HomeMy WebLinkAboutCDP 2019-0005; CARLSBAD CORPORATE PLAZA PARKING STRUCTURE; HYDROLOGY REPORT; 2020-08-18ccod Cozy
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HYDROLOGY STUDY
Parking Structure
Carlsbad Corporate Plaza
CDP 2019-0005 /spp aoi .. g
DWG 524-1 I GR 2020-013
6183 & 6185 Paseo Del Norte
Carlsbad, CA 92011
APN: 211-040-18
Prepared By:
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GregorijW.Iang, P.E.
Pasco Laret Suiter & Associates, Inc.
535 N. Highway 101, Suite A
Solana Beach, CA 92075
RICE 68075 EXP: 06-30-21
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c No RCE 68075
P A S C 0 L A R E T S U I T E R \\QsSN, ~zj
& ASSOCIATES
CIVIL ENGINEERING + LAND PLANNING + LAND SURVEYING
RECEIVED
AUG 272020
Prepared for:
NextMed Ill Owner, LLC
6125 Paseo Del Norte, Suite 210
Carlsbad, CA 92011
760-494-9216
LAND DEVELOPMENT Date: August 18, 2020
ENGINEERING
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
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 (sO Pervious Area (sfl 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 D0•5
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 (0) for a given basin in terms
of flow (typically in cubic feet per second (cfs). The RM equation is as follows:
0= CIA
Where:
Q= 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
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.
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 (sfl Q100 (cfs)
Existing 49,506 4,239 7.0
Proposed 47,104 6,641 6.8
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
Hydrologic Soil Group—San Diego County Area, California
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Map Scale: 1:1,1OOpintadciAIscape(11"x8S')shea
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Map pn)jedion: Web Meir Come- coordinatas: WGS84 Edge tics: tJrM Zone uN WGSS4
USDA Natural Resources Web Soil Survey
IFM Conservation Service National Cooperative Soil Survey
1/24/2019
Page 1 of 4
APPENDIX B