FLOOD08316

• County: Statewide
• Basin: Statewide
• Title: Flood Hazard News The 1997 Floods in the District
• Date: 2/11/1998
• Prepared For: State of Colorado
• Prepared By: UDFCD
• Floodplain - Doc Type: Educational/Technical/Reference Information

filter's surface. This relationship for a
sand fi1ter (i.e., FJgUlC 3) appears to be
not sensitive to the hydrau1ic surcharge
on the filter's surface and can be
expre nee' by Equation 6.
q=k.e-C-4. (6)

Inwbich,
K s empirical flow-through constant
(see Figure 3)
c s empirical expooential decay
conslllnt (see Figure 3)
L. s TSS load ,.,.,."nn,IAtNl on the
filter's surface in pounds per square foot

1'8S Load Removed By The Filter
Recognizing that not all runoff
during any given year will pass through
the fill<< in....u.mOD, the average annuaI
load removed by the fill<< faciIi1y can be
e,.pl cm~ by Equation 7.
E..
L~ =b'T.L. (7)
.
Inwbich,
Lqfr s average annual TSS load removed
by the filter in pounds
B s the fraction of all average annuaI
runoff volumes that is treated by the
fi1ter facility (i.e., not bypassed)
The ftaction of all runoff volume
from the tribntary area that will be
treated throngh the filter facility is, in
part, a function of the WQCV upstream
of the filter. D<yndn,g on whether the
basin is bypassed or overtopped will
also determine the amount of treatment
provided to the excess volumes during
large storms. If the IIU1Ximized capture
volume is provided, approximately 80%
to 90% of all runoffvolume can be
treated by the filter in....llArion If;
however, the mean capture volume is
used, approximately 65% to 70% of the
total annual runoffvolume will be fully
processed through the filter.
The filter will also need to be
maintAinetl to stay in operation. The
contaminotNl and clogged layers will
need to be removed and repIalled with
new media and eventuaI1y (say after five
to ten surface clP.Anillg.") the entire
media fill<< will need to be replaced.
Equatioo 8 can be used to ....mmAtP. the
TSS load removed by each sqpare foot
of the fill<< during each mlri.......nce
cycle.

L.

L~

Afio.m

(8)

In which,
L. = average TSS load removed by each
square foot of the filter during each
maint"""""", cycle in pounds per square
foot per D1l1intP.nance cycle
m = number of times per year the fi1ter
is cleaned and reconditioned. Use a
fraction (i.e., 0.5) if more than one year
between clP.Anillg."
Afio = surface area of the fi1terbased on
annuaI TSS load removed in square feet

Sizing TIle Filter
Equatioo 8 can be rearmnged to
....mtnAtP. the fi1ter's area based on TSS
removed.

Lqfr
A (9)
fio L.m
.
Equation 10 can be used to estimate this
area based on the desired drain time of
WQCV.
P. . Ac .43,560
A - (10)
JIo- q.T,.

In which,
Q s the design flow-through mte
throngh the sand filter's surface in
incheslhour
T. s the time it takes volume p. to drain
on! at mte q in hours
AJIo = surface area of the fi1ter based on
hydrau1ic sizing in square feet
The designer now has to find the
filter area that comes close to satisfying
both conditions and the following
design procedure to accomplish this:

Design Procedure
1. Determine E.. the average EMC of
TSS for the tributary catchment. Use
local TSS data when available. In
absence of local data, use the closest
regional averages reported in the
Nationwide Udlan RnnoffEvaluatioo
final report (EPA, 1983).
2. Calculate the average annual TSS
load in stormwaler runofffrom the
design calchment. Use Equatioo2 and
Equatioo 3 to estimate L,.
3. Selectjilter-detsntionlretention
conjiguration and preselect its desired
drain time. Cases 1 and 2 are Sl~
for ~clnnP.nts with more than one acre
of impervious surf'al:e, while Case 3 is
sugrtNl for smaller sites.
4. Estimate Et;t. the reduction in the
EMC ofTSS provided by the jilter if8elf.
Based on Case 1, 2 or 3 with a valne for
I"zl select a value from Table 1 for the

12

removals by the detention or retention
portion of the faciIi1y and use it in
Equation 4 or 5.
5. Estimate the average annual TSS
load re1ttOl1ed by the jiltsr. Use
Equation 7 to calculate a value for Lqfr
(assumeb s 0.90 ifWQCV= P,).
6. Determine the jiltsr 's annual
maintenance .fre~ncy. Typically one
cleaning per year is suggested as a
starting point
7. With the aid of Figure 3 select the
desired unit TSS load removed. L.per
each cleaning.
8. Set the WQCV for this installation. It
is recommended that, as a minimum, a
volume equal to the runoffbetween the
mean storm and the IIU1Ximizedvolume
be used for design. Use equations 1 and
2.
9. Makejirstestimatssofthejilter's
area. Calculate the filter's area,Ajio.
using Equation 9 and Equation 10.
10. Compare the two jilter areas
calculated in Step 9. If the two
calculations differ by more than 200/0;
average the two areas; calculate a new
value for the unit load removed by the
filter, L.; find a new q using Equation 7
and repeat Step 9. Otherwise use the
larger surface area of the two.

Design Eumples
RYAmnle 1. At a commercial site in
Denver the media fill<< will be preceded
by an upstream extended detention
basin. The known site conditions are:
S!ml:
Tributary Area,A. = 1.5 acres
Expected EMC of TSS, E. = 225 mgll
Average storm depth, p. = 0.43 inches
Average number of runoff storms per
year, n = 30
Catchment's total imperviousness,
1. = 85%
~: Using Equation 2 find its runoff
coefficient:
C = 0.66
Using Equation 3 calculate the annuaI
TSS load from the catchment:
1.. = 651lbs
~: Since the filter will be preceded
by an upstream extended detention
basin, we have Case 1 configuration
The WQCVwill drain in 12bours.
~: Using T. s 12 hours, Table 1
suggestsRD = 50%. j\....nnil1g 95"10
overa11 removal mte for the detention-
filter system, estimate using Equation 4

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