Debris Basins

Specially engineered and constructed basin for storing large amounts of sediment moving in an ephemeral stream channel. 

Purpose: Debris basins are constructed to treat either the loss of control of runoff and deterioration of water quality, or threats to human life and property. The design of debris basins must be to a standard that they provide immediate protection from flood water, floatable debris, sediment, boulders, and mudflows. They are usually constructed in stream systems with normally high sediment loads. Their purpose is to protect soil and water resources from unacceptable losses or to prevent unacceptable downstream damage. Debris basins are considered to be a last resort because they are extremely expensive to construct and require commitment to annual maintenance.

Relative Effectiveness: Not enough interviewees rated this treatment. In order for debris basins to function they must be able to trap at least 50 percent and preferably 70 to 80 percent of 100-year flows. A spillway needs to be constructed in the debris basin to safely release flow in excess of the design storage capacity. The downstream channel should be lined to prevent scour. In some instances excavated pits in ephemeral channels have been used as debris basins. These must be large enough to trap 50 to 90 percent of flood flow. They need to be cleaned annually until abandoned.

Implementation and Environmental Factors: Because debris basins are rather large, they require design by qualified engineers. They are built in depositional or runout areas that have large storage capacity. During construction it is important to maintain the channel gradient. Head cutting can result from improperly located or constructed debris basins.

Other Factors: Debris basins must be designed with large vehicle access to the basins so they can be cleaned out periodically. Maintenance is a key factor in effectiveness of this treatment. Although protection is immediate, maintaining debris basins may be a long-term commitment.