A consequence of extreme rain events is often that surface water gathers in depressions of the terrain, which potentially has catastrophic effects on housing and other assets within the depressions. Therefore, analyzing how surface water gathers in depressions of detailed terrain models is an important part of flood risk analysis.
The flash flood map identifies what part of each depression is below water after an event, as defined by a given amount of rain. In fact, it maps how much rain that has to fall before any given point on the terrain is below water, thus effectively analyzing how water collects in depressions for all possible amounts of rain. More precisely, the core of the flash flood map is a raster where every cell corresponds to a cell in the input terrain raster, and the cell contains a value indicating how many millimeters of rain is required before the corresponding terrain cell gets flooded from water rising in a depression. This mapping both considers the volume of water that needs to gather below the cell and the size of the watershed draining into the depression containing the cell. It also considers the dynamic flash flood events that occur as the watershed size of depressions grow when upstream depressions fill and start contributing water downstream.
Illustration of how water gathers in depressions of a terrain during an extreme rain event. In the top left figure none of the depressions are full and all depressions have their initial watershed given by colors on the terrain surface. In the top right figure the original green depression is full and all water contributing to the green depression is now contributing to the red depression. In the bottom left figure the red depression is also full and all water from this depression is now contributing to the orange depression. Finally, in the bottom right figure the purple depression is full and all water is contributed to the orange depression.
The flash flood map also shows how water flows between depressions of the terrain for any event. This makes it possible to analyze how depressions influence (spill into) each other, that is, how critical situations can occur when depressions fill with water resulting in dramatically increased watersheds for other depressions “downstream”. These results are available both in the form of flow lines and using the watershed tool.
The flash flood map can be queried in a number of powerful ways using the point query tool and the watershed tool. In the following examples we refer to the figure below.
Illustration of point queries in the flash flood map. Point u is in a depression and gives information about that depression, point v is on a flow path.