, Ponce,
Puerto Rico, was caused by heavy rainfall from
Tropical Storm Isabel in 1985. The mudflow destroyed more than 100 homes and claimed an estimated 300 lives. Heavy rainfall,
snowmelt, or high levels of
groundwater flowing through cracked bedrock may trigger a movement of soil or sediments in
landslides that continue as mudflows.
Floods and
debris flows may also occur when strong rains on hill or mountain slopes cause extensive
erosion and/or mobilize loose sediment that is located in steep mountain channels. The 2006
Sidoarjo mud flow may have been caused by rogue drilling. The point where a muddy material begins to flow depends on its
grain size, the water content, and the slope of the topography. Fine grained material like mud or sand can be mobilized by shallower flows than a coarse
sediment or a
debris flow. Higher water content (higher precipitation/overland flow) also increases the potential to initiate a mudflow. After a mudflow forms, coarser sediment may be picked up by the flow. Coarser sediment picked up by the flow often forms the front of a mudflow surge and is pushed by finer sediment and water that pools up behind the coarse-grained moving mudflow-front. Mudflows may contain multiple surges of material as the flow scours channels and destabilizes adjacent hillslopes (potentially nucleating new mudflows). Mudflows have mobilized boulders across in mountain settings. Some broad mudflows are rather
viscous and therefore slow; others begin very quickly and continue like an
avalanche. They are composed of at least 50% silt and clay-sized materials and up to 30% water. Because mudflows mobilize a significant amount of sediment, mudflows have higher flow heights than a clear water flood for the same water discharge. Also, sediment within the mudflow increases granular friction within the flow structure of the flow relative to clear water floods, which raises the flow depth for the same water discharge. ==Mudflows and landslides==