Debris flows and debris floods area a significant hazard on alluvial fans. Forested alluvial fans can significantly reduce the debris flow and flood hazards. A forest stand of stout trees can act as a stationary rake preventing large wood debris from spreading out on the fan surface. I have been to several alluvial fans where homes were saved by the presence of forest on either side of the stream on the fan, and I have seen homes destroyed that likely would not have been impacted if the forest had not been removed.
The debris flow path shown above came off of a very steep mountain slope. The debris flow initiated as a small shallow slide about 1,500 feet in elevation above this point. The slide mixing with creek water flushed out the accumulation of large wood within the steep channel and delivered the pile of logs with mud and water to the base of the mountain.
Forest along the side of the stream kept the wood debris as well as the stream confined to the existing stream route.
From a land use geology hazard policy, leaving forest stands on some alluvial fans can be a form of mitigation, but also a restraint on property owners from cutting trees and thereby increasing the risk to neighboring properties. I have recommended leaving trees on several development projects. At least one county has utilized restrictive covenants on permits based on those recommendations.
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