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Pocket Safety Guide for Dams and Impoundments

Dam Problems Continued

Livestock and Cattle Traffic

Probable Causes and Possible Consequences

  • Livestock paths and activities on the downstream dam face can damage slopes, especially when wet.
  • Livestock activities may reduce erosion protection and cause erosion channels.
  • Bare areas may allow water to stand and be susceptible to drying cracks.

Recommended Action

  • Fence the embankment area to keep out livestock.
  • Repair erosion protection by reestablishing grasses or by placing riprap.
Drawing of a path made by livestock going down the downstream side of a dam.
Figure 17—Livestock and cattle traffic.		 Image of cows standing on the side of a dam.
Figure 18—Livestock paths on dam faces can lead to dam erosion.

Transverse Cracking

Probable Causes and Possible Consequences

  • Uneven movement between adjacent segments of the embankment may cause transverse cracking.

  • Deformation caused by structural stress or instability may provide a path for seepage through the embankment cross section.

  • If not corrected, cracking creates an area of low strength within the embankment that may lead to future structural movement, deformation, or failure.

  • Cracks create a point for water to enter the embankment. Saturation of the adjacent embankment area may occur and could lead to localized failure.

Recommended Action

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

  • Inspect the crack and carefully record its location, length, depth, width, and other pertinent physical features. Stake out the crack limits.

  • Have a qualified engineer determine the cause of cracking and supervise all steps necessary to reduce danger to the dam and correct the condition.

  • Excavate the crest along the crack to a point below the bottom of the crack. Backfill the excavation using suitable material and correct construction techniques. This seals the cracks at the crest surface to prevent surface water infiltration. Monitor the crest routinely for evidence of future cracking.
Drawing of a large crack going though the middle of a dam.
Figure 19—Transverse cracking.		 Image showing large cracks in the ground.
Figure 20—Transverse cracks can be an indication of dam instability.

Longitudinal Cracking

Probable Causes and Possible Consequences

  • Uneven settlement between adjacent sections or zones within the embankment may cause longitudinal cracking.

  • Foundation failure causes loss of embankment support, which can result in embankment slide.

  • Settlement creates an area of low strength within an embankment, which may lead to future structural movement, deformation, or failure.

  • Cracks create a point for water to enter the embankment. Saturation of the adjacent embankment area may occur and could lead to localized failure.

Recommended Action

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

  • Inspect the crack and carefully record the location, length, depth, width, alignment, and other pertinent physical features. Immediately stake out the crack limits and monitor frequently.

  • Have a qualified engineer determine the cause of cracking and supervise the steps necessary to reduce danger to the dam and correct the condition.

  • Effectively seal the cracks at the crest surface to prevent surface water infiltration.

  • Monitor the crest routinely for evidence of further cracking.
Drawing of a longitudinal crack going through the crest of a dam.
Figure 21—Longitudinal crack.		 Image of someone measuring the depth of a longitudinal crack.
Figure 22—Longitudinal cracks can be an indication of dam instability.

Low Area in the Crest of a Dam

Probable Causes and Possible Consequences

  • Excessive settlement of the embankment or internal erosion of embankment material may cause a depression on the top of a dam.

  • Foundation spreading upstream and/or downstream, prolonged wind erosion, or improper final grading following construction may cause a low area in the crest of a dam.

  • Low areas can reduce the freeboard available to pass flood flows safely through the spillway.

Recommended Action

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

  • Establish survey monuments along the length of the crest to determine the exact amount, location, and extent of settlement in the crest.

  • Have a qualified engineer determine the cause of the low area and supervise all necessary steps to reduce any threat to the dam and correct the condition.

  • Use proper construction techniques to fill in the low area and reestablish a uniform elevation over the length of the crest.

  • Reestablish and routinely monitor survey monuments to detect any settlement across the crest of the dam.
Drawing of a low area in the crest of a dam.
Figure 23—Low area in the crest of a dam.		 A circle drawn on an image shows where the ground has sunk on the crest of a dam.
Figure 24—Low areas in the crest of a dam reduce the freeboard.

Excessive Quantity and/or Muddy Water Exiting From a Point

Probable Causes and Possible Consequences

  • Water has created an open pathway, channel, or pipe through the dam. The water is eroding and carrying embankment material.

  • Surface agitation of water and embankment materials exiting at one point may cause muddy water.

  • Rodents, frost action, or poor construction may allow water to create an open pathway or pipe through the embankment.

  • Continued flows can saturate parts of the embankment and lead to slides in the area.

  • Continued flows can further erode embankment materials and lead to dam failure.

Recommended Action

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

  • Begin measuring outflow quantity. Determine whether water is getting muddier, staying the same, or clearing up.
  • Have a qualified engineer inspect the condition and recommend further actions.
  • If the quantity of flow is increasing, lower the water level in the reservoir until the flow stabilizes or stops.
  • Search for an opening on the upstream side of the dam. Plug it if possible.
Drawing of water seeping through the dam where it should not be.
Figure 25—Excessive quantity and/or muddy water exiting from a point.		 Image of excess water seeping through the dam.
Figure 26—Excessive water piping through the dam at the toe of the embankment.

Seepage Water Exiting as a Boil in the Foundation

Probable Causes and Possible Consequences

  • Some part of the foundation material is supplying a flow path. This could be caused by a sand or gravel layer in the foundation.

  • Increased flows can lead to erosion of the foundation and dam failure.

Recommended Action

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

  • Examine the boil to see if foundation materials are being transported.

  • Have a qualified engineer inspect the condition and recommend further actions.

  • If soil particles are moving downstream, use sandbags or earth to create a dike around the boil. The pressures created by the water level may control flow velocities and temporarily prevent further erosion.

  • If erosion is increasing, the reservoir level should be lowered.
Drawing of a boil where water is seeping out at the downstream side of a dam.
Figure 27—Seepage water exiting as a boil in the foundation.		 Image of sandbags surrounding a boil.
Figure 28— Sandbags surrounding a boil on the downstream side of the dam.

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