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Pocket Safety Guide for Dams and Impoundments

Dam Problems

Sinkholes

Probable Causes and Possible Consequences

  • Internal erosion of embankment materials or the foundation (piping) can cause a sinkhole.

  • An eroded cavern (cave-in) can result in a sinkhole.

  • A small hole in the wall of an outlet pipe can develop into a sinkhole.

  • Water with sediment at the exit indicates erosion of the dam.

  • Piping can empty a reservoir through a small hole in the wall. It also can lead to dam failure as soil pipes develop and erode through the foundation or a pervious part of the dam.

Recommended Action

Report the suspected problem to the appropriate Federal or State agency official immediately.

  • Inspect other parts of the dam that may be susceptible to seepage or more sinkholes.
  • Check seepage and leakage outflows for muddy water.
  • Have a qualified engineer inspect the conditions, identify the exact cause of sinkholes, and recommend further actions.
  • Depending on the location of the sinkhole in the embankment, have the reservoir drawn down as necessary.
Drawing showing what a sinkhole looks like along with a crack in the foundation labeled 'Piping'.
Figure 5—Sinkholes.
Image of a whirlpool that has formed next to the bank of a dam.
Figure 6—Whirlpool action is an indication of advanced piping.

Slide, Slump, or Slip

Probable Causes and Possible Consequences

  • Foundation movement or a too steep slope can cause earth or rocks to move along a slip plane which can lead to a slump of the embankment.

  • Slide movements in the reservoir basin can lead to inlet obstruction or dam failure.

Recommended Action

Report the suspected problem to the appropriate Federal or State agency official immediately.

  • Evaluate the extent of the slide.
  • Monitor the slide.
  • Draw down the reservoir level if the safety of the dam is threatened.
  • Have a qualified engineer inspect the conditions and recommend further actions.
Drawing of a dam that shows where the side of the slope has slide away.
Figure 7—Slide, slump, or slip.
Image of where the side of a dam has slumped.
Figure 8—Slumping on the downstream face of a dam.

Broken Down or Missing Riprap

Probable Causes and Possible Consequences

  • Poor-quality riprap deteriorates and does not protect the slope.
  • Wave or ice action can displace riprap allowing erosion of the bank.
  • Similar-sized round rocks may roll downhill and expose the slope.
  • Wave action against these unprotected areas decreases the embankment width.

Recommended Action

  • Reestablish the normal slope.
  • Place bedding material and properly sized riprap to protect against wave action.
Drawing showing where riprap is missing or has broken down.
Figure 9—Broken down or missing riprap.
Image of the upstream side of a dam where wave erosion has caused damage due to not being properly protected by riprap.
Figure 10—Wave erosion on the unprotected face of
a dam.

Erosion

Probable Causes and Possible Consequences

  • Water from intense rainstorms or snowmelt carries surface materials down the slope and results in continuous troughs, which can be hazardous if allowed to continue.

  • Erosion can lead to deterioration of the downstream slope and failure of the structure.

Recommended Action

  • If erosion is detected early, add protective grasses that may resolve the problem.
  • Protect eroded areas; add rock or riprap, which is the preferred method.
Drawing of erosion of the downstream slope on the dam.
Figure 11—Erosion.
Image showing a section of erosion on the side of a dam.
Figure 12—Erosion on the downstream face of a dam.

Trees or Obscuring Vegetation

Probable Causes and Possible Consequences

  • Natural vegetation (bushes) obscures visual inspection and harbors rodents.
  • Large tree roots can create seepage paths.
  • Large trees can blow over during a storm and damage the dam, which may cause a breach.

Recommended Action

  • Control vegetation that obscures visual inspection of the embankment.
  • Remove all large, deep-rooted trees and shrubs on or near the embankment.
  • Backfill voids properly.
  • Remove trees at the toe of the dam to provide a 25-foot buffer.
Drawing of vegetation growing on the downstream side of a dam.
Figure 13—Trees or obscuring vegetation.
Image of the crest of a dam with trees growing all over it.
Figure 14—Trees growing on the crest and the faces of a dam.

Rodent Activity and Animal Impact

Probable Causes and Possible Consequences

  • Cattail-filled areas and areas where trees are close to the reservoir provide ideal habitat and foraging areas for animals.

  • An overabundance of rodents increases the chance of animal burrowing, which creates holes, tunnels, and caverns.

  • Tunnels may reduce the required length of the seepage path, which could cause a piping problem.

  • Tunnels can lead to the collapse of the dam crest and may cause dam failure.

Recommended Action

  • Start a rodent control program to reduce the population and prevent future damage to the dam.
  • Determine the exact location and extent of tunneling.
  • Backfill existing rodent holes with suitable well-compacted material to repair damages.
Drawing of holes in a dam made by rodents.
Figure 15—Rodent activity
Image of several rodent holes in the side of a dam.
Figure 16—Rodent holes in the dam face can cause dam failure.