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.

- Point query tool in depression
- The point query tool can be used to get information about depressions and watersheds. When you click on a point in a depression, say point
*u*in the figure above, the depression selected is highlighted in orange. The tool returns the amount of rain required until point*u*is flooded for the first time, the water depth at*u*(the length of the displayed dashed line) and the volume of the water currently in the highlighted depression. Note that the water depth and volume depend on the current event. - Point query tool on flow path
- When you use the point query on a flow path, e.g. point
*v*in the figure above, you get information about the upstream area of that point. In this case the green area to the left of*v*. Additionally you get the*depression storage*of the point, which is defined as the volume of rainwater stored in the depressions upstream of*v*. In our figure this is the volume of depression*B*and*C*which are both full and part of the watershed of*v*. The volume of depression*A*is not included since this depression is not in the watershed of*v*for this event. Finally, you get the*runoff volume*flowing through*v*, which is the volume of the rainwater upstream of*v*not stored in depressions, that is, the total volume of the rainfall in the green watershed minus the depression storage. If your point is in a depression the resulting values are that of the outlet of the depression. - Watershed tool
- The watershed tool returns the same information as the point query tool on a flow path, but additionally highlights the watershed of the query point in green. Furthermore the flow path downstream of the query point is shown, including downstream depressions. If the queried point drains to an active (non-full) depression at the current amount of rain, this final depression is highlighted in turquoise. In case the queried point is part of a depression, this depression is highlighted in red. If your point is in a depression the resulting values are that of the outlet of the depression.

Illustration of the watershed tool on a flash flood map. The queried point (blue pin) is in a depression (marked in red) whose watershed is shown in green. The downstream path passes through a depression (blue) before it reaches a depression that is not yet full (shown in turquoise)

The flash flood map can be downloaded in a number of different forms.

- Flash flood map
- A raster will be downloaded where each cell gives the minimal amount of rain for which it is flooded.
- Depth
- A raster will be downloaded where each cell gives the depth of water on the terrain surface at the current event.
- Vector
- For each of given depth thresholds, polygons will be downloaded in a vector format. Each polygon describes a flooded area where the depth of water is higher than the depth threshold at the current event.
- Flow paths
- A set of lines and a set of polygons will be downloaded in a vector format. The lines describe flow paths and the polygons describe flooded area at the current event. Each line contains
*flow*value. Each polygon contains*volume*and*upstream watershed size*. - Building information
- This download gives you information about standing water adjacent to buildings. Intuitively we can define the
*wetted perimiter*of a building as the part of the walls of the building that is under water (seen from the side) from depressions next to the building. In this download you will get a set of lines and a set of polygons. The lines are intersections between the sides of building and flooded areas at the current event, each of which contains the following information:*wetted-perimeter*: the length of the intersection.*wetted-area*: the sum of wetted area along the intersection.*max-depth*: the maximal water depth along the intersection.*coordinates*: the coordinates gives the maximal water depth.

*upstream size*of the corresponding building at the current event.