Dam Guidelines - Inspections
As previously noted, there are approximately 600 documented dams in the State of Nevada. Of those, 130 are high hazard, 120 are significant hazard and 350 are low hazard dams. All high hazard dams are inspected annually. Significant hazard dams should be inspected once in every three years and low hazard dams once in every 5 years. A dam inspection can be requested of the State Engineer's office at any time. Appendix H shows a sample inspection sheet that is used by the Division's engineers while inspecting a dam. It is also prudent for the owner to perform visual inspections on a regular basis and after storm and seismic events. There are reams of instruction literature on how to perform a dam inspection. If you are interested in this information, you can contact the State Engineer's office for a listing of some of the publications. The following is a brief list of items an owner can look for as part of a visual inspection and possible remedial actions:
- Heavy vegetation: Root systems for some vegetation can be quite extensive and actually provide a path for seep water. Also, as the plant, tree, etc. dies, the roots will rot, leaving a conduit for seep water. Large trees can blow over and cause a /LIeach in an earth embankment. Vegetation also provides cover and forage for rodents. Vegetation can be burned off, sprayed with herbicide, trimmed, etc.
- Rodent action: Burrowing by rodents (beaver, mole, mouse, squirrel, badger, vole, etc.) can provide conduits for seepage. If rodent holes become serious, rodents must be eradicated, their burrows /LIoken down and the holes backfilled with suitable compacted material.
- Outlet controls: The outlet works should be exercised from full closed to full open at least once a year to insure operability at high reservoir levels. Access to the outlet works is also important. It may be necessary to access the outlet works at high spillway flows to assist in the draining of the reservoir to prevent overtopping.
- De/LIis: The outlet conduit, spillway and outlet and spillway channels should be kept clear of de/LIis and vegetation so that they aren't choked closed during high flows, thus causing overtopping, or other damage to the dam. Fences should not be allowed in those areas as de/LIis can accumulate on the fence and cause clogging of the channels/conduits.
- Seeps: Look for wet spots along the toe of the dam, on the downstream face, on the ground downstream of the dam and along the abutments. You may not see wet spots but there may be an incongruous line or spot of vegetation. Any seeps that can be seen should be measured (gallons per minute) and the turbidity of the seep water should be noted. If the seep water is turbid with signs of embankment material mixed in it, there is a possibility of a piping problem. A piping problem can cause a dam failure very quickly.
- Cracks, slumps and settlement: Obviously any movement of the embankment after construction can be serious. A number of reasons could be responsible for embankment movement such as weak foundation conditions, poor compaction in areas, ice lenses during construction, earthquakes, excessive seeping etc. The condition should be monitored closely.
- Erosion protection: If the upstream face has riprap or some other type of armoring, it should be monitored. If the armoring is displaced, wave action will cause erosion of the embankment material.
- Beaching/Benching: Wave action on an unprotected embankment can erode the face of the dam causing a vertical face to form. This diminishes the ability of the dam to hold maximum storage and may lead directly to failure in a storm or even under good weather conditions. Benching should be avoided by armoring the upstream face with appropriate sized riprap. When the reservoir ices over, movement of the ice can displace riprap and lead to erosion or benching.
