Erosion prevention practices - Riprap

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Riprap is a permanent layer of large, angular stone, cobbles, or boulders that is typically used to armor, stabilize, and protect the soil surface against erosion and scour in areas of concentrated flow or wave energy. Riprap is typically placed along graded ditch, channel, and shoreline banks over geotextile, which prevents erosional undercutting. It can also be used with other mixed size rock to construct retention berms for sediment traps and check dams protecting high volume/velocity culvert inlets. The Minnesota Department of Transportation classifies riprap by type as random riprap or hand-placed riprap, depending on the method of placement and the stone size specified.

Purpose and function

Riprap is used to stabilize areas on a construction site with high erosive power by increasing surface roughness and slowing the velocity of runoff. Applicable areas on a site may include inlets and outlets of storm pipes and culverts, bridges, slopes drains, storm drains, and other areas where concentrated runoff may occur. Riprap is also effective for protecting and stabilizing slopes, channels, streambanks, and shorelines.

Applicability

As discussed above, there are many applications where riprap is more desirable than other erosion control practices. Although riprap is not often considered aesthetically pleasing, it can be one of the most effective methods of erosion prevention and is particularly desirable in areas where conditions prohibit establishment of vegetation (for example, areas where velocities are too great for vegetation to withstand). Compared to other erosion control practices, riprap is relatively simple to install and maintain. However, riprap is typically more expensive to install compared to vegetation and does not provide some of the secondary benefits provided by vegetated practices (e.g., habitat enhancement).

Site Applicability

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Riprap is useful in areas in which the powers of erosion outweigh the stabilization capacity of other erosion control practices such as vegetative control and mulching. As noted above, riprap is especially useful for armoring channel and ditch banks, lake shorelines, and for sediment trap berms and high-volume/velocity check dams. Because of the “hard” look of riprap, its higher overall cost, the growing popularity of vegetated solutions using turf reinforcement matting and other products, and the difficulty in removing it after installation, contractors should ensure that the post-construction site design specifically includes riprap before using it during construction. Riprap may be unstable on very steep slopes. For slopes steeper than 2:1, consider using materials other than riprap for erosion protection such turf reinforcement matting over seed, open-cell articulated concrete mats, or other slope protection geogrid products/matrices.

Permit applicability

Riprap as a stabilization measure

Riprap is often used for stabilization for construction sites. Section 25.30 of the MPCA Construction Stormwater General Permit (2018) defines stabilization and specifically identifies riprap as an appropriate stabilization measure, stating: “’Stabilize’, ‘Stabilized’, ‘Stabilization’ means the exposed ground surface has been covered by appropriate materials such as mulch, staked sod, riprap, erosion control blanket, mats or other material that prevents erosion from occurring. Grass seeding, agricultural crop seeding or other seeding alone is not stabilization. Mulch materials must achieve approximately 90 percent ground coverage (typically 2 ton/acre).” Because stabilization is required and essential for most construction activities, it is mentioned numerous times throughout the 2018 Permit. While “riprap” is not specifically mentioned in these instances, it is implicitly included due to its widespread acceptance as an effective stabilization practice. As noted above, before using riprap the site manager should check with the design engineer to ensure that the post-construction configuration includes riprap prior to its installation. In many cases it may be far less expensive to install the post-construction vegetation with whatever stabilization matrix is called for (e.g., turf reinforcement mat, articulated concrete mat, geogrid, etc.) during construction, rather than having to place and then later remove riprap. Several key locations in the permit that mention stabilization are noted below.

• (Erosion Prevention Practices) Section 8.4 states: “Permittees must stabilize all exposed soil areas, including stockpiles. Stabilization must be initiated immediately to limit soil erosion when construction activity has permanently or temporarily ceased on any portion of the site and will not resume for a period exceeding 14 calendar days. Stabilization must be completed no later than 14 calendar days after the construction activity has ceased. Stabilization is not required on constructed base components of roads, parking lots and similar surfaces. Stabilization is not required on temporary stockpiles without significant silt, clay or organic components (e.g., clean aggregate stockpiles, demolition concrete stockpiles, sand stockpiles) but permittees must provide sediment controls at the base of the stockpile.”
• (Erosion Prevention Practices) Section 8.5 states: “For Public Waters that the Minnesota DNR has promulgated "work in water restrictions" during specified fish spawning time frames, permittees must complete stabilization of all exposed soil areas within 200 feet of the water's edge, and that drain to these waters, within 24 hours during the restriction period.”
• (Erosion Prevention Practices) Section 8.6 states: “Permittees must stabilize the normal wetted perimeter of the last 200 linear feet of temporary or permanent drainage ditches or swales that drain water from the site within 24 hours after connecting to a surface water or property edge. Permittees must complete stabilization of remaining portions of temporary or permanent ditches or swales within 14 calendar days after connecting to a surface water or property edge and construction in that portion of the ditch temporarily or permanently ceases.”
• (Discharges to Special and Impaired Waters) Section 23.9 states: “Permittees must immediately initiate stabilization of exposed soil areas, as described in item 8.4, and complete the stabilization within seven (7) calendar days after the construction activity in that portion of the site temporarily or permanently ceases.”

Riprap as an energy dissipation measure

In additional to stabilization, riprap is also used as an energy dissipation measure to reduce the erosive force of concentrated stormwater and prevent scour. Section 25.8 of the MPCA Construction Stormwater General Permit (2018) defines energy dissipation as “methods employed at pipe outlets to prevent erosion caused by the rapid discharge of water scouring soils.” Energy dissipation is mentioned at several locations in the Permit:

• (Erosion Prevention Practices) Section 8.9 states: “Permittees must provide temporary or permanent energy dissipation at all pipe outlets within 24 hours after connection to a surface water or permanent stormwater treatment system.”
• (Temporary Sediment Basins) Section 14.7 states: “Permittees must provide energy dissipation for the basin outlet within 24 hours after connection to a surface water.”
• (Wet Sedimentation Basin) Section 18.6 states: “Permittees must design basin outlets to prevent short-circuiting and the discharge of floating debris. Basin outlets must have energy dissipation.”

Effectiveness

When properly designed and installed riprap can be an incredibly effective erosion control practice. Additionally, because riprap functions to increase surface roughness and decrease runoff velocity, secondary water quality benefits can be achieved for sediment control and total suspended solids removal due to settling.

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