Stormwater
Detention Volume

BMP Water
Quality Volume

Two Perforated

Aggregate
“Bottom Sand Filter

Overflow Collection
Lines

= — —F¥#
£ Sediment g4,<

WS ASS 4

se

Three Corrugated
Metal Perforated
Pipes

- Geotextile Filter

==

Fabric (Mirafi 700X

ine

or Equivalent)

Source: Fairfax County Soils Office, 1991.

FIGURE 2 INFILTRATION TRENCH WITH CONCENTRATED INPUT AND AUGMENTED PIPE
STORAGE

The drainage area slope determines the velocity of
the runoff and also influences the amount of
pollutants entrained in the runoff. Infiltration
trenches work best when the upgradient drainage
area slope is less than 5 percent (SEWRPC, 1991).
The downgradient slope should be no greater than
20 percent to minimize slope failure and seepage.

The trench surface may consist of stone or
vegetation with inlets to evenly distribute the runoff
entering the trench (SEWRPC, 1991, Harrington,
1989). Runoff can be captured by depressing the
trench surface or by placing a berm at the down
gradient side of the trench.

The basic infiltration trench design utilizes stone
aggregate in the top of the trench to promote
filtration; however, this design can be modified by
substituting pea gravel for stone aggregate in the
top 0.3 meter (1 foot) of the trench. The pea gravel
improves sediment filtering and maximizes the
pollutant removal in the top of the trench. When
the modified trenches become clogged, they can
generally be restored to full performance by
removing ard replacing only the pea gravel layer,
without replacing the lower stone aggregate layers.

Infiltration trenches can also be modified by adding
a layer of organic material (peat) or loam to the
trench subsoil. This modification appears to
enhance the removal of metals and nutrients through
adsorption. The trenches are then covered with an
impermeable geotextile membrane overlain with
topsoil and grass (Figure 2).

A vegetated buffer strip (6.1 to 7.6 meters, or 20-
25 feet, wide) should be established adjacent to the
infiltration trench to capture large sediment particles
in the runoff. The buffer strip should be installed
immediately after trench construction using sod
instead of hydroseeding (Schueler, 1987). The
buffer strip should be graded with a slope between
0.5 and 15 percent so that runoff enters the trench
as sheet flow. If runoff is piped or channeled to the
trench, a level spreader must be installed to create
sheet flow (Harrington, 1989). .

During excavation and trench construction, only
light equipment such as backhoes or wheel and
ladder type trenchers should be used to minimize
compaction of the surrounding soils. Filter fabric
should be placed around the walls and bottom of the
trench and 0.3 meters (1 foot) below the trench


5. Northern Virginia Planning District
Commission (NVPDC) and Engineers and
Surveyors Institute, 1992. Northern
Virginia BMP Handbook: A Guide to
Planning and Designing Best Management
Practices in Northern Virginia.

6. Schueler, T.R., 1987. Controlling Urban
Runoff: A Practical Manual for Planning
and Designing Urban Best Management
Practices. Metropolitan Washington
Council of Governments.

7. Schueler, T.R., 1992. A Current
Assessment of Urban Best Management
Practices. Metropolitan Washington
Council of Governments.

8. Southeastern Wisconsin Regional Planning
Commission (SEWRPC), 1991. Costs of
Urban Nonpoint Source Water Pollution
Control Measures. Technical Report No.
31.

9. U.S. EPA, 1991. Detention and Retention
Effects on Groundwater, Region V.

10. Washington, State of, 1992. Storm Water
Management Manual for the Puget Sound
Basin (The Technical Manual), Department
of Ecology.

ADDITIONAL INFORMATION

City of Alexandria, Virginia

Warren Bell

Department of Transportation and Environmental
Services

P.O. Box 178

Alexandria, VA 22313

Carroll County, Maryland

Martin Covington

Bureau of Developmental Review
225 North Center Street
Westminster, MD 21157-5194

King County, Washington

Dave Hancock

Department of Natural Resources, Water and Land
Resources Division, Drainage Services Section
700 5" Avenue, Suite 2200

Seattle, WA 98104

Montgomery County, Maryland

Rick Brush

Department of Permitting Services, Water Resource
Section

250 Hungerford Drive, Suite 175

Rockville, MD 20850

Southeastern Wisconsin Regional
Commission

Bob Biebel

916 N. East Avenue, P.O. Box 1607
‘Waukesha, WI 53187

Planning

The mention of trade names or commercial products
does not constitute endorsement or recommendation
for the use by the U.S. Environmental Protection
Agency.

For more information contact:

Municipal Technology Branch
U.S. EPA

Mail Code 4204

401 M St., S.W.

Washington, D.C., 20460

fe)
Exclence th complree through opt tec sky
MUNICIPAL TECHNOLOGY Trance


or in areas with fill. The trench should be located
well above the water table so that the runoff can
filter through the trench and into the surrounding
soils and eventually into the groundwater. In
addition, the drainage area should not convey heavy
levels of sediments or hydrocarbons to the trench.
For this reason, trenches serving parking lots must
be preceded by appropriate pretreatment such as an
oil-grit separator. This measure will make effective
maintenance feasible. Generally, trenches that are
constructed under parking lots must provide access
for maintenance.

An additional limitation on use of infiltration
trenches is the climate. In cold climates, the trench
surface may freeze, thereby preventing the runoff
from entering the trench and allowing the untreated
runoff to enter surface water. The surrounding soils
may also freeze, reducing infiltration into the soils
and groundwater. However, recent studies indicate
that if properly designed and maintained, infiltration
trenches can operate effectively in colder climates.
By keeping the trench surface free of compacted
snow and ice, and by ensuring that part of the trench
is constructed below the frost line, the performance
of the infiltration trench during cold weather will be
greatly improved.

Finally, there have been a number of concerns raised
about the long term effectiveness of infiltration
trench systems. In the past, infiltration trenches
have demonstrated a relatively short life span, with
over 50 percent of the systems checked having
partially or completely failed after 5 years. A recent
study of infiltration trenches in Maryland (Lindsey
et al, 1991) found that 53 percent were not
operating as designed, 36 percent were partially or
totally clogged, and another 22 percent exhibited
slow filtration. Longevity can be increased by
careful geotechnical evaluation prior to construction
and by designing and implementing an inspection
and maintenance plan. Soil infiltration rates and the
water table depth should be evaluated to ensure that
conditions are satisfactory for proper operation of
an infiltration trench. Pretreatment structures, such
as a vegetated buffer strip or water quality inlet, can
increase longevity by removing sediments,
hydrocarbons, and other materials that may clog the
trench. Regular maintenance, including the

replacement of clogged aggregate, will also increase
the effectiveness and life of the trench.

DESIGN CRITERIA

Prior to trench construction, a review of the design
plans may be required by state and local
governments. The design plans should include a
geotechnical evaluation that determines the
feasibility of using an infiltration trench at the site.
Soils should have a low silt and clay content and
have infiltration rates greater than 1.3 centimeters
(0.5 inches) per hour. Acceptable soil texture
classes include sand, loamy sand, sandy loam and
loam. These soils are within the A or B hydrologic
group. Soils in the C or D hydrologic groups
should be avoided. Soil survey reports published by
the Soil Conservation Service can be used to
identify soil types and infiltration rates. However,
sufficient soil borings should always be taken to
verify site conditions. Feasible sites should have a
minimum of 1.2 meters (4 feet) to bedrock in order
to reduce excavation costs. There should also be at
least 1.2 meters (4 feet) below the trench to the
water table to prevent potential ground water
problems. Trenches should also be located at least
30.5 meters (100 feet) upgradient from water supply
wells and 30.5 meters (100 feet) from building
foundations. Land availability, the depth to
bedrock, and the depth to the water table will
determine whether the.infiltration trench is located
underground or at grade. Underground trenches
receive runoff through pipes or channels, whereas
surface trenches collect sheet flow from the
drainage area.

In general, infiltration trenches are suitable for
drainage areas up to 4 hectares (10 acres)
(SEWRPC, 1991, Harrington, 1989). However,
when the drainage area exceeds 2 hectares (5 acres),
other BMPs should be carefully considered. The
drainage area must be fully developed and stabilized
with vegetation before constructing an infiltration
trench. High sediment loads from unstabilized areas
will quickly clog the infiltration trench. Runoff from
unstabilized areas should be diverted away from the
trench into a construction BMP until vegetation is
established.


surface. The filter fabric should overlap each side of
the trench in order to cover the top of the stone
aggregate layer (see Figure 1). The filter fabric
prevents sediment in the runoff and soil particles
from the sides of the trench from clogging the
aggregate. Filter fabric that is placed 0.3 meters (1
foot) below the trench surface will maximize
pollutant removal within the top layer of the trench
and decrease the pollutant loading to the trench
bottom, reducing frequency of maintenance.

The required trench volume can be determined by
several methods. One method calculates the volume
based on capture of the first flush, which is defined
as the first 1.3 centimeters (0.5 inches) of runoff
from the contributing drainage area (SEWRPC,
1991). The State of Maryland (MD., 1986) also
recommends sizing the trench based on the first
flush, but defines first flush as the first 1.3
centimeters (0.5 inches) from the contributing
impervious area. The Metropolitan Washington
Council of Governments (MWCOG) suggests that
the trench volume be based on the first 1.3
centimeters (0.5 inches) per impervious acre or the
runoff produced from a 6.4 centimeter (2.5 inch)
storm. In Washington D.C., the capture of 1.3
centimeters (0.5 inches) per impervious acre
accounts for 40 to 50 percent of the annual storm
runoff volume. The runoff not captured by the
infiltration trench should be bypassed to another
BMP (Harrington, 1989) if treatment of the entire
runoff from the site is desired.

Trench depths are usually between 0.9 and 3.7
meters (3 and 12 feet) (SEWRPC, 1991,
Harrington, 1989). However, a depth of 2.4 meters
(8 feet) is most commonly used (Schueler, 1987).
A site specific trench depth can be calculated based
on the soil infiltration rate, aggregate void spacey
and the trench storage time (Harrington, 1989).
The stone aggregate used in the trench is normally
2.5 to 7.6 centimeters (1 to 3 inches) in diameter,
which provides a void space of 40 percent
(SEWRPC, 1991, Harrington, 1989, Schueler,
1987).

A minimum drainage time of 6 hours should be
provided to ensure satisfactory pollutant removal in
the infiltration trench (Schueler, 1987, SEWRPC,
1991). Although trenches may be designed to

provide temporary storage of storm water, the
trench should drain prior to the next storm event.
The drainage time will vary by precipitation zone.
In the Washington, D.C. area, infiltration trenches
are designed to drain within 72 hours.

An observation well is recommended to monitor
water levels in the trench. The well can be a 10.2 to
15.2 centimeter (4 to 6 inch) diameter PVC pipe,
which is anchored vertically to a foot plate at the
bottom of the trench as shown in Figure 1 above.
Inadequate drainage may indicate the need for
maintenance.

PERFORMANCE

Infiltration trenches function similarly to rapid
infiltration systems that are used in wastewater
treatment. Estimated pollutant removal efficiencies
from wastewater treatment performance and
modeling studies are shown in Table 1.

Based on this data, infiltration trenches can be
expected to remove up to 90 percent of sediments,
metals, coliform bacteria and organic matter, and up
to 60 percent of phosphorus and nitrogen in the
runoff (Schueler, 1992). Biochemical oxygen
demand (BOD) removal is estimated to be between
70 to 80 percent. Lower removal rates for nitrate,
chlorides and soluble metals should be expected,

TABLE 1 TYPICAL POLLUTANT

REMOVAL EFFICIENCY
Pollutant Typical Percent
Removal Rates
Sediment . 90%
Total Phosphorous 60%
Total Nitrogen 60%
Metals 90%
Bacteria 90%
Organics 90%

Biochemical Oxygen 70-80%
Demand

re

Source: Schueler, 1992.


United States Office of Water EPA 832-F-99-019
Environmental Protection Washington, D.C. September 1999
Agency

EPA Storm Water
Technology Fact Sheet
Infiltration Trench


especially in sandy soils (Schueler, 1992).

Pollutant removal efficiencies may be improved by
using washed aggregate and adding organic matter
and loam to the subsoil. The stone aggregate
should be washed to remove dirt and fines before
placement in the trench. The addition of organic
material and loam to the trench subsoil will enhance
metals and nutrient removai through adsorption.

OPERATION AND MAINTENANCE

Infiltration, as with all BMPs, must have routine
inspection and maintenance designed into the life
performance of the facility. Maintenance should be
performed as indicated by these routine inspections.
The principal maintenance objective is to prevent
clogging, which may lead to trench failure.
Infiltration trenches and any pretreatment BMPs
should be inspected after large storm events and any
accumulated debris or material removed. A more
thorough inspection of the trench should be
conducted at least annually. Annual inspection
should include monitoring of the observation well to
confirm that the trench is draining within the
specified time. Trenches with filter fabric should be
inspected for sediment deposits by removing a small
section of the top layer. If inspection indicates that
the trench is partially or completely clogged, it
should be restored to its design condition.

When vegetated buffer strips are used, they should
be inspected for erosion or other damage after each
major storm event. The vegetated buffer strip
should have healthy grass that is routinely mowed.
Trash, grass clippings and other debris should be
removed from the trench perimeter and should be
disposed properly. Trees and other large vegetation
adjacent to the trench should also be removed to
prevent damage to the trench.

COSTS

Construction costs include clearing, excavation,
placement of the filter fabric and stone, installation
of the monitoring well, and establishment of a
vegetated buffer strip. Additional costs include
planning, geotechnical evaluation, engineering and
permitting. The Southeastern Wisconsin Regional
Planning Commission (SEWRPC, 1991) has

developed cost curves and tables for infiltration
trenches based on 1989 dollars. The 1993
construction cost for a relatively large infiltration
trench (i.e., 1.8 meters (6 feet) deep and 1.2 meters
(4 feet) wide with a 68 cubic meter (2,400 cubic
feet) volume) ranges from $8,000 to $19,000. A
smaller infiltration trench (i.e., 0.9 meters (3 feet)
deep and 1.2 meters (4 feet) wide with a 34 cubic
meter (1,200 cubic feet) volume) is estimated to
cost from $3,000 to $8,500.

Maintenance costs include buffer strip maintenance
and trench inspection and rehabilitation. SEWRPC
(1991) has also developed maintenance costs for
infiltration trenches. Based on the above examples,
annual operation and maintenance costs would
average $700 for the large trench and $325 for the
small trench. Typically, annual maintenance costs
are approximately 5 to 10 percent of the capital cost
(Schueler, 1987). Trench rehabilitation, may be
required every 5 to 15 years. Cost for rehabilitation
will vary depending on site conditions and the
degree of clogging. Estimated rehabilitation costs
run from 15 to 20 percent of the original capital
cost (SEWRPC, 1991).

REFERENCES

1. Fugill, R., 1991-1992. Fairfax County Soil
Science Office. Personal communication
with Lauren Fillmore, Parsons Engineering
Science, Inc.

2. Harrington, B.W., 1989. Design and
Construction of Infiltration Trenches in
Design of Urban Runoff Quality Control.
American Society of Civil Engineers.

3. Lindsey, G., L. Roberts, and W. Page, 1991.
Storm Water Management Infiltration.
Maryland Department of the Environment,
Sediment and Storm Water Administration.

4. Maryland Department of Natural Resources,
1986. Minimum Water Quality Objectives
and Planning Guidelines for Infiltration
Practices. Water Resources Administration,
Sediment and Storm Water Division.


United States
Environmental Protection
Agency

EPA 832-F-99-019
September 1999

Office of Water
Washington, D.C.

SEPA

Storm Water

Technology Fact Sheet
Infiltration Trench

DESCRIPTION

Urban development is significantly increasing
surface runoff and contamination of local
watersheds. As a result, infiltration practices, such
as infiltration trenches, are being employed to
remove suspended solids, particulate pollutants,

coliform bacteria, organics, and some soluble forms
of metals and nutrients from storm water runoff. As
shown in Figure 1, an infiltration trench is an
excavated trench, 0.9 to 3.7 meters (3 to 12 feet)
deep, backfilled with a stone aggregate, and lined
with filter fabric. A small portion of the runoff,
usually the first flush, is diverted to the infiltration

3 Ft.

9 INCH SQUARE STEEL FOOT PLATE

r-)
41 Ft. Gl Ba a
iN
GEOTEXTILE e
FILTER FABRIC we
7 o>
UNDISTURBED SOIL
&
MINIMUM INFILTRATION RATE s SP
OF 0.50 INCH PERHOUR * K
NY
TWAIN,

REMOVABVLE
WELL CAP
A
o& K
° a
°
°
of & ~ 4 FT, DEEP TRENCH
5 FILLED WITH 1-3 INCH
4 CLEAN STONE
° 6 INCH
° DIAMETER
© BK pvceire
og a.
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Ad

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1/2 INCH DIAMETER REBAR ANCHOR

Source: Reference 6

Source: Southeastern Wisconsin Regional Planning Commission, 1991.

FIGURE 1 TYPICAL INFILTRATION TRENCH


trench, which is located either underground or at
grade. Pollutants are filtered out of the runoff as it
infiltrates the surrounding soils. Infiltration trenches
also provide groundwater recharge and preserve
baseflow in nearby streams.

APPLICABILITY

Infiltration trenches are often used in place of other
Best Management Practices where limited land is
available. Infiltration trenches are most widely used
in warmer, less arid regions of the U.S. However,
recent studies conducted in Maryland and New
Jersey on trench performance and operation and
maintenance have demonstrated the applicability of
infiltration trenches in colder climates if surface
icing is avoided (Lindsey, et al, 1991).

Infiltration trenches capture and treat small amounts
of runoff, but do not control peak hydraulic flows.
Infiltration trenches may be used in conjunction with
another Best Management Practice (BMP), such as
a detention pond, to provide both water quality
control and peak flow control (Harrington, 1989).
Figure 2 is an example of such a combined
technology. This type of infiltration trench has a
concentrated input, as opposed to dispersed input
(as shown in Figure 1). This system stores the
entire storm water volume with the water quality
(BMP) volume connected to the infiltration system.
This is commonly achieved with a slow release of
the storm water management volume through an
orifice set at a specified level in the storage facility.
As a result the BMP water quality volume will equal
the storm water detention area below the orifice
level which must infiltrate to exit.

Runoff that contains high levels of sediments or
hydrocarbons (oil and grease) that may clog the
trench are often pretreated with other BMPs.
Examples of some pretreatment BMPs include grit
chambers, water quality inlets, sediment traps,
swales, and vegetated filter strips (SEWRPC, 1991,
Harrington, 1989).

ADVANTAGES AND DISADVANTAGES

Infiltration trenches provide efficient removal of
suspended solids, particulate pollutants, coliform
bacteria, organics and some soluble forms of metals
and nutrients from storm water runoff. The
captured runoff infiltrates the surrounding soils and
increases groundwater recharge and baseflow in
nearby streams. :

Negative impacts include the potential for
groundwater contamination and a high likelihood of
early failure if not properly maintained.

As with any infiltration BMP, the potential for
groundwater contamination must be carefully
considered, especially if the groundwater is used for
human consumption or agricultural purposes. The
infiltration trench is not suitable for sites that use or
store chemicals or hazardous materials unless
hazardous and toxic materials are prevented from
entering the trench. In these areas, other BMPs that
do not interact with the groundwater should be
considered. The potential for spills can be
minimized by aggressive pollution prevention
measures. Many municipalities and industries have
developed comprehensive spill prevention control
and countermeasure (SPCC) plans. These plans
should be modified to include the infiltration trench
and the contributing drainage area. For example,
diversion structures can be used to prevent spills
from entering the infiltration trench.

Because of the potential to contaminate
groundwater, extensive site investigation must be
undertaken early in the site planning process to
establish site suitability for the installation of an
infiltration trench. The use of infiltration trenches
may be limited by a number of factors, including
type of native soils, climate, and location of
groundwater tables. Site characteristics, such as
excessive slope of the drainage area, fine-particled
soil types, and proximate location of the water table
and bedrock, may preclude the use of infiltration
trenches. The slope of the surrounding area should
be such that the runoff is evenly distributed in sheet
flow as it enters the trench unless specifically
designed for concentrated input. Generally,
infiltration trenches are not suitable for areas with
relatively impermeable soils containing clay and silt