I have been doing some code/regulation language work in regards to geology hazards. Alluvial fan hazards have presented a challenge.
Alluvial Fan Hazard Areas: Alluvial fan hazard areas shall include those areas on alluvial fans potentially impacted by debris flows, debris floods, clear water floods, stream channel changes, and erosion. Watershed hydrology, geology, slope conditions, topography, land use, valley bottom and channel conditions upstream of an alluvial fan area are fundamental to potential hazards on alluvial fans.
It is one thing to recognize the hazard, but then What is the risk and What are the consequences? And every fan is a bit different and sometimes a lot different. I find writing code/regulations for alluvial fans ought not to be done without a very careful review of policy. Because like alluvial fans, every community is different.
I should add that one of the added hazards western Washington has on alluvial fans is the large wood entrained in debris flows and floods relative to the desert fans shown above.