Flash Flooding, Snowpack & Rainfall
The potential for flooding can be greatly enhanced during the winter and spring when heavy rainfall occurs on melting snow or when snowmelt is induced by warmer temperatures. There are several indicators—snowpack density, snow water equivalent (SWE) and landscape disturbance—to look for in gaging when snowpack is starting to melt, and if a rainstorm may trigger flash flooding.
Snowpack density is generally a good gage for the potential of snowmelt and is proportional to the amount of liquid water in the snowpack, or SWE. Higher-density readings are an indicator that the snow is getting ready to melt. A high-density snow (near or exceeding 40%) means that the snowpack is reaching a point where melting is imminent, therefore increasing the risk of flash flooding during rain-on-snow events.
Declining SWE can also allude to a heightened flash flood risk. If SWE is consistently declining, this also means the snowpack is beginning to melt. Generally, as SWE declines due to melting, the density tends to increase because the proportion of liquid water to solid ice is increasing.
Landscape disturbances, notably burned areas post-wildfire, also impact the risk and severity of flash floods. The risk depends on the severity of the wildfire, which impacts surface erosion and vegetation. Vegetation could take many years to grow back after a fire, thus leaving the surface exposed to debris flows during rainstorms. Each burned area poses its own unique risk due to many factors including proximity to population centers, burn severity, steepness of terrain, and size of the burned area. Typically, most burned areas are prone to flash floods and debris flows for at least two years post-burn.
Resources to Asses Risks and Prepare for Flash Floods
Snowpack, precipitation and streamflow monitoring, and burned-area mapping are essential for assessing flash-flood risks during rainstorms and rain-on-snow events. Below are some external resources that may assist in proper risk assessment if you live in an area that may be prone to flash floods during rain-on-snow events.
While the flooding risk cannot be eliminated entirely, there are steps to reduce and properly assess that risk. Learn how to prepare for a flood using the resources below: