HomeMy WebLinkAboutCT 02-07; EMERALD POINT ESTATES; PRELIMINARY RATIONAL METHOD HYDROLOGIC CALCULATIONS; 2002-04-17HUNSAKER CT() J..-D 1
&ASSOCIATES
;".::.-..-= 5 AND lEG 0, INC.
PLANNING
ENGINEERING
SURVEYING
IRVINE
RIVERSIDE
SAN DIEGO
DAVE HAMMAR
LEX WILLIMAN
ALiSA VIALPANDO
DANA SEGUIN
10179 Huennekens St.
San Diego, CA 92121
(858) 558-4500 PH
(858) 558-1414 F X
www.HunsakerSD.com
Info@HunsakerSD.com
April 17, 2002
Mr. Jack Henthorn
Jack Henthorn & Associates
5365 Avenida Encinas
Suite A
Carlsbad, CA. 92008
RECEIVED
MAY 08 2002
CITY OF CARLSBAD
PLANNING DEPT"
RE: Emerald Pointe Estates -BCS Site
Dear Mr. Henthorn:
Preliminary Rational Meth hyd ologic c
the Emerald Pointe Est s (BCS s·
10-year design storm e "-------e to existing condition. To mitigate the
3 cfs increase (from 8 cfs to 5 cfs), a detention facility has been proposed just
upstream of the site's connection to the existing Cobblestone Drive storm drain
system.
Per the Federal Highway Administration's "Urban .Drainage Design ManLiSI -
Hydraulic Engineering Circular No. 22" (November 1996), Figure 8..,5 (attached)
can' be used to estimate required detention storage volumes. Thefigure is based
on methodology presented in the Soil Conservation Service's TR...:55 Second
Edition Report, which describes a manual method for estimating require€! storage
volumes based on peak inflow and outflow rates.
The method is based on average storage and routing effects Observed for a" large
number of structures. A dimensionless figure relating the ratio of basin storage
volume (Vs) to the inflow runoff volume (Vr) with the ratio of peak outfloW (00) to
peak inflow (OJ) was developed as illustrated in Figure 8-5. This procedure for
estimating storage volume may have errors up to 25 p.ercent and should
therefore only be used for preliminary estimates.
Parameters for the proposed detention facility were determined as follows:
• Peak Inflow Rate = 8 cfs (from Rational Method analysis)
• Peak Outflow Rate = 5 cfs (to mitigate 3 cfs increase in 1 O-year flow)
• Inflow Runoff Volume = Runoff Coeff * 6-Hour Rainfall Volume
0.50 * 1.7 inches * 1 foot! 12 inches * 5 acres
~M.snun k 12403120021.101.doc
wo2403·1 4117/0nO-Z4AM
Jack Henthorn & Associates
Emerald Estates -BCS Site
April 17, 2002
Page 2
The ratio of the peak outflow discharge to the peak inflow discharge is 0.63 (5/8).
Using Figure 8-5, the ratio of storage volume to runoff volume for a Type I storm is
0.15. Since the inflow runoff volume was determine.d to be 0.35 acre-feet, the
storage volume was calculated to be 0.05 acre-feet. Per the TR-55 text, the storage
volume was increased by 25 percent to account for inherent errors in this preliminary
design, thus increasing the storage volume to 0.06 acre-feet.
In addition to the detention storage required, the basin must also serve qS a water
quality basin. The first flush runoff volume, determined to be 0.10 acre-feet, must be
contained in the bottom portion of the basin. This area may not be used for detention
storage calculations. Thus, the total water quality treatment and detention storage
volume required is 0.16 acre-feet.
The proposed basin has a bottom elevation of 232 feet and a top elevation of 240
feet. Calculations show that basin attains the required 0.16 acre-feet of storage
around elevation 237.5 feet. This leaves 2.5 feet of freeboard above the anticipated
1 O-year water surface elevation.
Should you have any questions regarding the material contained in this memo,
please contact me at (858) 558-4500.
Sincerely,
Hunsaker & Associates
San Diego, Inc.
!l,'1U I -
(Ip / fl!ft(r-
Eric Mosolgo, P.E.
Water Resources Department Manager
cc: Lex Williman -Hunsaker & Associates San Diego, Inc.
Dave Hammar -Hunsaker & Associates San Diego, Inc.
EM·smm k:\2403\2002\aOI.doc
wo.2403-1 4117/0210.24AM
/
l Jf /
WATER QUALITY BASIN SHEET.
EMERALD POINTE ESTATE ~F
. 1
J)
STAGE-STORAGE TABLE nE\I·~Qr.:D n \! &~ ... n_
EMERALD POINTE ESTATE
Elevation(ft) Area (acres) Incremental Volume (acre-ft:) Total Volurrie(acre':ft.)
232 0.012 0.00 ..
0.01
233 0.017 0.01
0.02
234 0.023 0.03
0.03
2;35 0.030 0.06
0.03
236 0.037 0.09
0.04
237 0.045 0.14
0.05
238 0.054 0.19
0.06
239 0.063 0.24
0.07
240 0.073 0.31
d.
ChapterS. Detelltioll and Retention Facilities
,,\ I
'\ I .5 '\ I
L-
.4 1,\
I'\. " '\ I-....
lYPES II &: III I
.3 i--.
" l"'-
f' ....
t-., lYPES I &: IA
.2 I"'-
.1
.1 .2 .3 .4 .5 .6 .7 .8
Peak Outflow Discharge (gJ Peak Inflow Discharge COil
Figure 8·5. SCS detention basin routing curves(13).
8.4.1.4 Regression Equation
An estimate of the storage volume required for a specified peak flow reduction can be obtained by using
the following regression equation procedure first presented by Wycoff & Singh in 1986 (48).
1. Determine the volume of runoff in the inflow hydrograph (Vr), the allowable peak outflow rate (Qo),
the time base of the inflow hydrograph (ti), and the time to peak of the inflow hydrograph (tp).
2. Calculate a preliminary estimate of the ratio V JVr using the input data from step 1 and the following
equation:
(~: )= [1.291 (1-Q, I Q, )0753]1[ (TJTp ) DAIl] (8-4)
3. Multiply the inflow hydrograph volume (Vr) times the volume ratio computed from equation 8-4 to
obtain an estimate of the required storage volume.
The following example problt:m demonstrates the use of some of these storage volume estimation methods.
8·9