HomeMy WebLinkAboutCT 2018-0001; WALNUT BEACH HOMES; FINAL HYDROLOGY STUDY; 2018-11-01FrNAL-
HYDRoLoGYSTUDY
FOR
GRADING PERMIT
WALNUT BEACH HOMES -362 WALNUT A VENUE
CT 2018-0001 I CDP 2018-0003 / PUD 2018-0001
DWG 513-lA / GP 2018-0029
CITY OF CARLSBAD, CA
PREPARED FOR:
RINCON REAL EST A TE GROUP
3005 S. EL CAMINO REAL
SAN CLEMENTE, CA 92672
PH: (
PREPARED BY:
PASCO LARET SUITER & ASSOCIATES, INC.
535 N. HIGHWAY 101 , SUITE A
SOLANA BEACH, CA 92075
PH: (858) 259-8212
C E
JAN O 7 2019
Prepared: January 2018
Revised: March 2018
Revised: May 2018
Revised: July 2018
Revised: September 2018
Final: November 2018
LAND DEVELOF iv:ENT
ENGINEERING
~z-G-. L-A-WSON, RCE 80356
/2-Z 'i--/t;
DATE
Executive Summary
Introduction
Existing Conditions
Proposed Project
Conclusions
References
Methodology
Introduction
TABLE OF CONTENTS
County of San Diego Criteria
City of Carlsbad Standards
Runoff Coefficient Determination
Hydrology & Hydraulic Model Output
Pre-Developed Hydrologic Model Output (100-Year Event)
Post-Developed Hydrologic Model Output (100-Year Event)
Detention Analysis (100-Year Event)
Appendix
Appendix A: Hydrology Support Material
PLSA 2808-02
SECTION
1.0
1.1
1.2
1.3
1.4
1.5
2.0
2.1
2.2
2.3
2.4
3.0
3.1
3.2
3.3
4.0
PLSA 2808-02
1.0 EXECUTIVE SUMMARY
1.1 Introduction
This Hydrology Study for the proposed development at 362 Walnut Avenue 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 100-
year, 6-hour storm.
1.2 Existing Conditions
The subject property is located between Tamarack A venue and Carlsbad Village Drive,
and is geographically settled between the Coastal Rail Trail and railroad tracks to the east,
and Lincoln Street, a public street, to the west. The site is bordered by Walnut Avenue to
the south, and existing single-and-multi-family residential developments to the north, east,
and west. The existing site consists of two single-family residences, with a driveway,
hardscape, and landscape improvements typical of this type of development. The property
is located within the Carlsbad watershed, and, more specifically, the Agua Hedionda
Hydrologic Area.
The existing site is comprised of approximately 0.574 acres. Based on a study of the
existing topography, the site generally drains from east to west, towards the southwest
corner of the property. There does not appear to be any existing storm drain infrastructure
located onsite to convey drainage, and runoff sheet flows through the site to the western
property line. To the north of the property, an existing berm prevents off site runon from
leaving the multi-family development to the north and entering the subject property.
Drainage does not enter the property from the west, as it is downstream of the subject
parcel. Lastly, curb, gutter, and sidewalk improvements in the right-of-way along Walnut
A venue -as well as a parkway sloped away from the site -prevent drainage from entering
the site to the south. Runoff primarily leaves the site to the west, where it eventually enters
the existing curb and gutter in Walnut Avenue. The existing curb and gutter conveys
drainage west to a sump inlet located approximately 100 feet away from the property. After
entering the curb inlet, the surface storm water enters the public storm drain in Walnut
Avenue and is conveyed east to the San Diego Northern Railway (SDNR) right-of-way.
Once outletting through a headwall, drainage will head south before ultimately discharging
in the Agua Hedionda Lagoon and the Pacific Ocean.
The subject property can be assumed to have one discharge location from the site in the
existing condition. Basin A is approximately 0.552 acres in size and 15.6% impervious.
Per the Web Soil Survey application available through the United States Department of
Agriculture, the area is generally categorized to have group B soils. Based upon soil type
and the amount of existing impervious area onsite, a runoff coefficient of 0.35 was
calculated for Basin A using the methodology described in section 3 .1.2 of the San Diego
County Hydrology Manual and the formula provided therein. Using the Rational Method
Page 1 of 13
PLSA 2808-02
Procedure outlined in the San Diego County Hydrology Manual, a peak flow rate and time
of concentration was calculated for each basin for the 100-year, 6-hour storm event. The
peak flow rate for the 100-year, 6-hour storm for Basin A was determined to be 0.92 cfs
with a time of concentration of 8.3 minutes, discharging to Walnut Avenue to the southwest
before entering a storm drain curb inlet where it is conveyed east via an existing public
storm drain to the railroad right-of-way.
It can be noted that a small strip of the site to the northeast providing access off Washington
Street is sloped to drain away from the property. While this portion of the property
discharges to a separate location, both flows are eventually conveyed to the railroad right-
of-way just downstream and continue south towards Tamarack A venue and the Agua
Hedionda Lagoon. Refer to pre-development hydrology calculations included in Section
3.1 of this report for a detailed analysis and the pre-development hydrology map included
in Appendix A of this report for existing drainage basin delineation and discharge
locations.
1.3 Proposed Project
The proposed project includes the demolition of all onsite structures and the construction
of eleven multi-family condominium units with a shared access driveway, and various
hardscape and landscape. Site grading along with drainage and utility improvements
typical of this type of residential development will also be constructed. The proposed pad
elevations range from elevation 54.0 at the northern end of the property to 55.0 at the
southerly end of the subdivision near Walnut Avenue.
Runoff from the proposed development will drain from south to north, and will be directed
to the proposed bioretention basin located at the northeast corner of the property. A
majority of the site and the entirety of the proposed impervious areas will be routed away
from proposed structures to a series of storm drain inlets located in the rear yards of each
unit. Runoff draining from the roof of structures will be directed over landscaping or the
pervious driveway to eliminate any directly connected impervious areas. Private drain
pipes will collect all runoff entering each inlet and will route to the storm water treatment
facility by means of 6" PVC storm drain pipes. The drive aisle is sloped in an inverse
crown to the center of the driveway, where a PCC ribbon gutter will collect and convey
drainage on the surface north through the site.
Similar to the existing condition, the analyzed watershed can be categorized into one
drainage basin with one discharge location from the subject property. As in the pre-
developed condition, Basin A is 0.552 acres in size, and is approximately 47% impervious.
Based upon soil type and the amount of proposed impervious area onsite, a runoff
coefficient of 0.56 was calculated for Basin A using the methodology described in section
3.1.2 of the San Diego County Hydrology Manual and the formula provided therein. Using
the Rational Method Procedure outlined in the San Diego County Hydrology Manual, a
peak flow rate and time of concentration was calculated for the post-development 100-
year, 6-hour storm event. The peak flow rate for the 100-year, 6-hour storm for Basin A
was determined to be 1.73 cfs with a time of concentration of 8.9 minutes, discharging
Page 2 of 13
PLSA 2808-02
from the site to the northeast by means of a PVC outlet pipe from the bioretention basin.
Treated storm water leaving the project in the post-developed condition flows on the
surface in Washington Street northeast toward Pine Avenue, where it reaches a cross gutter
that discharges to the railroad tracks. Thus, existing and proposed flows enter the NCTD
right-of-way approximately 500 feet apart, and eventually are conveyed to an existing RCP
storm drain on the east side of the railroad tracks. Per City of Carlsbad public record
drawing DWG 360-5, the existing RCP storm drain in NCTD right-of-way routes water
south to the Agua Hedionda Lagoon. Refer to post-development hydrology calculations
included in section 3.2 of this report for detailed analysis and the post-development
hydrology map in Appendix A of this hydrology report for proposed drainage basin
delineation and discharge locations.
While the proposed drainage basin A discharges from the site to the northeast toward
Washington Street -as opposed to Walnut Avenue to the south -this can be considered a
local micro-diversion in the Agua Hedionda Hydrologic Area. Based on a study by Pasco,
Laret, Suiter & Associates of the local drainage basin draining to Walnut Avenue, the
existing storm drain in Walnut is at or very near capacity in the current condition. By
routing existing flows away from Walnut Avenue, the project helps alleviate demand on a
system that is at full capacity. Based on a preliminary review of the existing surface
infrastructure on Washington Street and the amount of water the system is conveying, it is
also our professional opinion that the curb and gutter has the capacity to route peak flows
to the designated discharge location on Pine A venue. Lastly, after working with the City
of Carlsbad on all possible solutions, it was also deemed infeasible to construct drainage
facility "BB-1" as part of the City of Carlsbad's Drainage Master Plan.
In an effort to comply with the City of Carlsbad's storm water standards for Standard
Development Projects, the project site will implement source control and site design
BMP's where feasible and applicable in accordance with the City of Carlsbad's BMP
Design Manual, February 2016 edition. All runoff generated from proposed roofs and
hardscape areas will be directed to landscaped areas and grassy swales to disperse drainage
onto pervious surfaces. Modular permeable pavers are proposed for the drive aisle to
minimize proposed impervious area, soil compaction, and promote dispersion. Partial
infiltration and evapotranspiration in landscaped areas will assist in slowing peak
discharges and in reducing total volume generated during storm events. The drive aisle
and onsite landscaped areas will remove sediment and particulate-bound pollutants from
storm water prior to leaving the project site.
Page 3 of 13
PLSA 2808-02
1.4 Conclusions
Based upon the analysis included in this report, the proposed onsite drainage design is
adequate to capture and convey runoff from the proposed project and discharge to the
appropriate discharge locations, while complying with local and state level water quality
regulations. The micro-diversion of storm water from the existing to proposed condition
helps alleviate an already sub-standard drainage scenario by directing water away from the
storm drain system in Walnut Avenue that is currently at capacity.
1.5 References
"San Diego County Hydrology Manual ", revised June 2003, County of San Diego,
Department of Public Works, Flood Control Section.
"San Diego County Hydraulic Design Manual ", revised September 2014, County of San
Diego, Department of Public Works, Flood Control Section
"Engineering Standards, Volume 5: Carlsbad BMP Design Manual ", revised February
2016, City of Carlsbad
Soil Survey Staff, Natural Resources Conservation Service, United States Department of
Agriculture. Web Soil Survey. Available online at http://websoilsurvev.nrcs.usda.gov.
Page 4 of 13
PLSA 2808-02
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 Rational Method formula
estimates the peak rate of runoff based on the variables of area, runoff coefficient, and
rainfall intensity, and is the preferred method for drainage basins up to one square mile in
size. The rainfall intensity (I) is equal to:
Where:
I = 7.44 x P6 x o-0645
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:
Where:
Q=CIA
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
that 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.
Page 5 of 13
PLSA 2808-02
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 under
100 feet, and is dependent on land use and slope.
2.3 City of Carlsbad Standards
The City of Carlsbad requires a Preliminary Hydrology Study be completed for projects
larger than I-acre per Form P-2: Development Permits. The City of Carlsbad has additional
requirements for hydrology reports which are outlined in the Grading and Erosion Control
Ordinance. Please refer to this manual for further details.
2.4 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.
Page 6 of 13
PLSA 2808-02
3.0 HYDROLOGY MODEL OUTPUT
3.1 Pre-Developed Hydrologic Model Output (100 Year Event)
Pre-Development:
*Rational Method Equation Q=CIA
P100= 2.5 in * 100-Year, 6-Hour Rainfall Precipitation
Basin A (Onsite Drainage Basin)
Total Area= 24,020 sf -+ 0.552 Acres
Impervious Area = 3,742 sf -+ 0.086 Ac
Pervious Area = 20,278 sf -+ 0.466 Ac
Cn, Weighted Runoff Coefficient,
-0.25 , Cn value for natural ground, Type B Soils
*Per San Diego Hydrology Design Manual (SDHDM) Section 3 .1.2
-0.90, Cn value for developed/impervious surface
*Per SDHDM Section 3.1.2
Cn = 0.9 x 3,742 sf+ 0.25 x 20,278 sf= 0.35
24,020 sf
Tc = ti+ tt ti @ 65 '=1.0%
R-23 per Land Use Element => 6.1 min from Table 3-2
tt => L = I 5 5' dE = I. 6'
tt= [ { I l.9(L/5,280)"3 }/ dE]"0.385
tt= [ { 11.9(155/5,280)"3 }/I .6]"0.385 = 0.03 7
0.037 x 60 = 2.2 Min.
Tc= 6.1 + 2.2 = 8.3 Min
P6= 2.5
1 = 7.44 x P6 x D-0-645
I = 7.44 X 2.5 X 8.3-0645 ::::::4.75 in/hr
1100 :::::: 4. 75 in/hr
Q100 = C*l*A
Q100 = 0.35 x 4.75 in/hr x 0.552 Ac= 0.92 cfs
Page 7 of 13
PLSA 2808-02
3.2 Post-Developed Hydrologic Model Output (100-Y ear Event)
Post-Development:
*Rational Method Equation Q =CIA
P100 = 2.5 in * 100-Year, 6-Hour Rainfall Precipitation
Basin A (Onsite Drainage Basin -Draining to Bioretention)
Total Area = 24,020 sf-+ 0.552 Acres
Impervious Area = 10,770 sf-+ 0.247 Ac
*Pervious Pavement = 5,416 sf-+ 0.124 Ac
Pervious Area = 7,824 sf-+ 0.18 Ac
*Pervious Pavement (Permeable Pavers) assumed to behave as an impervious material
during the 100-year storm event
Cn, Weighted Runoff Coefficient,
-0.25, Cn value for natural ground, Type B Soils
*Per San Diego Hydrology Design Manual (SDHDM) Section 3.1.2
-0.90, Cn value for developed/impervious surface
*Per SDHDM Section 3.1.2
Cn = 0.9 x 16,186 sf + 0.25 x 7,824 sf= 0.69
24,020 sf
Tc = ti + tt ti @ 65 '=1.0%
R-23 per Land Use Element => 6.1 min from Table 3-2
tt => L = 225 ' ~E = 2.7'
tt= [{l l.9(L/5,280)"3}/~E]"0.385
tt= [ { 11.9(225/5,280)"3 }/2. 7]"0.385 = 0.046
0.046 x 60 = 2.8 Min.
Tc = 6.1 + 2.8 = 8.9 Min
P 6= 2.5
1 = 7.44 x P 6 x D-0645
I = 7.44 X 2.5 X 8.9-0645 ~ 4.55 in/hr
Q,oo = C*l*A
Q,oo = 0.69 x 4.55 in/hr x 0.552 Ac= 1.73 cfs
Page 8 of 13
Basin B (Panhandle Basin -Draining to Washington Street)
Total Area= 1,000 sf-+ 0.02 Acres
Impervious Area = 0 sf-+ 0.00 Ac
*Pervious Pavement= 1,000 sf-+ 0.02 Ac
Pervious Area = 0 sf-+ 0.00 Ac
Cn = 0.9 x 1,000 sf+ 0.25 x 0 sf= 0.90
1,000 sf
PLSA 2808-02
*Pervious Pavement (Permeable Pavers) assumed to behave as an impervious material
during the 100-year storm event
Tc = 5.0 min *Minimum Tc per SDCHM
Q100 = C*l*A
Q,oo = 0.90 x 6.59 in/hr x 0.02 Ac= 0.12 cfs
Pre-Development vs. Post-Development (Undetained -Basin A Onsite Runoff)
Pre-Development (Basin A)
Q,oo = 0.92 cfs
Post-Development (Basin A)
Q,oo = 1.73 cfs
Delta
0.81 cfs
*Total overall site runoff increased in proposed condition from pre-development site.
Refer to the following section, Section 3.3 , of this report for a discussion of detention
system proposed to mitigate site to existing conditions. Also refer to the Appendix of this
report for existing and proposed condition hydrology node maps showing discharge
locations, as well as the body of this report for justification for the local diversion of storm
water.
Page 9 of 13
PLSA 2808-02
3.3 Detention Analysis
The HMP bioretention basin provides pollutant control as well as hydromodification
management and mitigation of the 100-year, 6-hour storm event peak flow rate. The 100-
year, 6-hour storm event detention analysis was performed in accordance with Chapter 6
of the San Diego County Hydrology Manual. The results of the analysis provide a static
detention requirement by comparing the total volume produced by the 100-year, 6-hour
storm in the existing and developed conditions. The total volume can be calculated after
producing a hydrograph for each event (pre-and post-developed) with the results of the
Rational Method analysis. Calculations and results of this analysis can be shown below.
This method is considered to be conservative, as it does not take into account the routing
characteristics of the bioretention basin.
The HMP bioretention facility consists of a basin with an approximately 300 square foot
basin bottom, 18 inches of engineered soil and 30 inches of gravel. Runoff will be
biofiltered through the engineered soil and gravel layers, then collected in a series of small
PVC perforated drain pipes and directed to a catch basin located in the northeast corner of
the HMP bioretention basin. Once in the catch basin, runoff will be mitigated via a small
low-flow orifice to comply with HMP requirements. In larger, high-intensity storm events,
runoff not filtered through the engineered soil and gravel layers will be conveyed via an
overflow outlet structure consisting of a 36-inch by 36-inch grate located on top of the
catch basin. Runoff entering the outlet structure will be conveyed directly to a proposed
12-inch PVC drainpipe and will discharge northeast towards the SDNRright-of-way. Once
on the surface in the railroad right-of-way, runoff will eventually enter the large storm drain
located on the east side of railroad tracks and continue south towards the Agua Hedionda
Lagoon. Refer to Attachment 5 of the project's Storm Water Quality Management Plan
(SWQMP) for a general cross-section of the HMP biofiltration basin.
3.3.1 Static (No Routing) Detention Calculation
Pre-Development: 100-Year Event (Basin A)
Volume= C*P6*A *Equation 6-1 ofSDCHM
Volume= 0.35 * (2.5 in * 1 ft/ 12 in)* (24,020 SF)= 1,751 CF
Post-Development: 100-Year Event (Basin A)
Volume= C*P6*A *Equation 6-1 of SDCHM
Volume = 0.69 * (2.5 in* 1 ft/ 12 in)* (24 ,020 SF)= 3,453 CF
Required Detention Volume = Total Post-Development -Total Pre-Development
= Prop. Basin A -Pre Basin A
= 3,453 CF-1,751 CF= 1,702 CF
-> 1,702 CF static storage required for drainage basin A
Page 10 of 13
3.3.2 Basin Stage-Storage Volume Calculation
BMP Stage Storage -Basin INF-2
Basin Stage Total
Elevation Area Volume Volume
(sq-ft) (cu-ft) (cu-ft)
46.67 300 0 0
47.17 300 60 60
47.67 300 60 120
48.17 300 60 180
48.67 300 60 240
49.17 300 60 300
49.50 300 40.08 340.08
50.00 300 30 370.08
so.so 300 30 400.08
51.00 300 30 430.08
51.5 391.25 195.63 625.71
52 .0 482.5 241.25 866.96
52 .5 573.75 286.88 1153.83
53 .0 665 332 .5 1486.33
*Bottom of Gravel Layer
(40% Void Ratio)
PLSA 2808-02
*Top of Gravel Layer/Bot
of Pea Gravel
(20% Void Ratio)
*FG of Basin (Beneath mulch)
*Top of Ponding/ Weir EL
*Top of Slope
1,154 CF storage provided at ponding depth (1 ,486 CF storage total); use detention volume
in conjunction with storage provided in gravel layers of concrete driveway with gravel-
filled openings.
3.3.3 Pervious Pavement Drive Aisle
Detention Volume Provided = Driveway Area x 12" Gravel Layer x 40% Voids
= 4,320 SF* (12 in* 1 ft / 12 in)* 0.40
= 1,728 CF
VOID FILLER-118" TO 318"
(NO 8)AGGREGATE IN VOIDS
BEDDING COURSE -2" THICK OF 118"
TO 318" (NO 8)AGGREGATE
CHOKER COURSE · 4· THICK OF 318"
TO 1/2" (NO. 57) CRUSHED ROCK
MIN. 6" THICK OF 314" _/
CRUSHED ROCK
SOIL SUBGRADE
BELGARD MEGA-LAFITT GRANA
PERVIOUS PAVERS OR APPROVED
EQUAL WITH MIN 318" VOID
6" X24"PCC
FLUSH CURB
\
3" PVC UNDER DRAIN
PIPE@3' O.C. (BEYOND, TYP.)
TO CONNECT BETWEEN
GRAVEL LAYERS
1,486 CF storage from BMP basin+ 1,728 CF storage in gravel layers under PCC driveway
= 3,214 CF total > 1,702 CF static storage required for drainage basin A per SDCHM.
Page 11 of 13
PLSA 2808-02
3.4 Hydromodification Analysis
To satisfy the requirements of the MS4 Permit, a hydromodification management strategy
has been developed for the project based on the Final Hydromodification Management Plan
dated March 2011, (Final HMP). A continuous simulation model, the Environmental
Protection Agency (EPA) Storm Water Management Model (SWMM) version 5.1 , was
selected to size mitigation measures. The SWMM model is capable of modeling
hydromodification management facilities to mitigate the effects of increased runoff from
the post-development conditions and use changes that may cause negative impacts (i.e.
erosion) to downstream channels.
3.4.1 Hydromodification Criteria
Pursuant to the MS4 Permit, post-development runoff conditions (flow rates and durations)
must not exceed pre-development runoff conditions by more than I 0% (for the range of
flows that result in increased potential for erosion, or degraded instream habitat
downstream of the project. Based on the Final HMP:
• For flow rates between the pre-project lower flow threshold (10%, 30%, or 50%) of the
pre-project 2-year runoff event (0.1 Q2, 0.3Q2, or 0.5Q2) to the pre-project 10-year event
(Q10), the post-project discharge rates and durations may not deviate above the pre-
project rates and durations by more than 10% over more than 10% of the length of the
flow duration curve.
A channel screening analysis may be performed to determine a larger lower flow threshold,
however for this project a lower flow threshold of 0.1 Q2 (high susceptibility) is assumed.
· 3.4.2 SWMM Model Development
SWMM is a rainfall-runoff model used for single event or continuous simulation ofrunoff
quantity from primarily urban areas. SWMM calculates and routes runoff based on user-
specified input including precipitation data, subcatchment characteristics, soil data, routing
information, and BMP configuration. SWMM is capable of modeling various hydrologic
processes including but not limited to time-varying precipitation, evaporation, storage,
infiltration, and retention LID facilities.
3.4.3 SWMM Input
A pre-development and post-development model were created using the following global
information:
Parameter Input Source
Precipitation Oceanside Rainfall Data Proiect Clean Water
Evaporation Encinitas ETo Zone Data CIMIS ETo Zones Map
Soils B USDA Web Soil Survey Application
Page 12 of 13
PLSA 2808-02
The HMP biofiltration basin has been designed to comply with both pollutant control and
hydromodification management criteria. Refer to Section 3.5 for the pollutant control
calculations. The HMP biofiltration portion in the SWMM model is specified as an "LID
Control" within the "Subcatchment" to define the ponding depth, biofiltration soil layer
and gravel layer.
3.4.4 SWMM Processing and Output
The HMP sizing was determined assuming a completely pervious existing site condition.
The pre-development project 0.lQ2 and Q10 were determined to be 0.016 cfs and 0.35 cfs,
respectively. After routing through the HMP biofiltration basin, the post-development,
mitigated project 0.1 Q2 and Q10 were determined to be 0.006 cfs and 0.32 cfs leaving the
site, respectively. Refer to the SWMM output included in Attachment 5 of the project
SWQMP for detailed information and data from the SWMM model including input files,
rain gage and evaporation data, and flow duration and frequency curves. Drawdown
calculations are also included in Attachment 2 of the SWQMP.
3.5 Storm Water Pollutant Control
To meet the requirements of the MS4 Permit, the HMP biofiltration facility is designed to
treat onsite storm water pollutants contained in the volume of runoff from a 24-hour, 85th
percentile storm event by filtering runoff through an engineered soil layer and gravel layer.
3.5.1 Numeric Sizing Requirements for Pollutant Control BMPs
Pursuant to the MS4 Permit, Pollutant Control BMPs shall be designed to retain onsite
pollutants contained in the post-development Design Capture Volume (DCV). The DCV
is the volume of runoff resulting from the 24-hour, 85th percentile storm event. The DCV
calculations for the project are located in Attachment 1 of the project SWQMP. The
Pollutant Control BMP proposed for the project is an HMP biofiltration basin. The
proposed HMP biofiltration basin does not provide retention, therefore pursuant to the MS4
Permit, the HMP biofiltration basin is designed to biofilter 1.5 times the DCV and provide
non-routed storage volume equal to at least 0.75 times the portion of the DCV not retained
onsite.
Page 13 of 13
APPENDIX A
Hydrology Support Material
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Hydrology Manual
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I = 7.44 P5 D--0.645
I = Intensity (in/hr)
P5 = 6-Hour Precipitation (in)
D = Duration (min)
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Directions for Application:
(1) From precipitation maps determine 6 hr and 24 hr amounts
for the selected frequency. These maps are included in the
County Hydrology 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 lo the plotted lines.
(5) This line is the intensity-<luration curve for the location
being analyzed.
Application Fonn:
(a) Selected frequency~ year
p
(b) P6 = 3 in. p24 = 5.5 .-2. = 54.5 %(2) --, --'P24 --
(c) Adjusted P6<2l = _3_ in.
(d)lx = ~min.
(e) I = ___1L in./hr.
Note: This chart replaces the Intensity-Duration-Frequency
curves used since 1965.
I
P6 1 1.s 2 25 J 3.5_,_ 4 ~s s_ s.s &
Ourallonl 11 111 I I I I I
5 2.63-2:_9~5.27 6.59 7 909.22110.54 11.86 13.17 14.4!!j_1_5.8_1_
7 2.12 3.18 4.24 5.30 6.36 7.42. 8.48 9.54 10.60,. 11.66[..12.r.!
10 1.68 2.53 3.37 4.21 5.os s.90T s.14 1.sa ~ &~42 9.21 10.11
15 1.30 195~2.59 3.24 3.89 4.~ 5.19 584 6.49 7.13 778
20 1:oe 1.622.15 2.s9>,23:~n 4.31 4.~l-5·~-W-93 6.46
25 0.93 1.40 1.87 2.33 2.80 3.27 f;-!3 4.20 4.67 5. 13 5.60
30 0.83 1.24 1.66 2.07 2.49 2.90. 3.32 3.73 , 4.15 4.56 4.98
40 0.69 I 03 1.38, 1 n 2.07 241 I 276 3,10 , 3.45 3.79 4.13
50 0.60 0.90 1.19 1.49 1.79 2.09 2.39 2.69 2.98 3.28 3.58
60 0.53 080 1 06 1.33 1 59 1.86 2.12 2.39 2.65 2.92 3.18
90 0.41 0.61 0.82 1.02 1.23 I 43 1 63 1.84 2.04 2.25 2.45
120 0.34 0 51 0.68 0.85 1.02 1 19 1 36 1.53 1.70 1.87 2.04 · I J 150 0.29 '0.44 059~073 088-1.,.03 1.18 1.32 1.47 1.62 1.76
180 0.2610.39J0,52 0.65 0.78 0.91 1.04 1 18 1.31 1 44 1.57
240 0.22 10.3310.43~0.~ 065 0,76 0.87, 0.98 1.08 1.19 1.30 .!GO 0.19 o.2a10.J.8 0.47~0.56 _0.~-i-01sLQ.es o.94 1.03 113
360 0.17 0.25 0.33 042 0.50 0.58 087 l 0.75 0.84 0.92 1.00
FIGURE
Intensity-Duration Design Chart -Example I 3-2
San Diego County Hydrology Manual
Date: June 2003
Table 3-1
Section:
Page:
RUNOFF COEFFICIENTS FOR URBAN AREAS
Land Use I Runoff Coefficient "C"
Soil Type
NRCS Elements Coun Elements %IMPER. A 8
Undisturbed Natural Terrain (Natural) Permanent Open Space 0* 0.20 0.25
Low Density Residential (LOR) Residential, 1.0 DU/ A or less 10 0.27 0.32
Low Density Residential (LOR) Residential, 2.0 DU/A or less 20 0.34 0.38
Low Density Residential (LOR) Residential, 2.9 DU/A or less 25 0.38 0.41
Medium Density Residential (MOR) Residential, 4.3 DU/A or less 30 0.41 0.45
Medium Density Residential (MOR) Residential, 7.3 DU/A or less 40 0.48 0.51
Medium Density Residential (MOR) Residential, 10.9 DU/A or less 45 0.52 0.54
Medium Density Residential (MOR) Residential, 14.5 DU/ A or less 50 0.55 0.58
High Density Residential (HOR) Residential, 24.0 DU/ A or less 65 0.66 0.67
High Density Residential (HOR) Residential, 43.0 DU/A or less 80 0.76 0.77
Commercial/Industrial (N. Com) Neighborhood Commercial 80 0.76 0.77
Commercial/Industrial (G. Com) General Commercial 85 0.80 0.80
Commercial/Industrial (O.P. Com) Office Professional/Commercial 90 0.83 0.84
Commercial/Industrial (Limited I.) Limited Industrial 90 0.83 0.84
Commercial/Industrial (General I.) General Industrial 95 0.87 0.87
C
0.30
0.36
0.42
0.45
0.48
0.54
0.57
0.60
0.69
0.78
0.78
0.81
0.84
0.84
0.87
3
6 of26
D
0.35
0.41
0.46
0.49
0.52
0.57
0.60
0.63
0.71
0.79
0.79
0.82
0.85
0.85
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
N RCS = National Resources Conservation Service
3-6
San Diego County Hydrology Manual
Date: June 2003
Section:
Page:
3
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 Ti 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 (Ti)
Element* DU/ .5% 1% 2% 3% 5% 10%
Acre LM Ti LM Ti LM Ti LM Ti LM Ti LM Ti
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
33" g 23' N
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Map Scale: 1:1,140 f printed 00 A landscape (11" X 8.5") sheet ~-~;===~~-----======Meters gJ 0 15 :!) 8)
~-~;;==~~-----=======feet :nl 0 ~ ~ ~
Map projection: Web Merrator Comercroruinates: WGS84 Edge tics: UTM Zooe l lN WGS84
Natural Resources Web Soil Survey
Ni.'t;nn"'l Coof_'0 '"'';·,,e Soii,-C, '"";Y : ...... .,_~rvati!""' c~ice
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12/11/2017
33" g 23' N
33" '118' N
USDA r-
Hydrologic Soil Group-San Diego County Area, California
MAP LEGEND MAP INFORMATION
Area of Interest (AOI)
Area of Interest (AOI)
Soils
Soil Rating Polygons
D A
D AID
DB
D B/D
D C
D C/D
D D
D Not rated or not available
Soil Rating Lines
-.,, A
,...,,,. AID
,...,,,. B
,...,,,. BID
C
,...,,,. CID
,...,,,. D
-~ Not rated or not available
Soil Rating Points
■ A
■ AID
■ B
■ BID
Natural Resources
~""'~rvatt," c:4 _rvice
C
■
C
CID
■ D
□ Not rated or not available
Water Features
Streams and Canals
Transportation
+++ Rails
_, Interstate Highways
_, US Routes
Major Roads
Local Roads
Background
-Aerial Photography
Web Soil Survey Nt.::.::_:j Coor ""~'''le So~c:,,,.,,~y
The soil surveys that comprise your AOI were mapped at
1:24,000.
Warning: Soil Map may not be valid at this scale.
Enlargement of maps beyond the scale of mapping can cause
misunderstanding of the detail of mapping and accuracy of soil
line placement. The maps do not show the small areas of
contrasting soils that could have been shown at a more detailed
scale.
Please rely on the bar scale on each map sheet for map
measurements.
Source of Map: Natural Resources Conservation Service
Web Soil Survey URL:
Coordinate System: Web Mercator (EPSG:3857)
Maps from the Web Soil Survey are based on the Web Mercator
projection, which preserves direction and shape but distorts
distance and area. A projection that preserves area, such as the
Albers equal-area conic projection, should be used if more
accurate calculations of distance or area are required.
This product is generated from the USDA-NRCS certified data as
of the version date(s) listed below.
Soil Survey Area: San Diego County Area, California
Survey Area Data: Version 12, Sep 13, 2017
Soil map units are labeled (as space allows) for map scales
1 :50,000 or larger.
Date(s) aerial images were photographed: Nov 3, 2014-Nov
22,2014
The orthophoto or other base map on which the soil lines were
compiled and digitized probably differs from the background
imagery displayed on these maps. As a result, some minor
shifting of map unit boundaries may be evident.
12/11/2017
D:>.ge2 e.:,
Hydrologic Soil Group-San Diego County Area, California
Hydrologic Soil Group
Map unit symbol Map unit name Rating I Acres in AOI I Percent of AOI
MIC Marina loamy coarse
sand, 2 to 9 percent 1 slopes
I Totals for Area of lntere;--
Description
B I 2.4
+
Hydrologic soil groups are based on estimates of runoff potential. Soils are
assigned to one of four groups according to the rate of water infiltration when the
soils are not protected by vegetation, are thoroughly wet, and receive
precipitation from long-duration storms.
The soils in the United States are assigned to four groups (A, B, C, and D) and
three dual classes (AID, BID, and CID). The groups are defined as follows:
Group A Soils having a high infiltration rate (low runoff potential) when
thoroughly wet. These consist mainly of deep, well drained to excessively
drained sands or gravelly sands. These soils have a high rate of water
transmission.
Group B. Soils having a moderate infiltration rate when thoroughly wet. These
consist chiefly of moderately deep or deep, moderately well drained or well
drained soils that have moderately fine texture to moderately coarse texture.
These soils have a moderate rate of water transmission.
Group C. Soils having a slow infiltration rate when thoroughly wet. These consist
chiefly of soils having a layer that impedes the downward movement of water or
soils of moderately fine texture or fine texture. These soils have a slow rate of
water transmission .
Group D. Soils having a very slow infiltration rate (high runoff potential) when
thoroughly wet. These consist chiefly of clays that have a high shrink-swell
potential, soils that have a high water table, soils that have a claypan or clay
layer at or near the surface, and soils that are shallow over nearly impervious
material. These soils have a very slow rate of water transmission.
If a soil is assigned to a dual hydrologic group (AID, B/D, or C/D}, the first letter is
for drained areas and the second is for undrained areas. Only the soils that in
their natural condition are in group D are assigned to dual classes.
Rating Options
Aggregation Method: Dominant Condition
USDA Natural Resources
-iiiF Conservation Service
Web Soil Survey
National Cooperative Soil Survey
100.0%
100.0%
12/11/2017
Page 3 of 4
Hydrologic Soil Group-San Diego County Area, California
Component Percent Cutoff: None Specified
Tie-break Rule: Higher
USDA Natural Resources -ea Conservation Service
Web Soil Survey
National Cooperative Soil Survey
12/11/2017
Page 4 of 4
fF204 .132~iJ:oo • .__
1
RT/ON OF TRACT
OF THUM LANDS
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SUBDIVISION BOUNDARY
PROPERTY LINE I RIGHT-OF-WAY
CENTERLINE OF ROAD
EXISTING CONTOUR LINE
EXISTING PATH OF TRAVEL
--------
. ··--···--•
EXISTING MAJOR DRAINAGE BASIN BOUNDARY
EXISTING IMPERVIOUS AREA
BASIN A -AREA CALCULATIONS
TOT AL BASIN AREA
BASIN EXISTING IMPERVIOUS AREA
BASIN EXISTING PERVIOUS AREA
% IMPERVIOUS
Cn
24,020 SF (0.55AC)
3,742 SF /0.086 AC)
20,278 SF /0.466 AC}
15.6%
0.35
GRAPHIC SCALE 1" = 10'
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EXISTING HYDROLOGY EXHIBIT
WALNUT BEACH HOMES-362 WALNUT AVE
CITY OF CARLSBAD
PASCO LARET SUITER
& ASSOCIATES
CML ENBINEERINB + LAND PLANNINB + LAND 8URVEYINB
555 North lllpway 101, Ste A. Sol•ne Rnch, CA 92075
ph 151.259.12121 tz 1,a.2511,"12 I ~com
PLSA 2808-01
SEE RIGHT THIS SHEET AND
PROJECT GRADING PLAN FOR
OUTLET TO WASHINGTON
STREET RIGHT-OF-WAY -
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PORTION OF TRACT
222 OF THUM LANDS
MAP NO. 1681
BASIN A-AREA CALCULATIONS
TOTAL BASIN AREA
BASIN PROPOSED IMPERVIOUS AREA
.. BASIN PROPOSED PERVIOUS PAVEMENT
BASIN PROPOSED PERVIOUS AREA
% IMPERVIOUS
Cn
WP,LL
24,020 SF (0.55AC)
10.770 SF (0.247 AC)
5,416 SF (0.124AC)
7,824 SF (0.18 AC)
67.4%
0.69
,p.,
~' / .1<: i\i
PORTION OF TRACT
222 OF THUM LANDS
MAPNO. 1681
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NODEA1.2
54.7 TG
53.7 IE
PLAN VIEW -PROPOSED NODE MAP
SCALE: 1" = 10' HORIZONTAL
"PERVIOUS PAVEMENT CONSIDERED IMPERVIOUS FOR FLOOD CONTROL
CALCUlA TIONS I DETERMINING PEAK RUNOFF GENERA TED BY THE
100-YEAR. 6-HOUR STORM EVENT
BASIN B-AREA CALCULATIONS
TOTAL BASIN AREA
BASIN PROPOSED IMPERVIOUS AREA
"BASIN PROPOSED PERVIOUS PAVEMENT
BASIN PROPOSED PERVIOUS AREA
% IMPERVIOUS
Cn
1,000 SF (0.02 AC)
0 SF (O.OOAC)
1,000 SF (0.02 AC)
0 SF (OOOAC)
100.0%
0.90
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"PERVIOUS PAVEMENT CONSIDERED IMPERVIOUS FOR FLOOD CONTROL
CALCULATIONS I DETERMINING PEAK RUNOFF GENERATED BY THE
100-YEAR, !>-HOUR STORM EVENT
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LEGEND
SUBDIVISION BOUNDARY
PROPERTY LINE I RIGHT-OF-WAY
CENTERLINE OF ROAD
EXISTING CONTOUR LINE
PROPOSED CONTOUR LINE
PROPOSED PATH OF TRAVEL
--
• • • • • • •
PROPOSED MAJOR DRAINAGE BASIN BOUNDARY
PROPOSED IMPERVIOUS AREA
WASHINGTON STREET
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NODE 81.2
47.2 FS; 45.85 IE
0100 = 0.12 CFS
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APN: 204-132-26-00 APN: 204-132-16-00
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NODE 81.1
54.0FG
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PLAN VIEW -OUTLET TO WASHINGTON
SCALE: 1" = 10' HORIZONTAL
GRAPHIC SCALE 1" = 10'
10 0 10 20
PROPOSED HYDROLOGY EXHIBIT
WALNUT BEACH HOMES -362 WALNUT AVENUE
CITY OF CARLSBAD
30
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