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

Hydraulic Design of Sand Filters for Stormwater Quality
BeD R. Urboau, P.E

IDtrod1letioa
This article is an abbreviated version
of a full paper submitted for publication
in a profes$ionaljournaL It was
modified to fit the Denver area's
meteorology and the space available in
this F100d Ht1ZlI1'd News. The original
paper is based on research efforts by the
District, including filed data collection
and analysis, of the hydraulic
performance of sand filters under field
~ition'. Loca1 data were combined
with data from others in the U. S. to
suggest pollulaDl remova1 by sand
filters.

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De8lgn Hydrology aDd TSS Load
Because of the temporal variability
of stormwater runo1l; a media filter
needs a de(entjon volume upstream of it
to equalize the nmoff rates during a
rainstorm. This deteDtion volume has
be drained out (i.e., ful1y evacuated) in a
reasonable amount of time to provide
room for the next runoff event. Urbonas
and Ruzzo (1986) suggested a water
quality capture volume (WQCV) equal
to !h inch of nmoff from impervious
surfaces in the tributaJy watershed.
Subsequent studies of rainfall records in
the United States and field performance
ofBMPs now suggest that this WQCV
needs to be based on nmoff somewhere
between an average (i.e., mean) storm
depth (Driscoll, et aI., 1989) and the
1IlI1Ximlzed depth (GUll and Urbonas,
1996). Equation 1 is now suggested
(Urbonas, et aL, 1996a) for making the
first onler ~mAte of WQCV.

p. = a . C. p. (1)

C=Q8S&' 3 -Q7lli 2 +Q774i +QO
. . .
where(r2 =Q72) (2)

Inwhich,
A = coefficient for the maximized or
mean nmoffvolume from Figure 1
C = ""''''''''''''''s runoff coefficient found
using Equation 2
p. = average nmoff pro<l'~l1g storm
depth (0.43 incbes in the Denver Area)
Po = WQCVin inclJes
I. =1Jl00

1. = perceDl of the tota1 area covered by
impervious surfaces
The average amwa1load of tota1
suspended so1ids (TSS) in nmoff can be
~mAtM using:.

L. 02265 A" n p. E. (3)
In which,
L. = average amwa1 TSS load from the
tributaJy _r.hmP.l1( in pouuds
A. = area of tributlUy ""tr."""",,t in acres
p.. = average annual tota1 stormwater
nmoff from the ""lclllnent in incbes
n = average IDIIDber of nmoff producing
storms per year (n = 30 in Denver)
E. = average event mean concentration
(EMC) of TSS in stormwater in mgII
This annnalload of TSS, along with
the removal rates by the upstream
detentionhetention and by the filter
determines the size of a media filter.

Filter Configuratioas
Figure 2 !lr.h."...tica11y iI1ustrates
three basic arrangements of upstream
WQCV and the filter media. The
upstream WQCVequalizes stonnwater
nmoffrates to match the filteI's flow-
through capacity. When this capture
volume is exceeded by a large storm,
the excess runoff ponds on the smface
upstream of the filter, or it bypasses the
filter. In Case 1 the filter is preceded by
an extended detention basin. In Case 2
the filter is preceded by a retention pond
with a surcharge extended detention
above the permanent pool Forboth
cases the (J$ined volume is ~.~te<l
through an outlet designed to empty out
the volume over a desired time period,
name1y its drain time. !f the outlet is
oversized, the drain time is governed by
the flow-through rate of the filter itself.
This is the design condition shown as
Case 3, where at least a part of the
detenOOn volume is directly above the
filter's surface.
The detcntionlretention basin
upstream of the filter removes some of
the TSS in the nmoff. We need to
~mlIt'" how IDIIch TSS is removed this
way to know how IDIIch TSS is left for
removal by the filter. The intent of
these _mateo is to use reasonable,

11

somewhat conservative rates that will
resu1t in a realistic filter size. Table 1
provides the suggested TSS remova1
rates for desigoing media filters.
For Cases 1 and 2 defined in Figure
2 the concentration of TSS leaving the
filter fiIcili1y can be estimated using
Equation 4.

E =E .(14 -RD)
.. . 100

In which,
E.. = the dJaoge in suspended solids
concentration through the filter in mgII
RT = total system's average percent
remova1 rate of TSS (95%
recommended)
RD = the percent removal rate for the
retention or deteDtion basin upstream of
the filter bed from Table 1
For Case 3 the above analysis needs
to be modified. The water column 1hat
is above the filter's surface receives no
jl'ct.....,..oen! and all the TSSin this
water is subject to removal by the filter.
Thus, for Case 3 reduction in the EMC
of TSS by the filter inotallation can be
expressed by

E =E .[14-rR'RD]
Iff/". 100

In which,
I'R = (AW(AR+Ap), ratio of the retention
basin's surface area to the tota1 system's
surface area (When all detention storage
is above the filter, I'R = 0 and all the TSS
load is removed by the filter)
AR = surface area of the retention pond's
permanent pool in square feet
AI = surface area of the filter bed in
square feet

(4)

(5)

Filter', now Through Rate
The classic relationship for water
percolating through uniform soil media,
such as sand, breaks down for a slow
sand filter when fine ...timP.nt
llllCnmnlAtes on top of its surface. Field
observation and laboratory tests
(Neufeld, 1996; Urbonas et al., 1996b)
show that the flow-through rate for a
sand filter (and otheI" media as well)
quickly becomes a function of the
...timent being llllCnmntAt"'<l on the