HomeMy WebLinkAboutSDP 14-09; Tamarack Beach Custom Homes; Site Development Plan (SDP) (3)NOV 1 9 2014
PRELIMINARY HYDROLOGY STUDY
for
295 Chinquapin Ave., Carlsbad, CA
PUD 14-07/SDP 14-09/CDP 14-23/MS 14-09
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
Date: October 23,2014
PREPARED BY:
Pasco Laret Suiter & Associates
535 N. Highway 101, Suite A
Solana Beach, CA 92075
(858) 259-8212
YLER G LAWSON, RCE 80356 DATE
Preliminary Hydrology Study for Tamarack Beach Costume Homes
PISA 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 33°08'50" W 117"20'29". 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 comer of the project where it then flows onto the
neighboring westem property. Once on the neighboring property, the runoff enters a
private storm drain system and is routed to the west and south. Runoff then travels
through and adjacent southem property and ultimately to the Aaua 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.
Existina 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 pattem 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 eastem property line.
In an effort to reduce impervious surfaces where feasible, porous pavers 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 pavers are acceptable and
recommended for this site.
Excess runoff from this driveway will be collected in a curb & gutter and directed to the
southwest comer 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 comer. From there, runoff will travel to the existing storm drain system
located on the westem 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 pavers 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 Pracfices (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 pavers 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.
Prooosed Basin
area (sf)
Hardscape 4614
Landscape 3413
Pavers 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 pavers 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 OrderNo. 2009-0009-DWQ, "
Califomia 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 Pe X D-°^^
Where:
I = Intensity (in/hr)
Pe = 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 mnoff 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 fi'om 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 mnoff 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
Pioo=2.5
Basin 1
Total Area = 13,378 sf 0.31 Acres
Impervious Area = 4,683 sf ^ 0.11 Ac
Pervious Area = 8,695 sf 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 Min (min.)
Pe = 2.5
l = 7.44xPexD-°^^^
I = 7.44 X 2.5 X 5.0-*^^^ ~ 6.59 in/hr
liotf=^ 6.59 in/hr
l2~3.16 in/hr
II Q;^ 4.48 in/hr
Q2 = 0.55 x 3.16 in/hr x 0.31 Ac = 0.54 cfs
Q,o = 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
Pio=1.7
P 100=2.5
Basin 1
Total Area = 13,378 sf 0.31 Acres
Impervious Area = 4,614 sf ^ 0.11 Ac
Pervious Area = 8,764 sf 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 Min (min.)
Pe = 2.5
I = 7.44xP6xD-°^45
I = 7.44 X 2.5 X 5.0"°^^^ ~ 6.59 in/hr
Iioo- 6.59 in/hr
l2~ 3.16 in/hr
110^4.48 in/hr
Ql = 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
CONDITION DRAINAGE
AREA (ACRES) Q2(CFS) QIC (CFS) QIOO (CFS) TC(MINS.)
EXISTING 0.31 0.54 0.76 1.12 5
PROPOSED 0.31 0.53 0.75 LIO 5
EXISTING VS. PROPOSED
CONDITION COMPARISON -
BASIN A
LESS THEN
EXISTING
LESS THEN
EXISTING
LESS THEN
EXISTING
SAME AS
EXISTING
EXISTING VS. PROPOSED
CONDITION COMPARISON -
BASIN B
LESS THEN
EXISTING
LESS THEN
EXISTING
LESS THEN
EXISTING
SAME AS
EXISTING
Due to the decrease in the flow rate for the site, no detention is required.
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4.0 APPENDIX
10/23/2014
11
LEGEND
PROPERTY UNE
EXISmiG CONTOUR UNE
PROPOSED nxmjNe
a
5 S
5 S
5 S
5 g
PRE-DEVELOPMENT HYDROLOGY
TAMARACK BEACH HOMES
Existina Basin
area (sf)
Hardscape 4683
Landscape 8695
Total 13378
> •
APN: 206-080-14
N33-59"03"W 222.60'
, (N3£36'19"W 222.85'
NODE 2—
1 w \ I
/
\
\
N> O O) 1 1
o O)
o
1 - ^ u K) d d cn to CD cn
bJ U
(P o 5 5
K) K
z z.
N33'59'01''W 222.53'
(N34-36'19"W 22Z85')
APN: 206-080-38
/AP/V: 206-080-39
GRAPHIC SCALE 1''=10'
10 20 30
PASCO LARET SUITER
tm^^^^m & ASSOCIATES
civil ENQmEDIINa * LAND PUNHIIIS * lAMD aURVEnNe
513 Hwtk Blfkmr 111. ito A. Mn> Iwck, CA >M7S
J:\Actlw Jotn\2211 RINCON - CARLSSA0\CIML\REPORTSV<YDR0LOGY\pilstln9 hydro 221 l.dwg
LEGEND
PROPERTY UNE
EXISTING CONTOUR UNE
PROPOSED FLOWUNE
POST-DEVELOPMENT HYDROLOGY
TAMARACK BEACH HOMES
g g
s g
g g
H 1 il2-
g g
g g
g g
i g
g g
g g
g g
APN: 206-080-14
N33-59'03''W 222.60'
_ (N3«6'19"W 222.85')
,T7T7 l>.L . I
1 r TX
Eg
N33'59'orw 222.53' |/
(N34'36'19"W 222.85')
APN; 206-080-J8
GRAPHIC SCALE
10 10 20 30
TTT
T~T^
TTT
APN; 206-080-39
Proposed Basin
area (sf)
Hardscape 4614
Landscape 3413
Pavers 5351
Total 13378
7^
'88
88
nh.
Pf if! 18
1^ -RIP-RAP (Fa-50.2)
•a
K3 O 1 O) I o 00 o I
Ol
nn now DATB
PASCO LARET SUITER
^MMB^H & ASSOCIATES
CML ENaiNEOlim * LAND PlANNINa * LAND aURVEVINa
SS5HacttBI|k<rarlll, SMA. lobnaBwck.CAfa07S
15IJMJ»U I fc U*M»Ma
J:V^ctlv< JobBV211 RINCON - CARlS8AD\aVIL\REPORTSVIYDROLOGY\Proposad hydro 2211.dwg
ifwiBi^ Geotechnical Exploration, Inc.
SOIL AND FOUNDATION ENGINEERING • GROUNDWATER • ENGINEERING GEOLOGY
22 October 2014
Mr. Kevin Dunn Job No. 14-10523
RINCON REAL ESTATE GROUP, INC.
1520 N. El Camino Real, Unit 5
San Clemente, CA 92672
Subject: Permeable Pavers
Rincon Residential Project
295 Chinquapin Avenue
Carlsbad, California
Dear Mr. Dunn:
In accordance with the request of Mr. Tyler Lawson of Pasco, Laret, Suiter,
Geotechnical Exploration, Inc. has evaluated our findings with regard to the
suitability of permeable pavers 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 pavers. Perching conditions as described on page
16 of our May 15, 2014, report were not encountered within SVz feet on the lot and
do not exist at the shallow depths that would impact the performance of the
permeable pavers.
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 pavers. The subgrade
soils supporting the base layer should also be compacted to 95 percent relative
compaction.
7420 TRADE STREFT* SAN DIEGO, CA. 92121 • (858) 549-7222 • FAX: (858) 549-1604 • EMAIL: geotech@gel-sd.com
Rincon Residential Project
Carlsbad, California
Job No. 14-10523
Page 2
Thank you for this opportunity to be of service. Should you have any questions,
you may contact the undersigned. Reference to our Job No. 14-10523 will help to
expedite a reply to your inquiries.
Respectfully submitted,
GEOTECHNICAL EXPLORATI
L^Sne D. Reed, President
C.E.G. 999/P.G. 3391
Jaime A. Cerros, P.E.
R.C.E. 34422/G.E. 2007
Senior Geotechnical Engineer
CITY OF
CARLSBAD
STORM WATER
STANDARDS
QUESTIONNAIRE
E-34
Development Services
Land Deveiopment Engineering
1635 Faraday Avenue
760-602-2750
www.carlsbadca.gov
To address post-development pollutants that may be generated from development projects, the City requires that new development and
significant redevelopment priority projects incorporate Pemianent 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) at www.carlsbadca.qov/standards.
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 storni 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 detemiining 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 forthe 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 1 and follow the instructions. When completed, sign the fomi at the end and submit this with your
application to the city.
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 infonnation.
If you answered "no" to both questions, then go to Step:
E-34 Page 1 of 3 Effective 6/27/13
CITY OF
CARLSBAD
STORM WATER
STANDARDS
QUESTIONNAIRE
E-34
Development Services
Land Development Engineering
1635 Faraday Avenue
760-602-2750
www.carlsbadca.gov
To determine if your project is a priority development project, please answer the following questions: YES NO
1. 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. X
2. 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. X
3. 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.
X
4. 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 X
5. 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 pari<ing lot is a land area or facility for the
temporary parking or storage of motor vehicles used personally for business or for commerce. X
6. 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. X
7. 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
Area (ESA)? "Discharging Directly to" Includes flow that Is conveyed overiand 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).*
X
8. 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. X
9. 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. X
10. 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? X
11.1s 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%? X
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 tlie "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
CITY OF
CARLSBAD
STORM WATER
STANDARDS
QUESTIONNAIRE
E-34
Development Services
Land Deveiopment Engineering
1635 Faraday Avenue
760-602-2750
www.carlsbadca.gov
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
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.
My project meets PRIORITY DEVELOPMENT PROJECT (PDP) requirements and must comply with additional stomiwater
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: 295 Chinquapin Drive, Carlsbad, CA
Applicant Name:
Accessor's Parcel Number(s): 209-080-13
Applicant Title:
Applicant Signature: Date:
This Box for City Use Only
City Concurrence: YES NO
By:
Date:
Project ID:
* 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 F5ARE 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 Copermittees.
E-34 Page 3 of 3 Effective 6/27/13
San Diego County Hydrology Manual
Date: June 2003
Section:
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 Tj 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)
Element* DU/
Acre
.5% 1% 2% 3% 5% 10% Element* DU/
Acre LM T, LM Ti LM T, LM T, LM T, LM T.
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 7.4 100 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 50 8.2 65 7.4 80 6.5 90 6.0 100 5.4 100 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. Com 50 5.3 60 4.5 75 4.0 85 3.8 95 3.4 100 2.7
G. Com 50 4.7 60 4.1 75 3.6 85 3.4 90 2.9 100 2.4
O.P./Com 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 3.2 70 2.7 80 2.6 90 2.3 100 1.9
*See Table 3-1 for more detailed description
3-12
San Diego County Hydrology Manual
Date: June 2003
Section:
Page:
3
6 of 26
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. Com) 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. Com) Office Professional/Commercial 90 0.83 0.84 0.84 0.85
Commercial/Industrial (Limited I.) Limited Industrial 90 0.83 0.84 0.84 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
40 50 1
Duration
3 4
Hours
Directions for Appiication:
(1) From precipitation maps determine 6 hr and 24 hr amounts
forthe selected frequency. These maps are included in the
County Hydrok)gy Manual (10,50, and 100 yr maps included
in the Design and Procedure Manual).
(2) Adjust 6 hr precipitation (if necessary) so that it is within
the range of 45% to 65% of the 24 hr precipitation (not
applicaple to Desert).
(3) Plot 6 hr precipitation on the right side of the chart.
(4) Draw a line through the point parallel to the plotted lines.
(5) This line is the intensity-duration curve for the location
being analyzed.
-6 = 54.5 %(2)
24
Application Form:
(a) Selected frequency 50 year
(b) Pe = 3 in.. P24 = 5.5 ,
(c) Adjusted Pg^^' = 3 in.
(d) t„ = 20 min.
(e) I = 3.2 in./hr.
Note: This chart replaces the Intensity-Duration-Frequency
curves used since 1965.
P6
Duration
5
10
15
20
25
SO
60
1.5
I
2.5
j
3.5
I
43
I
5.5
1
90
120
ISO
JM
240
300
360
2.63
2; 12
3.9515.27
318 4 24
1.68
1.30
2.53
1.95
1.08
0.93
6:83
0.60
6^
0.41
0.34
0^29
1.62
1.40
1.24
3.37
2.59
6.59 7.90
5.30^8.361
4;2i_r5.M'
3.24 3.891
9.22! 10.54
7.42 i 8.48
5.901 6.74 •
2.15
1.87'
1^661
2.69 :3.23 3.77 4^
2.33 [2.861
.T,
3.271 3.73
lit. 2.07 2.49 2.90! 3.32
I.0311.38^ 1.72; 2.07
0.9b! 1/19il^'1.79
1.33"T59
1.02 1.23
0.26
0.22
0.19
0.17
0.80
0.61
d^si
0.44
0.39
033
1.06
0.82
0.68
0.52
0.43
0.28 0.38
0.85 1.02 i
OJSi 0.881
6.85T078l
6.54^065!
0.47 0.56
6.42T0.Sd
2.411 2.76
2.091 2.39
1.881 2.12
i 1.43' 1.63
1 19f 1.36
1.03^ 1.18
6.91' 1 04
0761 0.87
ase ' 6.75
0.58' 0.67
11.86
9.54
7.58^'
5.84
4Jj5"
4.20 '
3.73 •
3.10
2.69
2 39 •
1.84 •
1.53 '
1.32;
1.181 !
0 98;
0.85 •
0.75 '
13.17:
lo.eoj
8.42;
6.49 '
5^
4 67 ;
4.15 '
3.45 !
2.98 •
2.65 '
2.04 '
1.70 •
1.47 •
1.31 •
1 08 •
6.94 •
0.84 '
14.49 15.81
11.66112.72
9^27 '16,11
'7.78
6.46
7.13
5.93 •
5;i3;
4.56 i
3.79 :
3.28
2 .92 '
2.25'
1.87
1-62. i
1.44 :
1.19 '
lM
0.92 '
5.60
4.98
4.13
3.58
3.18
2.46
2.04
1.76
1 57
1-30
1.13
1.00
FIGURE
Intensity-Duration Design Chart - Example
117°30'0'W
I
117°15'0'W 117°0'0"W 116°45'0'W 116°30'0'W 116°15'0'W
Orange
County
llilllllilll
'llllllll—n—n—I—n—r-t—i—n—i—r 117°30'0'W 117°15'0'W 117°0'0"W
U CO
I I I I I I I I I I I
County of San Diego
Hydrology Manual
Soil Hydrologic Group
- z b o
Legend
•• IVlajor Roads
^ Incorporated City Bdy
HYDROLOGIC SOIL GROUP
Hydrologic Group Undefined
• Hydrologic Group A
Hydrologic Group B
Hydrologic Group C
" Hydrologic Group D
No Soil Data
Note: Soil Data Source
USDA/NRCS
SSURGO Soils 2007
N
A
I \ I
31.50 3 Miles
DPW
^GIS SanGIS
THIS MAP IS PROVIDED WITHOUT WARRANTY OF ANY KIND
EfTHER EXPRESS OR IMPLIED, INCLUDING, BUT NOT LIMrTED
TO. THE IMPLIED WARRANTIES OF MERCHANTABILITY AND
FfTNESS FOR A PARTICULAR PURPOSE
Copyright SanGIS All Rights Reserved
This product may contain information from the SANDAG Regional
Information System which cannot be reproduced without the wntten
permission of SANDAG
This product may contain infbnnation which has been reproduced
with permission granted by Thomas Brothers Maps
116°45'0'W 116°30'0'W 116°15'0'W
County of San Diego
Hydrology Manual
Rainfall Isopluvials
2 Year Rainfall Event - 6 Hours
Isopluvial (inches)
DPW GIS
We Have San Diego Ou'crcd!
THIS MAP IS PROVIDED WITHOUT WARRANTY OF ANY KIND, ErTHER EXPRESS OR IMPUED, INCLUDING, BUT NOT LIMrTED TO, THE IMPLIED WARRANTIES OF MERCHANTABIUTY AND FrrNESS FOR A PARTICULAR PURPOSE. Copyright SanGIS. All Rights Reserved.
This produds may contain information from the SANDAG Regional
Information System which cannot be reproAiced without the
'-' written permission of SANDAG.
This product may contain infomiation which has tjeen reproduced wtth
permission granted by Thomas Brottiers Maps.
3 Miles
County of San Diego
Hydrology Manual
Rainfall Isopluvials
10 Year Rainfall Event - 6 Hours
Isopluvial (inches)
DPW
GIS saiGis Wc Have .San Diego (k)vcrc<i!
THIS MAP IS PROVIDED WnXOUT WARRANTY OF ANY KIND, ErrHER EXPRESS
OR IMPUED, INCLUDING, BUT NOT UMITED TO, THE IMPLIED WARRANTIES
OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
Copyright SanGIS. All Rights Reserved.
This products may contain infonnation from the SANDAG Regional Information System which cannot tw reproduced without tha
written permission of SANDAG
This product may contain information which has been reproduced with
pemiission granted by Thomas Brothers Maps
3 Miles
County of San Diego
Hydrology Manual
Rainfall Isopluvials
100 Year Rainfall Event - 6 Hours
Isopluvial (inches)
DPW c^-s-i-xe
GIS SanGIS Csoatne™ ofP^tic 'A'ons Wc Have San Diego (-ovcrod!
THIS MAP IS PROVIDED WITHOUT WARRANTY OF ANY KIND, ErTHER EXPRESS
OR IMPLIED, INCLUDING, BUT NOT UMTTEO TO, THE IMPLIED WARRANTIES OF MERCHANTABIUTY AND FTTNESS FOR A PARTICULAR PURPOSE
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3 Miles
Vegetated Swale TC-30
'1 • ^' .iflJ -.' ^
Design Considerations
• TributaiyArea
• Area Required
• Slope
• Water Availability
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 mnoff 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
stomiwater mnoff. 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 mnoff Even in
the areas where the annual rainfall was only about lo 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 ofthe 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.
Targeted Constituents
0 Sediment
0 Nutrients •
0 Trash •
0 Metals
0 Bacteria •
0 Oil and Grease A
0 Organics •
Legend (Removal Effectiveness)
• Low • High
• Medium
January 2003 California StormwatEr BMP Handbook
New Development and Redevelopment
www.cabmphandbooks.com
lof 13
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.
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 fimction 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
rtmoff 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 charmels are nonnally recommended but other configurations, such as
parabohc, can also provide substantial water quality improvement and may be easier to mow
than designs with sharp breaks in slope.
• Swales constmcted 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 Maiming's Equation using a value of
0.25 fbr Maiming's n.
2 of 13 California Stormwater BMP Handbook January 2003
New Devebpmentand Redevelopment
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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 tHes;
stagger the ends of the tiles to prevent the formation of channels along the swale or strip.
• Use a roUer on the sod to ensure that no air pockets form between the sod and the soU.
• Where seeds are used, erosion controls will be necessary to protect seeds for at least 75 days
after the first rainfaU of the season.
Performance
The literature suggests that vegetated swales represent a practical and potentially effective
technique for controUing urban runoff quality. While limited quantitative performance data
exists for vegetated swales, it is known that check dams, sUght slopes, permeable soils, dense
grass cover, increased contact time, and small storm events all contribute to successful poUutant
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 mnoff 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.C, area and found no significant improvement in urban
mnoff quality for the poUutants 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 carefuUy designed artificial
swale that received runoff from a commercial parking lot. The project tracked 11 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 1). 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 charmel banks can
help to treat sheet flows entering the swale.
Only 9 studies have been conducted on aU grassed channels designed for water quality (Table 1).
The data suggest relatively high removal rates for some poUutants, but negative removals for
some bacteria, and feiir performance for phosphoms.
January 2003 California Stormwater BMP Handbook 3 of 13
New Development and Redevelopment
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TC-30 Vegetated Swale
Table 1 Grassed swale pollutant removal efficiency data
Removal Efficiencies (% Removal)
Study •res TP TN NO3 Metals Bacteria Type
Caltrans 2002 17 8 67 66 83-90 -33 dry swales
Goldberg 1993 67.8 4-5 -31-4 42-62 -100 grassed channel
Seattle Metro and Washington
Department of Ecology 1992 60 45 --25 2-16 -25 grassed channel
Seattle Metro and V^^ashington
Department of Eco logy, 1992 83 29 --25 46-73 -25 grassed channel
Wangetal., igSi 80 - --70-80 -dry swale
Dorman etal, 1989 98 18 -45 37-81 -dry swale
Harper, 1988 87 83 84 80 88-90 -dry swale
Kercher etal., 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 -wet swale
While it is difficult to distinguish between different designs based on the smaU amount of
available data, grassed channels generally have poorer removal rates than wet and dry swales,
although some swales appear to export soluble phosphoms (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 wiU depend on land use, size of the area serviced, soil type,
slope, imperviousness ofthe contributing watershed, and dimensions and slope ofthe 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, 1993)
m Comparable performance to wet basins
• Limited to treating a few acres
• Availability of water during dry 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 diy
periods, but may be necessaiy only to prevent the vegetation from dying.
4 of 13 California Stormwater BMP Handbook
New Devebpmentand Redevelopment
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January 2003
Vegetated Swale TC-30
The topography ofthe site should permit the design of a channel with appropriate slope and
cross-sectional area. Site topography may also dictate a need for additional stmctural controls.
Recommendations for longimdinal 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 ofthe 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,
Washirigton (Seattle Metro and Washington Department of Ecology, 1992), and is not well
supported. Analysis of the data coUected 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 (ColweU et al., 2000) has shown mowing frequency or
grass height has little or no effect on poUutant removal.
Summary qf Design Recommendations
1) The swale should have a length that provides a minimum hydrauUc 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 2/3rds the height of
the grass at the peak ofthe water quality design storm intensity. The channel slope
should not exceed 2.5%.
2) A design grass height of 6 inches is recommended.
3) Regardless of the recommended detention time, the swale should be not less than
100 feet in length.
4) The width ofthe swale should be determined using Marming's Equation, at the peak
ofthe design storm, using a Manning's n of 0.25.
5) The swale can be sized as both a treatment facility for the design storm and as a
conveyance system to pass the peak hydrauUc flows of the 100-year storm if it is
located "on-line." The side slopes should be no steeper than 3:1 (H:V).
6) 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.
7) 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
January 2003 California Stormwater BMP Handbook 5 of 13
New Development and Redevelopment
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TC-30 Vegetated Swale
estabUshment. Where runoff diversion is not possible, cover graded and seeded
areas with suitable erosion control materials.
Maintenance
The useful Ufe of a vegetated swale system is directiy 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 faciUty. Accumulated sediment should also be removed
manuaUy to avoid concentrated flows inthe 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 utiUzing 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 faU runoff to be sure the swale is ready for winter. However,
additional inspection after periods of heavy mnoff 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 detennined through periodic inspection, but litter should always be removed
prior to mowing.
• Sediment accumulating near culverts and in charmels 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.
6 of 13 California Stormwater BMP Handbook January 2003
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Vegetated Swale TC-30
Cost
CoTistruction Cost
Little data is avaUable 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^. This price does not include design costs or contingencies. Brown and Schueler
(1997) estimate these costs at approximately 32 percent of constmction 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 ft^, which compares
favorably with other stormwater management practices.
January 2003 California Stormwater BMP Handbook 7 of 13
New Development and Redevelopment
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TC-30 Vegetated Swale
Table 2 Swale Cost Estimate (SEWRPC, 1991)
Unit Cost Total Cost
Component Unit Extent Low Moderate High Low Moderate High
Mobil ization /
•emobillzatlon-Ugrit
Swale 1 $107 $274 $441 $107 1274 $441
Site Preparation
CloarinB"
Grubbing
GanenI
Excavatiorf
Lai/Bl and Till*
Acrs
Acra
Yd»
Yd'
O.G
a25
372
1,21C
$2,200
$3,600
$2.10
$0.20
$3,800
$5,200
$3.70
$0.35
$6,400
3e,eoo
$5.30
$0.50
$1,100
S950
$7B1
$Z4Z
$1,900
$1,300
$1,376
^24
$2.7m
$1,650
$1,972
$605
Silea Deve ioprrent
Salvaged Topsoil
Seed, and Mulch'..
Soda
Yd'
Yd'
1,210
1,21C
$0.40
$1.20
$1.00
$2 40
$1.60
$360
$454
$1,462
$1,210
$2,904
$1,936
$4,356
Subtotd -- -----15,116 $9,3^6 $13,660
Contingencies Swale 1 25% 25% 26% £1,279 $2,347 $3,415
ToM
•-
------$6 355 $11,735 $17,075
Note' MabiliTalionAlemabilzaticn lelers to ttia orgs niTatlcn and plarming inuolved in estsblishing a wegatativa swale
• Swale has a bottom width of 1.0 foot, a top width of 10 feet with 1:3 side slices, and a 1,000-foot length
"Area cleared - (tap width + 10 feet) x swale length
° Area grubbed - (tap width x swaie length).
"Volume excavated = (0.67 x top width x swate depth) x swale length (paraljollc cross-section).
"Area tilled = (top width + BIswale depth') x swale length (parabolic cross-section).
3ttop width)
'Area seeded = area cleared x 0.5.
> Area sodded = area cleared x 0.5.
8 of 13 California StormwatEr BMP Handbook
New Development and Redevetopment
www .cat»mphandboo ks.com
January 2003
Vegetated Swale TC-30
Table 3 Estimated Maintenance Costs CSEWRPC, 1991)
Swale Size
(Depth and Top VWdth)
Component Unit Cost 1.5 Foot Deplh, One-
Foot Bottom Width,
10-Foot Top Width
3-Foot Depth, 3-Fool
Bottom Width, 21-Foot
Top Width
Comment
Lawn Mowing $O BGn, QOO fl'/mowing $0.14 /linearfoot $0.21 / linear fool Lawn maintenance area=(tQp
width-H10 feet) X length Mow
eight times peryear
General Lawn Care 59.00 M,000fl»fyear $0.18 /linearfoot $0.28/linear foot Lawn maintenance area - Qop
wMlh +10 feat) x length
Swale Debris and Utter
Removal
S0.10/linear foot/year $0.10 / linearfoot $0.10/linear fooit -
Grass Reseeding with
Mulch and Fertilizer
$0.30/yd' $0.01 / linearfoot $0.01 / linearfoot Ansa revegetated equals 1 %
of lawn maintsnarcearea per
yeer
Program Admlnlstrstlor and
Swale Inspection
$0.15/ linear foot/year
plus $25/ inspection
$0.15 / linearfoot {0.15/linear foot 1 nspect four times per yes r
Total ~ {0.5t / linaar fool $0.75/lin frar foot -
January 2003 California Stormwater BMP Handbook
New Development and Redevelopment
www. ca bmptia nd twoks. CO m
9 of 13
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 fiinction of the mowing frequency. Unit costs developed by
SEWRPC are shown in Table 3. In many cases vegetated channels would be used to convey
mnoff and would require periodic mowing as well, so there may be Httle 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,
"Performance of vegetative controls for treating highway runoff," ASCE Journal of
Environmental Engineering, Vol. 124, No. 11, pp. 1121-1128.
Brown, W., andT. Schueler. 1997. The Economics of Stormwater BMPs 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., Horner, Richard R., and Booth, Derek B., 2000. Characterization of
Performance Predictors and Evaluation of Mowing Practices in BioJUtration 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. FederalHighway Administration, Washington, DC.
Gk)ldberg. 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. MassarelH. 1983. Grassy swales prove cost-effective for
water pollution control. Public Works, 16: 53-55.
Koon, J. 1995. Evaluation of Water (polity 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 Stonnwater Runoff Management: Disease Vectors Associated With Structural BMPs.
Stormwater 3(2): 24-39.Oakland, P H. 1983. An evaluation of stormwater pollutant removal
10of 13 California Stormwater BMP Handbook January 2003
New Development and Redevelopment
w WW .ca bmpha ndtwoks.com
Vegetated Swale TC-30
through grassed swale treatment. In Proceedings ofthe Intemational 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 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 /o wm/mtb / ve gswale. pdf Office of Water, Washington DC.
Wang, T., D. Spyridakis, B. Mar, and R. Horner. 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 Runoff Manual, Washington
State Department of Transportation, Olympia, Washington.
Welborn, C, and J. Veenhuis. 1987. Effects of Runoff 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 of Highway Contaminants By Roadside Swales. University of Central
Florida and Florida Department of Transportation, Orlando, FL.
Yu, S., S. Barnes, and V. Gerde. 1993. Testing of Best Management Practices for Controlling
Highway Runoff. FHWA/VA-93-R16. Virginia Transportation Research Council,
Charlottesville, VA.
Information Resources
Maryland Department ofthe Environment (MDE). 2000. Maryland Stormwater 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 BMP Handbook 11 of 13
New Development and Redevelopment
www .ca bmpha ndbooks .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 Specking Management Measures for Sources of Nonpoint 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
Stormwater 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 stormwater BMP Handbook January 2003
New Devebpmentand Redevelopment
www.cabmphandbooks.com
Vegetated Swale TC-30
ProY ide for sccHir
(>l'<«k'Ctioil.
(al Crxns SKtian or swale witii clwck dani.
Notation:
L - Ltnglh cf swale impeundrrwnl ar«« p«rclitck dam(tO (b>
Dg I>«|itti of Gheck dam (ft)
Ss = Bonom slpa cH swale (tt.tl]
W = Top width of check dam (ft)
Wg = Bottom width of ch»c<( dam (ft)
Zit2 ~ Ratio of horizontjil to vortical change in &-wal« s.ide slope (ft'Tt)
Dimensional t kn or swale Impoundmcnl area.
January 2003 California Stormwater BMP Handbook
New Development and Redevelopment
www.cabmphandbooks.com
13 of 13
Low Impact Development strategies use careful site design and decentralized stormwater management
to reduce the environmental footprint of new growth. This approach improves water quality, minimizes
the need for expensive pipe-and-pond stormwater systems, and creates more attractive developments.
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 fi-om 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 pavers" 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 successftilly in Norway,
incorporating design features to reduce frost heave.
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
Management Objectives
° Reduce stormwater runoff volume
from paved surfaces
° Reduce peak discharge rates.
° Increase recharge through
infiltration.
° Reduce pollutant transport through
direct infiltration.
° Improve site landscaping benefits
{grass pavers only.)
Cover: A driveway in Connecticut
built with manufactured pcwing
stones. Lower photo shows paving
stone detail. Photo: University of
Connecticut, Jordan Cove Urban
Monitoring Pro/ect
Above: A parking lot with concrete
grass paver parking stalls. Lower
photo shows grass paver detail.
Photos: tower 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
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.
13 Paving stones (aka unit pavers) 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.
• Grass pavers (aka turf blocks) are a type of open-cell unit paver in which the cells
are filled with soil and planted with turf. The pavers, 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.
Design Guidelines for
Porous Asphait with
Subsurface Infiltration
RIVERJACKS
OPEN INTO
RECHARGE BED
UNCOMPACTED
SUBGRADE IS
CRmCAL FOR PROPER
IKFILTRAHON
UNIFORMLY GRADED
STONE AGGREGATE
WfTH
40% VOID SPACE
FOR STORMWATER STORAGE
AND RECHARGE
oHii assoones OJBaJRFACEBED
Above: A parking lot with
conventional asphalt aisles and
paving stone parking stalls. Paving
stones are most appropriate for
low-speed, low-traffic areas. Photo:
tower Columbia River Estuary
Partnership
D Grass pavers can improve site appearance by providing vegetation where there
would otherwise be only pavement.
D Porous paving increases effective developable area on a site because portions of
the stormwater management system are located underneath the paved areas, and
the infiltration provided by permeable paving can significantly reduce the need
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 pavers. 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.
D 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 pavers 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.
Rlioto: GeoSynfBC Consultants, Inc.
Maintenance
n Post signs identifying porous pavement areas.
n Minimize use of salt or sand during winter months
• Keep landscaped areas well-maintained and prevent soil
firom being transported onto the pavement.
• Clean the surface using vacuum sweeping machines.
For paving stones, periodically add joint material (sand)
to replace material that has been transported.
D Monitor regularly to ensure that the paving surface
drains properly after storms.
o Do not reseal or repave with impermeable materials.
• Inspect the surface annually for deterioration.
D Grass pavers may require periodic reseeding to iiU in
bare spots.
Design Details
a For all permeable paving, base course is a reservoir
layer of I "-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 fi-iction to prevent
traffic compaction and rutting.
o 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.
o For grass pavers, 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 pavers
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. Consequendy, 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 pavers varies considerably and wUl 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 l%-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 pavers 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 instaUed
in the reinforced turf but are only used during very heavy
storms. Because the reinforced turf works so weU 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
facUity 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
wvm.epa.gov/ednnrmrl/repository/abstrac2/abstra2.htm
www.forester.net/sw_0503_advances.html
This publication is one component of the MassachuseUs Low Impact Development Toolkit, a production ofthe Metropolitan Area
Planmng Council, in coordination with the 1-495 MetroWest 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.