HomeMy WebLinkAboutTract 7239 Drainage StudyVesting Tentative Tract No. 7239
Drainage Study
2
Background
Kern Land Partners, LLC has been contracted to prepare drainage study for a proposed
residential development on Tract 7239 consisting of single family homes. This
residential project on approximately 10.74 acres is bound on the south by the Kern
River Canal, on the west by a retention basin, north by River Run Blvd., and on the east
by Elkhorn Creek Lane. The project site is flat with a surface soil of silty sand and sand.
The Final Drainage Study for Tract 5999 was prepared by Porter-Robertson
Engineering & Surveying, Inc. in September 27, 2000. Their study sized the storm drain
lines and inlets along Elkhorn Creek Lane. The existing storm drain pipes on Elkhorn
Creek Lane consist of 18” and 24” RCP lines which tie into a mainline, 36” RCP, on
River Run Boulevard. Three existing catch basins are located at the intersection of
Roaring River Avenue and Elkhorn Creek Lane. The analysis will focus on the existing
catch basin on the westerly side of Elkhorn Creek Lane. This study will focus on the
watershed contributing to the existing storm drain system. It will also verify the capacity
existing storm drain with flows generated from Tract 7239. A proposed storm drain line
tying into the retention basin will be located in the southwest corner of the tract. It will
require a 15’ easement between lots 11 and 12 for direct access to the basin.
Purpose
This study will present the findings pertaining to the anticipated storm water runoff for
the drainage areas tributary to and generated by the contributing to existing storm drain
line on Elkhorn Creek Lane. The study also provides analysis to size inlets and pipes by
quantifying the flow to each pipe and analyzing the hydraulic grade lines. In addition,
this study will show flows generated by the proposed residential lots.
Approach
The procedure used to determine the design flows are outlined in the City of Bakersfield
Subdivision & Engineering Design Manual (hereafter referred to as the Manual). The
pertinent steps are as follows:
1. Using a topographical map, the design grades, and the proposed development
layout, determine the sub-basin boundaries within the watershed. Record the
sub-basin characteristics in a tabular format similar to the one provided in the
Manual.
2. Determine the initial time of concentration, (Tc), based on Section 2.3.2.1 of the
Manual by using the roof gutter time and average velocity.
3. Establish the intensity based on the Intensity-Duration curves for a given event
using Sheet D-1 in the Manual.
4. Determine the flow to the concentration point.
5. Route the flow to the downstream concentration points and divide the study area
into defined subareas that incrementalized all flow contributions to the proposed
drainage inlets.
6. Estimate flows utilizing the City’s Rational Method in accordance with the
Subdivision Standards
Vesting Tentative Tract No. 7239
Drainage Study
3
7. Determine the rainfall depth for a 10-year, 5-day event to determine the sump
size requirements.
Assumptions
The following information was assumed to apply:
1. 127 - Cajon sandy loam, overblown, 0 to 2 percent slopes (approx. 99%) [Type A]
2. 174 - Kimberlina fine sandy loam, 0 to 2 percent slopes (approx. 1%) [Type B]
3. Starting HGL at the basin was estimated to be 4’ above the basin bottom.
4. The runoff coefficient of 0.42 is consistent over the residential zones and a runoff
coefficient of 0.95 is applied to offsite roadways.
5. These calculations are based on the current precise grading plans.
Summary of Results
The existing basin should be able to accommodate this residential development since
the site was initially zoned for a school. The weighted runoff coefficient of a proposed
school is greater than a R-1 development. Thus the City's required capacity of 11.37
acres-feet will not exceed the provided capacity of 15.69 acre-feet per the “Tract 5957 -
Sewer Plans Sump Detail” by SmithTech USA. The required FEMA capacity was
disregarded since the agency does not own and operate this basin. Refer to the Sump
Capacity Calculations in the Attachments.
Approximately three-quarters (8.17 acres) of the proposed drainage will drain in a
southwesterly direction. It will discharge into the proposed drainage system at the
knuckle of Waterfall Lane and Quiet Creek Way. Catch Basin #1 will collect the
discharge produced by Area 1 at the northeasterly corner of the knuckle. Drainage Area
2 will discharge into a catch basin (CB #2) located directly across from Catch Basin #1.
These inlets will tie directly into the existing retention basin via an 18” RCP line. These
areas will discharge 3.98 cfs into the existing basin.
The remaining part of this tract, Area A, will surface flow easterly towards cross gutters
along Elkhorn Creek Lane. The cross gutters will convey flow into an existing catch
basin on the west side of Elkhorn Creek Lane. The catch basin ties into an existing 18”
RCP on Elkhorn Creek Lane (EX. MH-1). This inlet accounts for the runoff from Areas
A, EX-1 and EX-2. Areas EX-1 and EX-2 are “Sub-Area 7” in the Porter-Robertson
Study but has been divided to better accurately represent the proposed development for
analysis purposes. A composite runoff coefficient of 0.43 was calculated for Areas A,
EX-1, and EX-2 (see appendix). These areas were combined into one large area
discharging into the existing catch basin for a total computed runoff of 2.76 cfs. The
area and time of concentration values were used from the Porter-Robertson Study for
the remaining inlets (Sub-Area 5 and Sub-Area 6) discharging into the existing manhole
(EX. MH-1). The values produced a discharge of 6.57 cfs coming from the inlets located
on the easterly side of Elkhorn Creek Lane. The confluence at the existing manhole
(EX MH-1) produces a discharge of 8.86 cfs. This storm drain system connects to a 24”
RCP stub out (EX. MH-3 to EX. MH-4) coming from the existing 36” mainline on River
Vesting Tentative Tract No. 7239
Drainage Study
4
Run Blvd. This system has adequate capacity to convey the 10-year storm runoff from
the proposed development. It also has enough capacity to handle the additional 0.98 cfs
from the proposed development, Area A.
The following table will summarize the results of the hydrology calculations. Refer to
the Hydrology Calculations for the complete results.
Drainage
Area
Area
(acres)
Tc
(mins)
Proposed
Q10 (cfs)
1 & 2 8.17 20.54 4.00
A, EX-1, &
EX-2 5.70 22.54 2.76
(SB 5 & 6) 16.83 29.00 8.86
The initial hydraulic grade line (HGL) was taken from the Porter-Robertson report at
existing manhole 4 (EX MH-4). The existing storm drain pipes were used to calculate
the upstream HGL (see appendix) using Hydraflow software. The proposed on-site
pipes were sized to keep the HGL’s within the pipe. The assumed starting HGL at the
basin was estimated to be 4’ above the basin bottom (Elev. 351.5) since no as-builts of
the basin were available. The proposed pipe sizes can be viewed in the Hydraulic
Calculations section of the Attachments or on the Drainage Exhibit.
There are 2 sag inlets proposed onsite which was sized using the Hydraflow software
(see appendix). It was determined that a 3.5 ft. wide opening was able to capture 100%
of the runoff. The inlets will be located in the southwest corner of the tract where
Waterfall Lane and Quiet Creek Way intersect.
Vesting Tentative Tract No. 7239
Drainage Study
5
Vicinity Map
Not to ScaleJ
Tract 7239
Vesting Tentative Tract No. 7239
Drainage Study
6
ATTACHMENTS
· SOIL EXHIBIT
· SUMP CAPACITY CALCULATIONS
· HYDROLOGY CALCULATIONS
· INLET SIZING CALCULATIONS
· HYDRAULIC CALCULATIONS AND PIPE PROFILES
· DRAINAGE EXHIBIT
SOIL MAP
Soil Map 127 Cajon sandy loam, overblown, 0 to 2 percent slopes 174 Kimberlina fine sandy loam, 0 to 2 percent slopes
Not to
Scale
Tract
7239
SOIL MAP
SUMP CAPACITY CALCULATIONS
TRACT 7239
BASIN SIZING CALCULATIONS
Provided 15.69 AC-FT Required FEMA 15.41 AC-FT
Required COB 11.34 AC-FT
TABLE OF RUNOFF COEFFICIENTS School R-1 C x A =
R-1, 6000 SF0.42 Area (AC)10.73 4.51
R-1, 7500 SF0.38
R-1, 10000 SF0.34
R-1, 15000 SF0.27
R-2 0.55
R-3, R-4, M-H0.8
Commercial0.9 Area (AC)5.37 4.83
Industrial 0.8
Parks 0.15 Area (AC)5.37 0.80
Grasslands, Type A Soil0.15
Grasslands, Type B Soil0.25
Grasslands, Type C Soil0.35
Grasslands, Type D Soil0.45
Pavement, drives & roofs0.95
Backyards0.05
Lawn-landscape 2% slope0.17
Lawn-landscape 2-7% slope0.22
Lawn-landscape 7% slope0.35
School
Required Sump Capacity = 0.15 x C x A =0.15 x 5.63 =0.84 AC-FT
R-1
Required Sump Capacity = 0.15 x C x A =0.15 x 4.51 =0.68 AC-FT
0.17AC-FT
Sump Capacity (per Tract 5957 - Sewer Plans Sump Detail)
Required Sump Capacity
Total Difference of
HYDROLOGY CALCULATIONS
CITY OF BAKERSFIELD RATIONAL METHOD
(In accordance with City of Bakersfield Standards)
TABLE OF RUNOFF COEFFICIENTS
R-1, 6000 SF 0.42
JOB TITLE:VTT 7239 - Onsite Improvements R-1, 7500 SF 0.38
DATE:R-1, 10000 SF 0.34
REVISION DATE:R-1, 15000 SF 0.27
Rational Values:R-2 0.55
Event:10YEAR Values: 5, 10, 50R-3, R-4, M-H 0.8
M.A.P.6 in./yr. Values: 6, 10, 15, 20, 25, 30Commercial 0.9
Industrial 0.8
Curve Values Parks 0.15
a:2.38I=a+bTc (Tc<20min.)Grasslands, Type A Soil0.15
b:-0.058Grasslands, Type B Soil0.25
P60:0.550I=K1*(6.02*Tc)^(0.17*LN(p60/K1)Grasslands, Type C Soil0.35
K1:40.00 (Tc>=20min.)Grasslands, Type D Soil0.45
Pavement, drives & roofs0.95
Backyards 0.05
Lawn-landscape 2% slope0.100.17
Lawn-landscape 2-7% slope0.150.22
Lawn-landscape 7% slope 0.20 0.35
SUBAREACIARslt TcLdHQV Trial Tc Roof toTmD
NAMERunoff Intensity TotalTimeLengthElev.FlowVel.MIN.GutterTravelPipe
Coef.(in./hr.)Area Conc.Feet Diff.Slope CFS FPS Time Time Dia.
AC.MIN.Feet%MIN.MIN.In.
PIPE SIZING CALCULATIONS
Area 1 0.421.253.9519.395012.850.57%2.08 1.9019.3915 4.39
CB #1 to CB #2 [3]531.18 0.7518
20.15
Area 2 0.421.204.2220.546313.000.48%2.12 1.9020.5415 5.54
CB #2 to MH-1 [2]0.421.208.17123 4.10 2.3220.54 0.8818
21.42
MH-1 to SUMP [1]0.421.168.1747 3.98 2.2521.42 0.3518
21.77
Area A 0.421.392.5717.114183.800.91%1.50 3.3017.1115 2.11
Area EX-1 (from Porter H/H Report)0.951.600.1410.003721.170.31%0.21 1.175.300 5.30
Area EX-2 (from Porter H/H Report)0.421.132.9922.209503.300.35%1.42 2.2022.2015 7.20
Area EX-1, Area EX-2, and Area A 0.431.125.7022.549503.300.35%2.76 2.1022.5415 7.54
EX. CB to EX. MH-1 [9]151.56 0.1618
22.70
Tract 5999 (from Porter H/H Report)
Sub-Area 5 and Sub-Area 6 0.420.9316.8329.00 6.57 3.3029.0015 14.00
EX. MH-1 to EX. MH-3 [7] & [8]0.420.9322.53346 8.86 5.0229.00 1.1518
30.15
EX. MH-3 to EX. MH-4 [9]0.420.9022.5368 8.62 2.7430.15 0.4124
30.56
September 19, 2013
P:\PROJECTS\12067.00-BOPRIVER\Studies\Drainage\Calculations\Hydrology_10yr.xls Page 1
INLET SIZING CALCULATIONS
Inlet Report
Hydraflow Express Extension for Autodesk® AutoCAD® Civil 3D® by Autodesk, Inc.Thursday, Sep 19 2013
Catch Basin #1 - Drainage Area 1
Curb Inlet
Location= Sag
Curb Length (ft)= 3.50
Throat Height (in)= 4.00
Grate Area (sqft)= -0-
Grate Width (ft)= -0-
Grate Length (ft)= -0-
Gutter
Slope, Sw (ft/ft)= 0.083
Slope, Sx (ft/ft)= 0.020
Local Depr (in)= 2.00
Gutter Width (ft)= 2.00
Gutter Slope (%)= -0-
Gutter n-value= -0-
Calculations
Compute by:Known Q
Q (cfs)= 2.08
Highlighted
Q Total (cfs)= 2.08
Q Capt (cfs)= 2.08
Q Bypass (cfs)= -0-
Depth at Inlet (in)= 5.04
Efficiency (%)= 100
Gutter Spread (ft)= 6.35
Gutter Vel (ft/s)= -0-
Bypass Spread (ft)= -0-
Bypass Depth (in)= -0-
Inlet Report
Hydraflow Express Extension for Autodesk® AutoCAD® Civil 3D® by Autodesk, Inc.Thursday, Sep 19 2013
Catch Basin #2 - Drainage Area 2
Curb Inlet
Location= Sag
Curb Length (ft)= 3.50
Throat Height (in)= 4.00
Grate Area (sqft)= -0-
Grate Width (ft)= -0-
Grate Length (ft)= -0-
Gutter
Slope, Sw (ft/ft)= 0.083
Slope, Sx (ft/ft)= 0.020
Local Depr (in)= 2.00
Gutter Width (ft)= 2.00
Gutter Slope (%)= -0-
Gutter n-value= -0-
Calculations
Compute by:Known Q
Q (cfs)= 2.12
Highlighted
Q Total (cfs)= 2.12
Q Capt (cfs)= 2.12
Q Bypass (cfs)= -0-
Depth at Inlet (in)= 5.07
Efficiency (%)= 100
Gutter Spread (ft)= 6.51
Gutter Vel (ft/s)= -0-
Bypass Spread (ft)= -0-
Bypass Depth (in)= -0-
HYDRAULIC CALCULATIONS
AND PIPE PROFILES
10-YEAR STORM
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4
0
.
3
6
0
.
0
3
3
CB
#
1
t
o
C
B
#
2
35
3
.
6
8
35
8
.
6
3
4.
9
3
1.
1
8
0.
0
2
35
3
.
7
0.
0
3
6
0.
0
3
8
0.
0
2
2
10.02
EX
I
S
T
I
N
G
C
O
N
D
I
T
I
O
N
(
O
N
E
L
K
H
O
R
N
C
R
E
E
K
L
A
N
E
)
PR
O
P
O
S
E
D
S
T
O
R
M
D
R
A
I
N
(
A
T
W
A
T
E
R
F
A
L
L
L
A
N
E
A
N
D
Q
U
I
E
T
C
R
E
E
K
W
A
Y
)
Ex. MH-4
Ex. MH-3
Ex. MH-2
Ex. MH-1 Ex. CB
LI
N
E
N
o
.
8
LI
N
E
N
o
.
7
LI
N
E
N
o
.
7
A
LINE No. 9
SUMP
MH-1
CB #2
CB #1
LI
N
E
N
o
.
1
LI
N
E
N
o
.
2
LINE No. 3
DRAINAGE EXHIBIT