Sediment control practices - Outlet energy dissipation

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Outlet Energy Dissipation (Scour Protection)

Definition

Permanent or temporary energy dissipators prevent erosion, turbulence, and turbidity where stormwater pipes or ditches discharge to unprotected areas, such as channel banks, slopes, or upslope outfall locations. Their main purpose is to reduce the speed of concentrated flows to prevent scour at conveyance outlets. This practice is also called Scour Protection or Outlet Protection. Common types of outlet protection devices include concrete aprons, riprap-lined basins, and settling (stilling) basins.

Example of riprap outlet protection

Purpose and Function

Energy dissipators intercept incoming flows from pipes and ditches and deflect, scatter, or otherwise neutralize the erosive force of concentrated, moving stormwater. These structures are intended to protect soil from turbulence and high velocities, which can otherwise cause scour erosion. They typically consist of a structural apron lining fabricated from riprap, concrete, turf reinforcement mats, or other structural materials. Certain types of basins (e.g., settling/stilling/impact basins) can also function as energy dissipators. Many of these techniques are effective and relatively inexpensive and easy to install. Energy dissipators require careful design based on the hydraulic forces of concentrated flows exiting pipes and ditches.

Applicability

Energy dissipators at pipe and ditch outlets are appropriate for any unpaved or otherwise non-armored location where concentrated flows are discharged to areas subject to erosion.

Site Applicability

Energy dissipators are used where concentrated flows are discharged against channel banks, on poorly stabilized slopes, or onto upslope areas that are not protected against erosive flows. They are appropriate at the outlets of ponds, pipe slope drains, culverts, ditches, or other conveyances, and where runoff is conveyed to a natural or manmade drainage feature such as a stream, wetland, lake, or ditch. The most common applications include riprap aprons at culvert outlets, ditches lined with turf reinforcement matting, and stilling or impact basins designed to accept and dissipate high energy stormwater flows. Some important considerations when selecting energy dissipators for a site include:

• It may be difficult to remove sediment trapped by the dissipator without removing and replacing the structure.
• Rock/riprap outlets with high velocity flows require frequent maintenance.
• These BMPs may not be aesthetically pleasing.

Permit Applicability

Section III.C.3 of the 2013 MPCA Construction Stormwater General Permit requires energy dissipation at the outlet of temporary and wet sedimentation basins. Section IV.B.3 of the permit states that the permittee(s) “must use erosion controls and velocity dissipation devices such as check dams, sediment traps, riprap, or grouted riprap at outlets within and along the length of any constructed stormwater conveyance channel, and at any outlet, to provide a non‐erosive flow velocity, to minimize erosion of channels and their embankments, outlets, adjacent stream banks, slopes, and downstream waters during discharge conditions.”

Section IV.B.5 stipulates that “(p)ipe outlets must be provided with temporary or permanent energy dissipation within 24 hours after connection to a surface water,” and Section IV.B.6 requires permittee(s) to use “velocity dissipation devices if necessary to prevent erosion when directing stormwater to vegetated areas.” Discharge points from the dewatering of sediment basins must be “adequately protected from erosion and scour,” and “(t)he discharge must be dispersed over natural rock riprap, sand bags, plastic sheeting, or other accepted energy dissipation measures” (Section IV.D.1).

Effectiveness

Properly installed temporary and permanent energy dissipators are effective in preventing channel bank scour, slope gullying, and plunge pool erosion where concentrated flows discharge into unarmored, poorly stabilized areas.

Expected performance for outlet energy dissipators
Link to this table

Water Quantity
Flow attenuation	Little or no design benefit
Runoff volume reduction	Little or no design benefit
Water Quantity
Pollution prevention
Soil erosion	Primary design benefit
Sediment control	Secondary design benefit
Nutrient loading	Little or no design benefit
Pollutant removal
Total suspended solids	Secondary design benefit
Total phosphorus	Little or no design benefit
Heavy metals	Little or no design benefit
Floatables	Little or no design benefit
Oil and grease	Little or no design benefit

Planning Considerations

A review, analysis, and evaluation of both the location and magnitude of concentrated flows associated with culvert, ditches, and channel discharges can help identify where scour and other erosion are likely to occur. Culvert outlets are common places where energy dissipators are needed, as are locations where ditches discharge to terraces or slopes that are not armored against erosive, concentrated flows. In cases where flow velocities are too high for the economical use of an apron, a stilling basin or impact basin may be more appropriate. These structures dissipate energy from high-velocity flows to an acceptable level before discharging to an outlet channel.

Energy dissipator design and installation instructions can usually be included in the construction/installation plans for pipes, ditches, and channels. For example, culverts can be outfitted with outlet riprap aprons during installation, and ditches discharging into larger channels can be designed to include appropriate turf reinforcement matting and/or other structural elements where small, high-velocity ditches discharge against poorly armored banks of larger channels.

Key planning considerations for large projects – especially those involving mass clearing and grading – include 1) minimizing the number and magnitude of concentrated flows during initial clearing and grading, to the extent possible; and 2) installing permanent drainage features (e.g., stabilized ditches, channels, energy dissipators, vegetated channel buffers, etc.) as early as possible, especially since installing, stabilizing, and maintaining temporary drainage infrastructure is expensive and time-consuming. Following these two recommendations can reduce construction costs and increase the overall efficiency of sediment and erosion control on a site.