Inlet protection is part of a system of sediment control devices and works best when upslope sources of sediment are reduced. For information on controlling upslope sources of sediment, see the following.
Inlet protection devices intercept and/or filter sediment before it can be transported from a site into the storm drain system and discharged into a lake, river, stream, wetland, or other waterbody. These devices also keep sediment from filling or clogging storm drain pipes, ditches, and downgradient sediment traps or ponds. Inlet protection may also include placement of a barrier to create a bypass of an inlet transferring flow downstream to a sediment trap, basin, or other inlet discharging to a non-critical area.
Inlet protection devices may be applied to any curb inlet, drop inlet, manhole, catch basin, or other entry point to the stormwater drainage system that might receive inflows with high sediment levels. Inlet protection is critical because it often is the last treatment measure before stormwater enters receiving waters.
Construction sites often drain toward the inlets, pipes, and ditches that comprise the stormwater drainage system. Inlet protection devices can help protect these surface and subsurface systems from high levels of sediment in runoff. Inlet protection in areas with only surface drainage ditches are usually protected by stabilization practices, perimeter controls, and sediment traps. Consider the following when evaluating the use of inlet protection for a site.
The 2018 MPCA Construction Stormwater General Permit states: Permittees must establish sediment control BMPs on all downgradient perimeters of the site and downgradient areas of the site that drain to any surface water, including curb and gutter systems. Permittees must locate sediment control practices upgradient of any buffer zones. Permittees must install sediment control practices before any upgradient land-disturbing activities begin and must keep the sediment control practices in place until they establish permanent cover.
See Section 9 of the Construction Stormwater permit for sediment control practice permit requirements.
Properly selected, installed, and maintained inlet protection devices can remove 25 to 35 percent of total solids and 15 to 25 percent of nutrients from incoming flows (Florida DOT, 2010). They work very well in keeping sandy and some silty soils out of storm drains, but have somewhat limited effectiveness with fine clay soils passing through the barrier. Supplementing downgradient inlet protection devices with upgradient soil stabilization and perimeter controls increases in importance as site soils become finer (e.g. clays). Filter fabric can be added to inlet protection devices using coarse stone or aggregate to enhance sediment removal. However, the possibility of increased ponding should be considered. The following table summarizes expected performance for an array of typical water quantity and quality target constituents for storm drain inlet protection BMPs that are properly designed, installed, and maintained.
Expected performance for storm drain inlet protection
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|Flow attenuation||Little or no design benefit|
|Runoff volume reduction||Little or no design benefit|
|Soil erosion||Little or no design benefit|
|Sediment control||Primary design benefit|
|Nutrient loading||Secondary design benefit|
|Total suspended solids||Primary design benefit|
|Total phosphorus||Secondary design benefit|
|Heavy metals||Secondary design benefit|
|Floatables||Little or no design benefit|
|Oil and grease||Little or no design benefit|
Install inlet protection devices prior to upslope soil disturbance occurs. In newly developed areas, inlet protection should be installed immediately after the storm sewer inlets are installed. It is critical that the storm sewer inlet not be completely blocked by inlet protection when public safety is a concern. Inlet protection should only be used in locations where sediment can be removed and temporary ponding will not create a public safety hazard or cause property damage. Blocking an inlet can cause streets to flood and sediment to build up. Select inlet protection devices that do not pond water where adjacent areas are open to traffic. Examples of these include low-profile filter berms and filters suspended in the inlet structure. Because inlet protection devices are high-maintenance BMPs, ensure that up-gradient soil stabilization, perimeter protection, and other erosion control practices are in place to minimize the amount of sediment in stormwater flowing to inlets.
Performance of all types of inlet protection depends on proper installation and use of materials. Materials should be checked for conformance with applicable MnDOT specifications. Inlet protection devices can be fabricated from clean aggregate, filter fabric, or other materials, or an appropriate device can be selected from the many commercial products available for construction site stormwater management. As noted above, devices that are bulky (e.g., concrete block) or that result in deeply ponded water and sediment deposits (e.g., rock and other berms) may not be appropriate for inlets on or immediately adjacent to public roadways.
Inlet protection can include
MnDOT classifies protection devices for field inlets, curb inlets, curb inlets without a curb head, and culvert inlets. When using manufactured inlet protection products, the MnDOT approved/qualified products list should be consulted to determine the acceptable materials and products to protect each type of inlet. The current list of approved/qualified products for inlet protection can be found on the MnDOT website.
Regardless of the type of inlet protection BMP selected, there are a few key design guidelines.
MnDOT Standard Plan 5-297.405 provides standard detail for storm drain inlet protection (effective date: 8/6/2014). Specific BMP types covered include “Filter Bag Insert”, “Rock Log/Compost Log”, “Pop-Up Head”, “Sediment Control Inlet Hat”, and “Silt Fence Ring and Rock Filter Berm”.
MnDOT Specification 2573.3.M (Storm Drain Inlet Protection) provides general guidance for implementation and maintenance of inlet protection BMPs. Specifications 3137 (Coarse Aggregate) and 3886 (Silt Fence) may also be applicable depending on the selected design. See the 2016 edition of the MnDOT Standard Specifications for Construction (page 494.)
Storm drain inlet protection is only as effective as the filter or barrier used around the inlet. Properly maintaining inlet protection is difficult and often inlets become clogged, causing erosion elsewhere. A typical inspection should include the following bulleted list. An example inspection form is illustrated on the right.
Inlet protection devices are high-maintenance BMPs, and typically require maintenance after nearly every precipitation event. Typical maintenance activities are summarized below.
The following table summarizes MnDOT’s workmanship and rework schedule (2016; version under development at the time of manual update). The table identifies common deficiencies for various types of temporary sediment control BMPs, including storm drain inlet protection, and corrective actions for these deficiencies. Once complete, the full, final version of this table will replace Table 2573-1 in MnDOT Standard Specifications for Construction (2016 edition).
Temporary sediment control: corrective actions
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|Item||Corrective Action Required if||Corrective Action1|
|Storm drain inlet protection||
1The number corresponds with the same number in the column "Corrective Action Required"
The following table summarizes estimated BMP costs based on MnDOT data summarizing average bid prices for awarded projects in 2014.
Average Bid Prices Storm drain for spec year 2014
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|Bid item||Item description||Units||Average price|
|2573.530/0010||Storm drain inlet protection||Each||$175.21|
|2573.531/0010||Storm drain inlet protection||Lump sum||$500.00|
Except where more stringent requirements are presented in this guidance, BMPs shall comply with MnDOT and other state requirements. Primary design references include:
The following is a list of additional resources that are not specific to Minnesota: