This data provides an illustration of the height above the nearest stream approach to flood inundation mapping based on the TauDEM vertical distance to stream function. This example uses a 10 m resolution National Elevation dataset for Onion Creek in Texas. Height above the nearest stream may be thought of as a “relative elevation function” which measures for every DEM cell in the landscape the difference in elevation between that cell and the cell to which it flows on the stream channel. This is like a “water depth” or “stage height” function defined using terrain analysis continuously across the landscape. This relative elevation function, combined with a depth in each stream reach provide a simplified terrain based approach to flood inundation mapping premised on the following:

1. Each reach has a water depth hw, from a hydraulic model such as SPRNT or RAPID.
2. Each reach has an ID
3. Each grid cell has the ID of the reach it connects to (catchment grid) and the height above the stream hs
4. Flood extent is “rapidly” mapped as
If(hw(id) > hs(id))
Inundation depth = hw(id) - hs(id)
Else
Inundation depth = 0

The data here can also be used to compute reach averaged hydraulic properties as follows
1. For each reach the stream network file gives reach length L.
2. For a series of water depths using the height above nearest stream intersected with catchment raster the innundation water volume V, surface area As and bed area Ab are obtained.
3. Reach average properties are then computed as
Cross section Area A = V/L
Wetted perimeter P = Ab/L
Top width = As/L
Hydraulic Radius = A/P

This approach is a simplification over finer scale hydraulics, and the inaccuracy due to introduction of this simplification still needs evaluation. This approach is also dependent on how well the DEM represents the channel and flooded area. This is expected to improve as we get better LIDAR DEMs and develop better ways to hydrologically condition DEMs that do not involve pit filling.