Channel Debris Clearing
Removal of woody debris from channels by heavy equipment or cable yarding.
Purpose: Channel clearing is the removal or size reduction of logs and other organic debris or the removal of sediment deposits to prevent them from being mobilized in debris flows or flood events or altering stream geomorphology and hydrology. This treatment has been done to prevent creation of channel debris dams which might result in flash floods, or aggravate flood heights or peakflows. Organic debris can lead to culvert failure by blocking inlets culverts, or reduce channel flow capacity. Excessive sediments in stream channels can compromise in-channel storage capacity and the function of debris basins.
Relative Effectiveness: Excellent-0% Good-71% Fair-0% Poor-29% (Replies = 7) Channel debris clearing was rated as “good” in effectiveness by the majority of the interviewees, but nearly a third rated its effectiveness to be “poor.” The latter rating came from situations where there was not enough post-fire organic debris in riparian areas or the channels to cause debris dam problems or stream hydrology was adversely altered by clearing. Because much of the debris from fire-killed trees does not enter channel system until 2 or 3 years later, this treatment was not considered by some to be a useful ESR treatment. Also, there has been a significant improvement in the understanding of the positive role of large woody debris in trapping sediment, dissipating the energy of flowing water, and providing aquatic organism habitat. In some instances the channel clearing has been more disruptive than the wildfire. So, in some areas the policy now is to avoid channel clearing.
Channel clearing is definitely an expensive, time-consuming operation, but it has been successful in certain situations such as locations where trash racks cannot be used to protect road culverts, where woody debris might move into reservoirs, and where sediment must be removed from debris basins and channels to provide adequate sediment storage capacity. Important factors in the relative effectiveness of channel clearing, when it is used, include a good analysis of risk and the value of resources at risk, knowledge of the size and quantity of material to remove, the clearing distances above roads needed to protect culverts, and understanding of the physical characteristics of the channels which might aggravate or reduce stormflows.
Implementation and Environmental Factors: Timing is an important factor which affects both the effective-ness and the assessment of the value of channel clearing. When sediment removal is the objective of channel clearing, operations must be done before sea-sons (usually winter) that produce the first or most significant stormflows. For large woody debris, the key question is if and when inputs of woody debris are likely to occur. In some areas, woody debris recruitment (greater than 2 years) may be beyond the timeframe of ESR projects. Crews conducting channel clearing must be well trained in order to recognize woody material that is too large to float or be firmly anchored, is part of the natural instream coarse woody debris load, or is a natural grade stabilizer. Where woody debris is cut up it must be sufficiently short to pass through culverts.
Other Factors: Channel debris clearing may produce significant, adverse riparian area impacts, destabilize the channel, reduce aquatic habitat, and alter stream hydrology. These side effects may negate any positive benefits derived from channel clearing in some situations.