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Inlet Protection

General

Stormwater inlet protection devices are temporary practices designed to prevent suspended sediment from entering drainage systems such as culverts, catch basins, and storm sewers. These devices reduce the velocity of runoff by forcing it through a filtering device, temporarily ponding the water and encouraging suspended particles to settle out. Inlet protection is critical because it is often the last treatment measure that the stormwater receives before it enters receiving waters.

Stormwater inlet protection devices are commonly used on sites where storm water drainage systems are operational prior to the permanent stabilization of the site. These devices vary according to the individual needs and characteristics of the site. Whichever type of protection is selected, it must be able to handle the runoff from the 10-year, 24-hour design storm.

Due to their temporary nature, stormwater inlet protection devices require frequent maintenance and are best used in conjunction with other management practices.

Advantages

  • Reduces the amount of sediment leaving the site
  • Easy to construct

Disadvantages

  • Requires frequent maintenance to prevent clogging
  • Ineffective for small particles and during large storm events
  • May cause flooding
  • Ineffective for drainage areas larger than 1 acre

Design

Framed Inlet Protection

Framed inlet protection has a rigid frame and attached fabric bag that sits completely below the inlet grate. Products are available for most standard sized curb and field inlets. This style is an improvement on Type-D inlet protection, with easier installation, more effective sediment trapping and less maintenance required.

Installed products must meet the following specifications:

  • All ASTM Standard D8057-17 requirements, including:
    • Bypass overflow that meets or exceeds inlet design flow
    • Frame and bag strong enough to handle full sediment load
  • No part of inlet protection projecting above the grate (e.g. bag fabric)
  • For combination inlets, capture of runoff entering both grate and curb opening
  • “Dual fabric” filter bag, with nonwoven bottom and woven top

Type A Inlet Protection

Wisconsin DNR Type A inlet protection is applicable to field inlets surrounded by unpaved surfaces. A frame (typically from 2" x 4" lumber) is constructed and staked to completely surround the inlet, with Type FF fabric attached and trenched into the ground.

Block and Gravel Drop Inlet Sediment Filter

Block and gravel sediment filters are applicable on a wide variety of sites with drainage areas of 1 acre or less, particularly those on which heavy volumes of runoff are expected. They may also be used when an overflow capacity is necessary to prevent excessive ponding on the site.

These structures are constructed with concrete blocks, laid lengthwise on their sides with a height of 12-24 inches. The blocks should be wrapped with a wire mesh, overlain with non-woven, pervious geotextile fabric, to prevent stone from washing through the openings in the block, which should be supported by 2 x 4 inch wood studs placed through the outer holes in the block. Geotextile fabric should be placed around the perimeter of the structure, extending at least 12 inches from the block. 1-3 inch washed stone should be placed on top of the fabric and against the block. The stone should be no more than 2 inches from the top of the concrete block with a slope of 2:1 or flatter.

Source: Adapted from Virginia Department of Environmental Quality

Excavated Drop Inlet Sediment Trap

Excavated drop inlet sediment traps are applicable for sheet flow on sites with drainage areas no larger than 1 acre. These devices consist of a rectangular shaped basin that surrounds the inlet with additional drainage openings for dewatering. The inlet is encompassed by 12 inches of ¾ to 1-inch stone with a slope of 3:1 or flatter, supported with a wire mesh and underlain with geotextile fabric.

Basin size and shape will depend upon individual site characteristics, but must be capable of handling the runoff from the 10-year, 24-hour storm event. The basin should have a depth of 1-2 feet (measured from the top of the drain structure) with a minimum storage capacity calculated at a rate of 67 cubic yards per acre. Side slopes should be designed 2:1 or flatter. The drainage openings in the inlet structure should be sized so that the basin will drain completely within 24 hours of the designed storm. These openings should be closed once construction is complete

The basin should be lined with erosion matting or vegetation tolerant of frequent inundation to prevent erosion of the basin. Native vegetation is encouraged; however, species should be carefully selected to ensure that exotic and invasive species are not planted.

Cross-Section of Excavated Drop Inlet Sediment Trap

Source: Adapted from Virginia Department of Environmental Quality

Manufactured Devices

Several types of pre-manufactured stormwater inlet protection devices are available for purchase and Dane County does not endorse any particular product. The products that have been included do not constitute a complete list of the products that are available for this purpose; rather, they represent a sample of pre-manufactured alternatives that are available for use on a case-by-case basis.

Construction

  • Stormwater inlet protection should be installed before the drain becomes operational or site disturbance occurs
  • Stormwater inlet protection should be removed after the site has been permanently stabilized and permanent BMPs have been implemented

Maintenance

  • Stormwater inlet protection devices must be inspected and repaired after each rainfall event
  • Accumulated sediment must be removed when it reaches ½ the original design depth
  • Sediment that is removed shall be placed in an area that is not susceptible to erosion
  • To prevent clogging, filter media should be cleaned or replaced periodically

Method to Determine Practice Efficiency

Stormwater inlet protection reduces the amount of suspended sediment in stormwater by reducing the flow velocity of water. The efficiencies for these practices vary by the type of device used and by manufacturer. Devices that are constructed on site will have an efficiency that is determined by calculating the settling efficiency for the device.

References