Four hazards are typically present on alluvial fans: 1) debris flows, 2) debris floods, 3) flooding, and 4) erosion. The debris flows and floods are the most worrisome because of their suddenness and violence. In many cases they are rare events which makes them more dangerous particularly in areas that have no previous recorded history of debris flow or debris flood events.
Another hazard on alluvial fans is the tendency for the stream channel to become filled with sediment and the stream then moving to another location on the fan. The new flow path causes flooding but also can cause significant erosion as the stream erodes into its new channel. This hazard can be mitigated, but in some cases it is a too costly a fight.
Last week I got some experience on an alluvial fan stream that has left its old channel of many decades and has begun the process of establishing a new channel on the fan surface.