Infiltration Trenches
Dave Briglio, P.E.
MACTEC
Mike Novotney
Center for Watershed Protection
Major Components
Infiltration Trench
1.
2.
3.
4.
5.
6.
Diversion for WQv
Stilling pool and
spreader
Sedimentation
channel or chamber
Overflow weir or
protective covering
layer
Infiltration trench with
gravel
Overflow & outlet
Pavement edges
< 5 acres drainage
≤ 2 day drawdown
No hotspot application
2-4 feet to water table –
close may need to do
mounding computation
 Setbacks for groundwater
protection
 WQv diverted into trench
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Design Steps
1.
2.
3.
4.
5.
6.
7.
8.
Compute WQv and if
applicable Cpv
Screen site
Screen local criteria
Compute Qwq
Size diversion
Size filtration area
Size pretreatment area
Size overflow




24-48 hour
drawdown
Fill time estimate of
2 hours is normal
Porosity = 0.32
If pretreatment
facility size for 25%
WQv
Gravel Trench Volume
A = WQv/(nd+kT/12)
= WQv/(0.32d+k/6)
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n=0.32
d=depth in feet
k=percolation rate
(in/hr)
T = fill time = 2
hrs.
Gravel Trench Area Per Acre
9000
Percent Impervious
Surface Area - sq ft
8000
0
7000
20
6000
40
5000
60
80
4000
100
3000
2000
1000
0
0
2
k=1 in/hr (sandy loam), n=0.32, T=2 hrs
4
6
Trench Depth - ft
8
10
12
Erosion & Sediment
Control Considerations


Ensure sediment is trapped before
entering filter area
Installation sequencing is important
– After adjacent areas are stabilized
– Converted temporary sediment basins
(Sd3)

Provide pretreatment with sediment
basin or filter strip
An example of Infiltration trench design
Taken from Appendix D4
Calculated Volumes….
Step 1. Determine if the site conditions are
appropriate
Step 1. Determine if the site conditions are
appropriate
Step 2. Calculate WQv peak discharge (Qwq)
WQv = 8102 cf, P = 1.2”, Qwv = WQv/Area = 0.74”
CN = 95…Ia=(1000/CN-10)…Ia/P…qu…Qwq
1.
Back out curve
number
½
C N = 1 0 0 0 /[1 0 + 5 P +1 0 Q w v - 1 0 (Q w v ² + 1 .2 5 Q w v P ) ]
2.
3.
Calculate unit peak
discharge using SCS
simplified peak
figures
Calculate peak
discharge as:
Q wq = qu * A * Q wv
Qwq = 2.2 cfs
Step 3. Size the infiltration trench
A = WQv/(nd+kT/12)
= WQv/(0.32d+k/6)

n=porosity
d=depth in feet
k=percolation rate
(in/hr)
T = fill time
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

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
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
n=0.32
D= 5 feet
k=1 (in/hr)
T = 2 hrs.
A=
8,102
(32.2x5)+(1x2/12)
A = 4,586 sf
Max. width = 25 ft
L = 4,586/25 = 183 ft
Step 3. Size Point A flow diversion structure
Q25 = 17cfs, WQv = 2.2 cfs
2/3 of the flow to Point A (1/3 to Point B)
Point A: Q25 = 5.7cfs, WQv = 0.73 cfs
Design orifice for low flow (0.73 cfs)
 Set max. head = 1.5’
 Q=CA(2gh)1/2…C=0.6…A=0.12 sf (4”dia.)
…use 6-inch pipe w/ gate valve
Step 3. Size Point A flow diversion structure
Q25 = 17cfs, WQv = 2.2 cfs
2/3 of the flow to Point A (1/3 to Point B)
Point A: Q25 = 5.7cfs, WQv = 0.73 cfs
Design weir for 25-yr flow (5.7 – 0.73 = 5cfs)
 Set max. head = 1.0’
 Q=CLH3/2…C = 3.1…L=1.6 ft
Coastal Challenges…
Challenges
Associated
with Using
Infiltration in Coastal GA
See
Handouts
for LID
Practices…
Site
Characteristic
Poorly
drained
soils, such as
hydrologic soil
group C and D
soils
How it Influences the
Use
of Infiltration Practices
Infiltration practices
cannot be used on
development sites that
have soils with infiltration
rates of less than 0.5
inches per hour (e.g.
hydrologic soil group C and
D soils).
Potential Solutions
Use
other low impact
development and
stormwater management
practices, such as
stormwater ponds (Section
8.4.1) and stormwater
wetlands (Section 8.4.2)
and underdrained
bioretention areas (Section
8.4.3)…
Coastal Challenges…
Challenges Associated with Using Infiltration in Coastal GA
See Handouts
for LID Practices…
How it Influences the
Site
Use
Potential Solutions
Characteristic
of Infiltration Practices
Well drained
Enhances the ability of
Use bioretention areas
soils, such as
infiltration practices to
(Section 8.4.3) or filtration
hydrologic soil
reduce stormwater runoff
practices (Section 8.4.4)
group A and B
rates, volumes and
with liners and underdrains
soils
pollutant loads, but may
at hotspot facilities and in
allow stormwater
areas with groundwater
pollutants to reach
recharge.
groundwater aquifers with In areas w/o groundwater
greater ease.
recharge, use infiltration
practices and nonunderdrained bioretention
areas (Section 8.4.3)
Coastal Challenges…
Challenges Associated with Using Infiltration in Coastal GA
See Handouts
for LID Practices…
How it Influences the
Site
Use
Potential Solutions
Characteristic
of Infiltration Practices
Flat terrain
Does not influence the use Where soils are sufficiently
of infiltration practices on
permeable, use infiltration
development and
practices and nonredevelopment sites. In
underdrained bioretention
fact, infiltration practices
areas (Section 8.4.3), to
should be designed with
significantly reduce
slopes that are as close to stormwater runoff volumes
flat as possible.
in these areas.
Coastal Challenges…
Challenges Associated with Using Infiltration in Coastal GA
See Handouts for LID Practices…
How it Influences the
Site
Use
Potential Solutions
Characteristic
of Infiltration Practices
Shallow water
May cause stormwater
Ensure at least 2 feet to
table
runoff to pond in the
the water table…
bottom of the infiltration
Use stormwater ponds
practice.
(Section 8.4.1), stormwater
wetlands (Section 8.4.2)
and wet swales (Section
8.4.6)...
Maximize the use of green
infrastructure…
Coastal Challenges…
Challenges Associated with Using Infiltration in Coastal GA
See Handouts
for LID Practices…
How it Influences the
Use
Site
Potential Solutions
Characteristic
of Infiltration
Practices
TidallyDoes not influence the
influenced
use of infiltration
drainage
practices on development
system
and redevelopment sites.
CSS Design Credits

7.4 Better Site Planning Techniques

7.5 Better Site Design Techniques

7.6 LID Practice

8.4 General Application BMPs
CSS Design Credits
Table 6.5: How Stormwater Management Practices Can Be Used to Help Satisfy the Stormwater Management Criteria
Stormwater Runoff
Reduction
Stormwater Management
Practice
Water Quality Protection
Aquatic Resource
Protection
Overbank Flood
Protection
“Credit”:
None
“Credit”:
Assume that a
stormwater pond
provides an 80%
reduction in TSS loads, a
30% reduction in TN loads
and a 70% reduction in
bacteria loads.
“Credit”:
A stormwater pond can
be designed to provide
24-hours of extended
detention for the aquatic
resource protection
volume (ARPv).
“Credit”:
A stormwater pond can
be designed to
attenuate the overbank
peak discharge (Qp25) on
a development site.
“Credit”:
A stormwater pond can
be designed to
attenuate the extreme
peak discharge (Qp100)
on a development site.
“Credit”:
None
“Credit”:
Assume that a
stormwater wetland
provides an 80%
reduction in TSS loads, a
30% reduction in TN loads
and a 70% reduction in
bacteria loads.
“Credit”:
A stormwater wetland
can be designed to
provide 24-hours of
extended detention for
the aquatic resource
protection volume (ARPv).
“Credit”:
A stormwater wetland
can be designed to
attenuate the overbank
peak discharge (Qp25) on
a development site.
“Credit”:
A stormwater wetland
can be designed to
attenuate the extreme
peak discharge (Qp100)
on a development site.
“Credit”:
Subtract 100% of the
storage volume provided
by a non-underdrained
bioretention area from
the runoff reduction
volume (RRv) conveyed
through the bioretention
area.
“Credit”:
Assume that a
bioretention area
provides an 80%
reduction in TSS loads, an
80% reduction in TN loads
and a 90% reduction in
bacteria loads.
“Credit”:
Although uncommon, on
some development sites,
a bioretention area can
be designed to provide
24-hours of extended
detention for the aquatic
resource protection
volume (ARPv).
“Credit”:
Although uncommon, on
some development sites,
a bioretention area can
be designed to
attenuate the overbank
peak discharge (Qp25).
“Credit”:
Although uncommon, on
some development sites,
a bioretention area can
be designed to
attenuate the extreme
peak discharge (Qp100).
Extreme Flood Protection
General Application Practices
Stormwater Ponds
Stormwater Wetlands
Bioretention Areas,
No Underdrain