above should be followed.
January 2003 California Stormwater BMP Handbook
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SD-20
Additional Information
Cost Considerations
Pervious Pavements
Permeable pavements are up to 25 % cheaper ( or at least no more e:,rpensive than the traditional
forms of pavement construction), when all construction and drainage costs are taken into
account. (Accepting that the porous asphalt itself is a more expensive surfacing, the extra cost of
which is offset by the savings in underground pipework etc.) (Niemczynowicz, et al., 1987)
Table 2 gives US cost estimates for capital and maintenance costs of porous pavements
(Landphair et al., 2000)
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-------------------Pervious Pavements
Table 2 Engineer's Estimate for Porous Pavement
Item lllli15 .Priee Cydtst Qll311t,1
Year AcreWS
Graditlg SY $2.00 804
Paving SY $1$.00 212
Exc1;1vation CY $3.60 201
FUtet Fabric SY $1.1$ 700
Stooe Fill CY $16.00 201
Sane;! CY $7.00 100
Sight Well EA $300.00 2
S-oodin9 LF $0.05 644
Ch-ackDam CY $35.00 0
T-otal ~ion Co!Ui
Con$1:roctk,n Cogts. AmortizGd
for2GYears
ltl!m Uml.~ Prii;e ('.yde5I Quant.l
Year Acn\\'S
Sweeping AC $250.00 6 1
Washi»g AC $250.00 6 1
lt'1$peetlon MH $20.00 5 5
O&epClean AC $450.00 0.5 1
Total Annual Mai~ce &&)$Mt
January 2003
Porous Pavement
'total ~anl.2 Total Qllaflt.J
AcreWS AcreWS
$1,208 1209 $2,418 1812
$4,028 424 $8,0$6 638
$124 403 $1,451 604
$805 ·i.100 $1,610 2000
$3,216 403 $6,448 604
$100 200 $1,400 300
$600 3 $900 4
$32 1288 $64 1932
$0 0 $0 0
$10,105 $1t,92$
$505 $9$6
Annual Maintenance Expen$e
Toqil Quant.2 T1,t.l Qwmt,3
AcreWS AcreWS
$1,500 2 $$,000 3
$1,,500 2 $:MOO 3
$100 5 $100 5
$22$ 2 $450 3
$3i~60 $7,792
California Stormwater BMP Handbook
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SD-20
Total Qollant, 4 Tolal Quaut.5 Total AcreWS A~reWS
$3,624 2419 $4,838 3020 $6,040
$12,0t:W 848 $16;112 1060 $20.140
$2,114 806 $2JJ02 1008 $3,6:29
$2,SOO 2800 $3,220 :3600 $4, 14(1
$9,664 806 $12J~~6 1008 $16,128
$2,100 400 $2,8()0 500 $3,5(10
$1,200 1 $2,100 1 $2,100
$97 2576 $129 3:220 $161
$0 0 $0 0 $0
$29.619 $40,158 $@,'193
$1t481 $2,008 $2,490
Tnt;d Quant.◄ Tola! Qnant.~ 'fol~ AcreWS AtreWS
$4,500 4 $6,000 f,i $1,500
$4,,500 4 $6,000 $ $7,500
$100 5 $100 5 $100
$675 3.9 $878 5 $1.12$
$11,651 $1$,483 $1$,370
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SD-20 Pervious Pavements
Other Resources
Abbott C.L. and Comino-Mateos L. 2001. In situ pe1formance monitoring of an infiltration
drainage system andfield testing of current design procedw·es. Journal CIWEM, 15(3), pp.198-
202.
Construction Industiy Research and Information Association (CIRIA). 2002. Source Control
using Constructed Pervious Swfaces C582, London, SW1P 3AU.
Construction Industry Research and Infonnation Association (CIRIA). 2000. Sustainable urban
drainage systems -design mammlfor Scotland and Northern Ireland Report C521, London,
SW1P3AU.
Construction Industly Research and Information Association (CIRIA). 2000 C522 Sustainable
urban drainage systems -design manualfor England and Wales, London, SW1P 3AU.
Construction Industry Research and Information Association (CIRIA). RP448 Manual of good
practice for the design, construction and maintenance of infiltration drainage systems for
stormwater runoff control and disposal, London, SW1P 3AU.
Dierkes C., Kuhlmann L., Kandasamy J. & Angelis G. Pollution Retention Capability and
Maintenance of Permeable Pavements. Proc 9th International Conference on Urban Drainage,
Portland Orego~ September 2002.
Hart P (2002) Permeable Paving as a Stonnwater Source Control System. Paper presented at
Scottish Hydraulics Study Group 14th Annual seminar, SUDS. 22 March 2002, Glasgow.
Kobayashi M., 1999. Stonnwater runoff control iu Nagoya City. Proc. 8th Int. Conf. on
Urban Storm Drainage, Sydney, Australia, pp.825-833.
Landphair, H., McFalls, J., Thompson, D., 2000, Design Methods, Selection, and Cost
Effectiveness of Stormwater Quality Structures, Texas Transportation Institute Research Report
1837-1, College Station, Texas.
Legret M, Colandini V, Effects of a porous pavement with reservior strucutre on runoff
water:water quality and the fate of heavy metals. Laboratoire Central Des Pouts et Chaussesss
Macdonald K & Jefferies C. Performance Comparison of Porous Paved and Traditional Car
Parks. Proc. First National Conference on Sustainable Drainage Systems, Coventry June 2001.
Niemczynowicz J, Hogland W, 1987= Test of porous pavements performed in Lund, Sweden, in
Topics in Drainage Hydraulics and Hydrology. BC. Yen (Ed.), pub. Int. Assoc. For Hydraulic
Research, pp 19-80.
Pratt C.J. SUSTAINABLE URBAN DRAINAGE - A Review of published material on the
performance of various SUDS devices prepared for the UK Environment Agency. Covent1y
University, UK December 2001.
Pratt C.J., 1995. Infiltration drainage-case studies of UK practice. Project Report
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Pervious Pavements SD-20
22,ConstJ.uction Industry Research and Information Association, London, SW1P 3-AU; also
known as National Rivers Authority R & D Note 485
Pratt. C. J., 1990. Permeable Pavements for Stormwater Quality Enhancement. In: Urban
Stormwater Quality Enhancement -Source Control, retrofitting and combined sewer
technology, Ed. H.C. Torno, ASCE, ISBN 087262 7594, pp. 131-155
Raimbault G., 1997 French Developments in Reservoir StJ.•uctures Sustainable water resources I
the 21st centmy. Malmo Sweden
Schluter W. & Jefferies C. Monitoring tl1e outflow from a Porous Car Park Proc. First National
Conference on Sustainable Drainage Systems, Coventry June 2001.
Wild, T.C., Jefferies, C., and D'Arcy, B.J. SUDS in Scotland -the Scottish SUDS database
Report No SR( 02)09 Scotland and Northern Ireland Forumfor Environmental Research,
Edinburgh. In preparation August 2002.
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TC-32 Bioretention
be required since clogging may result, particularly if the BMP receives runoff with high
sediment loads (EPA, 1999).
■ Bioretention is not a suitable BMP at locations where the water table is within 6 feet of the
ground smface and where the surrounding soil stratum is unstable.
• By design, bioretention BMPs have the potential to create very attractive habitats for
mosquitoes and other vectors because of highly organic, often heavily vegetated areas mixed
with shallow water.
■ In cold climates the soil may freeze, preventing runoff from infiltrating into the planting soil.
Design and Sizing Guidelines
■ The bioretention area should be sized to capture the design storm runoff.
• In areas where the native soil penneabilityis less than 0.5 in/hr an underdrain should be
provided.
■ Recommended minimum dimensions are 15 feet by 40 feet, although the preferred width is
25 feet. Excavated depth should be 4 feet. •
■ Area should drain completely within 72 hours.
■ Approximately 1 tree or shrub per 50 ft2 of bioretention area should be included.
■ Cover area with about 3 inches of mulch.
Construction/Inspection Considerations
Bioretention area should not be established until contributing watershed is stabilized.
Performance
Bioretention removes stonuwater pollutants through physical and biological processes,
including adsorption, filtration, plant uptake, microbial activity, decomposition, sedimentation
and volatilization (EPA, 1999 ). Adsorption is the process whereby particulate pollutants attach
to soil ( e.g., clay) or vegetation smfaces. Adequate contact time between tlie surface and
pollutant must be provided for in the design of the system for this removal process to occur.
Thus, the infiltration rate of the soils must not exceed those specified in the design criteria or
pollutant removal may decrease. Pollutants removed by adsorption include metals, phosphorus,
and hydrocarbons. Filtration occurs as runoff passes through the bioretention area media, such
as the sand bed, ground cover, and planting soil.
Common particulates removed from stormwater include pa1ticulate organic matter,
phosphorus, and suspended solids. Biological processes that occur in wetlands result in
pollutant uptake by plants and microorganisms in the soil. Plant growth is sustained by the
uptake of nutrients from the soils, with woody plants locking up these nutrients through the
seasons. Microbial activity within tlie soil also contributes to the removal of nitrogen and
organic matter. Nitrogen is removed by nitrifyiug and denitrifying bacteria, while aerobic
bacte1·ia are responsible for the decomposition of tlie organic matter. Microbial processes
require oxygen and can result in depleted oxygen levels if the bioretention ru:ea is not adequately
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Bioretention TC-32
aerated. Sedimentation occurs in the swale or ponding area as the velocity slows and solids fall
out of suspension.
The removal effectiveness of bioretention has been studied during field and laboratory studies
conducted by the University of Maryland (Davis et al, 1998). During these experiments,
synthetic stormwater runoff was pumped through several laboratory and field bioretention areas
to simulate typical storm events in Prince George's County, MD. Removal rates for heavy metals
and nuh·ients are shown in Table 1.
Table 1 Laboratory and Estimated
Bioretention Davis et al. {1998);
PGDER (1993)
Pollutant Removal Rate
Total Phosphorus 70-83%
Metals {Cu, Zn, Pb) 93-98%
TKN 68-80%
Total Suspended Solids 90%
Organics 90%
Bacteria 90%
Results for both the laboratory and field experiments were similar for each of the pollutants
analyzed. Doubling or halving the influent pollutant levels had little effect on the effluent
pollutants concentrations (Davis et al, 1998).
The microbial activity and plant uptake occurring in the bioretention area will likely result in
higher removal rates than those determined for infilh·ation BMPs.
Siting Criteria
Bioretention BMPs are generally used to treat stormwater from impervious surfaces at
commercial, residential, and industrial areas (EPA, 1999). Implementation ofbioretention for
stormwater management is ideal for median strips, parking lot islands, and swales. Moreover,
the runoff in these areas can be designed to either divert directly into the bioretention area or
convey into the bioretention area by a curb and gutter collection system.
The best location for bioretention areas is upland from inlets that receive sheet flow from graded
areas and at areas that will be excavated (EPA, 1999 ). In order to maximize treatment
effectiveness, the site must be graded in such a way that minimizes erosive conditions as sheet
flow is conveyed to the treatment area. Locations where a bioretention area can be readily
incorporated into the site plan without further environmental damage are preferred.
Furthermore, to effectively minimize sediment loading in the treatment area, bioretention only
should be used in stabilized drainage areas.
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TC-32 Bioretention
Additional Design Guidelines
TI1e layout of the bioretention area is determined after site constraints such as location of
utilities, underlying soils, existing vegetation, and drainage are considered (EPA, 1999 ). Sites
with loamy sand soils are especially appropriate for bioretention because the excavated soil can
be backfilled and used as the planting soil, thus eliminating the cost of importing planting soil.
The use of bioretention may not be feasible given an unstable surrounding soil stratum, soils
with clay content greater than 25 percent, a site with slopes greater than 20 percent, and/ or a
site with mature trees that would be removed during construction of the BMP.
Bioretention can be designed to be off-line or on-line of the existing drainage system (EPA,
1999). The drainage area for a bioretention area should be benveen 0.1 and 0-4 hectares (0.25
and 1.0 acres). Larger drainage areas may require multiple bioretention areas. Furthermore,
the maximum drainage area for a bioretention area is determined by the expected rainfall
intensity and runoff rate. Stabilized areas may erode when velocities are greater than 5 feet per
second (1.5 meter per second). The designer should determine the potential for erosive
conditions at the site.
TI1e size of the bioretention area, which is a function of the drainage area and the runoff
generated from the area is sized to capture the water quality volume.
The recommended minimum dimensions of the bioretention area aTe 15 feet (4.6 meters) wide
by 40 feet (12.2 meters) long, where the minimum width allmvs enough space for a dense,
randomly-distributed area of trees and shrubs to become established. Thus replicating a natural
forest and creating a microclimate, thereby enabling the bioretention area to tolerate the effects
of heat stress, acid rain, runoff pollutants, and insect and disease infestations which landscaped
areas in urban settings typically are unable to tolerate. TI1e preferred width is 25 feet (7.6
meters), with a length of twice the width. Essentially, any facilities wider than 20 feet ( 6.1
meters) should be twice as long as they are ·wide, which promotes the distribution of flow and
decreases the chances of concentrated flow.
In order to provide adequate storage and prevent water from standing for excessive periods of
time the ponding depth of the bioretention area should not exceed 6 inches (15 centimeters).
Water should not be left to stand for more than 72 hours. A restriction on the type of plants that
can be used may be necessmy due to some plm1ts• water intolerance. Furthennore, if water is
left standing for longer than 72 hours mosquitoes and other insects may strut to breed.
The appropriate planting soil should be backfilled into the excavated bioretention area. Planting
soils should be sandy loam, loamy sand, or loam texture ,-vith a clay content ranging from 10 to
25 percent.
Generally the soil should have infiltration rates greater than o .5 inches ( 1.25 centimeters) per
hour, which is typical of sm1dy loams, loamy sands, or loams. The pH of the soil should range
between 5.5 and 6.5, where pollutants such as organic nih·ogen and phosphorus can be adsorbed
by the soil and microbial activity can flourish. Additional requirements for the planting soil
include a 1.5 to 3 percent organic content and a maximum 500 ppm concentration of soluble
salts.
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Bio retention TC-32
Soil tests should be performed for eve1y 500 cubic yards (382 cubic meters) of planting soil,
,-vith the exception of pH and organic content tests, which are required only once per
bioretention area (EPA, 1999). Planting soil should be 4 inches (10.1 centimeters) deeper than
the bottom of the largest root ball and 4 feet (1.2 meters) altogether. Tiris depth Vlrill provide
adequate soil for the plants' root systems to become established, prevent plant damage due to
severe wind, and provide adequate moisture capacity. Most sites ·will require excavation in
order to obtain the recommended depth.
Planting soil depths of greater than 4 feet ( 1.2 meters) may require additional construction
practices such as shoring measures (EPA, 1999). Planting soil should be placed in 18 inches or
greater lifts and lightly compacted until the desired depth is reached. Since high canopy trees
may be destroyed during maintenance the bioretention area should be vegetated to resemble a
terrestrial forest community ecosystem that is dominated by u11derst01y trees. TI1ree species
each of both trees and shrubs are recommended to be planted at a rate of 2500 trees and shrubs
per hectare (1000 per acre). For instance, a 15 foot (4.6 meter) by 40 foot (12.2 meter)
bioretention area (600 square feet or 55.75 square meters) would require 14 trees and shrubs.
The shrub-to-tree ratio should be 2:1 to 3:1.
Trees and shrubs should be planted when conditions are favorable. Vegetation should be
watered at the end of each day for fomteen days follm,ving its planting. Plant species tolerant of
pollutant loads and varying wet and dry conditions should be used in the bioretention area.
The designer should assess aesthetics, site layout, and maintenance requirements ·when
selecting plant species. Adjacent non-native invasive species should be identified and the
designer should take measures, such as providing a soil breach to eliminate the threat of these
species invading the bioretention area. Regional landscaping manuals should be consulted to
ensure that the planting of the bioretention area meets the landscaping requirements
established by the local authorities. The designers should evaluate the best placement of
vegetation within the bioretention area. Plants should be placed at irregular intervals to
replicate a natural forest. Trees should be placed on the perimeter of the area to provide shade
and shelter from the wind. Trees and shrubs can be sheltered from damaging flows if they are
placed away from the path of the incoming runoff. In cold climates, species that are more
tolerant to cold winds, such as evergreens, should be placed in windier areas of the site.
Following placement of the trees and shrubs, the ground cover and/or mulch should be
established. Ground cover such as grasses or legumes can be planted at tl1e beginning of tl1e
growing season. Mulch should be placed immediately after trees and shrubs are planted. Two
to 3 inches (5 to 7.6 cm) of commercially-available fine shredded hardwood mulch or shredded
hardwood drips should be applied to the bioretention area to protect from erosion.
Maintenance
TI1e primary maintenance requirement for bioretention areas is that of inspection and repair or
replacement of the treatment area's components. Generally, this involves nothing more than the
routine periodic maintenance that is required of any landscaped area. Plants that are
appropriate for the site, climatic, and watering conditions should be selected for use in the
bioretention cell. Appropriately selected plants will aide in reducing fertilizer, pesticide, water,
and overall maintenance requirements. Bioretention system components should blend over
time through plant and root growth, organic decomposition, and the development of a natural
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TC-32 Bioretention
soil horizon. These biologic and physical processes over time will lengthen the facility's life span
and reduce the need for extensive maintenance.
Routine maintenance should include a biannual health evaluation of the trees and shrubs and
subsequent removal of any dead or diseased vegetation (EPA, 1999). Diseased vegetation
should be treated as needed using preventative and low-toxic measures to the e>..1:ent possible.
BMPs have the potential to create very attractive habitats for mosquitoes and other vectors
because of highly organic, often heavily vegetated areas mixed with shallow water. Routine
inspections for areas of standing water within the BMP and corrective measures to restore
proper infiltration rates are necessary to prevent creating mosquito and other vector habitat. In
addition, bioretention BMPs are susceptible to invasion by aggressive plant species such as
cattails, which increase the chances of water standing and subsequent vector production if not
routinely maintained.
In order to maintain the treatment area's appearance it may be necessary to prune and weed.
Fmthermore, mulch replacement is suggested when erosion is evident or when the site begins to
look unattractive. Specifically, the entire area may require mulch replacement every twu to
three years, although spot mulching may be sufficient when there are random void areas. Mulch
replacement should be done prior to the start of the wet season.
New Jersey's Depaitment of Environmental Protection states in their bioretention systems
standards that accumulated sediment and debris removal ( especially at the inflow point) will
normally be the primary maintenance function. Other potential tasks include replacement of
dead vegetation, soil pH regulation, erosion repair at inflow points, mulch replenishment,
unclogging the underdrain, and repairing overflow structures. TI1ere is also the possibility that
the cation exchange capacity of the soils in the cell will be significantly reduced over time.
Depending on pollutant loads, soils may need to be replaced witl1in 5-10 years of construction
(LID, 2000).
Cost
Construction Cost
Construction cost estimates for a bioretention area are slightly greater than those for the
required landscaping for a new development (EPA, 1999). A general rule of thumb (Coffman,
1999) is that residential bioretention areas average about $3 to $4 per square foot, depending on
soil conditions and the density and types of plants used. Commercial, industrial and
institutional site costs can range bet\veen $10 to $40 per square foot, based on the need for
control structures, curbing, storm drains and underdrains.
Retrofitting a site typically costs more, averaging $6,500 per bioretention area. The higher costs
are attributed to the demolition of existing concrete, asphalt, and existing structmes and the
replacement of fill material with planting soil. The costs of retrofitting a commercial site in
Maryland, Kettering Development, with 15 bioretention areas were estimated at $111,600.
In any bioretention ai·ea design, the cost of plants varies substantially and can account for a
significant portion of the expenditures. vVhile these cost estimates are slightly greater than
those of typical landscaping treatment ( due to the increased number of plantings, additional soil
excavation, backfill material, use of underdrains etc.), those landscaping expenses that would be
required regardless of the bioretention installation should be subtracted when determining tl1e
net cost.
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Bio retention TC-32
Perhaps of most importance, however, the cost savings compared to the use of traditional
structural stonnwater conveyance systems makes bioretention areas quite attractive :financially.
For example, the use of bioretention can decrease the cost required for constructing storm water
conveyance systems at a site. A medical office building in Maryland was able to reduce the
amount of storm drain pipe that was needed from 800 to 230 feet - a cost savings of $24,000
(PGDER, 1993). And a new residential development spent a total of approximately $100,000
using bioretention cells on each lot instead of nearly $400,000 for the traditional storrnwater
ponds that were originally planned (Rappahanock, ). Also, in residential areas, storrnwater
management controls become a part of each property owner's landscape, reducing the public
burden to maintain large centralized facilities.
lUaintenance Cost
TI1e operation and maintenance costs for a bioretention facility will be comparable to those of
typical landscaping required for a site. Costs beyond the normal landscaping fees ·will include
the cost for testing the soils and may include costs for a sand bed and planting soil.
References and Sources of Additional Information
Coffman, L.S., R. Goo and R Frederick, 1999: Low impact development: an innovative
alternative approach to stormwater management. Proceedings of the 26th Annual Water
Resources Planning and Management ConferenceASCE, June 6-9, Tempe, Arizona.
Davis, A.P., Shokouhian, M., Sharma, H. and Mina.mi, C., "Laboratory Study of Biological
Retention (Bioretention) for Urban Stormwater Management," Water Environ. Res., 73(1), 5-14
(2001).
Davis, A.P., Shokouhian, M., Sharma, H., Minami, C., and Winogradoff, D. "Water Quality
Improvement through Bioretention: Lead, Copper, and Zinc," Water Environ. Res., accepted for
publication, August 2002.
Kim, H., Seagren, E.A., and Davis, AP., "Engineered Bioretention for Removal of Nitrate from
Stormwater Runoff," WEFTEC 2000 Conference Proceedings on CD ROM Research
Symposium, Nitrogen Removal, Session 19, Anaheim CA, October 2000.
Hsieh, C.-h. and Davis, A.P. "Engineering Bioretention for Treatment of Urban Stormwater
Runoff," Watersheds 2002, Proceedings on CDROM Research Symposium, Session 15, Ft.
Lauderdale, FL, Feb. 2002.
Prince George's County Department of Environmental Resources (PG DER), 1993. Design
Manual for Use of Bioretention in Stormwater Management. Division of Environmental
Management, Watershed Protection Branch. Landover, MD.
U.S. EPA Office of Water, 1999. Sto1mwater Technology Fact Sheet: Bioretention. EPA 832-F-
99-012.
Weinstein, N. Davis, A.P. and Veeramachaneni, R. "Low Impact Development (IJD) Stonnwater
Management Approach for the Control of Diffuse Pollution from Urban Roadways," 5th
International Conference DijJuse/Nonpoint Pollution and Watershed Management
Proceedings, C.S. Melching and Emre Alp, Eds. 2001 International Water Association
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Section 6
Long-term Maintenance of BMPs
6.1 Introduction
The long-term performance of BMPs hinges on ongoing and proper maintenance. In order for
this to occur detailed maintenance plans are needed that include specific maintenance activities
and frequencies for each type of BMP. In addition, these should include indicators for assessing
when "as needed" maintenance activities are required. The fact sheets included in this volume
contain the basic information needed to develop these maintenance plans, but municipalities
and other regulatory agencies also need to identify the responsible party and potentially to
address funding requirements. The following discussion is based primarily on data developed
by Horner et al. (1994) and information available at http://www.stormwatercenter.net/
6.2 Critical Regulatory Components
Critical regulatory components identified by Horner et al. (1994) include:
■ Regulations should officially designate a responsible party, frequently the development site
owner, to have ultimate responsibility for the continued maintenance of stormwater
facilities. This official designation provides the opportunity for appropriate preparation and
budgeting prior to actually assuming responsibilities. It also facilitates enforcement or other
legal remedies necessary to address compliance or performance problems once the facility
has been constructed.
■ Regulations should clearly state the inspection and maintenance requirements. Inspection
and maintenance requirements should also comply with all applicable statutes and be based
on the needs and priorities of the individual measure or facility. A clear presentation will
help owners and builders comply and inspectors enforce requirements.
■ Regulations should contain comprehensive requirements for documenting and detailing
maintenance. A facility operation and maintenance manual should be prepared containing
accurate and comprehensive drawings or plans of the completed facility and detailed
descriptions and schedules of inspection and maintenance.
■ The regulations should delineate the procedure for maintenance noncompliance. This
process should provide informal, discretionary measures to deal with periodic, inadvertent
noncompliance and formal and severe measures to address chronic noncompliance or
performance problems. In either case, the primary goal of enforcement is to maintain an
effective BMP -the enforcement action should not become an end in itself.
■ Regulations should also address the possibility of total default by the owner or builder by
providing a way to complete construction and continue maintenance. For example, the
public might assume maintenance responsibility. If so, the designated public agency must
be alerted and possess the necessary staffing, equipment, expertise, and funding to assume
this responsibility. Default can be addressed through bonds and other performance
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Section 6
Long-term Maintenance of BMPs
guarantees obtained before the project is approved and construction begins. These bonds
can then be used to fund the necessary maintenance activities.
■ The regulations must recognize that adequate and secure funding is needed for facility
inspection and maintenance and provide for such funding.
6.3 Enforcement Options
A public agency will sometimes· need to compel those responsible for facility construction or
maintenance to fulfill their obligations. Therefore, the maintenance program must have
enforcement options for quick corrective action. Rather than a single enforcement measure, the
program should have a variety of techniques, each with its own degree of formality and legal
weight. The inspection program should provide for nonconforming performance and even
default, and contain suitable means to address all stages.
Prior to receiving construction approval, the developer or builder can be forced to provide
performance guarantees. The public agency overseeing the construction can use these
guarantees, usually a performance bond or other surety in an amount equal to some fraction of
the facility's construction cost, to fund maintenance activities.
Enforcement of maintenance requirements can be accomplished through a storm water
maintenance agreement, which is a formal contract between a local government and a property
owner designed to guarantee that specific maintenance functions are performed in exchange for
permission to develop that property (http:/ /www.stormwatercenter.net/). Local governments
benefit from these agreements in that responsibility for regular maintenance of the BMPs can be
placed upon the property owner or other legally recognized party, allowing agency staff more
time for plan review and inspection.
6.4 Maintenance Agreements
Maintenance agreements can be an effective tool for ensuring long-term maintenance of on-site
BMPs. The most important aspect of creating these maintenance agreements is to clearly define
the responsibilities of each party entering into the agreement. Basic language that should be
incorporated into an agreement includes the following:
1. Performance of Routine Maintenance
Local governments often find it easier to have a property owner perform all maintenance
according to the requirements of a Design Manual. Other communities require that property
owners do aesthetic maintenance (i.e., mowing, vegetation removal) and implement pollution
prevention plans, but elect to perform structural maintenance and sediment removal
themselves.
2. Maintenance Schedules
Maintenance requirements may vary, but usually governments require that all BMP owners
perform at least an annual inspection and document the maintenance and repairs performed.
An annual report must then be submitted to the government, who may then choose to perform
an inspection of the facility.
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Section 6
Long-term Maintenance of BMPs
commitment to inspection and maintenance difficult and subject to political pressures.
Nevertheless, tax revenues remain a popular funding source because the collection and
disbursement system is already in place and familiar.
Utility Charges
Using utility charges to fund inspection and maintenance is a somewhat recent application of an
already established financing technique. In addition, several municipalities and counties
throughout the country have runoff management, drainage, and flood control authorities or
districts to provide residents with runoff related services.
Using utility chc!-rge financing has several advantages. By addressing only runoff needs and
benefits, utility funding avoids competing with other programs and needs. Utility funding also
demonstrates a direct link between the funding and the services it provides. This approach can
require an entirely new operating system and organization that needs legal authorization to
exist, operate, and assess charges. The effort required to create such an entity can deter many, . .
although the continued success of established authorities and growth of new ones have done
much to allay concerns over the effort required.
In a runoff utility, the user charges are often based on the need for services rather than the
benefits derived from them. While charges are based on actual costs to inspect and maintain
runoff facilities and measures within the service area, the assessed rate structure should relate to
site characteristics. These include property area size, extent of impervious coverage, and other
factors with a direct and demonstrable effect on runoff. To be fair, the rate structure should also
remain simple and understandable to the ratepayer.
To finance the stormwater utility in Prince William County, Virginia, residential and
nonresidential owners of developed property pay based on the amount of impervious area
(rooftops, paved areas, etc.) on their property. Residents pay $10.38 billed twice a year ($20.76
total annual fee) for detached singe-family homes. Town home and condominium owners will
pay $7.785 billed twice a year ($15.57 total annual fee). Nonresidential property owners pay
$0.84 per 1,000 ft2 of impervious area per month. Fee adjustments or credits may be available
if a stormwater management system is already in place. The fee will be on the real estate bills.
Fees for the stormwater utility in Austin, Texas are higher with residential users billed
$5.79/mo, while commercial users pay $94.62/mo/acre of impervious cover. These fees cover
not only maintenance of existing BMPs, but also capital improvement projects related to the
drainage infrastructure.
Permit Fees
Collecting permit fees to finance runoff inspection and maintenance is a long standing funding
procedure. Most governmental entities local, county, and state can establish and collect fees
and other charges to obtain operating funds for programs and services. Many inspection
services, most notably the construction inspection of both ESC measures and permanent
drainage and runoff management facilities, are financed at least in part through fees collected by
permitting agencies. Unlike taxes or some utility charges, inspection costs are borne by those
who need them.
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Section 6
Long-term Maintenance of BMPs
The permit fee collection program should have a demonstrable link to the runoff management
or drainage systems. The public agency should demonstrate a direct link between the permit
fees collected and the permitted project one method is using dedicated accounts for individual
projects and facilities. Finally, the rate structure should reflect site characteristics such as area
size or imperviousness that directly relate to the measure or facility by affecting runoff or
erosion.
Dedicated Contributions
Public agencies at times have used developer contributions to fund long-term facility
maintenance. This approach is particularly appropriate in single-family residential
subdivisions, where numerous individual property owners served by a single runoff facility can
result in confusion over who has maintenance responsibility.
The exact funding technique depends on many factors, including community attitude and
knowledge, economic and political viability, and program needs and costs. Some techniques,
including permit fees and dedicated contributions, may be more appropriate for short-term
activities, such as construction inspection. Others utility charges and specialized tax revenues
may apply to all phases of an inspection and maintenance program but require considerable
effort and special legal authorization to operate.
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PRELIMINARY STORM WATER MANAGEMENT PLAN
ROBERTSON RANCH, PA 22
APPENDIX I: APPLICABLE MANUFACTURER'S BMP INFORMATION
Please see attached.
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PRELIMINARY STORM WATER MANAGEMENT PLAN
ROBERTSON RANCH, PA 22
APPENDIX J: MAP EXHIBITS
Please see attached.
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PRELIMINARY STORM WATER MANAGEMENT PLAN
PA22
BIORETENTION SIZING CALCULATIONS:
Sub-Basin A (ac.) C I (in./hr.) Q (treat) = CIA
A2 0.73 0.82 0.2 0.12
B1 0.59 0.82 0.2 0.10
B2 0.21 0.82 0.2 0.03
B3 0.29 0.82 0.2 0.05
B4 0.17 0.82 0.2 0.03
B5 0.15 0.82 0.2 0.02
B7 0.07 0.82 0.2 0.01
BB 0.05 0.82 0.2 0.01
B9 0.53 0.82 0.2 0.09
B10 0.83 0.82 0.2 0.14
Required Surface Area
(sq.-ft.) = C*A*4%
1043
843
300
414
243
214
100
71
757
1186
G:\011014\SWMP\PA 22\PSWMP-PA22\Bioretention.xls
Provided Surface Area (sq.
ft.)
1785
910 (600 sf pervious pvmt)
1790
510 (305 sf pervious ovmt)
1093
830
440
350
2400
3260