Outlet Sediment Trap
Description
An Outlet Sediment Trap is a small basin lined with riprap or other suitable non-erosive lining, and located at the end of an outlet pipe, paved waterway, or channel outlet. The Outlet Sediment Trap is designed similar to a plunge pool, to dissipate the energy of incoming runoff. It is also sized to detain the runoff for initial settling of coarse particulates. Outlet Sediment Traps may be used for pretreatment of runoff before it discharges to another BMP, or they may be used as a BMP at the outlet of a drain system without further downstream treatment. This device can be employed where insufficient space is available to install another type of BMP, and where the device can be inspected and cleaned on a regular basis.
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Design Criteria
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Design Parameter
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Criteria
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Applicable DEP Stormwater Management Policy Performance Standards
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Standard #4 (TSS Removal)
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TSS Removal
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DEP Credit:
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25% for “Sediment Traps/Forebays” meeting a specified sizing rule. An alternative sizing method is provided in this fact sheet.
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Estimated Range from Literature:
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Data Not Available
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Estimated Range :
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35% to 45% (based on modeling with P-8 Urban Catchment Model)
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Relative Cost
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Construction: Low
Maintenance: Low to moderate
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Potential Constraints to Use
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Other Considerations
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- Recommended for use where space requirements preclude the use of other BMPs
- Should be designed for stability as an energy dissipation device.
- Where feasible, design to minimize short-circuiting between inlet and outlet ends of the trap.
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Maintenance Requirements
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- Inspect annually;
- Removal of debris from outlet structures as needed;
- Remove and dispose of accumulated sediment based on inspection.
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Design References
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- Current MassHighway design manuals, applicable sections on design of plunge pools.
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Contributing Drainage Area
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No minimum or maximum drainage area specified for this device.
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Minimum Area (Top of Pool)
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100 square feet per acre of contributing impervious area.
Minimum area of 50 square feet.
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Minimum Depth
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24 inches
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Interior slopes
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2:1 or flatter recommended.
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Other
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- Trap should also be designed as a plunge pool for dissipating flow velocities at the drainage system outlet (refer to MassHighway Design Manual and Drainage Manual).
- Stabilize interior surface of trap with properly designed riprap or other suitable lining for stability under anticipated flow conditions
- Exit velocities from the trap shall be non-erosive.
- Maintenance access should be provided
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Example of Outlet Sediment Trap