HomeMy WebLinkAboutCT 16-05; QUARRY CREEK; HYDROLOGY STUDY; 2020-08-18HYDROLOGY STUDY
NextMed Parking Structure
Carlsbad Corporate Plaza
CDP 2019-0005
DWG 524-1 / GR 2020-00013
6183 & 6185 Paseo Del Norte
Carlsbad, CA 92011
APN: 211-040-18
Prepared By:
Q.
GregonTW.(1ang, P.E. V RCE 68075
Pasco Laret Suiter & Associates, Inc.
535 N. Highway 101, Suite A
Solana Beach, CA 92075
EXP: 06-30-21
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\ No. RCE 68075 J * P. 06-30-2 PASCO LARET SUTER'
J & ASSOCIATES
CIVIL ENGINEERING + LAND PLANNING + LAND SURVEYING
RECEIVED
AUG 27 2020
LAND DEVELOPMENT
ENGINEERING
Prepared for:
NextMed Ill Owner, LLC
6125 Paseo Del Norte, Suite 210
Carlsbad, CA 92011
760-494-9216
Date: August 18, 2020
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TABLE OF CONTENTS
1.0 INTRODUCTION .......................................................................................................................1
1.1 Project Description.........................................................................................................................1
1.2 Existing Conditions .........................................................................................................................1
1.3 Proposed Conditions ......................................................................................................................2
2.0 METHODOLOGY.......................................................................................................................2
2.1 Rational Method.............................................................................................................................3
2.2 County of San Diego Criteria ..........................................................................................................3
3.0 HYDROLOGIC ANALYSIS............................................................................................................5
3.1 Existing Condition...........................................................................................................................6
3.2 Proposed Condition........................................................................................................................7
4.0 CONCLUSION ...........................................................................................................................8
APPENDICES
Appendix A: Hydrology Support Material
Appendix B: Existing and Proposed Hydrology Maps
1.0 INTRODUCTION
This Hydrology Study for the proposed NextMed Parking Garage project has been prepared to analyze
the hydrologic characteristics of the proposed project site. This report presents both the methodology
and the calculations used for determining the storm water runoff from the project site in the post-
developed (proposed) conditions produced by the 100-year, 6-hour storm event.
1.1 Project Description
The NextMed Parking Garage project site is approximately 3.8 acres and is located at 6183 & 6185 Paseo
Del Norte in the City of Carlsbad. The site currently consists of buildings and a surface parking lot with
landscaped islands and an access drive located along the northeastern boundary of the site.
The proposed project is a Priority Development Project (PDP) and is subject to storm water pollutant
control and hydromodification management (HMP) flow control requirements as set forth by the
California Regional Water Quality Control Board San Diego Region municipal storm water permit (Order
No. 139-2013-0001, referred to as MS4 Permit) and the City of Carlsbad BMP Design Manual. For detailed
pollutant control and HMP calculations refer to the report titled "Priority Development Project Storm
Water Quality Management Plan for NextMed Parking Garage" dated March 2019, prepared by Pasco
La ret Suiter & Associates.
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1.2 Existing Conditions
In the existing condition storm water runoff from the site flows overland across the parking lot in a
northwesterly direction and is collected in the existing storm drain located along the northern boundary
of the parking lot. Runoff from the existing buildings is conveyed in roof drains to the same existing
storm drain line. All flow continues northeasterly in the storm drain where it discharges onsite to a
grouted rip rap pad, then re-enters a short segment of storm drain prior to discharging via curb outlet to
Paseo Del Norte. Runoff flows northerly as gutter flow along Paseo Del Norte, enters a curb inlet and
discharges to Encinas Creek which flows westerly and discharges to the Pacific Ocean. No offsite runoff
is conveyed through the site.
Per the United States Department of Agriculture Web Soil Survey, the project site is underlain with
Hydrologic Soil Group D. Refer to Appendix A for soil information.
1.3 Proposed Conditions
The project proposes to construct a new parking structure within the existing parking lot. The parking
lot will remain at the same grade with some of the parking area being reconfigured including demolition
of existing islands and asphalt for parking structure footings and the addition of HMP biofiltration basins
for stormwater treatment and flow control. Utility re-routing will be required for the public waterline
and private storm drain onsite due to the impact of the proposed parking structure footprint. A new
easement dedication will be required for the waterline re-routing coupled with vacation of the
abandoned portion of the waterline easement.
Storm water runoff from the proposed parking garage and portions of the surrounding surface parking
lot including disturbed areas will be collected and conveyed to the HMP biofiltration basins. Flow from
the basins will discharge to the onsite existing storm drain and continue as it does in the existing
condition. Areas that can not be collected and conveyed to the biofiltration basins will be routed to
pervious areas with amended soils for impervious area dispersion. These amended soil areas will be
depressed planters within parking islands. Overflow from the amended soils areas will be collected in
area drains and routed to the storm drain system.
Using the Rational Method Procedure outlined in the San Diego County Hydrology Manual dated June
2003 (SDCHM), the 100-year, 6-hour storm event peak flow rate was calculated for the project site in
the existing and proposed condition. In the proposed condition, the drainage pattern mimics the existing
condition and the amount of proposed impervious area is slightly less than the existing, therefore the
hydrologic results are the same. The table below summarizes the hydrologic analysis.
Summary of 100-yr Storm Event Hydrologic Analyses
Condition Impervious Area (sf) Pervious Area (sf) Q100 (cfs)
Existing 49,506 4,239 7.0
Proposed 47,104 6,641 6.8
Refer to the hydrologic calculations included in Section 3.1 and 3.2.
2.0 METHODOLOGY
Pursuant to the San Diego County Hydrology Manual dated June 2003 (SDCHM), the Rational Method is
recommended for analyzing the runoff response from drainage areas up to approximately 1 square mile
in size. The proposed project and associated watershed basins are less than 1 square mile, therefore the
Rational Method was used to analyze the project's hydrologic characteristics in the existing and
proposed conditions.
2.1 Rational Method
The Rational Method (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:
= 7.44 x P6 x D °
Where:
= Intensity (in/hr)
P6 = 6-hour precipitation (in)
D = duration (mm - use Tc)
Using the Time of Concentration (Tc) 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). The RM equation is as follows:
0= CIA
Where:
0= flow (cfs)
C = runoff coefficient, ratio of rainfall that produces storm water runoff
(runoff vs. infiltration/evaporation/absorption/etc)
= average rainfall intensity for a duration equal to the Tc for the Area (in/hr)
A = drainage area contributing to the basin (ac)
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.
2.2 County of San Diego Criteria
The County of San Diego has developed 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). The 100-year 6-hour storm event rainfall isopluvial map is included in Appendix A.
One of the variables of the RM equation is the runoff coefficient (C) which is dependent upon land use
and soil type. Table 3-1 Runoff Coefficients for Urban Areas in the SDCHM 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 for each soil type class. A composite runoff
3
C
coefficient can also be calculated for an area based on soil type and impervious percentage using the
following equation from Section 3.1.2 of the SDCHM:
C = 0.90 x (% Impervious) + Cp x (1 - % Impervious)
Where: Cp = Pervious Coefficient Runoff Value for the soil type
(shown in Table 3-1 as Undisturbed Natural Terrain/Permanent Open
Space, 0% Impervious)
The calculations contained herein figure a composite runoff coefficient for the project area based on the
percentage of impervious area and the percentage of pervious or landscape area. Refer to Section 3.0
for the composite runoff coefficient calculations.
4
3.0 HYDROLOGIC ANALYSIS
The table below summarizes the hydrologic calculations provided in Sections 3.1 and 3.2.
Summary of 100-yr Storm Event Hydrologic Analyses
Condition Impervious Area (sf) Pervious Area (sf) Q100 (cfs)
Existing 49,506 4,239 7.0
Proposed 47,104 6,641 6.8
3.1 Existing Condition
3048 NextMed Parking Garage
Existing Condition
Q = CIA
I = 7.44 x P6 x D-0•645
P6 (in) 2.5
Area (so ft) C value
Impervious 49506 0.9 44555.4
Pervious 4239 0.35 1483.65
Liptal (sf) 53745 46039.05
Total (ac)
Tc = (assumed)
I (in/hr) =
Q (cfs) =
1.23
5.0
6.6
7.0 I
6
A x C value Weighted C
3.2 Proposed Condition
3048 NextMed Parking Garage
Proposed Condition
Q = CIA
I = 7.44 x P6 x
P6 (in) 2.5 I
Area (so ftl C value Ax C value Weighted C
Impervious 47104 0.9 42393.6
Pervious 6641 0.35 2324.35
Total (sf) 53745 44717.95
Total (ac)
Tc = (assumed)
I (in/hr) =
Q (cfs) =
1.23
5.0
6.6 I
6.8
7
0.83
[]
4.0 CONCLUSION
Based upon the analyses included in this report, the proposed condition 100-year peak discharge is less
than the existing, therefore the project will not impact the existing storm drain system. The proposed
storm water HMP biofiltration basins meet the requirements of the MS4 Permit for both pollutant
control and hydromodification management resulting in no negative downstream impacts.
APPENDIX A
Rainfall Isopluvials
100 Year Rainfall Event — 6 Hours
IsopIuvial (inches)
32°45'
32°30
33°OO' -
32°45'
32°30
Orange
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County of San Diego
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Hydrology Manual
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Hydrologic Soil Group—San Diego County Area, California
470340 470400 47042 470440 470140 470440 479070 479070 479040 47J 470580 470400
3707ON
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470380 4704(1) 470470 470440 470440 470440 4710 479070 479040 470540 470540 470
Map Scale: 1:1,10Oif printed onA landscape (11"x8S')sheet
Meters o N 0 15 40 40 92
0 50 100 200 300
Map projection: Web Mercator Corner cnordinat: WGS84 Edge tics: UTM Zone uN WG584
USDA Natural Resources Web Soil Survey 1/2412019
Conservation Service National Cooperative Soil Survey Page 1 of 4
Hydrologic Soil Group—San Diego County Area, California
MAP LEGEND MAP INFORMATION
Area of Interest (AOl) 0 C The soil surveys that comprise your AOl were mapped at
Area of Interest (AOl) D C/D 1:24,000.
Soils D 1 Warning: Soil Map may not be valid at this scale.
Soil Rating Polygons
A 0 Not rated or not available Enlargement of maps beyond the scale of mapping can cause
misunderstanding of the detail of mapping and accuracy of soil
AID Water Features line placement. The maps do not show the small areas of
B Streams and Canals contrasting soils that could have been shown at a more detailed
Transportation scale.
BID Rails Please rely on the bar scale on each map sheet for map C Interstate Highways measurements.
CID US Routes Source of Map: Natural Resources Conservation Service
D Web Soil Survey URL:
Major Roads Coordinate System: Web Mercator (EPSG:3857)
Not rated or not available Local Roads Maps from the Web Soil Survey are based on the Web Mercator
Soil Rating Lines Background projection, which preserves direction and shape but distorts
-.. A Aerial Photography distance and area. A projection that preserves area, such as the
Albers equal-area conic projection, should be used if more
. ND accurate calculations of distance or area are required.
B This product is generated from the USDA-NRCS certified data as
BID of the version date(s) listed below.
C Soil Survey Area: San Diego County Area, California
Survey Area Data: Version 13, Sep 12, 2018
CID
Soil map units are labeled (as space allows) for map scales
D 1:50,000 or larger.
- Not rated or not available Date(s) aerial images were photographed: Nov 3, 2014—Nov
Soil Rating Points 22, 2014
• A The orthophoto or other base map on which the soil lines were
compiled and digitized probably differs from the background
• AID imagery displayed on these maps. As a result, some minor
• B shifting of map unit boundaries may be evident.
• BID
USDA Natural Resources Web Soil Survey 112412019
Conservation Service National Cooperative Soil Survey Page 2 of 4
Hydrologic Soil Group—San Diego County Area, alifornia S
Hydrologic Soil Group
Map unit symbol Map unit name Rating [ Acres in AOl 7-percent of AOl
CfB Chesterton fine sandy
loam, 2 to 5 percent
slopes
D 2.6 84.0%
CfC Chesterton fine sandy
loam, 5 to 9 percent
slopes
D 0.5 16.0%
Totals for Area of Interest 3.1 100.0%
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 0) 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, 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 0 are assigned to dual classes.
USDA Natural Resources Web Soil Survey 1/24/2019 Conservation Service National Cooperative Soil Survey Page 3 of 4
APPENDIX B
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CAAL.SBAD TRACT NO. 72-2.1
MAP NO. 7303
LEGEND
EASTNG STOMA DRM
EVSTNQ RA.8CA.PEI WIRAOUS AFEA
DRA1LAE MAT
AREA (At BOUNDARY
OVEI=AjL2 FLOW DI€CTION AW
srof1i DRAW R.DW DUECTION AW
SOIL INFORMATION
rrro
SUE INFORMATION
TOTAL M'MOt. AREA: 49,508 9=
TOTAL FXiS AFEA 4,23P 9=
TOTAL AA 5797459=
V
PARCEL 2
PM 3415
7'
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EXISTING
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BUILDING A
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EXISTING
BUILDING
- - - - -
I
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PLAN WEW
SCALE. 1'- 30'
30' 15' 0 30' 60
SCALE: 1. = 30'
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PASCO LARET
& ASSOCIATES
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NEXTMED PARKING STRUCTURE
EXISTING HYDROLOGY EXHIBIT
CARLSBAD CALIFORNIA
SHEET I OF 1
V
PARCEL 2
PM 3415
EXISTING
BUILDING A
PLAN VIEW
SCALE I" = 30'
15.
Know what's below.
Call before you dig.
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IIIIIUUIIMUIUUNIIM*NIIUHRI
0
LEGEND
PROPOSED STORM DRAW
PROPOSED HAIOSCAPE/ WERVIOLIS
AREA
PROPOSED LA818CAPt F(81 AREA
DRAINAGE MAitA4T
AREA (DMA) OXMAW
we BAM BOUNDARY
TFEAW
OVERLAND FLOW DFE-arM ARROW
9tc6VI DRAPI HOW G4ECTKIN ARROW
TREATMENT CONTROL BMPS
OFIT9ADR WITH
WEIVAEABLE LPAM
SOIL INFORMATION
SOIL:TYPE 0
TOTAL .F1CX,81 AREA: 47,508 9Z
TOTAL PM0t.81 A.'EAF 4228 9R
TOTAL AREA 51.745
TOTAL P.7Ot AREA 14 2&292 SF
TOTAL P1WXE AREA 14 4288 9B
TOTAL AREA IA 2578
TOTAL AVER0008 ACE B 24812
TOTAL PERV"S AREA B Z335
TOTAL AREA S 27,187
PAsco I A DEl s., NEXTMED PARKING STRUCTURE
L1I PROPOSED HYDROLOGY EXHIBIT
& ASSOCIATES CARLSBAD CALIFORNIA
535 North Highway 101. SUIte A Solana F3eah. ('A 92075 BArR
ph 85525982! 21 Ix 855.259,4812 1pkacngineeriog.Coln SHEET 1 OF 7