HomeMy WebLinkAboutCT 06-01; LAGUNA BREEZE; DRAINAGE REPORT; 2015-03-26-
I DRAINAGE REPORT
1 :
FOR:
1 CTO6-Oi/RP14-1O S
Laguna. Breeze
I Carlsbad, California
PREPARED FOR:
I
. Zephyr Partners
10505 Sorrento Valley Road
S San Diego, Ca 92121 S
I 858-705-0242
1 S
PREPARED BY:
I LANDMARK CONSULTING S
9555 GENESEE AVENUE, SUITE 200
S
.5 SAN DIEGO, CA 92121 ..
TEL: 858-587-8070' / N 0
REV. DATE: 3-26-2015
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No.. 6271
I DAVID YEH, RCE 62717, EXP 6-30-16 RECEIVED
S .. . 5 ... APR 072015
S
1 LAND DEVELOPMENT
ENGINEERING
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TABLE OF CONTENTS.
I VICINITY MAP 5
I. INTRODUCTION 7
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METHOD OF ANAYSIS : 9-12
100-YEAR HYDROLOGY CALCULATIONS 13
I SUMMARY 15
I APPENDIX.AND HYDROLOGY MAPS 17
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CITY OF OCEANSIDE
HIGHWAY 78
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I INTRODUCTION
I The property is located at the south side of Laguna Drive at Madison Street in the city of Carlsbad, State of
California.
I The project site is located in a developed area l000ft southeast of the Buena Vista Lagoon. The
westerly potion of the project site is a barren pad that sheet flows westerly onto the existing Madison
Street. Runoff from Madison Street flows into an existing curb inlet just south of the intersection of
Laguna Drive and Madison Street. The runoff is conveyed via an existing storm drain system westerly
along existing Laguna Drive.
The eastern portion of the project site consists of a single-family residence. The runoff from the
driveway sheet flows onto the existing Laguna Drive, which is conveyed by a curb and gutter into the
existing curb inlet on the corner of Laguna Drive and Madison Street. The curb inlet is connected to a
storm drain system that flows westerly along the existing Laguna Drive.
Under the proposed conditions, the existing single-family residence will be demolished and four
I condominiums will be constructed. The runoff from the buildings will be conveyed through roof drains
onto the proposed permeable paver driveway. The runoff will then be conveyed westerly and enters the
existing storm drain on Madison Street. Any runoff from the proposed driveway will sheet flow onto
landscaped area and enter an area drain which conveys runoff into the storm drain system
The proposed development does not alter the existifig drainage pattern.
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I METHOD OF ANAYSIS
HYDROLOGY
I 3.1 THE RATIONAL METHOD
I The Rational Method (RM) is a mathematical formula used to determine the maximum
runoff rate from a given rainfall. It has particular application in urban storm drainage, where
it is used to estimate peak runoff rates from small urban and rural watersheds for the design I of storm drains and small drainage structures. The RM is recommended for analyzing the
runoff response from drainage areas up to approximately 1 square mile in size. It should not
be used in instances where there is a junction of independent drainage systems or for
drainage areas greater than approximately I square mile in size. In these instances, the
Modified Rational Method (MRM) should 'be used for junctions of in drainage
systems in watersheds up to approximately I square mile in size (see Section 3.4); or the
NRCS Hydrologic Method should be used for watersheds greater than approximately 1
1 square mile in size (see Section 4).
The RM can be applied using any design storm frequency (e.g., 100-year, 50-year, 10-year,
etc.). The local agency determines the design storm frequency that must be used based on
the type Of project and specific local requirements. A discussion of design storm frequency
is provided in Section 2.3 of this manual. A procedure has been developed that converts the
6-hour and 24-hour precipitation isopluvial map data to an Intensity-Duration curve that can
be used for the rainfall intensity in the RM formula as shown in Figure 3-1. The RM is
applicable to a 6-h6ur storm duration because the, procedure uses Intensity-Duration Design
Charts that are based on a 6-hour storm duration.
3.1.1 Rational Method Formula
The RM formula estimates the peak rate of runoff at any location in a watershed asa function
of the drainage area (A), runoff coefficient (C), and rainfall intensity (1). for a duration equal
to the time of concentration (Ta), which is the time required for water to
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flow from the most remote point of the basin to the location being analyzed. The RM I
formula is expressed as follows:
Q=CIA I
Where: Q = peak discharge, in cubic feet per second (cf) I
C = runoff coefficient, proportion of the rainfall that runs off the surface (no
units) I I = average rainfall intensity for a duration equal to the T for the area, in
inches per hour (Note: If the computed T is less than 5 minutes, use 5
minutes for computing thepeak discharge, Q)
A = drainage area contributing to the design location, in acres
Combining the units for the expression CIA yields:
(l acre x inch'1 43,560 ft' (121nches)
_lfot ( ihour '1 1.008,C&hour ) t acre ) 3,600 seconds)
For practical purposes the unit conversion coefficient difference of 0.8% can be ignored.
The RM formula is based on the assumption that for constant rainfall intensity, the peak
discharge rate at a point will occur when the raindrop that falls at the most upstream point in
the tributary drainage basin arrives at the point of interest. I
Unlike the MRM (discussed in Section 3.4) or the NRCS hydrologic method (discussed in
Section 4), the RM does not create hydrographs and therefore does not add separate subarea
bydrographs at collection points. Instead, the RM develops peak discharges in the main line
by increasing the T as flow travels downstream. I
Characteristics of, or assumptions inherent to, the RM are listed below:
The discharge flow rate resulting from any I is maximum when the I lasts as long as or
longer than the T. I
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I The storm frequency of peak discharges is the same as that of I for the given I.
I The fraction of rainfall that becomes runoff (or the runoff coefficient, C) is independent
of I or precipitation zone number (PZN) condition (PZN Condition is discussed in
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Section 4.1.2.4).
The peak rate of runoff is the only information produced by using the RM.
3.1.2 Runoff Coefficient
Table 3-1 lists the estimated runoff coefficients for urban areas. The concepts related to the
runoff coefficient were evaluated in a report entitled Evaluation, Rational Method "C"
Values (Hill, 2002) that was reviewed by the Hydrology Manual Committee. The Report is
available at San Diego County Department of Public Works, Flood Control Section and on
the San Diego County Department of Public Works web page
The runoff coefficients are based on land use and soil type. Soil type can be determined from
the soil type map provided in Appendix A. An appropriate runoff coefficient (C) for each
type of land use in the subarea should be selected from this table and multiplied by the
percentage of the total area (A) included in that class. The sum of the products for all land
uses is the weighted runoff coefficient (E[CA]). Good engineering judgment should be used
when applying the values presented in Table 3-I, as adjustments to these values may be
appropriate based on site-specific characteristics. In any event, the impervious percentage
(% Impervious) as given in the table, for any area, shall govern the selected value for C. The
runoff coefficient can also be calculated for an area based on soil type and impervious
percentage using the following formula:
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C = 0.90 x (% Impervious) + C, x (1 - % Impervious)
Where: C, = Pervious Coefficient Runoff Value for the soil type (shown in
Table 3-1 as Undisturbed Natural Terrain/Permanent Open Space,
0% Impervious). Soil type can be determined from the soil type map
provided in Appendix A.
The values in Table 3-1 are typical for most urban areas. However, if the basin contains rural
or agricultural land use, parks, golf courses, or other types of nonurban land use that are
expected to be permanent, the appropriate value should be selected based upon the soil and
cover and approved by the local agency.
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I HYDROLOGY CALCULATIONS
I Pre-Development Conditions
Determination of dwelling units per acre:
I 1 single-family residence on 0.24 Ac project site = 4.2 DU/A
Based on County of San Diego Hydrology Manual (Medium Density Residential) for type 'B' soil,
I C=0.45.
Determination of intensity:
I Based on the medium density residential classification and gently sloping terrain, t1 = 9.6 min.
Using t1 = 10 min and 6-hour precipitation of 2.5 in, the intensity = 4.2 in/hr.
AREA 1
(AC) C I(IN) TC(MIN) Q(CFS)
100-YEAR 0.24 0.45 4.2 . 10 0.45
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Post-Development Conditions
Determination of dwelling units per acre:
5 condominium units on 0.24 Ac project site = 20.7 DU/A
Based on County of San Diego Hydrology Manual (High Density Residential) for type 'B' soil,
C= 0.67.
Determination of intensity:
Based on the high density residential classification and gently sloping terrain, t1 = 6.1 mm.
Using t1 = 6 min and 6-hour precipitation of 2.5 in, the intensity = 5.8 in/hr.
AREA 1
(AC) C I(IN) TC(MIN) Q(CFS)
100-YEAR 0.24 0.67 5.8 6 0.93
Increase in flow (cfs)
Area (AC) Post-Development (CFS) Pre-Development (CFS) Increase of Q (CFS)
0.24 1 0.93 0.45 0.48
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I SUMMARY
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Based on the hydrology calculations presented in this report, the proposed development will increase the
I runoff from the site by 0.48cfs. The proposed development increases the density of the dwelling units
and also the impervious area on the project site, therefore the runoff is expected to increase. However,
the runoff is discharged into an existing underground storm drain system. Therefore, the slight increase
of 0.48cfs is not expected to have any appreciable effect on the existing system.
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EX. LAGUNA DRIVE
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\\.. A= 04C
\ Tc= 10 MIN
\ \ \ \ ' 1= 4.2 IN/HR \\9) \ Q100=i 0.45 CFS
LEGEND
HYDROLOGIC SUB—BASIN - — -
DIRECTION OF FLOW
SCALE - 30'
EXISTING
LE INDMARK
CONSULT I N G Planning Engineering Surveying 9555 Genesee Avenue, Suite 200 San Diego, CA 92121, (858) 587-8070
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EX. LAGUNA DRIVE \
_____________
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1= 5.8 IN/HR
Q100= 0.93 CFS
.4
LEGEND
LIMIT OF DISTURBANCE
HYDROLOGIC SUB-BASIN - - -
DIRECTION OF FLOW
SCAL JO'
— — — — mm — — — — — — — — — — — —
- - - - - - - - -- - - - - - - - - San Diego County Hydrology Manual
. . Section: . 3 Date: June 2003
Page: 6 of 26
Table 3-1
RUNOFF COEFFICIENTS FOR URBAN AREAS.
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Land Use . Runoff Coefficient "C" -
Soil Type
NRCS Elements County Elements % IMPER. A B C D Undisturbed Natural Terrain (Natural) Permanent Open Space 0" 0.20. 0.25 0.30 0.35 Low Density Residential (LDR) Residential, 1.0 DU/A or less 10 0.27 0.32 0.36 - 0.41 Low Density Residential (LDR) Residential, 2.0 DU/A or less 20 0.34 . 0.38 0.42 0.46. Low Density Residential (LDR) . Residential, 2.9 DU/A or less ' . 25 0.38 0.41 0.45 0.49 Medium Density Residential (MDR) Residential, 4.3 DU/A or less 30 - 0.41 0.48 0.52 Medium Density Residential (MDR) Residential, 7.3 DU/A or less . 40 . 0.48 0.51 0.54 0.57 Medium Density Residential (MDR) Residential, 10.9 DU/A or less 45 0.52 0.54 0.57 0.60 Medium Density Residential (MDR) Residential, 14.5 DU/A or less . . 50 . 0.55 0.58 0.60 . 0.63 High Density Residential (HDR) . . Residential, 24.0 DU/A or less 65 0.66 Pb 0.69 0.71, High Density Residential (HDR) Residential, 43.0 DU/A or less 80 0.76 . 0.77 0.78 0.79 Commercial/Industrial (N. Corn)
- Neighborhood Commercial 80 0.76 0.77 . 0.78 0.79 Commercial/Industrial (G. Corn) General Commercial 85 0.80 0.80 0.81 0.82 Commercial/Industrial (O.P.Com) Office Professional/Commercial -. 90 0.83 0.84 0.84 0.85 Commercial/Industrial (Limited I.) Limited Industrial 90 0.83 0.84 0.84 0.85 Commercial/Industrial (General. I.) 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 Secti
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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
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3-6 . .
San Diego County Hydrology Manual Section: 3 Date: June 2003 Page: 12of26
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.
Tablc 3-2 provides limits of the length (Maximum Length (LM)) 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 Tj LM Ti LM Ti LM T1 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 12.2 1 70 11.5 1 85 10.0 1 100 .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' 8.8 95 8.1 100 7.0 100 5.6
MDR 4.3 50 10.2 70 80 8.1 95 7.8 100 63 100 53
MDR 7.3 50 9.2 1 65 8.4 80 7.4 1 95 7.01 100 6.01 100 4.8
MDR 10.9 50 8.7 65 7.9' 80 6.9 90 6.41 100 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 1 50 6.7 65 . . 5.1 90 4.9 95 43 100 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 60 4.5 75 4.01 85 3.8 1 95 3.41 100 2.7
G. Corn' 50 4.7 . 60 4.1 75 3.6 85 3.4 90 2.9 100 1.2.4
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 1 701 3.1 80 1 2.9 901 2.6 100 2.2
General I. 50 1 3.71 60 3.2 1 70 1 2.7 1 80 1 2.61 90 1 2.31 100 1.9
*See Table 3-1 for more detailed description
3-12
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