HomeMy WebLinkAboutCT 2018-0008; GRAND JEFFERSON; HYDROLOGY STUDY FOR GRAND JEFFERSON; 2020-07-01HYDROLOGY STUDY
FOR.
THE GRAND JEFFERSON
GRADING PERMIT
786 GRAND AVENUE, CARLSBAD, CA
DWG 523-7A I GR 2020-0010
CF 2018-0008 / SDP 2018-0010
CITY OF CARLSBAD, CA
PREPARED FOR:
RINCON REAL ESTATE GROUP
5315 AVENIDA ENCINAS, SUITE 200
CARLSBAD, CA 92008
PH: (949) 438-5494
PREPARED BY:
PASCO LARET SUITER & ASSOCIATES, INC.
.535 N. HIGHWAY 101, SUITE A
SOLANA BEACH, CA 92075
PH: (858) 259-8212
Prepared: March 2020
Revised: July 2020
Z -
*LER 1AWSON. RCE 80356 DATE
PLSA 2863-01
TABLE OF CONTENTS
SECTION
Executive Summary 1.0
Introduction 1.1
Existing Conditions 1.2
Proposed Project 1.3
Conclusions 1.4
References 1.5
Methodology 2.0
Introduction 2.1
County of San Diego Criteria 2.2
Runoff Coefficient Determination 2.3
Hydrology & Hydraulic Model Output 3.0
Pre-Developed Hydrologic Model Output (100-Year Event) 3.1
Post-Developed Hydrologic Model Output (100-Year Event) 3.2
Detention Analysis (100-Year Event) . 3.3
Building Drainage System Discussion 3.4
Hydromodification Analysis 3.5
Pollutant Control 3.6
Appendix 4.0
Appendix A: Hydrology Support Material
PLSA 2863-01
1.0 EXECUTIVE SUMMARY
1.1 Introduction
This Hydrology Study for the proposed development at 786 Grand Avenue, at the
intersection of Grand Avenue and Jefferson Street, 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 west of Interstate-5 between Carlsbad Village Drive and
the Buena Vista Lagoon, and is geographically settled between Jefferson Street to the east,
Grand Avenue to the south, and existing single-family developments to the west and north.
The existing site consists of an existing single-family residence, with a driveway, sheds
and ancillary structures, and landscape improvements typical of this type of development.
The property is located within the Carlsbad watershed, and, more specifically, the Buena
Vista Creek Hydrologic Area.
The existing site is comprised of approximately 0.206 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 toward the
western property line and Grand Avenue to the south. To the north of the property, an
existing concrete driveway prevents run-on from leaving the 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 Jefferson Street - as well as a parkway sloped away from the site -
prevent drainage from entering the site to the east. Runoff primarily leaves the site to the
west, where it eventually enters the existing curb and gutter in the public right-of-way. The
existing curb and gutter conveys drainage west along Grand Avenue to what appears to be
a sump inlet located at the intersection of Grand and Madison Street. After entering the
existing curb inlet, the surface storm water enters the public storm drain and is conveyed
to its ultimate discharge location in 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 0.206 acres in size and 39.0% 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
B and the amount of existing impervious area onsite, a runoff coefficient of 0.50 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 each basin for the 100-year, 6-hour storm event. The
Page! of 12
PLSA 2863-01
peak flow rate for the 100-year, 6-hour storm for Basin A was determined to be 0.53 cfs
with a time of concentration of 7.4 minutes, discharging to Grand Avenue to the southwest
before entering a storm drain curb inlet downstream at the intersection of Madison and
Grand Avenue. From there, it is conveyed by way of an existing public storm drain to the
Pacific Ocean. Refer to pre-development hydrology calculations included in section 3.1 of
this report for detailed analysis and the pre-development hydrology map in Appendix A of
this hydrology 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 a 7-unit mixed-use building, with six multi-family residential condominium units and
one commercial condominium unit, along with a covered parking garage and various
hardscape and landscape improvements. Site grading along with drainage and utility
improvements typical of this type of residential development will also be constructed.
Runoff from the proposed development will be collected in a roof drain system and will be
directed to a series of biofiltration BMPs / raised planter areas on the upper floors of the
building for treatment and detention. Private drain pipes and catch basins located in the
biofiltration planters will collect all runoff entering each planter and will route offsite to
the Jefferson Street and Grand Avenue rights-of-way by means of sidewalk under drain
piping or a curb outlet structure. The covered portion of the parking garage is directed to
drain to a series of inlets located at the center of the drive aisle, and will be plumbed to the
public sewer main in Jefferson Street, bypassing any storm water treatment prior to being
routed offsite.
Similar to the existing condition, the analyzed watershed can be categorized into one major
drainage basin with one eventual discharge location from the subject property toward the
southwest corner of the lot. As in the pre-developed condition, Basin A is 0.206 acres in
size, and is approximately 89% impervious. Based upon soil type B and the amount of
proposed impervious area onsite, a runoff coefficient of 0.83 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 1.13 cfs with a time of
concentration of 5.0 minutes (minimum time of concentration per San Diego County
Hydrology Manual used to calculate rainfall intensity for drainage basin of this size and
amount of impervious area) discharging from the site. This increase of 0.6 cfs due to the
proposed development and additional hardscape is mitigated by the onsite biofiltration
BMPs / raised planter areas. The mitigated peak flow rate, after taking into account the
detention characteristics of the raised planters, for the 100-year, 6-hour storm event is 02.
cfs, which is less than the pre-project peak flow of 0.53 cfs. Refer to section 3.3 of this
report for additional discussion and analysis of the detention provided onsite for the 100-
year storm event, including a summary of the reduced peak runoff leaving the site after
Page 2 of 12
PLSA 2863-01
being routed through the raised planters. Refer also to Appendix A of the report for the
results of the detention routing analysis performed using Hydraflow Hydrographs software.
Treated storm water leaving the project in the post-developed condition flows on the
surface in Jefferson Street before transitioning to the Grand Avenue curb and gutter through
an existing cross gutter at the intersection. Several discharge locations are proposed from
the raised planters to the existing right-of-way, but all drainage leaving the site eventually
is conveyed to the southwest corner similar to the pre-developed condition. 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.
In an effort to comply with the City of Carlsbad's storm water standards for Standard
Development Projects, in addition to Priority Development Project requirements, 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 raised planters prior to leaving the project site.
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. There is both no diversion and no increase of flows, and water leaving any
portion of the site eventually is conveyed to the southwestern portion of the property along
Grand Avenue on the surface by existing drainage infrastructure.
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://websoilsurvey.nrcs.usda.gov.
Page 3 of 12
PLSA 2863-01
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:
I = 7.44 x P6 x D 065
Where:
I = Intensity (in/hr)
P6 = 6-hour precipitation (inches)
D = duration (minutes - use Tc)
Using the Time of Concentration (Tc), which is the time required for a given element of
water that originates at the most remote point of the basin being analyzed to reach the point
at which the runoff from the basin is being analyzed. The RM equation determines the
storm water runoff rate (Q) for a given basin in terms of flow (typically in cubic feet per
second (cfs) but sometimes as gallons per minute (gpm)). The RM equation is as follows:
Q=CIA
Where:
Q = flow (in cfs)
C = runoff coefficient, ratio of rainfall that produces storm water
runoff (runoff vs. infiltration/evaporation/absorption/etc)
I = average rainfall intensity for a duration equal to the Tc for the
area, in inches per hour.
A = drainage area contributing to the basin in acres.
The RM equation assumes that the storm event being analyzed delivers precipitation to the
entire basin uniformly, and therefore the peak discharge rate will occur when a raindrop
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 4 of 12
PLSA 2863-01
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 Runoff Coefficient Determination
As stated in section 2.2, the runoff coefficient is dependent only upon land use and soil
type and the County of San Diego has developed a table of Runoff Coefficients for Urban
Areas to be applied to basin located within the County of San Diego. The table, included
at the end of this section, categorizes the land use, the associated development density
(dwelling units per acre) and the percentage of impervious area.
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PLSA 2863-01
3.0 HYDROLOGY MODEL OUTPUT
3.1 Pre-Developed Hydrologic Model Output (100 Year Event)
Pre-Development:
Q=CIA
Pioo= 2.5 in
Basin A (Onsite Drainage Basin)
Total Area = 8,966 sf4 0.206 Acres
Impervious Area = 3,497 sf 4 0.08 Ac
Pervious Area = 5,469 sf4 0.13 Ac
*Rational Method Equation
*I 00-Year, 6-Hour Rainfall Precipitation
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 3,497 sf+ 0.25 x 5,469 sf= 0.50
8,966 sf
Tc = ti + tt ti @ 65'=1.0%
29 DU/Ac density in accordance with VC District => 6.1 min from Table 3-2
tt=>L=60' tE=0.4'
tt= [(1 1.9(L/5,280)'3}/AE]'O.385
tt= [{ 11.9(60/5,280)"3}/O.4]"0.385 = 0.037
0.021 x 60 = 1.3 Mm.
Tc=6.1+ 1.3 =7.4Min
P6 = 2.5
I = 7.44 x P6 x D 05
I = 7.44 x 2.5 x 740.645. 5.13 in/hr
Loo 5.13 in/hr
Qioo = C*I*A
Qioo = 0.50 x 5.13 in/hr x 0.206 Ac = 0.53 cfs
Page 6 of 12
PLSA 2863-01
3.2 Post-Developed Hydrologic Model Output (100-Year Event)
Post-Development:
Q=CIA *Rational Method Equation
Pioo= 2.5 in *I 00-Year, 6-Hour Rainfall Precipitation
Basin A (Onsite Drainage Basin - Prior to Biofiltration Treatment)
Total Area = 8,966 sf 4 0.206 Acres
Impervious Area = 7,964 sf4 0.183 Ac
Pervious Pavement =0 sf 4 0.11 Ac
Pervious Area = 1,002 sf 4 0.023 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 7.964 sf+ 0.25 x 1.002 sf= 0.83
8,966 sf
Tc = 5.0 Mm
P6 = 2.5
I = 7.44 x P6 x D 05
I = 7.44 x 2.5 x 5.0 0 6.59 in/hr
Qioo = C*I*A
Qioo = 0.83 x 6.59 in/hr x 0.206 Ac = 1.13 cfs
Pre-Development vs. Post-Development (Undetained - Basin A Onsite Runoff)
Pre-Development (Basin A) Post-Development (Basin A) Delta
Qioo= 0.53 cfs Qioo = 1.13 cfs 0.60 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 from the subject property.
Page 7 of 12
PLSA 2863-01
3.3 Detention Analysis
The onsite biofiltration basins and raised planters provide pollutant control as well as
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 biofiltration basins. At the request of the City of Carlsbad, an
additional routing analysis was performed to determine the detained peak flow rate and
detained time of concentration to peak. A discussion can be found in Section 3.3.3.
The biofiltration / raised planter facilities consist of a total area of approximately 340
square feet, 18 inches of engineered soil and 12 inches of gravel. Runoff will be bioflitered
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 a corner of each biofiltration
basin / raised planter. 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 12-inch by 12-inch grate located on top of the catch basin. Runoff entering
the outlet structure will be conveyed directly to a proposed PVC outlet drainpipe and will
discharge towards the Jefferson Street or Grand Avenue right-of-way, discharging on the
surface to the existing curb and gutter system by way of sidewalk under drain pipes per
SDRSD D-27. Once on the surface in the right-of-way, runoff will continue west as it does
in the existing condition before eventually entering public storm drain infrastructure and
discharging to the Pacific Ocean.
3.3.1 Static (Non-Routing) Detention Calculation
Pre-Development: 100-Year Event (Basin A)
Volume = C*P6*A *Equation 6-1 of SDCHM
Volume = 0.50 * (2.5 in * 1 ft/ 12 in) * (8,966 SF) = 934 CF
Post-Development: 100-Year Event (Basin A)
Volume = C*P6*A *Equation 6-1 of SDCHM
Volume = 0.83 * (2.5 in * 1 ft/ 12 in) * (8,966 SF) = 1,550 CF
Required Detention Volume = Total Post-Development - Total Pre-Development
= Prop. Basin A - Pre Basin A
= 1,550 CIF —934 CIF =616CF
-> 616 CF static storage required for drainage basin A to
mitigate to pre-development conditions
Page of 12
PLSA 2863-01
3.3.2 Basin Stage-Storage Volume Calculation
BMP Stage Storage - Basin INF-2
Elevation
Basin
Area
(sq-ft)
Stage
Volume
(cu-ft)
Total
Volume
(cu-ft)
0 312 0 0
0.5 312 62.4 62.4
1 312 62.4 124.8
1.5 312 62.4 187.2
2 312 62.4 249.6
2.5 312 62.4 312
2.6 312 31.2 343.2
2.7 1 312 31.2 374.4
2.8 312 31.2 405.6
2.9 312 31.2 436.8
3 312 31.2 468.0
3.1 312 31.2 499.2
3.2 312 31.2 530.4
3.3 312 31.2 561.6
3.4 312 31.2 592.8
3.5 -312 31.2 624.0
3.6 312 31.2 655.2
3.7 312 31.2 686.4
*Boom of Gravel Layer
(40% Void Ratio)
*Top of Gravel Layer/Bot of Soil
(40% Void Ratio)
Planter Surface
*Top of Ponding / Grate Elevation
*Top of Planter Wall
530 CF storage provided at ponding depth (686 CF storage total); detention volume
provided in raised planter / biofiltration basin areas sufficient to mitigate 100-year, 6-hour
storm event.
3.3.3 Dynamic (Routing) Detention Analysis and Calculations
As demonstrated in the above section, the biofiltration planters proposed for this project
provide enough dead storage to account for the increase in volume produced during a 100-
year storm event by the proposed development in accordance with the methodology
described in Section 6 of the San Diego County Hydrology Manual. However, an
additional analysis was conducted at the request of the City of Carlsbad to determine the
mitigated peak discharge (Qioo) leaving the project site, taking into account the routing
characteristics of the planters.
The dynamic 100-year storm event detention analysis was performed using Hydraflow
Hydrographs Extension Software with AutoCAD Civil 3D. Hydraflow Hydrographs has
the ability to route the 100-year, 6-hour storm event inflow hydrograph through the BMP
planter facilities. Based on the BMP cross-sectional geometry and volumetric stage-
Page 9 of 12
PLSA 2863-01
storage and outlet structure data, Hydraflow Hydrographs calculates the detained peak flow
rate and detained time of concentration at the resultant, mitigated peak flow.
For the project site, one total inflow hydrograph was generated for the proposed private
building roof areas and impervious surfaces. The inflow runoff hydrograph was modeled
using RatHydro, which is a Rational Method Design Storm Hydrograph software that
creates a hydrograph using the results of the Rational Method calculations. In Hydraflow
Hydrographs, the model assumes the various planters proposed on the different floors of
the building act as one planter with a combined surface area receiving the storm event in
the form of the inflow hydrograph. It was determined that mitigation for the 100-year
storm event peak flow rate is provided, detaining the proposed condition to 0.25 cfs, which
is below the existing condition Qioo of 0.53 cfs. Refer to Appendix A of this report for the
results of the Hydraflow Hydrographs analysis including a detailed pond report,
calculations, and graphical support data.
3.4 Building Drainage System Discussion
The onsite building drainage system to route storm water from each area of the roof to the
biofiltration planter BMP's located onsite has been prepared by GMEP Engineers and can
be found in the project building plan set under separate cover. Although the main and
overflow roof drains have been designed in accordance with the California Plumbing Code
(CPC) Section 1103.0, a quick check has been provided to ensure capacity of the proposed
building drainage system to convey the 100-year storm event peak flow rate for each
portion of the roof (as calculated using the Rational Method procedure for estimating
surface runoff). A schematic diagram of the building drainage has been included in this
section as well for reference.
3.4.1 Building Drainage Capacity Check for 100-Year Storm Event
Peak Flow Conveyed to Each BMP:
To BMP #1 Peak Flow: 0.83 x 6.59 in/hr x (1,415 SF / 43,560 SF/AC) = 0.18 cfs
To BMP #2 Peak Flow: 0.83 x 6.59 in/hr x (2,086 SF / 43,560 SF/AC) = 0.26 cfs
To BMP #3 Peak Flow: 0.83 x 6.59 in/hr x (1,613 SF / 43,560 SF/AC) = 0.20 cfs
To BMP #4 Peak Flow: 0.83 x 6.59 in/hr x (536 SF / 43,560 SF/AC) = 0.07 cfs
To BMP #5 Peak Flow: 0.83 x 6.59 in/hr x (605 SF / 43,560 SF/AC) = 0.08 cfs
To BMP #6 Peak Flow: 0.83 x 6.59 in/hr x (762 SF / 43,560 SF/AC) = 0.10 cfs
To BMP #7 Peak Flow: 0.83 x 6.59 in/hr x (811 SF / 43,560 SF/AC) = 0.10 cfs
Pipe Capacity Calculations:
PVC @ 1.0% = 0.115 cfs capacity (See Appendix for Channel Report)
PVC @ 1.0% = 0.241 cfs capacity (See Appendix for Channel Report)
Page 10 of 12
PLSA 2863-01
Capacity Check to Each BMP:
To BMP #1 Pipe Capacity: 3" PVC @ 1.0% primary, 3" PVC ® 1.0% overflow
= 0.23 cfs capacity > 0.18 cfs peak flow -> OK
To BMP #2 Pipe Capacity: 4" PVC @ 1.0% primary, 4" PVC @ 1.0% overflow
= 0.48 cfs capacity > 0.26 cfs peak flow -> OK
To BMP #3 Pipe Capacity: 3" PVC @ 1.0% primary, 3" PVC @ 1.0% overflow
= 0.23 cfs capacity > 0.20 cfs peak flow -> OK
To BMP #4 Pipe Capacity: 3" PVC @ 1.0% primary, 3" PVC @ 1.0% overflow
= 0.23 cfs capacity > 0.07 cfs peak flow -> OK
To BMP #5 Pipe Capacity: 3" PVC @ 1.0% primary, 3" PVC @ 1.0% overflow
= 0.23 cfs capacity > 0.08 cfs peak flow -> OK
To BMP #6 Pipe Capacity: 3" PVC @ 1.0% primary, 3" PVC @ 1.0% overflow
= 0.23 cfs capacity > 0.10 cfs peak flow -> OK
To BMP #7 Pipe Capacity: 3" PVC ® 1.0% primary, 3" PVC @ 1.0% overflow
= 0.23 cfs capacity > 0.10 cfs peak flow -> OK
3.4.2 Schematic Diagram of Storm Water Routing
ROOFTOP
ROOFDRAINJ
BMP I~
'~
COMNECT iv 'SEE SECTION
A48ELOWTHLS
SHEET
FOURTHF
:i
I FROM
ROOF DRAW
I
AREA (SF)
BMP2 'CONNECT TO 11 CATCH I
OASINBELOI THIRD FLOOR
ROOF DRAIN - BMP3
AREA (SF) • 72
BMP4
&MP5 I II 9W7 I L I BMP6
AREA(SF)2I I CA I AREA =2O I I
..
I I4S25 1BASINBELOW
I FIRSTFLOORI I
PER 0.27 FACE PER 0.27
GRAND AVE JEFFERSON ST
Page!! of 12
PLSA 2863-01
3.5 Hydromodification Analysis
This project is exempt from hydromodification management criteria pursuant to the
Watershed Management Area Analysis performed for this area of the City of Carlsbad in
the Buena Vista Creek hydrologic area of the Carlsbad watershed. Refer to Attachment 2
of the project Storm Water Quality Management Plan (SWQMP) prepared by Pasco, Laret,
Suiter & Associates under separate cover for material to support the hydromodification
management criteria exemption.
3.6 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.
Refer to the project Storm Water Quality Management Plan (SWQMP) prepared by Pasco,
Laret, Suiter & Associates under separate cover for discussion of pollutant control
measures.
Page 12 of 12
APPENDIX A
Hydrology Support Material
County of San Diego
33°30' 1
Hydrology Manual
Rainfall Isopluvials
33°15
100 Year Rainfall Event — 6 Hours
IsopIuvial (inches)
33°OO'
32°45
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N THIS MAP IS PROVIDED WITHOUT WARRANTY OF ANY KIND. EITHER EXPRESS
w*
OR IMPLIED, INCLUDING BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
Copyright SAnGIS. All Rights Reserved.
This products roey Wotaro infccrnatim Iroor the SANDAG Regional
E Infonrraton System wlrHA cannot be reproduced without the
written p rnrrswin,r ut SANDAlS
This product way contain nlomraton which has been reproduced with
permission granted by Thonres Brothers Maps.
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San Diego County Hydrology Manual Section: 3
Date: June 2003 Page: 6of26
Table 3-1
RUNOFF COEFFICIENTS FOR URBAN AREAS
Land Use - -- Runoff Coefficient "C"
Soil Type
NRCS Elements County Elements I % IMPER. A B C D
Undisturbed Natural Terrain (Natural)
Low Density Residential (LDR)
Low Density Residential (LDR)
Low Density Residential (LDR)
Medium Density Residential (MDR)
Medium Density Residential (MDR)
Medium Density Residential (MDR)
Medium Density Residential (MDR)
High Density Residential (HDR)
High Density Residential (HDR)
Commercial/Industrial (N. Corn)
Commercial/Industrial (G. Corn)
Commercial/Industrial (O.P. Corn)
Commercial/Industrial (Limited 1.)
Commercial/Industrial (General I.)
Permanent Open Space 0* 0.20 0.25 0.30 0.35
Residential, 1.0 DU/A or less 10 0.27 0.32 0.36 0.41
Residential, 2.0 DU/A or less 20 0.34 0.38 0.42 0.46
Residential, 2.9 DU/A or less 25 0.38 0.41 0.45 0.49
Residential, 4.3 DU/A or less 30 0.41 0.45 0.48 0.52
Residential, 7.3 DU/A or less 40 0.48 0.51 0.54 0.57
Residential, 10.9 DU/A or less 45 0.52 0.54 0.57 0.60
Residential, 14.5 DU/A or less 50 0.55 0.58 0.60 0.63
Residential, 24.0 DU/A or less 65 0.66 0.67 0.69 0.71
Residential, 43.0 DU/A or less 80 0.76 0.77 0.78 0.79
Neighborhood Commercial 80 0.76 0.77 0.78 0.79
General Commercial 85 0.80 0.80 0.81 0.82
Office Professional/Commercial 90 0.83 0.84 0.84 0.85
Limited Industrial 90 0.83 0.84 0.84 0.85
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
San Diego County Hydrology Manual
Date: June 2003
Section: 3
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 (Li)) of sheet flow to be used in
hydrology studies. Initial T1 values based on average C values for the Land Use Element are
also included. These values can be used in planning and design applications as described
below. Exceptions may be approved by the "Regulating Agency" when submitted with a
detailed study.
Table 3-2
MAXIMUM OVERLAND FLOW LENGTH (LM)
& INITIAL TIME OF CONCENTRATION (T;)
Element* DU/
Acre
.5% 1% 2% 3% 5% 10%
LM T1 LM T1 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 1 50 12.21 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 10.2 701 9.6 801 8.1 95 1 7.8 1001 6.7 1001 5.3
MDR 7.3 1 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. Corn 50 5.3 1 60 4.5 1 75 4.01 85 3.81 95 3.4 1 100 2.7
C. Corn 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. 501 3.7 1 60 3.2 70 1 2.7 1 80 2.6 90 1 2.3 1 100 1.9
*See Table 3-1 for more detailed description
3-12
- Hydrologic Soil Group—Sa ---.o County Area, California
4675aJ 467,990 467€00 467610 467620 467630 167CAO 407J 467610 467670 46760 467690 46779) 407710
33- 46 N
I
33- 946"N
je
I Ilk
33' 943' N
I r4J p 11 I V r1;))
33' 9'43N
467553 4670213 467546 45 467710
6:
MapScae: 1:636 ff pirdionAIarcspe(11'x8,5')sheet
A
: 12D 18D
Feet
Map projection: Web Mercator Come- coordinates: WGS84 Edge tics: IJIM Zone uN WGS84
USDA Natural Resources Web Soil Survey 7/12/2018
Conservation Service National Cooperative Soil Survey Page 1 of 4
Hydrologic Soil Group—San Diego County Area, California
Hydrologic Soil Group
Map unit symbol Map unit name Rating Acres in AOl Percent of AOl
MIC Marina loamy coarse
sand, 2 to 9 percent
slopes
B 0.7 100.0%
Totals for Area of interest 0.7 100.0%
Description
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 (ND, 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 (ND, BID, or CID), 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
DA Natural Resources Web Soil Survey 7/12/2018
Conservation Service National Cooperative Soil Survey Page 3 of 4
Hydrologic Soil Group—San Diego County Area, California
Component Percent Cutoff. None Specified
Tie-break Rule: Higher
Natural Resources Web Soil Survey 7/12/2018
Conservation Service National Cooperative Soil Survey Page 4 of 4
EXISTING HYDROLOGY EXHIBIT
GRAND JEFFERSON — 786 GRAND AVENUE
LEGEND
PROPERTY BOUNDARY
CENTERLINE OF ROAD
ADJACENT PROPERTY LINE/RIGHT-OF-WAY
EXISTING CONTOUR LINE
EXISTING PATH OF TRAVEL
EXISTING MAJOR DRAINAGE BASIN BOUNDARY
EXISTING IMPERVIOUS AREA
BASIN -AREA CALCULATIONS
APN 203-110-33-00 LOT 48
MAP 1722
NODE A. I
(572FG)
iE
00000, ..
______ APN 203-302-04-00
dV '
LOTS 13, 14, 15 & 16
FlEr iI7
NODEA.2 MAP 535
(56.2 FG)
000000 APN: 203-302-03-00
TOTAL BASIN AREA 8,966 SF (0.21 AC)
BASIN EXISTING IMPERVIOUS AREA 3,497 SF (0.08 AC)
**BASIN EXISTING PERVIOUS PAVEMENT 0 SF
BASIN EXISTING PERVIOUS AREA 5,469 SF (0.13 AC)
% IMPERVIOUS 39.0%
Cn 0.50
GRAPHIC SCALE V`= 10'
10 0 10 20 30
GRAND
AVENUE
EXISTING HYDROLOGY EXHIBIT
GRAND JEFFERSON - 786 GRAND AVENUE
C/i'! OF CARLSBAD
PLAN VIEW - EXISTING NODE MAP
SCALE: 1"- 10'HOR!ZONTAL
& ASSOCIATES
CIVIL ENGINEERING + LAND PLANNING + LAND SURVEYING
535 North Highway 101, Ste A, Solana Beach, CA 92075
ph 858.259.8212 I ft 858.259.4812 I pl.aenglneering.com
PLSA 2863-01
PROPOSED HYDROLOGY EXHIBIT
LEGEND
GRAND JEFFERSON - 786 GRAND AVENUE
PROPERTY BOUNDARY - -
CENTERLINE OF ROAD
ADJACENT PROPERTY LINE! RIGHT-OF-WAY
EXISTING CONTOUR LINE
PROPOSED CONTOUR LINE
PROPOSED PATH OF TRAVEL
PROPOSED MAJOR DRAINAGE BASIN BOUNDARY
PROPOSED MINOR DRAINAGE BASIN BOUNDARY
ROOFAREADELINEATION/RIDGELINE
PROPOSED IMPERVIOUS AREA
PROPOSED BMP PLANTER AREA
BASIN -AREA CALCULATIONS BASIN Al - PEAK DISCHARGE CALCULATIONS
BASIN A
AREA = 8,966 SF
(0.21 AC)
Cn=0.83
L0T48 4
MAP 1722
=
TOTAL SUB-BASIN TO POINT OF DISCHARGE #1
A = 5,866 SF(0.135AC) 'AREA OF SUB-BASIN A I
Cn = 0.83
P100 = 2.5 IN *RAINFALL DEPTH PER SDCHM ISOPLUVIAL MAP
Tc = 5.0 MIN *MINIMUM PER SDCHM (FOR SMALL AREAS)
I = (7.44) (2.5) (5.0)&0.645 = 6.59 IN/HR
QIOO (UNMITIGATED) = CIA
Q 10 = (0.83) (6.59 IN/HR) (0.135 AC) = 0.74 CFS
TOTAL BASIN AREA 8,966 SF (0.206 AC)
BASIN PROPOSED IMPERVIOUS AREA 7,964 SF (0.18 AC)
BASIN PROPOSED PERVIOUS AREA 1,002 SF (0.023)
% IMPERVIOUS 88.8%
On 0.83
0100(UNMITIGATED) 1.13 CFS
*QIOO (MITIGATED) 0.25 CFS
*SEE RESULTS OF HYDROGRAPHS HYDRAFLOWANALYSIS INCLUDED INAPPENDIXA
OF HYDROLOGY REPORT.
BASINAI
AREA = 5,866 SF
(0. 135
BASIN A3 - PEAK DISCHARGE CALCULATIONS
TOTAL SUB-BASIN TO POINT OF DISCHARGE #3
A = 1,220 SF (0.028 AC) *AREA OF SUB-BASIN A3
Cn=0.83
P100 = 2.5 IN *RAINFALL DEPTH PER SDCHM
Tc = 5.0 MIN *MINIMUM PER SDCHM (FOR SMALL AREAS)
I = (7.44) (2.5) (5.0)A0.645 = 6.59 IN/HR
QIOO (UNMIT(GATED) = CIA
Q 10 = (0. 83) (659 IN/HR) (0.028 AC) = 0.15 CFS
BASIN A2 - PEAK DISCHARGE CALCULATIONS
TOTAL SUB-BASIN TO POINT OF DISCHARGE #2
A =800 SF (0OI8AC) *AREA OF SUB-BASIN A2
Cn=0.83
P100 = 2.5 IN *RAINFALL DEPTH PER SDCHM ISOPLUVIAL MAP
Tc = 5.0 MIN *MINIMUM PER SDCHM (FOR SMALL AREAS)
I = (7.44) (2.5 IN) (5.0 MIN)&0.645 = 6.59 IN/HR
Q 10 (UNMITIGATED) = CIA
Q 10 = (0.83) (6.59 IN/HR) (0.018 AC) = 0.10 CFS
LPOINTOF LL I -
\DISCHARGE #1
L I QIOO=0.74CFS
AREA = 236 SF -
(UNMITIGATED)
(0.005 AC)
I 4
N -
,. 4'XI0'X4' TREE N
- WELL #1; SEE DETAIL
DWG 523-7 (40 SF,
/7 10'DIA TREE)
POINT OF
_
4
/ DISCHARGE #2
-
ROOFTOP
AREA(SF) - 72 i_H TO
-Aw1
FOURTH FLOOR BASIN BELOtV
*SEE SECTION
A-A BELOW THIS
SHEET
BASIN A3
AREA 1,220SF BASIN A2
- I / - / L_
BMP4
_J
N / N \BMP5
(FLOOR 1)-' 'L.(FLObR 1)
N 21 .çF II 20 SF /
FROM . __ I
ROOF DRAIN BMP2 1 *CONNECT TO cI AREA (SF) =72
- CATCH I
THIRD FLOOR BASIN BELOI1V
10
- -
- (UNMITIGATED)
-
BMP7
-(FLOOR 1)
30 SF
p Cl ROOF DRAIN BMP
AREA (SF) =72 CATCH
SECOND
FLOOR BASIN BELOI1V
PLAN VIEW - PROPOSED NODE MAP
SCALE: 1"- 10'HORIZONTAL
Cl)
FROM
FROM ROOF DRAIN
ROOF DRAIN I
BMP4 *CONNECT BMP5 BMP7 BMP6
AREA (SF) = 21
BAS
CATCH AREA (SF) = 20 AREA (SF) = 30 AREA (SF) = 25 GRAPHIC SCALE 1"= 10'
IN BELOW
FIRST FLOOR
F7,"7 10 0 10 20 30
TO CURB TO CURB TO CURB
FACE PER D-27 FACE PER D-27 FACE PER D-27 PROPOSED HYDROLOGY EXHIBIT
GRAND AVE JEFFERSON ST GRAND JEFFERSON - 786 GRAND AVENUE
CITY OF CARLSBAD
PASCO LARET SUITEN SCHEMATIC DIAGRAM OF STORM WATER ROUTING
& ASSOCIATES
CIVIL ENGINEERING + LAND PLANNING + LAND SURVEYING
$35 North Highway 101, St. A, Solana Beech, CA 92075
ph 858.259.8212 I ft 858.259.4812 1 plurniglneering.cont
UL)t 4vI)..1L' I