Selected BMPs: Inlet Protection–Fabric Insert

Definition: A fabric insert used as inlet protection drop used as inlet protection is a temporary fabric sack suspended inside a catchbasin. barrier placed around a drop inlet.

Purpose: The purpose of this practice is to help prevent sediment from entering storm drains during construction operations. This practice allows early use of the storm drainage system.

Application: A fabric insertdrop type of inlet protection may be used where storm drain inlets are to be made operational before permanent stabilization of the disturbed drainage area. This method of inlet protection is effective for areas with vehicular traffic, in particular, because the fabric insert is located inside the catchbasin. Fabric inserts must be maintained frequently.where the inlet drains a small, nearly level area with slopes generally less than 5 percent and where shallow sheet flows not exceeding 1 cubic feet per second (cfs) are expected. The immediate land area around the inlet should be relatively flat (less than a 1 percent slope) and located so that accumulated sediment can be removed easily. This method should not be used in areas receiving concentrated flows, such as in street or highway medians.

Recommended Design Criteria:

Requirements for Regulatory Compliance

(none specified)

Performance-Based Guidelines

Storm drain inlet protection consists of several types of inlet filters and traps. Each type differs in application depending on site conditions and type of inlet. Not all designs are appropriate in all cases. The user must select a design suitable for the needs and site conditions carefully.

Inlet protection devices are used for drainage areas of less than 1 acre. Runoff from areas of more than 1 acre should be routed through a properly designed practice, such as SEDIMENT BASIN or TEMPORARY SEDIMENT TRAP.

In some instances, a wire mesh may be needed to reinforce the fabric and supporting posts. Reinforcement should be used in areas where concentrated flows may occur or where timely maintenance may be a concern. If wire mesh is used, it should have a maximum opening of 6 inches.

A temporary berm may need to be constructed downstream of the inlet protection device to prevent sediment from bypassing.

The best way to prevent sediment from entering the storm sewer system is to stabilize the disturbed area of the site as quickly as possible, preventing erosion and stopping sediment at its source.

Operation and Maintenance: The following operation and maintenance guidelines should be followed:

• Inspect the fabric insert barrier after each rain event and make repairs as necessary.
• If there have been no major rain events, inspect the fabric insert every 2 to -3 weeks.
• Clean out the filter insert before it fills halfway.
• Remove sediment from the insertpool, as necessary, to provide adequate storage volume and filtering capacity for the next rain event. Take care not to damage or undercut the fabric when removing sediment.
• When the contributing drainage area has been adequately stabilized, remove all materials and unstable sediment and dispose of them properly. Bring the disturbed area to the grade of the drop inlet and smooth and compact it. Appropriately stabilize all bare areas around the inlet.

Considerations: In developing areas, streets and storm sewer networks usually are installed before homes, businesses, or other developments are constructed. While the streets and storm sewers are being constructed and during subsequent phases of construction, unprotected soil is susceptible to erosion. Storm sewers that are operational before their drainage areas are stabilized often carry large amounts of sediment to lakes, detention ponds, streams, or other natural or constructed drainageways. As a result, the water quality of the receiving body of water is detrimentally affected. In cases of extreme sediment loading, the storm sewer may clog completely or lose most of its capacity. To avoid these problems, sediment must be prevented from entering the system at the inlets.

References:

Atlantic Construction Fabrics, Siltsack product literature, Richmond, VA.

Commonwealth of Massachusetts, Department of Environmental Protection. Nonpoint Source Management Manual. Publication No. 17356-500-500G/93-67.00. Boutiette, L., and C. Duerring, authors. 1994.

State of Illinois, Soil Conservation Service. Conservation Practice Standards. Inlet Protection–Fabric Drop, Code 860.

Specification and Methodology: The plans and specifications for installing fabric insertdrop inlet protection must include the following information:

• Location of inlet

• Location of inlet
• Fabric material requirements
• Detail around inlet structure
• Installation, inspection, and maintenance schedules with responsible party identified

The following design criteria should be used:

• Maximum drainage area that is tributary to the catchment should not exceed 1 acre.
• Minimum depth of 3 feet is recommended for the filter insert to provide enough volume for filtering and sediment storage.

The maximum drainage area must not exceed 1 acre per inlet.

A minimum depth of 3 feet is recommended for the filter insert to provide enough volume for filtering and sediment storage.The maximum height of fabric above the crest of the drop inlet must be 1.5 feet. This height allows a shallow temporary desilting pool to form behind the fabric but limits the pressure against the fabric if overtopping occurs. The selected height of the top of the barrier should allow overflow to the drop inlet and not let overflow bypass the inlet to unprotected lower areas. A temporary dike may need to be built on the downslope side of the structure to prevent flows from bypassing.

For fabric barriers, use stakes of 2- by 4-inch wood (preferred) or equivalent metal with a minimum length of 3 feet. Space the stakes a maximum of 3 feet apart, and securely drive them into the ground to a depth of approximately 18 inches.

Drive the stakes close to the drop inlet so that overflow will fall directly into the structure and not on unprotected soil.

For added stability to the installation, make a frame around the stakes a maximum of 1.5 feet above the top of the drop inlet. The frame will serve as a stable crest for overflow during rainfall. Place the bottom 12 inches of the fabric in a trench and backfill the trench with 12 inches of compacted soil or 6 inches of crushed gravel.

Fasten fabric securely by staples or wire to the stakes and frames. Joints should be overlapped to the next stake.

Performance can be improved and sediment storage volume increased by excavating the area. See BMP description for INLET PROTECTION—EXCAVATED DRAIN.

The fabric must meet the requirements as shown in Material Specification ____ with an equivalent opening size (EOS) of at least 30 for nonwoven fabric and 50 for woven fabric.