HomeMy WebLinkAboutCDP 2019-0009; AKIN RESIDENCE; DRAINAGE STUDY; 2020-06-24-
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Drainage Study
Akin Residence
Lot 42, Map No. 2696
5290 Carlsbad Blvd
Carlsbad, California 92008
CDP 2019-0009
GR 2020-0003
DWG 522-5A
Prepared for:
Sadri and Rose Akin
8839 San Badger Way
Elk Grove, CA 95624
Prepared by:
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Christensen Engineering & Surveying
7888 Silverton A venue, Suite "J"
San Diego, CA 92126
(858) 271-9901
June 24, 2020
~ECORD COPY
d,([ b/JJ/9-1
Initial Date
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Introduction
This project involves construction of single-family residence on Lot 42 of Map
No. 2696, in the City of Carlsbad. The project is located at 5290 Carlsbad Blvd.
Other improvements include landscaping to treat runoff, walkways and keystone
walls.
The attached drainage area map is from a topographic survey prepared by Baker
Land Surveys Inc. dated February 7, 2019.
No runoff is tributary to the project parcel, only runoff generated from the site, is
the subject of this study. Currently, runoff from the site flows onto Carlsbad Blvd.
Following construction, the same pattern will persist with runoff flowing by
surface flow to Carlsbad Blvd.
The Rational Method was used to calculate the anticipated flow for the 100-year
storm return frequency event.
Antony K. Christensen
RCE 54021
Exp. 12-31-21
JN A2019-121
06.24.20
Date
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Calculation
Coefficient Determination
For residential area
From Table 3-1
Soil Type= B
Cp = 0.25
Pre-construction / Impervious area = 4,291 sf
Soil Group B (see appendix A)
Hydrologic Soil Group-San Diego
County Area, California
Imperviousness pre-construction = ( 4,29116,604) 100% = 65%
Post-construction
Impervious area= 3,659 sf ✓
Imperviousness post-construction= (3,659/6,604) x 100% = 55.4%
C = 0.90 x (%Impervious)+ Cp x (l-%Impervious)
Pre-construction
C = 0.90 x 0.65 + 0.25(1-0.65)
C = 0.67
Post-construction
C= 0.90 x 0.554 + 0.25(1-0.554)
C= 0.61
Intensity Calculation
(From San Diego County Hydrology Manual, Figure 3-3)
Tc= Time of concentration
= 1.8{1. l-C)Dist. 112
( s 1/3 )
Pre-construction slope over the area with the greatest elevation change
is (57.5'-54.8')/150' = 1.8% over the site and the distance traveled is
150 feet and the runoff coefficient is 0.67 pre-construction.
Post-construction slope over the area with the greatest elevation change
is (58.1-54.8')/155' = 2.13% over the site and the distance traveled is
155 feet and the runoff coefficient is 0.61 post-construction.
Tc= 7.8 minutes pre-construction
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Tc= 8.5 minutes post-construction
Ix = 7.44 P6 Duration ·0·645
Since D= 7.8 min pre-construction and D = 8.5 min post-construction
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Ix= 1.98 P6 pre-construction
Ix= 1.87 P6 post-construction
from: San Diego County Hydrology Manual Rainfall Isopluvial
Map for 100 year Rainfall Event -6 Hour
P6 c100> = 2.5, P24 (100) = 4.0
Adjusted P6= 2.5
So:
l100 = 1.98 (P6)
1100 = 1.98 (2.5)
1100 = 4.95 inches pre-construction
1100 = 1.87 (P6)
1100 = 1.87 (2.5)
1100 = 4.67 inches post-construction
Volume calculations
Q=CIA
Areas of Drainage
Pre-Construction
Area of site flowing to street
Post-Construction
Area of site flowing to street
A= 0.152 Acres
PCA = 0.152 Acres
Entire lot runoff flows to street by surface flow.
Q100 (A)= (0.67) ( 4.95) (0.152)
Q100 (PCA) = (0.61) ( 4.67) (0.152)
Q100 <A>= 0.50 cfs pre-construction ✓
~ • ✓ Q100 (PCA) = 0.43 c1s post-construction
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4. Discussion
The site drains to Carlsbad Blvd. before and after development. There is
a calculated decrease of 0.07 ~ for the 100-yr return frequency event
following development.
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APPENDIX ..
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~E
Feet
5000
4000
Tc
L
i6.E
= Time of concentration (hours)
= Watercourse Distance (miles)
= Change in elevation along
effective slope line (See Figure 3-5)(feet)
3000
2000
300
200
30
20
10
5
SOURCE: California Division of Highways (1941) and Kirpich (1940)
L
MIies Feet
' 3000
0.5 '
2000
1800
1600
1400
500
300
200
L
'
Nomograph for Determination of
' '
Tc
Hours Minutes
' ' '
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 'llatersheds
F I G U R E
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I-w w LL z
w (.) z <(
I-Cf) a w
Cf) a:: :.,
0 (.) a:: w ~ $
I I i t i t • I I I I I I I •
2.50% slope
1001 ~:~ I #U/L '.:'1 27 217 2?' I ~ 130
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LL
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0::: w > 0
--------i'---___ ...__ ___ ..__ ___ ..__ ___ ......_ _______ _,__ ____ o
EXAMPLE:
Given: Watercourse Distance (0) = 70 Feet
Slope (s) = 1.3%
Runoff Coefficient (C) = 0.41
Overland Flow Time {T) = 9.5 Minutes
T = 1.8 (1.1-C) Vfi
3Vs
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SOURCE: Airport Drainage, Federal Aviation Administration, 1965
FIGURE
Rational Formula -Overland Time of Flow Nomograph 3.3
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EQUATION
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I I ""l "'loo -~ ~ ~ p 4.o 1 : i"k.. , I ""' l ' .. 6 = 6-Hour Precipitation {in)
'-· ' I l""'
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15 20 30 40 50 1 5 6 7 8 9 10 . • . .
Minutes
Duration
l
2 3
Hours
4
I m
11:
II:
5 6
Intensity-Duration Design Chart -Example
t I I I • I •
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 range 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
p
(b) Ps = 3 in. P24 = 5.5 _§_ = 54.5 %<2) --, --'P24 --
(c) Adjusted p6(2) = _3_ in.
(d) tx = --22_ min.
(e) I= _±L_in./hr.
Note: This chart replaces the Intensity-Duration-Frequency
curves used since 1965.
PS 1 1.5 2 -. 2.5-. 3 3.5 4 4.5 5 5.5 . 6
Duration . I . I I I I ·-I --· f T -I i . i ··---··s 2.63 3.9s s.21 6.s9 1.90 9.22 10.54 11.as 13.11 14.49 1s.81 1 -fi2 ·s.1,n.2.f s.so · s.36 1.42: 8.48 · 9'.54 ·10.so· 11.ss·1:f12 ·10 -1:ss ·2.s3· :t:i1··4::21· s.os·s:90· 6.74 · i.sa · a.i2 '9.21 ·10.11
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 1.62 2.15 2.69. 3.23 3.77 4.31 4.85 5.39 , 5.93 . 6.46
25 0.93 1.40· 1.ai· 2.33.iso···:327 3.73 4.20 4.67 -··is:fa-· 5.60 30 o:83 . 1.24' 1.66. 2.07. i49. 2.90· 3.32-· 3.73 '. 4.15 . 4.56 .. 4:98
40 o.69 ·1.03· 1.JS· 1.12·2.01·2.4,: 2:1s · 3:10 · 3.4s · 3.79 ·-4.13
so 0.60 · o.90 · 1.19 · , .49 · 1. 79 · 2.09 · 2.39 · 2.ss · 2.98 · 3.2s iss 60 0.53 ·o.eo· i.06· (33' f59 1.ss· 2.12. fag·· 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 . 2.45
-·,20 if34 '.o.s1·0.sa·,:i:as·1.02·1.19· 1.36·1.ss· 1.10·1.a1·2.04
150 0.29 ·0_44·0.sg·o.73·0.ss·,.os· 1.18· 1.32· 1.41· 1.s2: 1.76 -1so --o:2G-·o.ss' o:S2·0.ss··o.18·0.9i' 1.04 · 1.18 · 1."31 · 1.«··1.si
·-240 ·ci.22 ·0_33·0-.43·0.54-o.ss· o.1s· o.a1 · o.98· ·-,-_os-·1.19 · ,:30
·300 -o:;g ·o.2s·o:31f 0:41 · o:ss· o.ss · 0.1s · o:ss · o.94 · f03--·,~1':3 360 0.17 -0.25: 0:33· 0.-12·0.so. 0.58. 0.67 -0.75°~-o:i34-· 0.92 r·foo
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• I , i ' ' I I t
San Diego County Hydrology Manual
Date: June 2003
j • • • I • I I • I I I I
Section:
Page:
Table 3-1
RUNOFF COEFFICIENTS FOR URBAN AREAS
Land Use I Runoff Coefficient "C"
Soil Tree
NRCS Elements Coun Elements % IMPER. A B
Undisturbed Natural Terrain (Natural) Permanent Open Space 0* 0.20 0.25
Low Density Residential (LOR) 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 (HDR) Residential, 24.0 DU/A or less 65 0.66 0.67
High Density Residential (HDR) 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 ProfessionaVCommercial 90 0.83 0.84
Commercial/Industrial (Limited I.) Limited Industrial 90 0.83 0.84
Commercial/Industrial { Qeneraj_1_} General Industrial 95 0.87 0.87
I I
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
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County of San Diego
Hydrology Manual
Rainfall Jsopluvials
100 Year Rainfall Event-6 Hours
lsopluvial (inches)
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County of San Diego
Hydrology Manual
Rainfall lsopluvials
100 Year Rainfall Event-24 Hours
lsopluvial (Inches)
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DRAINAGE AREA MAPS
PRE-DEVELOPMENT
DRAINAGE AREA MAP
I + 54.s
~
lit I
+54.4
+54.4
54.~..5 +
~"· --
/'
PRE-CONSTRUCTION DRAINAGE
AREA MAP
-·---------
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54.0
53.6 +M.4 + 55.7'
l l
AREA == 0.152 AC
FLOWS TO CARLSBAD AVENUE
LEGEND:
a IMPERVIOUS AREA (4,291 SF)
·5'/.5 I
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..
..
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-
..
..
..
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-
..
..
-
...
•
-
-
POST-DEVELOPMENT
DRAINAGE AREA MAP
~ at I
I ~
d
q
.t'J
LEGEND:
POST-CONSTRUCTION DRAINAGE
AREA MAP
~ IMPERVIOUS AREA (3,659 SF)
--~
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AREA= 0.152AC
FLOWS TO CARLSBAD AVENUE