Selected BMPs: Inlet Protection–Excavated Drain

Definition: An excavated drain is a type of inlet protection in which an area at the approach to a storm drain
drop inlet or curb inlet is excavated.

Purpose: An excavated drain is used to help prevent sediment from entering storm drains during the period before contributing watershed is stabilized. This practice allows uninterrupted use of the storm drain system.

Application: An excavated drain type of inlet protection may be used where storm drain drop inlets are to be made operational before the disturbed drainage area is permanently stabilized. This method of inlet protection applies where relatively heavy flows are expected and overflow capability and ease of maintenance are desired. Excavated drains must be maintained frequently and the excavated area will flood temporarily.

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.

Stone is used as the main ponding or filtering agent in many types of inlet protection. Various types of coarse aggregates are able to filter out sediment mainly by slowing down flows directed to the inlet. The flows are slowed down by creating an increased flow path for the stormwater (through void space in the respective stone). The stone-filtering medium does not slow stormwater flow rate as efficiently as filter cloth, and therefore, cannot give the same degree of filter efficiency when smaller silt and clay particles are introduced into stormwater flows. However, excessive ponding in busy areas adjacent to stormwater inlets is unacceptable.

In most instances, inlet protection using stone should not be the sole control measure. A temporary berm may need to be constructed downstream of the inlet protection device to prevent sediment from bypassing. When the storm sewer inlet and associated appurtenances become operational, areas adjacent to the structures most likely are at final grade or will not be altered for extended periods; this is the time when BMPs such as INTERIM SURFACE STABILIZATION, should be implemented to help mitigate sediment loss. In addition, by varying stone sizes used in the construction of inlet protection, more sediment can be removed. As an option, filter cloth can be used with the stone to further improve sediment removal. The potential inconvenience of excessive ponding must be examined with these choices. In designs that use stone with a wire-mesh support as a filtering mechanism, the stone can be completely wrapped with the wire mesh to improve stability and to make cleaning easier.

Filter fabric may be added to any of the devices that use coarse aggregate stone to improve sediment removal. For assistance in specifying fabric, consult Appendix E, Approach for Developing Material SpecificationsGuide to Specifying Construction Materials (Appendix E__). Because fabric increases filter efficiency significantly, a larger range of stone sizes may be used with such a configuration. Larger stone will help keep larger sediment masses from clogging the cloth.

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

• Inspect, clean, and properly maintain the excavated basin after every storm until the contributing drainage area has been permanently stabilized. To provide satisfactory basin efficiency, remove sediment when the volume of the basin has been reduced by one-half. Spread all excavated material evenly over the surrounding land area or stockpile and stabilize it appropriately.
• After the contributing drainage area has been stabilized, remove the gravel and accumulated sediment, plug the weepholes, and backfill to final grade.

Considerations: In developing areas, streets and storm sewer networks usually are installed before homes, businesses, or other developments are constructed. During this and 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.

Field experiences have shown that inlet protection that causes excessive ponding in an area of high construction activity may become so inconvenient that it is removed or bypassed, thus transmitting sediment-laden flows unchecked. In such situations, a structure with an adequate overflow mechanism should be used.

References:

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.

U.S. Department of Agriculture, Natural Resources (formerly Soil) Conservation Service. National Handbook of Conservation Practices (latest edition).

State of Illinois, Soil Conservation Service. Conservation Practice Standards. Inlet Protection–Excavated Drain, Code 85.

Specifications and Methodology: Specifications for installing excavated drain inlet protection should include the following information:

• Location of inlet
• Length, width, and depth of basin
• Detail around inlet structure
• Installation, inspection, and maintenance schedules with responsible party identified

The following design criteria should be used:

• Limit the drainage area to 1 acre. The minimum depth should be 1 foot and the maximum depth should be 2 feet as measured from the crest of the inlet structure.
• Maintain side slopes around the excavated area no steeper than 2H:1V.
• Use a minimum volume of excavated area around the drop inlet of approximately 135 cubic yards per acre of drainage area.
• Shape the basin to fit site conditions, with the longest dimension oriented toward the longest inflow area for maximum trapping efficiency. Where an inlet will receive concentrated flows, such as in a highway median, the basin should be rectangular in a 2:1 length-to-width ratio, with the length being oriented in the direction of the flow.
• Install provisions, such as weep holes, for draining the temporary pool to improve trapping efficiency for small storms and to avoid problems from standing water after heavy rains.
• To improve filtering efficiency, PADOTAASHTO No. 1 coarse aggregate (or coarser material) may be placed next to the storm drain inlet structure (PADOT specifications, Section 805).
• When coarse aggregate is used, cover the weep hole with screen wire or hardware cloth to prevent the gravel from entering the storm drain