Probable Causes and Possible Consequences
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Recommended Actions
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- Overgrazing, poor weed control, concentrated runoff, and wave erosion are all common causes for poor vegetative cover and erosion on a dam.
- Bare soil or sparse vegetative cover are especially susceptible to damage and erosion.
- Heavy rainfall, high winds, and flood events can cause extensive damage and costly repairs to unvegetated areas.
- In spillways, lack of grass cover, obstructions, and concentrated runoff can lead to erosion.
- Once erosion begins in the spillway, the erosion can grow quickly in size during a large flood and result in dam failure.
- Over time, wave action will erode unprotected earthen slopes decreasing the width of the dam.
- Waves can erode soil through holes between large pieces of riprap.
- In earthen spillways, farming, livestock grazing, and vehicle paths remove protective grass cover.
- Lack of grass cover can lead to massive erosion and dam failure when water flows through the spillway.
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- Areas of sparse vegetation should be reseeded with perennial grasses each spring or fall.
- Control weeds by mowing or application of herbicides.
- If erosion is detected early, add protective grass vegetation that may resolve the problem.
- It is best to keep livestock from grazing on earthen dams and in vegetated spillways. At a minimum, livestock grazing should be extremely limited. Protect areas by installing fencing. Fences should not be installed across spillways where the flow velocities could be high.
- Redirect surface runoff away from steep areas by constructing swales and terraces.
- Check for erosion in earthen spillways after major storm events.
- Reestablish the normal embankment slope by placing compacted soil in eroded areas.
- Place filter material such as crushed rock, gravel, or filter fabric under rock riprap to prevent wave erosion.
- Do not plant crops in vegetated, earthen spillways.
- Do not create paths by repeatedly driving vehicles over the same path through the spillway.
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Rock riprap wave protection over filter material such as gravel or filter fabric on upstream slope. The size of the riprap needed depends on several factors, but rocks that are 24 inches in diameter are suitable for most situations. To fill the holes between the larger rocks, it is important to include a mixture of smaller stones ranging in size from 6 to 24 inches.
(Diagram source: North Carolina DEQ)
