HomeMy WebLinkAboutCDP 2020-0017; SAREM RESIDENCE; HYDROLOGY STUDY; 2020-05-27HYDROLOGY STUDY
4005 Skyline Road, Carlsbad, CA
City of Carlsbad Project No.: CDP 2020-0017
Prepared for:
Mr. & Mrs. Scott Sarem
6684 Lemon Leaf Drive
Carlsbad, CA 92011
Tel: (760) 533-2470
Prepared by:
Toal Engineering, Inc.
139 Avenida Navarro
San Clemente, CA 92672
Tel: (949) 492-8586
Fax: (949) 498-8625
May 27, 2020
······························································· Caleb 0. Rios R.C.E. 57587
JN 20036
1.0 PURPOSE
This report has been prepared to accompany the Grading Plan for the proposed construction of a
new single-family residence at 4005 Skyline Road (Por. Lot 24, Map 2647 / Carlsbad Heights)
(City of Carlsbad Project No.: CDP 2020-0017).
2.0 DESCRIPTION OF SITE
The subject lot is a partially-developed, rectangular-shaped 0.572-acre (24,904 sq. ft.) residential
property. The building and appurtenant features associated with the existing residential
development are primarily located on the easterly half of the property, with the westerly half
consisting of a brush covered slope. The existing on-site impervious area measures 5,000 sq. ft.
(or ~20% of the project site). The property is bounded on the North, South, and West by similar
residential development, and to the East by Skyline Road (public street).
3.0 EXISTING DRAINAGE
All site runoff, and an ~10' wide section of the Skyline Road right-of-way (~1,000 sq. ft. total;
55% impervious), flows across the surface of the project property in a westerly direction and onto
the neighboring properties to the west; total existing project drainage area to Sunnyhill Drive =
25,904 sq. ft. (0.595 ac), with 5,550 impervious (21 %). The remaining portion of Skyline Road
flows in a southerly direction along both sides of the asphalt roadway. Site runoff draining onto
the adjacent westerly properties ultimately reaches Sunnyhill Drive, then flows northerly ( from the
street high point at the south corner of 4020 Sunny hill Drive) before entering the City storm drain
system via curb inlet catch basin near the intersection of Sunnyhill Drive and Alder A venue.
4.0 PROPOSED DEVELOPMENT
The developer proposes to construct a 4,800-sq. ft. residence with ~7,100 sq. ft. of additional
impervious surfaces (driveway, walkways, stairs, etc.) across the entire project. The impervious
surface area within the parcel is 11,500 sq. ft. (46% of parcel area). The proposed improvements
are shown on the construction plans for the project, including the Grading Plan referenced above.
5.0 PROPOSED DRAINAGE
A ridge will be created at the Skyline Road right-of-way intended to keep all off-site storm water
in the street. All runoff from the subject property will be collected, treated (see separate SWQMP
report), and ultimately discharged to Sunnyhill Drive via surface flow across 4020 Sunnyhill Drive
(existing drainage pattern). Drainage improvements are designed to limit the post-project
discharge rate to match the existing condition, thereby eliminating any impact to downstream City
storm drain improvements. The proposed drainage system is designed to address the 100-yr storm,
however, emergency overflow features will be provided at ground level along the westerly edge
of the project property in the event of on-site system failure.
6.0 METHODOLOGY
The purpose of the calculations presented herein is to analyze the pre-and post-project drainage
characteristics for 4005 Skyline Road.
For both conditions, we performed a rational method hydrologic analysis based on the San Diego
County Hydrology Manual. Post-project peak runoff quantities at the westerly edge of the project
property were compared to the pre-project values.
See Appendix A for tables and charts from the Hydrology Manual. The Drainage Map in
Appendix B graphically depicts the project watershed and data relevant to the runoff calculations.
7.0 RESULTS AND ANALYSIS
Our hydrologic and hydraulic analysis of 4005 Skyline Road yielded the following results:
Table 1. Peak Runoff.
Condition C Tc min. I in./hr A acres 100 cfs
0.38 6.8 5.8 0.595 1.31
0.54 5.0 7.1 0.572 2.19
Condition
Peak Pre-Pro·ect
Peak Post-Pro·ect
Post-Pro·ectPeak-lTc
The values for 'C' in Table 1 are representative of the impervious surface areas within the
drainage boundary for each location (see Table 3-1). The 'Tc' information in Table 1 is based
upon initial times of concentration per Table 3-2, and the Tc equation on Figure 3.4 ( existing
condition only); as the developed condition Tc is 4.5 minutes, which is less than the minimum
allowable Tc of 5 minutes, the post-project Tc shall be set at 5.0 minutes. The values for 'I' in
Table 1 are from Figure 3-1, and are based upon times of concentration 'Tc'. See Appendix A
for all calculations, tables, and figures.
As shown in Table 2, only the peak of the post-project storm exceeds the flowrate of the existing
100-yr storm event (Post-project peak-1 Tc calculated per Section 6 of the San Diego County
Hydrology Manual). To match the peak pre-project flowrate of 1.31 cubic feet per second ( cfs ),
the project drainage system must employ flow control measures and provide 264 cu. ft. of water
storage (minimum) during the 100-yr peak.
8.0 CONCLUSIONS
Toal Engineering, Inc. proposes the use of a flow control device to limit off-site discharge to 1.31
cfs, along with off-line storage on the project site to detain the peak volume of 264 cu. ft. that
exceeds the controlled discharge rate
APPENDIXA
Calculations
HYDROLOGY CALCULATIONS
4005 Skyline Road, Carlsbad, CA
Pre-proiect Condition:
Project Information:
Total drainage area= 25,904 sq. ft. (0.595 ac; incl. 1,000 sq. ft. of Skyline Road right-of-way)
Impervious area= 5,500 sq. ft. (21 %)
Land Use= Low Density Residential (LDR)-2.0 DU/A or less
Soil Type = NRCS Type B
Runoff Coefficient (C) = 0.38 -Table 3-1 of San Diego County Hydrology Manual (SDCHM)
Surface slope across first 100 feet = ~ 10%
Initial Time of Concentration= 5.8 minutes -per Table 3-2 of SDCHM
6-Hour Precipitation (P6) = 2.7 inches -per 100-yr, 6-Hour Isopluvial in SDCHM Appendix B
Calculations:
Runoff Travel Time (Tt) = [11.9 * (195 ft. * 1 mi./5,280 ft.)3 / 25 ft.]0385 * 60 min./hr. = 1 min.
Per Equation on Figure 3-4 of SDCHM
Tc = Initial Tc+ Tt = 5.8 minutes+ 1 minute = 6.8 minutes
! = 7.44 * 2.7 inches* (6.8 minutes)-0·645 = 5.8 in./hr. -per Equation on Figure 3-1 of SDCHM
Qioo = C * I* A = 0.38 * 5.8 in./hr. * 0.595 ac = 1.31 cfs
Post-Proiect Condition:
Parcel Information:
Total drainage area = 24,904 sq. ft. (0.572 ac; no Skyline Road right-of-way)
Impervious area= 11,500 sq. ft. (46%)
Land Use= Medium Density Residential (MDR)-10.9 DU/A or less
Soil Type = NRCS Type B
Runoff Coefficient (C) = 0.54-Table 3-1 of San Diego County Hydrology Manual (SDCHM)
Surface slope across first 100 feet = ~ 10%
Initial Time of Concentration= 4.5 minutes -per Table 3-2 of SDCHM
6-Hour Precipitation (P6) = 2.7 inches -per 100-yr, 6-Hour Isopluvial in SDCHM Appendix B
Calculations:
Runoff Travel Time (Tc) = 5 min. (minimum per SDCHM; used due to initial Tc< 5 minutes).
I = 7.44 * 2.7 inches* 5.0 minutes)-0·645 = 7.1 in./hr. -per Equation on Figure 3-1 of SDCHM
.Qi.oo_ = C *I* A = 0.54 * 7.1 in./hr. * 0.572 ac = 2.19 cfs
Verify that Q100 only exceeds the pre-project condition at the hydrograph peak by checking
the unit immediately preceding the peak (Q100 -1 Tc) per Section 6 of the SDCHM:
Pn = 0.124 * 2.7 inches * (1 * 5 minutes)°-355 = 0.59 inches
PT2 = 0.124 * 2.7 inches* (2 * 5 minutes)°-355 = 0.76 inches
P2 = PT2 -Pn = 0.76 inches -0.59 inches = 0.17 inches
h = 60 * (P2 / Tc) = 60 * (0.17 inches / 5 in./hr) = 2.04 in./hr.
Q2 = C * A * h = 0.54 * 0.572 ac * 2.04 in./hr = 0.63 cfs ...... < 1.31 cfs .... OK
Calculate storage volume required to maintain peak discharge at 1.31 cfs (Q100 Pre)
V = (Q100 Post -Q100 Pre)* Tc = (2.19 cfs -1.31 cfs) * 5 min. * (60 sec.II min.) = 264 cu. ft.
San Diego County Hydrology Manual
Date: June 2003
Table 3-1
Section:
Page:
RUNOFF COEFFICIENTS FOR URBAN AREAS
Land Use Runoff Coefficient '"C''
Soil Type
NRCS Elements Coun Elements %IMPER. A B
Undisturbed Natural Terrain (Natural) Pennanent Open Space o• 0.20 0.25
Low Density Residential (LOR) Residential, 1.0 DU/A or less JO 0.27 0.32
Low Density Residential (LOR) Residential, 2.0 DU/A or less 20 0.34 0.38
Low Density Residential (LOR) Residential, 2.9 DU/A or less 25 0.38 0.41
Medium Density Residential (MOR) Residential, 4.3 DU/A or less 30 0.41 0.45
Medium Density Residential (MOR) Residential, 7 .3 DU/ A or less 40 0.48 0.51
Medium Density Residential (MOR) 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
C
0.30
0.36
0.42
0.45
0.48
0.54
0.57
0.60
0.69
0.78
0.78
0.81
0.84
0.84
0.87
3
6 of26
D
0.35
0.41
0.46
0.49
0.52
0.57
0.60
0.63
0.71
0.79
0.79
0.82
0.85
0.85
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:
Page:
3
12 of26
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 (LM)) of sheet flow to be used in
hydrology studies. Initial Ti 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.
MAXIMUM OVERLAND FLOW LENGTH (LM)
& INITIAL TIME OF CONCENTRATION (Ti)
Element* DUI .5% 1% 2% 3% 5% 10%
Acre LM Ti LM Ti 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
LOR 1 50 12.2 70 11.5 85 10.0 100 9.5 100 8.0 100 6.4
LOR 2 50 11.3 70 10.5 85 9.2 100 8.8 100 7.4 100 5.8
LOR 2.9 50 10.7 70 10.0 85 8.8 95 8.1 100 7.0 100 5.6
MOR 4.3 50 10.2 70 9.6 80 8.1 95 7.8 100 6.7 100 5.3
MOR 7.3 50 9.2 65 8.4 80 7.4 95 7.0 100 6.0 100 4.8
MOR 10.9 50 8.7 65 7.9 80 6.9 90 6.4 100 5.7 100 4.5
MOR 14.5 50 8.2 65 7.4 80 6.5 90 6.0 100 5.4 100 4.3
HOR 24 50 6.7 65 6.1 75 5.1 90 4.9 95 4.3 100 3.5
HOR 43 50 5.3 65 4.7 75 4.0 85 3.8 95 3.4 100 2.7
N.Com 50 5.3 60 4.5 75 4.0 85 3.8 95 3.4 100 2.7
G.Com 50 4.7 60 4.1 75 3.6 85 3.4 90 2.9 100 2.4
0.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. 50 3.7 60 3.2 70 2.7 80 2.6 90 2.3 100 l.9
*See Table 3-1 for more detailed description
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I = Intensity (in/hr)
P5 = 6-Hour Precipitation (in)
I D = Duration (min)
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Intensity-Duration Design Chart -Template
Directions for Application:
(1) From precipitation maps determine 6 hr and 24 hr amounts
for the selected frequency. These maps are included in the
County Hydrology Manual (10, 50, and 100 yr maps included
in the Design and Procedure Manual).
(2) Adjust 6 hr precipitation (if necessary) so that it is within
the ra nge of 45% to 65% of the 24 hr precipitation (not
applicaple to Desert).
(3) Plot 6 hr precipitation on the right side of the chart.
(4) Draw a line through the point parallel to the plotted lines.
(5) This line is the intensity-duration curve for the location
being analyzed.
Application Form:
(a) Selected frequency ___ year
(b) P5 = ---in., P24 :: ---, ;s = ---%(2)
24
(c) Adjusted P6(2l = ___ in.
(d) Ix = ___ min.
(e) I = ___ in./hr.
Note: This chart replaces the Intensity-Duration-Frequency
curves used since 1965.
PG 1 1.5 2 2.5 3 3.5 4 4,5 5 5,5 6
Oura1ion I I I I I I I I I I I
5 2.63 3.95 5.27 6.59 7.90 9.22 10 54 11 .86 13.17 1449 15.81
7 2.12 3.18 4.24 5.30 6.36 7.42 8.48 9.54 10.60 11.66 12.72
10 1.68 2.53 3 37 4.21 5 OS 5.90 6.74 7.58 8.42 9.27 10 II
15 1.30 1.95 ·2_59 3.24 3.89 4.54 5.19 5.84 6.49 7.13 7.78
20 1.08 1.52· 2 15 2.69 3.23. 3.77 4.31 4.85 5.39 5.93 6.'16
25 0.93 1.40 1.87 2.33 2 80 3.27 3.73 4.20 4.67 5.13 5.60
30 0.83 1.24 1.66 2.07 2.49 2.90 3.32 3.73 4.15 4.56 4.98
40 0.69 1.03 1.38 1.72 207 2.41 2.76 3.10 3.45 3.79 4.13
50 0.60 0.90 1.19 1.49 1.79 2 09 2.39 2.69 2.98 3.28 3.58
60 0.53 0.80 1.06 1.33 1.59° 1.86 2.12 2.39 2.65 2.92 3.18
90 0.41 0.61 0.82 1.02 1.23 1.43 1.63 1.84 2.04 2.25 245
120 0.34 0.51 0.68 0.85 1.02 1.19 1.36 1.53 1.70 1.87 2.0-I
150 0.29 0.44 0.59 0. 73 0.88 1.03 1.18 1.32 1.47 1.62 1 76
180 0.26 0.39 0.52 0.65 0. 78 0.91 104 1.18 . 1.31 1.44 I 57
240 0.22 0.33 0.43' 0.54 0.65 0.76 0 87 0 98 1.08 1.19 1.30
300 0.19 o 2e· o.38 0.47 0.56 0.66 0.75 0.85 0.94 I 03 1.13
360 0.17 0.25, 0.33 0.42 0.50 0.58 067 0.75 0.84 092 1.00
FIGURE ~
,6. E
Feet
5000
4000
Tc
Tc
L
.6.E
=
=
=
=
EQUATION
(1~~3)°.385
Time of concentration (hours)
Watercourse Distance (miles)
Change in elevation along
effective slope line (See Figure 3-S)(feet)
3000
2000
400
300
200
100
30
20
10
5
.6. E
SOURCE: California Division of Highways (1941) and Kirpich (1940)
L
Miles Feet
3000
0.5 '
2000
1800
1600
1400
1200
1000
900
800
700
600
500
400
300
200
L
'
Nomograph for Determination of
' '
Tc
Hours Minutes
4
3
2
' ' '
Tc
50
40
30
20
18
16
14
12
10
9
8
7
6
5
4
3
Time of Concentration (Tc) or Travel Time (Tt) for Natural Watersheds
F I G l ' R E
~
33''30'-r~n'hiir-::~Wt---+-------+--------l------~l-------_J_ ___ 33'30'
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32'30'
County of San Diego
Hydrology Manual
Rainfall Isop/uvials
100 Year RainfaU Event -6 Hours
lsopluvial (klches)
DPW GIS "--"-.... ---N ,,...,..~-.mcM'WMfWm'O,_,...,..,,.~
4 =~~,.::..~~~ o..,,w......_,.,_..__,_ llllt,...,. ________ ........ ...,_
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,,. ........................ -.. ........ .. ................ ,__ ..... __ s
3 0 3 MIies ,.._
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z
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116"45'0-W
Legend
-MljorRoada
U lnco,p-od City Bely
HYDROLOGIC SOIL GROUP
N
Hydrologlc Group Undefinod
• H)ldro&ogic Gro14> A
Hydrologlc Group B
Hydrologlc Group C
• Hydrologlc Gro'4) D
□No sou Oat■ ..
Note: Soil Data Source
USDA/NRCS
SSURGO Soils 2007
A 31.50 3 Miles
THIS t,w> IS PROVIDEOWmiOUTWMRANTYOF ~ Kip,(),
Em-ER EXPRESS a:t MA.ED, INClUDING, BUT" NOT Ut,ITEO
TO, THE a.RJED WARRANTIES OF MEROIAHTABIUTY AND ffTMESS FOR A PARTlCUI.AR PURPOSE. CoJ¥lghl SanCJS. Al Rights RNr,,ed.
Tl'N p,od.Jd "8Yoonlllln lnformllllon frcm1he SANOAGRaglonll
~ormalonSyaemWhl:ilewwmt>erep,uducedwlllOY4thewlttlln
pem,lalon of SANOAG.
TN1p,ockdnaycoitalnlnfolrNlllonwN1t1hub&11111r9t)IOCI~ will ~ongtarted ~Thcmu Eftllhets Mapa.
APPENDIXB
Drainage Map
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0
0 20 40 ----SCALE: 1 ·= 20·
LEGEND
(]) SUBAREA DESIGNAllON
0.253 N:, MIA IN N:.RES
ap-0.4# PERYIOUS MIA FRACTION
-RUNOFF FLOW DIRECTION
1 -1 SUBAREA BOUNDAA'Y
L..-...J
I
j
I
FIGURE
PRE-PROJECT tM>ROLOGY MAP A.1 o
\
\
\
\
\
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POOL svon,eRS
BIOFl.TRATION BMP (
DRAINAGE BOtJNDMY
AND PROPERlY LINE
DRMWlE BOUNDARY AND PROPERTY LINE
0
0 20 40 ----SCALE: 1 •= 20'
Y.AIN LEVEL
rsLAB-..291 a2
PA!),,.297.24
PROPOSED
RESIDENCE
LEGEND
® 0.253 ,c
Clp=0.444 -
SUBAR£A DESIGN4roN
N1EA IN 1CRES
PERYIOIJS N1EA FRACTION
RUNOFF Fl.OW OIRECTlON
r--1 SUIIARE"A BOUNDARY
L. -....J
•
NODE 10
.•310.0
I
I
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FIGURE POST-PROJECT H'tDROLOG'( MAP e.1 o