HomeMy WebLinkAboutGPA 09-01; ROBERTSON RANCH PA 22; PRELIMINARY DRAINAGE STUDY; 2009-01-08I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
PRELIMINARY DRAINAGE STUDY
ROBERTSON RANCH PA 22
TABLE OF CONTENTS
DECLARATION OF RESPONSIBLE CHARGE ............................................................. 1
1.0 INTRODUCTION .................................................................................................. 2
1.1 PURPOSE OF STUDY ....................................................................................... 2
1.2 SITE CHARACTERISTICS ............................................................................... 2
1.2.1 Existing Conditions ..................................................................................... 2
1.2.2 Proposed Conditions ................................................................................... 2
1.3 SOIL CLASSIFICATION .................................................................................. 3
1.4 SITE VICINITY MAP ........................................................................................ 4
2.0 RATIONAL METHOD DESCRIPTION ............................................................... 5
3.0 RUNOFF CALCULATIONS ................................................................................. 5
3.1 EXISTING CONDITION ................................................................................... 5
3.2 PROPOSED CONDITION ................................................................................. 6
3.3 EXISTING AND PROPOSED RUNOFF COMPARISON ............................... 6
4.0 CONCLUSION ....................................................................................................... 7
5.0 REFERENCES ....................................................................................................... 8
APPENDICES:
APPENDIX A -EXHIBITS
• EXISTING CONDITION HYDROLOGY MAP
• PROPOSED CONDITION HYDROLOGY MAP
• HYDROLOGIC SOILS GROUPS
APPENDIX B -HYDROLOGY CHARTS
• RAINFALL ISOPLUVIALS
• INTENSITY DURATION DESIGN CHARTS
• NOMOGRAPH FOR DETERMINATION OF TIME OF
CONCENTRATION
• RUNOFF COEFFICIENTS FOR URBAN AREAS
i
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
PRELIMINARY DRAINAGE STUDY
ROBERTSON RANCH PA 22
DECLARATION OF RESPONSIBLE CHARGE
I, hereby declare that I am the Civil Engineer of work for this project, that I have
exercised responsible charge over the design of the project as defined in section 6703 of
the business and professions code, and that the design is consistent with current design.
I understand that the check of the project drawings and specifications by the regulating
agency is confined to the review only and does not relieve me, as engineer of work, of
my responsibilities for project design.
George O'Day, RCE 32014 Date
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
PRELIMINARY DRAINAGE STUDY
ROBERTSON RANCH PA 22
1.0 INTRODUCTION
1.1 PURPOSE OF STUDY
The purpose of this preliminary hydrology study is two-fold:
1) to determine the potential hydrologic impacts of the proposed development;
2) to verify the capacities of the existing drainage facilities to ensure
accommodation of the proposed drainage conditions.
This report compares the proposed development hydrology condition with the
existing hydrology condition using the Rational Method as outlined in the County
of San Diego Hydrology Manual, June 2003 (SD County 2003). This report
analyzes 100-year six-hour storm event flows for the existing and proposed site
conditions. The calculated Q values are used to determine the capacity of existing
and proposed drainage structures in accordance with the City of Carlsbad
Engineering Standards, 2004 to determine adequacy to accommodate run-off.
1.2 SITE CHARACTERISTICS
1.2.1 Existing Conditions
The site currently consists of a rough graded pad with a temporary sediment
basin. The pad area is approximately 3.64 acres and runoff flows from east to
west into the temporary sediment basin via overland flow. An existing 24-inch
RCP storm drain pipe then conveys runoff into an existing depollutant swale
downstream. (See Appendix A for Existing Condition Hydrology Map).
Approved drainage study Drainage Study for Robertson Ranch East Village, C.T.
02-16 by O'Day Consultants dated November 30, 2006 analyzed the existing
rough graded pad as future multifamily development. The analysis used a runoff
coefficient (C) of 0.71 producing a Q 100 of 12.7-cfs. This study reanalyzed the
rough graded pad area with a C value of 0.82 for the commercial office site and
RV storage, producing a Q100 of 14.2-cfs. (See Existing Runoff Calculations in
Section 3).
1.2.2 Proposed Conditions
The proposed site will consists of an office commercial building with on grading
parking and a RV storage area for Robertson Ranch East Village. The existing
rough graded pad will be extended to the east. Runoff generated in the developed
area will be collected and conveyed by a proposed private storm drain system to
an existing depollutant basin. The increase in pad area will result in an increase
Q100 from 14.2-cfs (existing) to 20.4-cfs (developed). The exiting downstream 24"
RCP storm drain can adequately convey the developed Q100. (See hydraulic
calculations in Section 3).
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
PRELIMINARY DRAINAGE STUDY
ROBERTSON RANCH PA 22
1.3 SOIL CLASSIFICATION
The Soil Hydrologic Group for this preliminary drainage study is Type 'D' soil as
determined by using the County of San Diego Hydrology Manual Soil Hydrologic
Groups Map. A full size and project specific copy of the Soils Hydrologic
Groups Maps are provided in Appendix A.
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
PRELIMINARY DRAINAGE STUDY
ROBERTSON RANCH PA 22
2.0 RATIONAL METHOD DESCRIPTION
The rational method, as described in the SD County 2003 manual, was used to
generate surface runoff flows, which were then used to size the proposed drainage
facilities and to verify the capacity of the existing drainage facilities.
Rational equation: Q = CIA
Q = peak discharge in cubic feet per second ( cfs)
C = runoff coefficient (varies with surface)
I= intensity (varies with time of concentration)
A = Area in acres
The design storm for this project is the 100-year event; the corresponding 6-hour
rainfall amount cP6) and 24-hour rainfall amount (P24) were found using the
Isopluvial Charts in Appendix B. The P6 and P24 values are given in the following
table.
6-hour and 24-hour Rainfall
p6 P24 P61P24 (%)
2-year 1.2 1.9 63.2
10-year 1.7 3.1 54.8
100-year 2.6 4.5 57.8
Runoff coefficients for each area were obtained from the table included in
Appendix B.
3.0 RUNOFF CALCULATIONS
3.1 EXISTING CONDITION
The existing drainage areas are illustrated on the Existing Hydrology Map in
Appendix A. Corresponding runoff coefficients (C) and rainfall intensity (I)
values were derived using the SD County 2003 manual. The table of coefficients
and rainfall isopluvials are included in Appendix B. These values were used to
calculate the peak flow for the 2-year, 10-year, and 100-year flows (Q) using the
Rational Method. The drainage areas and corresponding Q values are summarized
below.
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
PRELIMINARY DRAINAGE STUDY
ROBERTSON RANCH PA 22
Existing Drainage Condition
Basin No Area
A 3.64 acres
3.2 PROPOSED CONDITION
C Imo Q100
4.76
0.82 in./hr. 14.2 cfs
The proposed hydrology conditions for the project have been designed to preserve
the existing drainage patterns to the maximum extent practicable. The proposed
site drainage conditions are shown in Proposed Hydrology Map in Appendix A.
The proposed drainage areas, and corresponding Q values are summarized below.
Basin No Area C Imo Q100
A 1.14 acres 0.82 6.85 in./hr. 6.4 cfs
B 3.21 acres 0.82 5.72 in./hr. 15.1 cfs
Peak Runoff (confluence) 4.35 acres 0.82 20.4 cfs
3.3 EXISTING AND PROPOSED RUNOFF COMPARISON
The difference between the existing and proposed hydrology conditions are
shown below.
Condition
Q 100 Existing 14.2 cfs
Q 100 Proposed 20.4 cfs
Difference +6.2 cfs
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
100 7l.i 10: f'VF/'.tjt,·vo SUF?Fr· F;/e1t,u}
/.8 ( I.I -c )J7: I-~ ,.'J
~ ,.a( 1.1-0.82.)!i: I t11~ :;' 3.6 t11tJu1'Fs
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
NOD! /o( 7r. /.:1/ (Ov[t!All{) :::HffT FtoW /!Zo:F Top)
I-;; So ·t-i
S.:::: 2X
c:: ),O
L...::-10,o ·r1 \ S= IX I D;, 12-1Na1es I Te--:-s.o ;1ivv,./'ff;_.
c~ 0.32 \ :r: 6.85 u-'·/11~) A~ O,,fO /J,c_,
Q:: 2,2 c"ts > V= S. 7 ~s
1-61 ~ L/11-::. ).4 Ml/l)Vi.f''S
IL, <fE, /\JOO£ /o't; :;, 5,0 MlfVv1eS°
Q <e /lJOP£ I Jo"7> ~ G x t }I. A::: CJ. !?,2 x 6, f35x {1 14::: ~,4 ~
T lr/P 0 NOD{ /0 ~ ~ tf, 6? .NV/'ll.'£
I I
~ASJN A = /, /4 Ac. 1 ~~ ~-SZ
A ., A A q.
U( {00 f$A-'$/N A ;: C X i /(. .: 6, Pi ,f'J.$
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
Not>f &,,.,, 7e1 2.02. ( OVE!ZJ..AND -F.tow)
L;;; qO -t-t > ~ -::: Z /.
6..E ~ I. g -F-l
T, . i,
NODf "202. {0 2,03 (PIF'E-rt-oGt,/)
L = 1.2.0 -+-b I s "" /;; J -re;; S.o MWv1'f-5
A"' o. sq AG .. 1.>= a .~z. 1 1 = r;. 6 8 ,,,v.f1"<.
Q = ?,2. c-15
V = t. ~ -rps ( srr ,;~11A~11,1u,. !2f.f;:Y1rr • N;J1;;,t .:J:-::z ---; • • 20 ::-)
Ji2 "' L/V:; 2:Z.C> ~ /t.3 -~s. ;;;.. (J. 6 }Awv7F'~
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
Q7
1
::· !21 .i. (T; /Tz) () z =-t . .+ 4--(S/G.6) 15. / = 17. -~ cti
Q -r 2. ~ -(';J ~ * ~Tr'! /1~ )l), :: 15, I -t ( s. 72/,.ss ) ,A :-_zo.4 c fs
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
PRELIMINARY DRAINAGE STUDY
ROBERTSON RANCH PA 22
4.0 CONCLUSION
Runoff generated in the developed area will be collected and conveyed by a
proposed private storm drain system to an existing depollutant basin. The increase
in pad area will result in an increase Q 100 from 14.1-cfs (existing) to 20.4-cfs
(developed). The exiting downstream 24" RCP storm drain can adequately
convey the developed Q100.
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
PRELIMINARY DRAINAGE STUDY
ROBERTSON RANCH PA 22
5.0 REFERENCES
City of Carlsbad, Engineering Standards, 2004.
San Diego County Hydrology Manual, County of San Diego Department of Public Works
Flood Control Section, 2003.
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
PRELIMINARY DRAINAGE STUDY
ROBERTSON RANCH PA 22
APPENDIX A -EXHIBITS
• EXISTING CONDITION HYDROLOGY MAP
• PROPOSED CONDITION HYDROLOGY MAP
• HYDROLOGIC SOILS GROUPS
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
PRELIMINARY DRAINAGE STUDY
ROBERTSON RANCH PA 22
APPENDIX B -HYDROLOGY CHARTS
• RAINFALLISOPLUVIALS
• INTENSITY DURATION DESIGN CHARTS
• NOMOGRAPH FOR DETERMINATION OF TIME OF
CONCENTRATION
• RUNOFF COEFFICIENTS FOR URBAN AREAS
-------------------
San Diego County Hydrology Manual
Date: June 2003
Table3-l
Section:
Page:
RUNOFF COEFFICIENTS FOR URBAN AREAS
Land Use Runoff Coefficient "C"
Soil TyPe
NRCS Elements Coun Elements %IMPER. A B
Undistwbed Natural Terrain (Natural) Permanent Open Space 0* 0.20 0.25
Low Density Residential {LDR) Residential, 1.0 DU/ A or less 10 0.27 0.32
Low Density Residential {LDR) Residential, 2.0 DU/A or less 20 0.34 0.38
Low Density Residential {LDR) Residential, 2.9 DU/A or less 25 0.38 0.41
Medium Density Residential {MDR) Residential, 4.3 DU/A or less 30 0.41 0.45
Medium Density Residential {MDR) Residential, 7.3 DU/A or less 40 0.48 0.51
Medium Density Residential {MDR) Residential, 10.9 DU/A or less 45 0.52 0.54
Medium Density Residential {MDR) 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
3
6 of26
C D
0.30 0.35
0.36 0.41
0.42 0.46
0.45 0.49
0.48 0.52
0.54 0.57
0.57 0.60
0.60 0.63
0.69 0.71
0.78 0.79
0.78 0.79
0.81 0.82
0.84 0.85
0.84 0.85
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