HomeMy WebLinkAboutCT 01-01; CALAVERA HILLS VILLAGE L2; HYDROLOGY STUDY VILLAGE H; 2003-09-17HYDROLOGY STUDY
FOR
VILLAGE 'H'
FOR
CALAVERA HILLS
CARLSBAD TRACT 01-01
Carlsbad, California
September 17, 2003
PREPARED BY
O'Day Consultants, Inc.
2710 Loker Avenue West, Suite 100
Carlsbad, CA 92008
(760) 931-7700
981020
/lyeJf^Uj looA
Contents
Runoff Under Existing Conditions 1
Runoff Under Proposed Conditions 2-3
Design ofDesilting Basin "A" 4
Outlet Calculations 5
San Diego County Isopluvial Maps 6-7
San Diego County Hydrology Manual Reference Tables 8-11
Basin Design Reference* 12-17
*Goldnian, Erosion & Sediment Control Handbook, 1986, McGraw-Hill
Runoff Under Existing Conditions
Qioo=CIA
C=0.45 (Soil Group D, Residential-Rural)
P6(ioo)=2.7" (see page 6)
P24(ioo)=5.0" (see page 7)
P6/P24=2.7/5.0=.54 .45<.54<.65 OK
Tc=[(11.9(L)VZ]°-^^^ (seepage 10)
Tc=[(l 1.9(785/5280)^)/! 02]°
Tc=.048 hr=2.9 min+10 min=12.9 min
I=7.44(P6)(Te)-^^^ (see page 9)
I=7.44(2.7")(12.9)-^^^
1=3.86 in/hr
A=7.13 acres
Qioo=(-45)(3.86)(7.13)= 12.38 cfs
Runoff Under Proposed Conditions
Qioo=CIA
C=0.45 (Soil Group D, Residential-Rural)
P6(ioo)=2.7"
P24(100)=5.0"
P6/P24=2.7/5.0=.54 .45<.54<.65 OK
Basin A
Tc=[(1.8(l.l-C)(D) Vs"] (seepage 11)
Tc=[(1.8(l.l-.45)(390)V2 ]
Tc=18.3 min
-.645 I=7.44(P6)(Tc)-
I=7.44(2.7")(18.3)-^'*^
1=3.08 in/hr
A=2.55 acres
Qioo=C45)(3.08)(2.55)= 3.53 cfs
Tc=[(1.8(l.l-C)(D)VS"]
Tc=[(1.8(l.l-.45)(185)V29.7"]
Tc=5.1 min= 10 min (minimum)
I=7.44(P6)(Tc)-^^^
I=7.44(2.7")(10)-^''^
1=4.55 in/hr
A=0.59 acres
Qioo=(-45)(4.55)(0.59)= 1.21 cfs
Basin B
Basin C
Tc=[(1.8(l.l-C)(D)Vsf]
Te=[(1.8(l.l-.45)(560)Vl5- ]
Tc=11.2min
I=7.44(P6)(Tc)-^'
I=7.44(2.7")(11.2)"-^^
1=4.23 in/hr
A=3.99 acres
Qioo=C45)(4.23)(3.99)= 7.59 cfs
Qioo Proposed=3.53+1.21+7.59=12.33 cfs
12.33<12.38
Qproposed'^QExisting
Design of Desilting Basin "A"
Sizing of Desilting Basin
Pl0-yr,6-hr=1.9 in
iAva=l.9/6=0.32 in/hr (see page 12)
QAVG=0.45*0.32*2.55=0.37 cfs
Vs=.00096 ft/sec (.02 mm particle) (see page 15)
As=1.2Qavg/Vs (seepage 14)
As=1.2(0.37)/(.00096)=463 sf 39'xl2'
Dewatering Holes
Ao=As(2h)V3600(T)(Cd)(g)-' (see page 17)
Ao=Surface Area of Orifice (sf)
As=Basin Area (sf)
h=Head of Water (ft)
T=Time (Hrs.)
Cd=0.6 (Sharp Edged Orifice)
g=Acceleration of Gravity (32.2 ft/s^)
Ao=468(2*2) V3600(40)(0.6)(32.2)^=0.002 sf=0.31 in^
USE 3/4" Dia. Hole
Standpipe Sizing
Q=Ao(Cd)(2g*h)-^ (see page 16)
Ao=Surface Area of Orifice (sf)
h=HeadofWater(fl)
Cd=0.6 (Sharp Edged Orifice)
g=Acceleration of Gravity (32.2 ft/s^)
Q=Qioo=3.59 cfs (see page 2)
3.53=Ao(0.6)(2*32.2*l)-^
Ao=0.73sf MinDia=l'
O'Day Consultants Inc.
2710 Loker Avenue West, Suite 100
Carlsbad, CA 92008
Tel: (760) 931-7700 Fax: (760) 931-8680
Inside Diameter
( 15.00 in.)
AAAAAAAAAAA/NAAAAAAAAA
Water
{ 3.52 in.)
{ 0.293 ft.)
I
I
V
Circular Channel Section
Desilting Basin Discharge Pipe
Depth of Flow at lOO-year Storm
Flowrate 3.530 CFS
Velocity 16.097 fps
Pipe Diameter 15.000 inches
Depth of Flow 3.519 inches
Depth of Flow 0.293 feet
Critical Depth 0.758 feet
Depth/Diameter (D/d) 0.235
Slope of Pipe 20.500 %
X-Sectional Area 0.219 sq. ft,
Wetted Perimeter 1.264 feet
AR'^(2/3) 0.068
Mannings 'n' 0.013
Min. Fric. Slope, 15 inch
Pipe Flowing Full 0.299 %
O'Day Consultants Inc.
2710 Loker Avenue West, Suite 100
Carlsbad, CA 92008
Tel: (760) 931-7700 Fax: (760) 931-8680
Inside Diameter
( 12.00 in.)
*
* *
* *
AAAAAAAAAAAAAAAAAAAAA
Water * |
I I
I
4.33 in.
0.361 ft.
I
Circular Channel Section
12" Diameter Culvert Under Trail
Depth of Flow During lOO-year Storm
Flowrate 3.600 CFS
Velocity 14.112 fps
Pipe Diameter 12.000 inches
Depth of Flow 4.329 inches
Depth of Flow 0.361 feet
Critical Depth 0.806 feet
Depth/Diameter (D/d) 0.361
Slope of Pipe 13.200 %
X-Sectional Area 0.255 sq. ft.
Wetted Perimeter 1.288 feet
AR^(2/3) 0.087
Mannings 'n' 0.013
Min. Fric. Slope, 12 inch
Pipe Flowing Full 1.022 %
f32-30>
County of San Diego
Hydrology Manual
Rainfall Isopluvials
10 Year Rainfall Event - 6 Hours
Isopluvial (inchas)
,D.PW
Wo Hnvc .Sar. iJitgri {Ucred;
TtJSMW (S PHOWUCO WMOyr WMRANTV (W ANV IM. UTWA EWWM
lmu>. MCLMNNG. MIT NOT lAWiP TO. TW IMMO WMWW
County of San Diego
Hydrology Manual
Rainfall Isopluvials
10 Year Rainfall Event - 24 Hours
Isopluvial (inches)
,'jan.(jilj> fcGIS
IMS •>VU> M M0^«>EO VmHOUT WAIHiWTY Of ANY MNO. emcR EXMWn on IMPLED.INClUOMj. BUT NOT UMRB) TOt IMEMnltO WMHUNTlit OF tCnCMWTMUrr AND FITNiM Kft AnUftKULMMWOii.
UHOW. »• m^i llM«r i»C
-3 0 3 Miles
Tjjuma
County of San Diego
Hydrology Manuai
Rainfall Isopluvials
100 Year Rainfall Event - 6 Hours
/V'' Isopluvial (inches)
Map Notes
Slateplane Prqjectica Zones. NAD83
Cnalicn Dale: June 22.20O1
NOT TOBEUSEOFOR DESIGN CALCULATIONS
ame d3>
T
Orange
Comty
^>-"-.i--#.'^ A>(ZA2rr']A\ s.n Dft^o-a^.§v \ \ \ v^xxx r\ """^^^ToTv"^
1(10,''
I / I
'< I • / /13.0 XT
i i
/ A5-V ./
UJollait
Padicaudi
MuionBflidi
4J)—
County of San Diego
Hydrology Manual
Rabtfall Isopluvials
100 Year Rainfall Event • 24 Hours
Isopluvial (inches)
Map Notes
Slateplane Prejedioa ZoneC NAOea
Crealian Dale: JKie 22,2ixn
NOTTO BE USED FOR DESIGN CAUCULATIONS
amecP
TABLE 2
RUNOFF COEFFICIENTS (RATIONAL METHOD)
DEVELOPED AREAS (URBAN)
Coefficient. C
Soi1 Group (1)
Land Use
Residential: - t. B C 0
Si ngle Family AO .^5 .50 .55
Multi-Units A5 .50 .60 .70
Mob i1e homes A5 .50 .55 .65
Rural (lots greater than 1/2 acre) .30 .35 AO AS
Commercial(2)
80% Impervious
.70 .75 .80 .85
Industrial (2)
90% Impervious
.80 .85 .90 .95
NOTES
(1) Soil Group maps are available at the offices of the Department of Public Works.
f2)where actual conditions deviate significantly from the tabulated impervious-
ness values of 80% or 90%, the values given for coefficient C, may be revised
by multiplying 80% or 90% by the ratio of actual imperviousness to the
tabulated imperviousness. However, in no case shall the final coefficient
be less than 0.50, For example: Consider commerciai property on D soil.group.
Actual imperviousness = 50%
Tabulated imperviousness = 80%
Revised C = 50 ^ 0.85 = 0.53
80
IV-A-9
APPENDIX IX-B
Rev. 5/81
Duration
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 Oesign 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.
, year
'P. 24
= %<2)
In.
Application Form:
(a) Selected frequency
(b) P6= in..P24^
(c) Adjusted Pgl^) =
(d) tj5 = min.
(e) I = ln./hr.
Note: This chart replaces the Intensity-Duratlon-Frequency
curves used since 1965.
P6
Duraiion
5
^7
JO
IS
ao
"25
^30
40
50
60
90 120
JSO JM
240
job
360
,.,1,1 I
2.5 3 I 3.5 4.5
I I •!
5.5
2.63
2.T2
1.60
"1.30
1.08
0.03
0.83'
0.69
0.60
0.53
d;41
0.34'
0.29-
0.26"
6^2' 619"
0.17 0.25
3.95
3.18
2.53
1.95
1.82
1.40
1.24
1.03
0.90
0.80
0.61
0.51
6744
6!39
0.33
a28
5.27
4724
3.37
2.59
2:15
1.87
1.66
1.38
1.19
1.06
0.82
0.68
0^59
0!52
0.43
6.38
0.33
6.59
5.30
4.21
'3.24
2.69
2.33
2.07
'{.72
i.49
1.33 l!62
0.85
0.73"
aes
0.54
0.47
0.42
7.90
&36
5.05
3j0
3.23
2760
2.49
207
1.70
l759
'1.23
1.02
0.88
6.78
0.65
6.56
9.22
7742
5.90
"4.54
3777
3.27
2:96
£41
2'09
T.06
1.43
l.i 9
1.03
0.91
0.76
•6.66
"0758
10.54111.86,13.17
8.48
6.74
6.'19
4.3T
_
3.32
2.76
2.39"
2.12
1.63
1^36
"i.16
l704"
0.87
6775
0.67"
9.54 ,10.60
7.58 : 8.42
5.84X6.49
'4.85 I 5^39
'4.'20T4.67
3.73 1 4.15
3^10 1 3.45
2.69 ; 2.98
2.39 ,
1.84 !
1.53"
1.32 1
1.18!
2.65
2.04
1.70
1.47
1.31
0.98 [ 1.08
0.85 I 6.94"
0.75 ! 0.84
14.49115.81
fi.66jl272
9.27 ib.fi
7.78
6.46
5.60
4.98
4.13
3.58
3.18
2.45
7.13
"5.93
5.13
4.56
3.79
3.28
2.92
2.25
1.87 ] 2.04
1.62"l 1.76
1.44 j" 1.57
1.19 I 1.30
i7o3i i7i3 0.92! rob
Intensity-Duration Design Chart - Template
HazMal/County h, .geology Manual/lnt.Our Oosign Chart.FHS A-
FIGURE
3-1
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200'
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90
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Tr
SAN DIEGO COUNTY
DEPARTMENT OF SPECIAL DISTRICT SERVICES
DESIGN MANUAL
APPROVED • .^rX<^^y^^ A-AXk^
NOMOGRAPH FOR DETERMINATfON
OF TIME OF CONCENTRATION (Tc)
FOR NATURAL WATERSHEDS
DATE APPENDIX X-A
A-10 Rev. 5/81
EXAMPLE:
Given: Watercourse Distance (D) = 250 Feet
Slope (s) = 0.5%
Rimoff Coefficient (C) = 0.70
Overiand Flow Time (T) = 14.3 Minutes
SOURCE: Airport Drainage, Federal Aviation Administration, 1965
1.8 (1.1-C) VD"
FIGURE
Rational Formula - Overland Time of Flow Nomograph
HazMat/County Hydrogeology ManuaVOverland Flow.FHS
Estimating Runoff 4.5
tenance and repair costs. Such a policy should be based on whether and where
such high-risk areas exist. An appropriately longer storm retum interval is
advised in high-risk areas.
4.1c Use of Qpeak
The peak runoff, which is normally used to size drainage systems, is calculated
when the capacity of a channel or other conveyance structure must be sufficient
to carry all of the flow. In erosion control work, Qp„i, is important not only to
size conveyance facilities but also to:
• Check for potentially erosive velocities in unlined channels
• Select channel linings that will not erode
• Design outlet protection
In these cases, the rational method is applied by using a peak precipitation
intensity:
Qp-k = C X ip«a, X A
Peak precipitation intensity is determined by estimating the time of con-
centration for the drainage area and then finding the maximum rainfall intensity
for that time duration and design storm retum interval. For example, if the time
of concentration for a watershed is 1 hr, you should use the peak 1-hr rainfall
intensity in your calculations. The procedure for determining this time is
explained in Sec. 4.1g.
4.1d UseofQ.vg
An average flow Q,,,, rather than peak flow, is used to find the required surface
area of sediment basins and traps. The rational formula is still applied, except
that an average precipitation intensity instead of the peak intensity is used:
Qavf — C X i,vf X ^
Average precipitation intensity is determined by taking the total rainfall
for a specified storm return period and duration (e.g., 10-year, 6-hr storm) and
dividing that total by the number of hours of duration:
_ total 6-hr rain
~ 6
A 6-hr storm duration is suggested. Sediment basins designed with a 6-hr storm
strike a reasonable compromise between being somewhat undersized during
storm peaks and being somewhat oversized during the rest of the storm.
8.14 Erosion and Sediment Control Handbook
—L.
0.2 0.3 0.4 0.6 0.8 }.0 2 \
Surface area adjustment tactor
Fi(f. 8.14 Effect of turbulence on sediment basin efficiency. (3, 7)
-L_i
4 5 6
upon performance, there is little guidance on reducing turbulence and increasing
efficiency to that predicted for ideal conditions. More research needs toTeTr
formed on sediment basms before significant improvements in design can be pro-
posed. However, the following practices can influence turbulence tTsome degree
and reduce the surface area adjustment multiplier: ^
. Reduce water velocities through the basin. The ideal surface area is computed
by usmg the equation A - Q/V.. Increasing the surface area increZ the
cross-sectional area of the flow through the basin and decreases the LorizLw
flow velocity, and thus turbulence. An altemative way to reduce EnS
IMI '^'^ • '^'^ on72c!n,
. Eliminate unnecessary angles or doglegs in the flow. When water flow changes
directon random currents that inhibit particle settling are set up S
straight basins are best ^ ^'
. Reduce the effect of wind-induced turbulence. Large open water surfaces are
affected by wind, which produces cross- and counterculrents that Eder s^t!
ling and may resuspend bottom deposits. Using several smaller bas ns with a
total capacjty equal to the capacity of one large basin should imprZcapture
efficiency. Note: Multiple basins must be placed in parallel, not ZserfTlf
tTuf wXe^m-pS' ^""''^ ^^'^^^
Surface Area Formula
The basin designer should select a surface area adjustment factor based nn .if»
conditions. A U.S. Environmental Protection Agency publLatLTon elln
trol for surface mining (6) proposes a surface area Jd^^flZT^.^l
Sediment Retention Structures 8.1S
factor is based on the assumption that the basin is well designed in all key
aspects (shape, outlet location, riser design, etc.) but that turbulence and other
nonideal conditions will reduce the basin's efficiency. The resulting sediment
basin surface area sizing formula becomes:
. 1.2Q
where A, is the appropriate surface area for trapping pairticles of a certain size
and V, is the settling velocity for that size particle.
8.2b Basin Efficiency
The trapping efficiency of a baain is a function of the particle size distribution
of the inflowing sediment. Assuming ideal settling conditions, all particles with
size and density equal to or larger than those of the design particle will be
retained in the basin. In addition, some smaller particles will be captured while
, the basin is dewatering and the overflow rate has decreased. The additional cap-
ture ranges from 2 to 8 percent of the total sediment load. For our purposes the
increase is not important enough to include in the calculations, particularly since
the increase is offset by reduced capture efficiency of the basin when flow exceeds
the design value.
Therefore, ideal basin efficiency corresponds to the percent of soil equal to or
larger than the design particle size. For example, if a sediment basin on a site is
designed to capture the 0.02-mm particle and 64 percent of the particles on this
site are greater than or equal to 0.02 mm. the maximum efficiency of the basin
is 64 percent. The only practical way to increase this efficiency is to increase the
surface area of the basin.
8.2c Design Particle Size
The equation A, => 1.2Q/V, defines the relation between size of particle to be
captured and the surface area required for the basin. By applying this equation
with the settling velocities V, of various particle sizes from Fig. 8.12. Table 8.1
of surface areas per unit discharge is derived.
From Table 8.1 it is clear that the surface area required increases very rapidly
as the particle size decreases. To capture the 0.02-mm particle, the area must be
6.5 times larger than the area required to capture the 0.05-mm particle. To cap-
ture the 0.01-mm particle, the basin area must be 4 times larger than for the 0.02-
mm particle and 25 times larger than for the O.OS-mm particle. For partictes
smaller than 0.02 mm. the surface area requirement increases dramatically.
Where soils have a high content of clay or fine silt, increasing the size (and
thus cost) of a basin will not bring about a proportional increase in basin effi-
ciency. For example, a typical soil in the San Francisco Bay Area is 62 percent
by weight composed of particles 0.02 mm and larger, but it is only 5 percent by
8.16 Erosion and Sediment Control Handbook
TABLE 8.1 Surface Area Requirements of Sediment Traps and Basins
Particle size, mm Settling velocity,
ft/sec (m/sec)
0.5
0.2
0.1
0.05
0.02
0.01
(coarse sand)
(medium sand)
(fine sand)
(coarse silt)
(medium silt)
(fine silt)
0.005 (clay)
0.19 (0.058)
0.067 (0.020)
0.023 (0.0070)
0.0062 (0.0019)
0.00096(0.00029)
0.00024 (0.000073)
0.00006 (0.000018)
Surface area requirements.
ft» per ftVsec (m» per m'/sec
discharge discharge)
6.3
17.9
52.2
193.6
1.250.0
5,000.0
20,000.0
(20.7)
(58.7)
(171.0)
(635.0)
(4,101.0)
(16,404.0)
(65,617.0)
weight composed of particles in the 0.01- to 0.02-mm range. A surface area 4
times larger would be needed to capture 5 percent more of this soU
A balance between the cost-effectiveness of a certain basin size and the desire
to capture fine particles must be achieved. It is desirable to capture the very
small 80,1 particles (clays and fine silts) because they cause turbidity and other
water quality problems. However, Table 8.1 shows that a basin would have to be
ve^ large to capture pdicies smaUer than 0.02 mm. particularly clay particles
0^5 mm and smaUer. Because ofthe high cost of trapping very smaU particles!
the authors recommend 0.02 as the design particle size for sediment baSns
except m areas with coarse soils, where a larger design particle may be used. The
0.02-mm particle is classified as a medium silt by the AASHTO soU classification
system.
8.2d Basin Discharge Rate
The peak discharge, calculated by the rational or another approved method, is
used to size the basm nser. Durmg any major storm, a sediment basin should fiU
with water to the t»p of its riser and then discharge at the rate of inflow to the
basm. A sediment b^m is not designed with a large water storage volume as is
a reservoir W the inflow exceeds the design peak flow used to size the riser, the
overflow should discharge down an emergency spillway.
8.2e Design Runoff Rate
i«rJhl!Tf " discharge rate Q is a
variable to be chosen by the designer. Tfie above discussion of b^in discharge
rate shows that the discharge rate is, to a large extent, equal to the inflow The
nser is sized to handle the peak inflow to the basin. The authors suggest deter
mming the surface area by the average runoff of a lO-year. G-hr storm instead
Sediment Retention Structures 8.23
Owmnrinf r Btair
holt
\Axk
Siphon
Oi
Dew ittr infl
hote virith ^
PMhraud dnln pip. In Irandt in bottom of b«in for ttawatirinR lodlmMrt
Fig. 8.17 Basin-dewatering methods. Methods (e) and (/) are poor; (g), (h), and (i) are
best.
top. This hole will dewater the settling zone but wilt not dewater the sediment
storage zone. The equation that describes fiow through an orifice is:
Q = Ci X A„V2jh
where Q = flow, ftVsec (mVsec)
Cd =• coefficient of contraction for an orifice, approximately 0.6 for a
sharp-edged orifice
Ao = surface area of the orifice, ft* (m^)
g =• acceleration of gravity, 32.2 ft/sec' (9.81 m/sec')
h =• head of water above the orifice, ft (m)
The time required for an orifice to dewater a basin can be obtained by inte-
grating the change in basin volume over time as a function of depth of water.
The resulting equation is
^•^^ iBrosion and Sediment Control Handbook
J, _ A.\/2h
3600 X A„ X CjVg
where T = time, hr
A, = surface area of basin, ft' (m')
Conversely, the size of orifice needed to dewater a basin within a specified time
Tis
A
3600 X T X CjVg
Thus a typical 10.000-ft» (930-m') basin would need a 0.068-ft» (0.0063-m') or
3.5-in- (9-cm-) diameter orifice to dewater a 2-ft (0.6-m) settling zone in 24 hr
Several orifices adding up to 0.068 ft' (0.0063 m") in surface area could be used.
Figure 8.176 presents the same riser with a shield tack-welded over the orifice.
The shield greatly reduces clogging of the orifice, but it will somewhat lower the
discharge rate through the orifice.
Figure 8.17c and d shows a siphon used to dewater a basin to the sediment
level. This method, as presented in the SCS Maryland handbook (9), calls for a
4-in- (10-cm-) diameter siphon. With the system shown in Pig. 8.17c, runoff from
small storms and base flow would flow through the siphon virith no backing up
and storage. In the system of Fig. 8.17d smaU flows would back up and discharge
in a batch fill and drain process. This would allow some settling of the sediment.
Neither system would dewater below the siphon.
The method shown in Fig. 8.17e has been frequently observed at Califomia
construction sites. It is potentially the worst possible to be employed. Although
the basin would dewater rapidly, most runoff would flow straight across the basin
and out the multiple holes. Not only would there be no backing up of the water
with associated particle settling, but the straight channel flow through the basin
would resuspend previously deposited solids. The discharge from such basins
has been found by the authors to contain more sediment than the runoff into the
basins.
The dewatering hole shown in Fig. 8.17/ can dewater the entire basin, includ-
ing the sediment zone. However, this system is poor for two reasons. First any
orffice sized to dewater the basin quickly would directly pass low flows associated
with the more common storms and provide either no treatment at all or negative
treatment. Second, any sediment accumulating near the riser would be quickly
washed through the orifice.
A number of consultants have suggested the systems shown in Figs. 8.17^ and
h. The gravel piled around the base of the riser acts as a coarse filter to keep
sediment from escaping. These systems have not been monitored to assess their
effectiveness. AdditionaUy, the system shown in Fig. 8.17i has been suggested for
the dewatering of sediment in large basins. Performance of this system also has
not been verified. In all cases, the dewatering hole must be sized to prevent
smaller storms from draining without any retention time in the basin.
Figure 4.2 shows that, within a typical 6-hr storm, for 4 hr the flow rate will
be about 60 percent or less of the average rate. If, for example, the basin were
designed for a 10-year interval storm that drops 2.6 in (66 mm) in 6 hr a much