HomeMy WebLinkAboutMS 14-09; TAMARACK BEACH CUSTOM HOMES; HYDROLOGY STUDY FOR 295 CHINQUAPIN AVE., CARLSBAD, CA; 2015-01-20HYDROLOGY STUDY
for
295 Chinquapin Ave., Carlsbad, CA
Grading Permit
Tamarack Beach Custom Homes
City of Carlsbad, CA
PREPARED FOR:
RINCON REAL ESTATE GROUP, INC.
1520 N. EL CAMINO REAL, UNIT 5
SAN CLEMENTE, CA 92672
(949)637-3354
MIN
Date: January 20, 2015 JAN 20 2Ui5
PREPARED BY:
Pasco Laret Suiter & Associates
535 N. Highway 101, Suite A
Solana Beach, CA 92075
(858) 259-8212
No. 80356 Exp.
TYLER G LAWSON, RCE 80356 DATE
Preliminary Hydrology Study for Tamarack Beach Costume Homes
PLSA 2211
TABLE OF CONTENTS
SECTION PAGE
Executive Summary 1.0 3
Introduction 1.1 3
Existing Conditions 1.2 3
Proposed Project 1.3 4
Summary of Results and Conditions 1.4 5
Conclusions 1.5 5
References 1.6 5
Methodology 2.0 6
Introduction 2.1 6
County of San Diego Criteria 2.2 7
Runoff coefficient determination 2.3 7
Hydrologic Analyses 3.0 8
Pre-Developed Hydrologic Analysis 3.1
Post-Developed Hydrologic Analysis 3.2
Appendix 4.0
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1.0 EXECUTIVE SUMMARY
1.1 Introduction
This Hydrology Study for the Tamarack Beach Custom Homes project has been prepared
to analyze the hydrologic and hydraulic characteristics of the existing and proposed
project site. This report intends to present both the methodology and the calculations
used for determining the runoff from the project site in both the pre-developed (existing)
conditions and the post-developed (proposed) conditions produced by the 2-year, 10-year,
and 100-year, 6-hour storm.
1.2 Existing Conditions
The property is geographically located at N 33008'50" W 117020'29". The site is
bordered by a residential development on all sides with denser multifamily developments
to the north and south. The project site is located in San Marcos Creek Hydrologic Area
and more specifically, the Agua Hedionda Creek Hydrologic Sub-Area (904.21). The
project is located at 295 Chinquapin Ave.
The existing site consists of a single family residence, an accessory structure in the rear
and hardscape patios, walkways and driveways throughout. The site generally slopes
from north to south away from Chinquapin Avenue. Drainage from the existing site sheet
flows towards the south west corner of the project where it then flows onto the
neighboring western property. This drainage pattern can be seen in the appendix, on the
City of Carlsbad's drawing 387-8A. Once on the neighboring property, .the runoff enters
a private storm drain system and is routed to the west and south. The water is then
released out of the storm drain and flows through a rip rap energy dissipater before
entering the property to the south. The runoff then travels through the adjacent southern
property. This drainage pattern can be seen in the appendix on the City of Carlsbad's
drawing 438-4A. The water is then routed to the public storm drain system on Date
Avenue. The storm water ultimately drains to the Agua Hedionda Lagoon and Pacific
Ocean.
Based on a site specific hydrology node map, it was determined that the basin has an
approximate area of 0.31 acres and has a runoff coefficient of 0.55. Using the Rational
Method Procedure, the peak flow rate and time of concentration calculated for the
drainage basin for the 100-year, 6-hour storm event is 1.12 CFS and 5.0 minutes. See
section 3.1 for pre-development calculations.
Existing Basin
area (sf)
Hardscape 4683
Landscape 8695
Total 13378
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1.3 Proposed Project
The intent of proposed project is to develop the site into a 3-unit detached condo map
with associated landscaping and hardscape improvements. The project proposes minimal
grading to construct the building and the construction of all underground utilities
typically associated with residential development.
The project was designed to mimic the existing drainage pattern where feasible. As with
the pre-developed condition, runoff collected on site will be directed to travel from north
to south away from Chinquapin Avenue. Runoff from roofs, patios and hardscape areas
will be directed to flow towards a vegetated swale bordering the eastern property line.
In an effort to reduce impervious surfaces where feasible, porous payers are proposed for
the common driveway. Per the site specific geotechnical investigation, low permeability
soils may exist at relatively shallow depths and the site could be subjected to shallow
'perched' water conditions. However, the geotechnical investigation goes on to state that
no such conditions were encountered on the site. Additionally, per the supplemental
geotechnical letter dated October 22, 2014 porous payers are acceptable and
recommended for this site.
Excess runoff from this driveway will be collected in a curb & gutter and directed to the
southwest corner of the site. Once collected in either the vegetated swale or curb &
gutter, runoff will then be discharged to a riprap energy dissipater located in the
southwest corner. From there, runoff will travel to the existing storm drain system
located on the western property. Runoff will then be directed south and west through an
adjacent property before discharging to the Agua Hedionda Lagoon and ultimately the
Pacific Ocean.
Based on a site specific hydrology node map, it was determined that the basin has an
approximate area of 0.31 acres and has a runoff coefficient of 0.54. Using the Rational
Method Procedure, the peak flow rate and time of concentration calculated for the basin
for the 100 year, 6 hour storm event is 1.10 CFS and 5.0 minutes. See section 3.2 for
post-development calculations.
By reducing the impervious area and proposing pervious payers and landscaping where
feasible, the proposed project reduces the overall impervious area from the pre-
development condition. This reduction in impervious area results in a net reduction of
runoff.
Based on the City of Carlsbad's Storm Water Standards Questionnaire E-34, the project
is not a priority development project. To address the storm water quality goals
established for this development, proposed Best Management Practices (BMP's) and
Low Impact Developments (LID's) methods will be incorporated into the storm water
runoff design. The proposed project includes vegetated swales to naturally treat storm
water runoff, and porous payers to help reduce the impervious area of the proposed
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development. Both of these items were included as part of the site design to comply with
the City of Carlsbad's SUSMP. Additionally, per the City's SUSMP, all impervious
surfaces will be directed to pervious BMP areas prior to discharging the site.
Proposed Basin
area (sf)
Hardscape 4614
Landscape 3413
Payers 5351
Total 13378
1.4 Summary of Results
Upon preparing hydrologic calculations for both the pre-development and post-
development, the following results were produced.
The pre-development condition indicates that the 2 year peak flow rate is 0.54 cfs, the 10
year peak flow rate is 0.76 cfs, and the 100-year peak flow rate is 1.12 cfs with a time of
concentration of 5 min based on an area of 0.31 AC.
The post-development condition indicates that the 2 year peak flow rate is 0.53 cfs, the
10 year peak flow rate is 0.75 cfs, and the 100-year peak flow rate is 1.10 cfs with a time
of concentration of 5 min based on an area of 0.31 AC.
Because the proposed project reduces the impervious area and the project is a not a
priority development project, the project is exempt from HMP requirements. The site has
been designed to treat the minimum LID. The site has been layout to drain the
impervious area into the pervious areas, and the proposed road is pervious payers to help
decrease the impervious area.
1.5 Conclusions
Based on the discussion in this report it is the professional opinion of Pasco Laret Suiter
& Associates, Inc. that the existing drainage system on the corresponding Tentative
Parcel Map will function to adequately intercept, contain and convey flow to the
appropriate points of discharge.
1.6 References
"San Diego County Hydrology Manual", revised June 2003, County of San Diego,
Department of Public Works, Flood Control Section.
"California Regional Water Quality Control Board Order No. 2009-0009-D WQ,"
California Regional Water Control Board, San Diego Region (SDRWQCB).
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2.0 METHODOLOGY
2.1 Introduction
The hydrologic model used to perform the hydrologic analysis presented in this report
utilizes the Rational Method (RM) equation, Q=CIA. The RM formula estimates the
peak rate of runoff based on the variables of area, runoff coefficient, and rainfall
intensity. The rainfall intensity (I) is equal to:
I = 7.44 x P6 x D.0645
Where:
I = Intensity (in/hr)
P6 = 6-hour precipitation (inches)
D = duration (minutes - use Tc)
Using the Time of Concentration (Tc), which is the time required for a given element of
water that originates at the most remote point of the basin being analyzed to reach the
point at which the runoff from the basin is being analyzed. The RM equation determines
the storm water runoff rate (Q) for a given basin in terms of flow (typically in cubic feet
per second (cfs) but sometimes as gallons per minute (gpm)). The RM equation is as
follows:
Q=CIA
Where:
Q= flow (in cfs)
C = runoff coefficient, ratio of rainfall that produces storm water
runoff (runoff vs. infiltration/evaporation/absorption/etc)
I = average rainfall intensity for a duration equal to the Tc for the
area, in inches per hour.
A = drainage area contributing to the basin in acres.
The RM equation assumes that the storm event being analyzed delivers precipitation to
the entire basin uniformly, and therefore the peak discharge rate will occur when a
raindrop falls at the most remote portion of the basin arrives at the point of analysis. The
RM also assumes that the fraction of rainfall that becomes runoff or the runoff coefficient
C is not affected by the storm intensity, I, or the precipitation zone number.
In addition to the above Rational Method assumptions, the runoff coefficients utilized for
this report are based on type "B" soils per San Diego County Hydrology Manual.
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2.2 County of San Diego Criteria
As defined by the County Hydrology Manual dated June 2003, the rational method is the
preferred equation for determining the hydrologic characteristics of basins up to
approximately one square mile in size. The County of San Diego has developed its own
tables, nomographs, and methodologies for analyzing storm water runoff for areas within
the county. The County has also developed precipitation isopluvial contour maps that
show even lines of rainfall anticipated from a given storm event (i.e. 100-year, 6-hour
storm).
One of the variables of the RM equation is the runoff coefficient, C. The runoff
coefficient is dependent only upon land use and soil type and the County of San Diego
has developed a table of Runoff Coefficients for Urban Areas to be applied to basin
located within the County of San Diego. The table categorizes the land use, the
associated development density (dwelling units per acre) and the percentage of
impervious area. Each of the categories listed has an associated runoff coefficient, C, for
each soil type class.
The County has also illustrated in detail the methodology for determining the time of
concentration, in particular the initial time of concentration. The County has adopted the
Federal Aviation Agency's (FAA) overland time of flow equation. This equation
essentially limits the flow path length for the initial time of concentration to lengths of
100 feet or less, and is dependent on land use and slope.
2.3 Runoff Coefficient Determination
As stated in section 2.2, the runoff coefficient is dependent only upon land use and soil
type and the County of San Diego has developed a table of Runoff Coefficients for Urban
Areas to be applied to basin located within the County of San Diego. The table, included
at the end of this section, categorizes the land use, the associated development density
(dwelling units per acre) and the percentage of impervious area.
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3.0 HYDROLOGIC ANALYSES
3.1 Pre-Developed Hydrologic Model Output (100 Year Event)
Pre-Development:
Q=CIA
P2=1.2
Pio=1.7
P100=2.5
Basin 1
Total Area= 13,378 sf4 0.31 Acres
Impervious Area = 4,683 sf4 0.11 Ac
Pervious Area = 8,695 sf 4 0.20 Ac
Cn, Weighted Runoff Coefficient,
- 0.35, Cn value for natural ground, San Diego Hydrology Design Manual (SDHDM)
- 0.90, Cn value for developed/impervious surface, SDHDM
Cn = 0.9 x 4,683 sf+ 0.35 x 8,795 sf= 0.55
13,378 sf
Tc =D= 5.0 Mm (mm.)
P6 = 2.5
I = 7.44 x P6 D 0645
I = 7.44 x 2.5 x 5•045 6.59 in/hr
Iwo 6.59 in/hr
I2 3.16 in/hr
Iio 4.48 in/hr
Q2 = 0.55 x 3.16 in/hr x 0.31 Ac = 0.54 cfs
Qio = 0.55 x 4.48 in/hr x 0.31 Ac = 0.76 cfs
Qioo = 0.55x 6.59 in/hr x 0.31 Ac = 1.12 cfs
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3.2 Post-Developed Hydrologic Model Output (100 Year Event)
Post-Development:
Q=CIA
P2=1.2
P10=1 .7
P100=2.5
Basin 1
Total Area = 13,378 sf 4 0.31 Acres
Impervious Area = 4,614 sf4 0.11 Ac
Pervious Area = 8,764 sf 4 0.20 Ac
Cn, Weighted Runoff Coefficient,
- 0.35, Cn value for natural ground, San Diego Hydrology Design Manual (SDHDM)
- 0.90, Cn value for developed/impervious surface, SDHDM
Cn = 0.9 x 4.614 sf+ 0.35 x 8,764 sf= 0.54
13,378 sf
Tc =D= 5.0 Mm (mm.)
P6 = 2.5
I = 7.44 X P6 x D.645
I = 7.44 x 2.5 x 5.005 6.59 in/hr
Iioo 6.59 in/hr
I2 3.16 in/hr
Iio 4.48 in/hr
Q2 = 0.54 x 3.16 in/hr x 0.31 Ac = 0.53 cfs
Qio = 0.54 x 4.48 in/hr x 0.31 Ac = 0.75 cfs
Qioo = 0.54 x 6.59 in/hr x 0.31 Ac = 1.10 cfs
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3.3 Detention Volume Calculation
PEAK DRAINAGE FLOW COMPARISON
DRAINAGE
CONDITION 02(CFS) Q10(CFS) Q100(CFS) TC(MINS.)
AREA (ACRES)
EXISTING 0.31 0.54 0.76 1.12 5
PROPOSED 0.31 0.53 0.75 1.10 5
EXISTING VS. PROPOSED
LESS THEN LESS THEN LESS THEN SAME AS
CONDITION COMPARISON
- EXISTING EXISTING EXISTING EXISTING
BASIN A
EXISTING VS. PROPOSED
LESS THEN LESS THEN LESS THEN SAME AS CONDITION COMPARISON -
EXISTING EXISTING EXISTING EXISTING BASIN B
Due to the decrease in the flow rate for the site, no detention is required.
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4.0 APPENDIX
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11
GHRfl Geotechnical Exploration, Inc.
SOIL AND FOUNDATION ENGINEERING ó GROUNDWATER 0 ENGINEERING GEOLOGY
22 October 2014
Mr. Kevin Dunn
RINCOW REAL ESTATE GROUP, INC.
1520 N. El Camino Real, Unit 5
San Clemente, CA 92672
Subject: Permeable Payers
Rincon Residential Project
295 Chinquapin Avenue
Carlsbad, California
Dear Mr. Dunn:
Job No. 14-10523
In accordance with the request of Mr. Tyler Lawson of Pasco, Laret, Sulter,
Geotechnical Exploration, Inc. has evaluated our findings with regard to the
suitability of permeable payers for the proposed driveway areas at 295 Chinquapin
Avenue. The subject site is more particularly referred to as Assessor's Parcel No.
206-080-13-00, Lot 6 of Block T, according to Recorded Map 1803, in the City of
Carlsbad, County of San Diego, State of California. We previously conducted a
geotechnical investigation and issued a "Report of Preliminary Geotechnical
Investigation," dated May 15, 2014.
Based on our site observations and laboratory testing, it is our opinion that the silty
sand fill soils and underlying medium dense silty sand formational soils are well-
suited for the use of permeable payers. Perching conditions as described on page
16 of our May 15, 2014, report were not encountered within 51/2 feet on the lot and
do not exist at the shallow depths that would impact the performance of the
permeable payers.
It is recommended that a minimum 6-inch-thick base layer of crushed
miscellaneous rock material, compacted to at least 95 percent relative compaction,
be placed below a 1-Inch-thick leveling sand layer under the payers. The subgrade
soils supporting the base layer should also be compacted to 95 percent relative
compaction.
7420 TRADE STREE70 SAN DIEGO, CA. 92121 (858) 549-7222.'FAX: (858) 549-1604* EMAIL: geoted@gel-sd.com
,0f
Carlsbad
STORM WATER
STANDARDS
QUESTIONNAIRE
E-34
Development Services
Land Development Engineering
1635 Faraday Avenue
760-602-2750
www.carlsbadca.gov
IEI INSTRUCTIONS: 1. I
To address post-development pollutants that may be generated from development projects, the City requires that new development and
significant redevelopment priority projects incorporate Permanent Storm Water Best Management Practices (BMP's) into the project
design per the City's Standard Urban Stormwater Management Plan (SUSMP). To view the SUSMP, refer to the Engineering Standards
(Volume 4, Chapter 2).
Initially this questionnaire must be completed by the applicant in advance of submitting for a development application (subdivision,
discretionary permits and/or construction permits). The results of the questionnaire determine the level of storm water standards that
must be applied to a proposed development or redevelopment project. Depending on the outcome, your project will either be subject to
'Standard Stormwater Requirements' or be subject to additional criteria called 'Priority Development Project Requirements'. Many
aspects of project site design are dependent upon the storm water standards applied to a project.
Your responses to the questionnaire represent an initial assessment of the proposed project conditions and impacts. City staff has
responsibility for making the final assessment after submission of the development application. If staff determines that the questionnaire
was incorrectly filled out and is subject to more stringent storm water standards than initially assessed by you, this will result in the return
of the development application as incomplete. In this case, please make the changes to the questionnaire and resubmit to the City.
If you are unsure about the meaning of a question or need help in determining how to respond to one or more of the questions, please
seek assistance from Land Development Engineering staff.
A separate completed and signed questionnaire must be submitted for each new development application submission. Only one
completed and signed questionnaire is required when multiple development applications for the same project are submitted concurrently.
In addition to this questionnaire, you must also complete, sign and submit a Project Threat Assessment Form with construction permits
for the project.
Please start by completing Step I and follow the instructions. When completed, sign the form at the end and submit this with your
application to the city.
STEP-11,
TO BE COMPLETED FOR ALL PROJECTS
To determine if your project is a priority development project, please answer the following questions:
YES NO
Is your project LIMITED TO constructing new or retrofitting paved sidewalks, bicycle lanes or trails that meet
the following criteria: (1) Designed and constructed to direct storm water runoff to adjacent vegetated areas, or
other non-erodible permeable areas; OR (2) designed and constructed to be hydraulically disconnected from
paved streets or roads; OR (3) designed and constructed with permeable pavements or surfaces in
accordance with USEPA Green Streets guidance?
Is your project LIMITED TO retrofitting or redeveloping existing paved alleys, streets, or roads that are
designed and constructed in accordance with the USEPA Green Streets guidance?
If you answered "yes" to one or more of the above questions, then your project is NOT a priority development project and therefore is
NOT subject to the storm water criteria required for priority development projects. Go to step 4, mark the last box stating "my
project does not meet PDP requirements" and complete applicant information.
If you answered "no" to both questions, then go to Step 2.
E-34 Page 1 of 3 Effective 6/27/13
STORM WATER
STANDARDS
QUESTIONNAIRE
E-34
Development Services
Land Development Engineering
1635 Faraday Avenue
760-602-2750
www.carlsbadca.gov
STEP2
To:BEcMpLETEb FoR ALLNEW OR REDEVELOPMENTPRojEcts:.
To determine if your project is a priority development project, please answer the following questions: YES NO
Is your project a new development that creates 10,000 square feet or more of impervious surfaces collectively
over the entire project site? This includes commercial, industrial, residential, mixed-use, and public
development projects on public or private land.
Is your project creating or replacing 5,000 square feet or more of impervious surface collectively over the entire
project site on an existing site of 10,000 square feet or more of impervious surface? This includes commercial,
industrial, residential, mixed-use, and public development projects on public or private land.
Is your project a new or redevelopment project that creates 5,000 square feet or more of impervious surface
collectively over the entire project site and supports a restaurant? A restaurant is a facility that sells prepared
foods and drinks for consumption, including stationary lunch counters and refreshment stands selling prepared
foods and drinks for immediate consumption.
Is your project a new or redevelopment project that creates 5,000 square feet or more of impervious surface
collectively over the entire project site and supports a hillside development project? A hillside development
project includes development on any natural slope that is twenty-five percent or greater.
Is your project a new or redevelopment project that creates 5,000 square feet or more of impervious surface
collectively over the entire project site and supports a parking lot. A parking lot is a land area or facility for the
temporary parking or storage of motor vehicles used personally for business or for commerce.
Is your project a new or redevelopment project that creates 5,000 square feet or more of impervious surface
collectively over the entire project site and supports a street, road, highway freeway or driveway? A street,
road, highway, freeway or driveway is any paved impervious surface used for the transportation of
automobiles, trucks, motorcycles, and other vehicles.
Is your project a new or redevelopment project that creates or replaces 2,500 square feet or more of
impervious surface collectively over the entire site, and discharges directly to an Environmentally Sensitive
"Discharging Area (ESA)? Directly to" includes flow that is conveyed overland a distance of 200 feet or less
from the project to the ESA, or conveyed in a pipe or open channel any distance as an isolated flow from the
project to the ESA (i.e. not commingles with flows from adjacent lands).*
Is your project a new development that supports an automotive repair shop? An automotive repair shop is a
facility that is categorized in any one of the following Standard Industrial Classification (SIC) codes: 5013,
5014, 5541, 7532-7534, or 7536-7539.
Is your project a new development that supports a retail gasoline outlet (RGO)? This category includes RGO's
that meet the following criteria: (a) 5,000 square feet or more or (b) a project Average Daily Traffic (ADT) of
100 or more vehicles per day.
Is your project a new or redevelopment project that results in the disturbance of one or more acres of land and
are expected to generate pollutants post construction?
Is your project located within 200 feet of the Pacific Ocean and (1) creates 2,500 square feet or more of
impervious surface or (2) increases impervious surface on the property'by more than 10%?
If you answered "yes" to one or more of the above questions, you ARE a priority development project and are therefore subject to
implementing structural Best Management Practices (BMP's) in addition to implementing Standard Storm Water Requirements such
as source control and low impact development BMP's. A Storm Water Management Plan (SWMP) must be submitted with your
application(s) for development. Go to step 3 for redevelopment projects. For-new projects, go to step 4 at the end of this
questionnaire, check the "my project meets PDP requirements" box and complete applicant Information.
If you answered "no" to all of the above questions, you ARE NOT a priority development project and are therefore subject to
implementing only Standard Storm Water Requirements such as source control and low impact development BMP's required for all
development projects. A Storm Water Management Plan (SWMP) is not required with your application(s) for development. Go to step
4 at the end of this questionnaire, check the "my project does not meet PDP requirements" box and complete applicant
Information.
E-34 Page 2 of 3 Effective 6/27/13
STORM WATER
STANDARDS
QUESTIONNAIRE
E-34
Development Services
Land Development Engineering
1635 Faraday Avenue
760-602-2750
www.carlsbadca.gov
STEP3 .
TO BE COMPLETED FOR REDEVELOPMENT PROJECTS THAT ARE PRIORITY DEVELOPEMENT PROJECTS ONLY
Complete the questions below regarding your redevelopment project: YES NO
Does the redevelopment project result in the creation or replacement of impervious surface in an amount of less than
7 50% of the surface area of the previously existing development?
If you answered "yes," the structural BMP's required for Priority Development Projects apply only to the creation or replacement of
impervious surface and not the entire development. Go to step 4, check the 'my project meets PDP requirements" box and
complete applicant information.
If you answered no," the structural BMP's required for Priority Development Projects apply to the entire development. Go to step 4,
check the "my project meets PDP requirements" box and complete applicant information.
STEP4 .. .
CHECK THE APPROPRIATE BOX AND COMPLETE APPLICANT INFORMATION
U My project meets PRIORITY DEVELOPMENT PROJECT (PDP) requirements and must comply with additional stormwater
criteria per the SUSMP and I understand I must prepare a Storm Water Management Plan for submittal at time of application.
I understand flow control (hydromodification) requirements may apply to my project. Refer to SUSMP for details.
My project does not meet PDP requirements and must only comply with STANDARD STORMWATER REQUIREMENTS per
the SUSMP. As part of these requirements, I will incorporate low impact development strategies throughout my project.
Applicant Information and Signature Box
Address: S Accessor's Parcel Number(s): 295 Chinquapin Drive, Carlsbad, CA 209-080-13
Applicant Name: Applicant Title:
Kevin Dunn For, Rincon Real Estate Group, Inc. Director
Applicant Signature: . Date:
This Box for City Use Only
nce: YES NO
F
* 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
quivalent under the Multi Species Conservation Program within the Cities and County of San Diego; and any other equivalent
environmentally sensitive areas which have been identified by the Coperrnittees.
E-34 Page 3 of 3 Effective 6/27/13
San Diego County Hydrology Manual Section: 3
Date: June 2003 Page: 12 of 26
Note that the Initial Time of Concentration should be reflective of the general land-use at the
upstream end of a drainage basin. A single lot with an area of two or less acres does not have
a significant effect where the drainage basin area is 20 to 600 acres.
Table 3-2 provides limits of the length (Maximum Length (LM)) of sheet flow to be used in
hydrology studies. Initial T1 values based on average C values for the Land Use Element are
also included. These values can be used in planning and design applications as described
below. Exceptions may be approved by the "Regulating Agency" when submitted with a
detailed study.
Table 3-2
MAXIMUM OVERLAND FLOW LENGTH (LM)
& INITIAL TIME OF CONCENTRATION (T)
Element* DU/
Acre
.5% 1% 2% 3% 5% 10%
LM Tj LM Tj LM T1 LM T1 LM Tj LM T1
Natural 50 13.2 70 12.5 85 10.9 100 10.3 100 8.7 100 6.9
LDR 1 50 12.2 70 11.5 85 10.0 100 9.5 100 8.0 100 6.4
LDR 2 50 11.3 70 10.5 85 9.2 100 8.8 100 7.41 100 5.8
LDR 2.9 50 10.7 70 10.0 85 8.8 95 8.1 100 7.0 100 5.6
MDR 4.3 50 10.2 70 9.6 80 8.1 95 7.8 100 6.7 100 5.3
MDR 7.3 50 9.2 65 8.4 80 7.4 95 7.0 100 6.0 100 4.8
MDR 10.9 50 8.7 65 7.9 80 6.9 90 6.4 100 5.7 100 4.5
MDR 14.5 1 50 8.21 65 7.41 80 6.5 1 90 6.01 100 5.4 1001 4.3
HDR 24 50 6.7 65 6.1 75 5.1 90 4.9 95 4.3 100 3.5
HDR 43 50 5.3 65 4.7 75 4.0 85 3.8 95 3.4 100 2.7
N. Corn 50 5.3 60 4.5 75 4.0 85 1 3.8 95 1 3.4 100 2.7
G. Corn 50 1 4.71 60 4.1 1 75 3.6 85 3.4 90 2.91 100 2.4
O.P./Corn 50 4.2 60 3.7 70 3.1 80 2.9 90 2.6 100 2.2
Limited I. 50 4.2 60 3.7 70 3.1 80 2.9 90 2.6 100 2.2
General I. 50 3.7 60 1 3.2 70 2.7 80 1 2.6 90 2.3 100 1.9
*See Table 3-1 for more detailed description
3-12
San Diego County Hydrology Manual Section: 3
Date: June 2003 Page: 6of26
Table 3-1
RUNOFF COEFFICIENTS FOR URBAN AREAS
Land Use Runoff Coefficient "C"
Soil Type
NRCS Elements County Elements % IMPER. A B C D
Undisturbed Natural Terrain (Natural) Permanent Open Space 0* 0.20 0.25 0.30 0.35
Low Density Residential (LDR) Residential, 1.0 DU/A or less 10 0.27 0.32 0.36 0.41
Low Density Residential (LDR) Residential, 2.0 DU/A or less 20 0.34 0.38 0.42 0.46
Low Density Residential (LDR) Residential, 2.9 DU/A or less 25 0.38 0.41 0.45 0.49
Medium Density Residential (MDR) Residential, 4.3 DU/A or less 30 0.41 0.45 0.48 0.52
Medium Density Residential (MDR) Residential, 7.3 DU/A or less 40 0.48 0.51 0.54 0.57
Medium Density Residential (MDR) Residential, 10.9 DU/A or less 45 0.52 0.54 0.57 0.60
Medium Density Residential (MDR) Residential, 14.5 DU/A or less 50 0.55 0.58 0.60 0.63
High Density Residential (HDR) Residential, 24.0 DU/A or less 65 0.66 0.67 0.69 0.71
High Density Residential (HDR) Residential, 43.0 DU/A or less 80 0.76 0.77 0.78 0.79
Commercial/Industrial (N. Corn) Neighborhood Commercial 80 0.76 0.77 0.78 0.79
Commercial/Industrial (G. Com) General Commercial 85 0.80 0.80 0.81 0.82
Commercial/Industrial (O.P. Corn) Office Professional/Commercial 90 0.83 0.84 0.84 0.85
Commercial/Industrial (Limited I.) Limited Industrial 90 0.83 0.84 084 0.85
Commercial/Industrial (General I.) General Industrial 95 0.87 0.87 0.87 0.87
The values associated with 0% impervious may be used for direct calculation of the runoff coefficient as described in Section 3.1.2 (representing the pervious runoff
coefficient, Cp, for the soil type), or for areas that will remain undisturbed in perpetuity. Justification must be given that the area will remain natural forever (e.g., the area
is located in Cleveland National Forest).
DU/A = dwelling units per acre
NRCS =National Resources Conservation Service
3-6
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Targeted Constituents
El Sediment A
El Nutrients S
El Trash
El Metals A
El Bacteria
El Oil and Grease A
El Organics A
Legend (Removal Effectiveness)
Low • High
A Medium
Vegetated Swale TC-30
A.
-I -
tL ur
- -1
Description
Vegetated swales are open, shallow channels with vegetation
covering the side slopes and bottom that collect and slowly
convey runoff flow to downstream discharge points. They are
designed to treat runoff through filtering by the vegetation in the
channel, filtering through a subsoil matrix, and/or infiltration
into the underlying soils. Swales can be natural or manmade.
They trap particulate pollutants (suspended solids and trace
metals), promote infiltration, and reduce the flow velocity of
stormwater runoff. Vegetated swales can serve as part of a
stormwater drainage system and can replace curbs, gutters and
storm sewer systems.
California Experience
Caltrans constructed and monitored six vegetated swales in
southern California. These swales were generally effective in
reducing the volume and mass of pollutants in runoff. Even in
the areas where the annual rainfall was only about 10 inches/yr,
the vegetation did not require additional irrigation. One factor
that strongly affected performance was the presence of large
numbers of gophers at most of the sites. The gophers created
earthen mounds, destroyed vegetation, and generally reduced the
effectiveness of the controls for TSS reduction.
Advantages
If properly designed, vegetated, and operated, swales can
serve as an aesthetic, potentially inexpensive urban
development or roadway drainage conveyance measure with
significant collateral water quality benefits.
Design Considerations
Tributary Area
Area Required
Slope
Water Availability
ASQ1
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TC-30 Vegetated Swale
Roadside ditches should be regarded as significant potential swale/buffer strip sites and
should be utilized for this purpose whenever possible.
Urn itat ions
. 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.
They 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 normally 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 part of a regularly irrigated landscaped area.
The width of the swale should be determined using Manning s Equation using a value of
o. 25 for Manning's n.
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Vegetated Swale TC-30
Construction/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.
Where seeds are used, erosion controls will be necessary to protect seeds for at least 75 days
after the first rainfall of the season.
Performance
The literature suggests that vegetated swales represent a practical and potentially effective
technique for controlling urban runoff quality. While 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 performed by the Nationwide Urban Runoff Program (NURP) monitored
three grass swales in the Washington, D. G, area and found no significant improvement in urban
runoff quality for the pollutants analyzed. However, the weak performance 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. The project tracked ii storms and
concluded that particulate 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.
The effectiveness of vegetated swales can be enhanced by adding check dams at approximately
17 meter (50 foot) increments along their length (See Figure i). These 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 have been conducted on all grassed channels designed for water quality (Table 1).
The data suggest relatively high removal rates for some pollutants, but negative removals for
some bacteria, and fair performance for phosphorus.
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TC-30 Vegetated Swale
rable 1 Grassed swale pollutant removal efficiency data
Removal Efficiencies (96 Removal)
Study ThS TP TN NO3 Metals Bacteria Type
Callrans 2002 i' 8 67 66 83-90 -33 dry swales
Goldberg 1993 67.8 4.5 - 31.4 42-62 -ioo grassed channel
Seattle Metro and Washington 60 45 Department of Ecology 1992 - - 2-16 - grassed channel
Seattle Metro and Washington
Department of Ecology, 1992 83 29 - -25 46-73 - grassed channel
Wang et at, 1981 80 - - - 70-80 - dry swale
Dorman et al., 1989 98 18 - 45 37-81 - dry swale
Harper, 1988 87 83 84 80 88-90 - dry swale
Kercher et al., 1983 99 99 99 99 99 - dry swale
Harper, 1988. 81 17 40 52 37-69 - wet swale
Koon, 1995 67 39 - 9 -35 to 6 1 - Pet swale
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 thy swales,
although some swales appear to export 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 of less 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, iç))
Comparable performance to wet basins
Limited to treating a few acres
Availability of water during thy 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 thy
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 between 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 Washington Department of Ecology, 1992), and is not well
supported. Analysis 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 ofDesign Recommendations
i) The swale should have a length that provides a minimum hydraulic residence time of
at least 10 minutes. The maximum bottom width should not exceed 10 feet unless a
dividing berm is provided. The depth of flow should not exceed 2/3rd5 the height of
the grass at the peak of the water quality design storm intensity. The channel slope
should not exceed 2.5%.
A design grass height of 6 inches is recommended.
Regardless of the recommended detention time, the 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 the peak hydraulic flows of the 100-year storm if it is
located "on-line." The side slopes should be no steeper than 3:1 (H:V).
Roadside ditches should be regarded as significant potential swale/buffer 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 the soil surface. For
general purposes, select fine, close-growing, 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. Where runoff diversion is not possible, cover graded and seeded
areas with suitable erosion control materials.
Maintenance
The useful life of a vegetated swale system is directly proportional 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 shorter 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. The 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
debris 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 winter. 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, particularly 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
Consivuction 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 ft-2. This 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 with other practices. A
more realistic estimate would be a total cost of approximately $0.50 per ft2, which compares
favorably with other stormwater management practices.
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TC-30 Vegetated Swale
Table 2 Swale Cost Estimate (SEWRPC, 1991)
Unit Cost V Total Cost
Low Moderate High LOW Moderate High Component Unit Extent
Mobization/ Swale 1 8107 $74 $441 $107 $274 $441 Demobilization-Light
Sille Preparation Cloaringb Acre 0.5 $,200 $3,800 $8,400 £1,100 $1,900 $2,700
Awe 0.25 $3,800 $5,200 $6,600 $050 $1,300 $1,650 GenarBI
E,tiOfld.... Yd' 372 52.10 $3.70 $5.30 . $761 $1,376 $1,972
Laval and TOO ........ .1,210 $020 $0.35 $0.50 $242 $424 $805
Sits Development
Salvaged Topsoil V
Seed, and Mulch' Yd' 1,210 $0.40 $1.00 $1.60 $484 $1,210 $1,936
Sod'. ..................... Yd' 1,210 $120 $.40 $360 *1452 $2,904 $4,356
Subtotal -- - -. - -- $5116 $9,366 $13,660
Contingencies Swale 1 25% 25% 26% $1270 $2,347 $3,415
Total -- - -- - -- $6,365 $11,735 817.075
Some: (SEWRPC, 1991)
Note: MobilizatoriPderrobilation rfersto the orga niiai and planning involved in establishing a vegetative swab.
Swale has a bottom width of 1.0 foot, a top width of 10 feet with 1:2 side slopes, and a 1,000-root length.
bArea cleared = (top width + 10 feet) x swabe length.
C Area grubbed = (top width x swie length).
dvolume excavated = (0.67 x top width x swale depth) x male length (parabolic crass-section).
Area tilled = (top width + B(swale deoth x swale length (parabolic cross-section).
3(top width)
'Area seeded = area cleared x 0.5.
'Area sodded = area cleared x 0.5.
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Table 3 Estimated Maintenance Costs (SEWRPC, 1991)
Swats Size
(Depth and Top Width)
Component Unit Cost Comment 1,S Foot Depth, One- 3Foot Depth, 3-Fool
Foot Bottom Width, Bottom Width, 21-Foot
106Foot Top Width : Top Width
Lawn Mowing $0.8511,000ft21nnoing $0.141lirirfoot $0.21 /linear foot Lawn maintenance area=(top
widlh + l0 feet) x length. Mow
eight times per year
General Lawn Care $0.00 /1,000 fig/ year $0.1 B I ilniearfoot $0.28 /linear foot Lawn maintenance area = (top
width + 10 feet) x length
Swele Debris and Lltisr $0.10 lhnear foot I year $0.10 !Ilnieerfoot $0.10 lllnear foot -
Removal
Graoe Reseeding with $0.0 Iyd2 $0.011 linearfact $0.01! linear foot Area regaled equals 1%
Mulch and Fertilizer of lawn maintenance area per
year
Program Administration and $0.15! linear foot! year, WAS I lineerfoot $0.15 /linger foot Inspect four times per year
Swab Inspection plus $25! inspection
Total -_ $0.581 linear fool $0.75lIin.artoat
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TC-30 Vegetated Swale
Maintenance 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 mowing 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 mowing 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, RandallJ, 1998,
"Performance of vegetative controls for treating highway runoff;" ASCE Journal of
Environmental Engineering, Vol. 124, No. ii, pp. 1121-1128.
Brown, W., and T. Schueler. 1997. The Economics ofStormwaterBMPs in the Mid-Atlantic
Region. Prepared for the Chesapeake Research Consortium, Edgewater, MD, by the Center for
Watershed 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., Homer, Richard R., and Booth, Derek B., 2000. Characterization of
Performance Predictors and Evaluation of Mowing Practices in Biofiltratio n Swales. Report
to King County Land And Water Resources Division and others by Center for Urban Water
Resources Management, Department of Civil and Environmental Engineering, University of
Washington, Seattle, WA
Dorman, M.E., J. Hartigan, R.F. Steg, and T. Quasebarth. 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 of Stormwater 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. Massareffi. 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.Oaldand, P.H. 1983. An evaluation of stormwater pollutant removal
10 of 13 California Stormwater BM P Handbook January 2003
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Vegetated Swale TC-30
though 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 RunoffProject. 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 Watershed Project. Ontario Ministry 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 Department 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 report no. 31. Southeastern
Wisconsin Regional Planning Commission, Waukesha, WI.
U.S. EPA, 1999, Stormwater Fact Sheet: Vegetated Swales, Report # 832-F-99-006
http://www.epa.gov/owm/mtb/vegwale.pdf Office of Water, Washington DC.
Wang, T., D. Spyridakis, B. Mar, and R. Homer. 1981. Transport, Deposition and Control of
Heavy Metals in Highway Runoff. FHWA-WA-RD-39-10. University of Washington,
Department of Civil Engineering, Seattle, WA.
Washington State Department of Transportation, 1995, Highway RunoffManual, 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 Report
No. 87-4004. U.S. Geological Survey, Reston, VA.
Yousef, Y., M. Wanielista, H. Harper, D. Pearce, and R. Tolbert. 1985. Best Management
Practices: Removal ofHighway Contaminants By Roadside Swales. University of Central
Florida and Florida Department of Transportation, Orlando, FL
Yu, S., S. Barnes, and V. (Jerde. 1993. Testing ofBest Management Practices for Controlling
Highway Runoff. FHWA/VA-93-R16. Virginia Transportation Research Council,
Charlottesville, VA.
Information Resources
Maryland Department of the Environment (M DE). 2000. Maryland Stormwate.r Design
Manual. www.mde.state.md.us/environment/wma/stormwatermanual. Accessed May 22,
2001.
Reeves, E. 1994. Performance and Condition of Biofilters in the Pacific Northwest. Watershed
Protection Techniques 1(3):117-119.
January 2003 - California Stormwater BlIP Handbook 11 of 13
New Development and Redevelopment
www.cabmphandbookc.com
TC-30 Vegetated Swale
Seattle Metro and Washington Department of Ecology. 1992. Biofiltration Swale Performance.
Recommendations and Design Considerations. Publication No. 657. Seattle Metro and
Washington Department of Ecology, Olympia, WA.
USEPA 1993. Guidance Specifying Management Measures 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, Maintenance, and Management of
Storrnwater Management Systems. Prepared for U.S. Environmental Protection Agency, Office
of Water. Washington, DC, by the Watershed Management Institute, Ingleside, MD.
12 of 13 - California Stormwatr BMP Handbook January 2003
New Development and Redevelopment
www.cabmphandbooks.com
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MASSACHUSETTS LOW IMPACT DEVELOPMENT TOOLKIT
FACT SHEET #6
PERMEABLE PAVING
Overview
Since impervious pavement is the primary source of stormwater runoff,
Low Impact Development strategies recommend permeable paving for
parking areas and other hard surfaces. Permeable paving allows rainwater
to percolate through the paving and into the ground before it runs off.
This approach reduces stormwater runoff volumes and minimizes the
pollutants introduced into stormwater runoff from parking areas.
All permeable paving systems consist of a durable, load bearing, pervious surface overlying
a crushed stone base that stores rainwater before it infiltrates into the underlying soil.
Permeable paving techniques include porous asphalt, pervious concrete, paving stones, and
manufactured "grass payers" made of concrete or plastic. Permeable paving may be used for
walkways, patios, plazas, driveways, parking stalls, and overflow parking areas.
Applications and Design Principles
Permeable paving is appropriate for pedestrian-only areas and for very low-volume, low-speed areas
such as overflow parking areas, residential driveways, alleys, and parking stalls. It can be constructed
where the underlying soils have a permeability of at least 0.3" per hour. Permeable paving is an
excellent technique for dense urban areas because it does not
require any additional land. With proper design, cold climates are not a
major limitation; porous pavement has been used successfully in Norway, Management Objectives incorporating design features to reduce frost heave. . . rr Keauce stormwoter runort volume
from paved surfaces
Reduce peak discharge rates.
Increase recharge through
infiltration.
Reduce pollutant transport through
direct infiltration.
a Improve site landscaping benefits
(grass payers only.)
Permeable paving is not ideal for high traffic/high speed areas because it has
lower load-bearing capacity than conventional pavement. Nor should it be
used on stormwater "hotspots" with high pollutant loads because stormwater
cannot be pretreated prior to infiltration. Heavy winter sanding may clog
joints and void spaces.
Metropolitan Area
Planning Council innovation
Cover: A driveway in Connecticut
built with manufactured paving
stones. Lower photo shows paving
stone detail. Photo: University of
Connecticut, Jordan Cove Urban
Monitoring Project
Above: A parking lot with concrete
gross paver parking stalls. Lower
photo shows grass paver detail.
Photos: Lower Columbia River
Estuary Partnership
Right: A schematic cross section of
permeable paving. In some
applications, the crushed stone
reservoir below the paving is
designed to store and infiltrate
rooftop runoff as well. Image: Cahill
Associates, Inc. 2004
Design Guidelines for
Porous Asphalt with
Subsurface Infiltration
Three Major Types of Permeable Paving
Porous asphalt and pervious concrete appear to be the same as traditional asphalt or
concrete pavement. However, they are mixed with a very low content of fine sand, so
that they have 10%-25% void space and a runoff coefficient that is almost zero.
Paving stones (aka unit payers) are impermeable blocks made of brick, stone, or
concrete, set on a prepared sand base. The joints between the blocks are filled with
sand or stone dust to allow water to percolate downward. Runoff coefficients range
from 0.1 - 0.7, depending on rainfall intensity, joint width, and materials. Some
concrete paving stones have an open cell design to increase permeability.
a Grass payers (aka turf blocks) are a type of open-cell unit paver in which the cells
are filled with soil and planted with turf. The payers, made of concrete or synthetic,
distribute the weight of traffic and prevent compression of the underlying soil.
Runoff coefficients are similar to grass, 0.15 to 0.6.
Each of these techniques is constructed over a base course that doubles as a reservoir
for the stormwater before it infiltrates into the subsoil. The reservoir should consist
of uniformly-sized crushed stone, with a depth sufficient to store all of the rainfall
from the design storm. The bottom of the stone reservoir should be completely flat
so that infiltrated runoff will be able to infiltrate through the entire surface. Some
designs incorporate an "overflow edge:' which is a trench surrounding the edge of
the pavement. The trench connects to the stone reservoir below the surface of the
pavement and acts as a backup in case the surface clogs.
Benefits and Effectiveness
Porous pavement provides groundwater recharge and reduces stormwater runoff
volume. Depending on design, paving material, soil type, and rainfall, permeable
paving can infiltrate as much as 70% to 80% of annual rainfall.
Porous pavement can reduce peak discharge rates significantly by diverting
stormwater into the ground and away from the pipe-and-pond stormwater
management system.
r map-
L
; f
Above: A parking lot with a Grass payers can improve site appearance by providing vegetation where there
conventional asphalt aisles and would otherwise he only pavement.
paving stone parking stalls. Paving
stones are most appropriate for Porous paving increases effective developable area on a site because portions of
low-speed, low-traffic areas. Photo: the stormwater management system are located underneath the paved areas, and
Lower Columbia River Estuary the infiltration provided by permeable paving can significantly reduce the need
Partnership for large stormwater management structures on a site.
Limitations
Permeable paving can be prone to clogging from sand and fine sediments that fill
void spaces and the joints between payers. As a result, it should be used carefully
where frequent winter sanding is necessary because the sand may clog the surface
of the material. Periodic maintenance is critical, and surfaces should be cleaned
with a vacuum sweeper at least three times per year.
In cold climates, the potential for frost heave may be a concern for the use of
permeable paving. Some design manuals recommend excavating the base course
to below the frost line, but this may not be necessary in rapidly permeable soils.
In addition, the dead air and void spaces in the base course provide insulation so
that the frost line is closer to the surface.
a Permeable paving should not receive stormwater from other drainage areas,
especially any areas that are not fully stabilized.
Permeable paving can only be used on gentle slopes (<5%); it cannot be used in
high-traffic areas or where it will be subject to heavy axle loads.
Snow plows can catch the edge of grass payers and some paving stones. Rollers
should be attached to the bottom edge of a snowplow to prevent this problem.
Above: A handicap-accessible park pathway made of
permeable paving stones.
Photo: GeoSyntec Consultants, Inc.
Maintenance
D Post signs identifying porous pavement areas.
Minimize use of salt or sand during winter months
Keep landscaped areas well-maintained and prevent soil
from being transported onto the pavement.
a Clean the surface using vacuum sweeping machines.
For paving stones, periodically add joint material (sand)
to replace material that has been transported.
a Monitor regularly to ensure that the paving surface
drains properly after storms.
Do not reseal or repave with impermeable materials.
Inspect the surface annually for deterioration.
a Grass payers may require periodic reseeding to fill in
bare spots.
Design Details
For all permeable paving, base course is a reservoir
layer of 1 "-2" crushed stone; depth to be determined by
storage required and frost penetration.
Permeable paving require a single-size grading of base
material in order to provide voids for rainwater storage;
choice of materials is a compromise between stiffness,
permeability, and storage capacity. Use angular crushed
rock material with a high surface friction to prevent
traffic compaction and rutting.
The design may also include a 2" thick filter course of
0.5" crushed stone, applied over the base course. A
geotextile fabric may be laid at the top of the filter layer
to trap sediment and pollutants.
For grass payers, use deep-rooted grass species whose
roots can penetrate the reservoir base course. Irrigation
may be required but should be infrequent soakings so
that the turf develops deep root systems. Grass payers
are not suitable for every day, all day parking because
the grass will get insufficient sunlight. Better for use as
occasional overflow parking.
The introduction of dirt or sand onto the paving surface,
whether transported by runoff from elsewhere or carried by
vehicles, will contribute to premature clogging and failure
of the paving. Consequently, permeable paving should
be constructed as one of the last items to be built on a
development site, after most heavy construction vehicles are
finished and after the majority of the landscaping work is
completed.
Cost
On most sites, permeable paving costs more than
conventional asphalt or cement paving techniques. In the
case of porous asphalt and pervious concrete, construction
costs may be 50% more than conventional asphalt and
concrete. Construction costs of paving stones and
grass payers varies considerably and will depend on the
application. As with any site improvement or stormwater
management structure, property owners should provide a
budget for maintenance of permeable paving, at an annual
rate of 1%-2% of construction costs.
Permeable paving reduces the need for stormwater
conveyances and treatment structures, resulting in cost
savings elsewhere. Permeable paving also reduces the
amount of land needed for stormwater management and
may satisfy requirements for greenspace, allowing more
development on a site.
Local Case Study
West Farms Mall - West Hartford, CT
Grass payers were installed at the West Farms Mall off of 1-84
at exit 40, to handle peak-season overflow parking associated
with a mall expansion. Over four acres of reinforced turf was
designed to accommodate 700 spaces of overflow parking for
the peak shopping seasons. There are a few drains installed
in the reinforced turf but are only used during very heavy
storms. Because the reinforced turf works so well the existing
storm drainage system did not have to be enlarged for the
additional parking. The overflow parking area needs to be
mowed on a regular basis and treated like a regular lawn. The
area also needs to be plowed as any parking would be. Rollers
were fit to the bottom of the snow plow so the reinforced
turf would not be damaged. The manager of the Westfarms
facility is satisfied with the turf.
Websites
www.unh.edu/erg/cstev/index.htm
www.invisiblestructures.com/GP2/whole_lotof_turf.htm
www.uni-groupusa.org/case.htm
www.nemo.uconn.edu/
www.lowimpactdevelopment.org/epa03/pavespec.htm
www.epa.gov/ednnrmrl/repository/abstrac2/abstra2.htm
www.forester.net/sw...0503_advances.html
This publication is one component of the Massachusetts Low Impact Development Toolkitj, a production of the Metropolitan Area
Planning Council, in coordination with the 1-495 Metro West Corridor Partnership, with financial support from US EPA.
The Massachusetts Low Impact Development Interagency Working Group also provided valuable input and feedback on the LID Toolkit.
FOR MORE INFORMATION, VISIT: WWW.MAPC.ORG/LID AND WWW.ARC-OF-INNOVATION.ORG.