Steve,
Depending on the quality of your DEM and streams, this could be an "easy" exercise. If your DEM and the flow direction match the placement and elevation of your streams, then you can directly use the HAND (Height Above Nearest Drainage) approach. You will need the following rasters (I suggest that you use .tif format for your rasters):
- DEM (i'll call it DEM.tif). Should be in correct horizontal/vertical units - e.g. meters.
- Flow direction raster derived from the DEM (FDR.tif) that "flows" into your streams.
- Stream raster (STR.tif). This can have value of 1 where stream cells are and nodata elsewhere. The elevation of the streams (thalweg) should be reflected in the DEM.
The steps are then (these are all core Spatial Analyst functions - you do not need Arc Hydro):
- Use "Flow Distance" (Hydrology toolset) to calculate the HAND (HAND.tif) surface (that is the "output raster". Specify inputs stream raster (STR), surface raster (DEM) and flow direction raster (FDR). Keep all other parameters at their default values.
- Once you have the HAND surface, to get the depth and extent of flooding for a specific depth (D) in the same vertical units as the DEM, use the simple conditional statement (use CON SA function from “Conditional” toolset) and specify:
- HAND.tif as your conditional raster.
- Expression as: Value is less than or equal to 3 (D is your flooding depth – in your case it is 3).
- 3 as your true raster (D is your flooding depth – in your case it is 3).
- Leave false raster entry empty (meaning all values outside of your flood extent will be nodata).
- Specify output raster (I like to put name of the depth in the output name – e.g. fe3m.tif – for flood extent of 3m, but it can be anything).
That’s it. In the output raster, where you have values, area is flooded and where it is nodata, it is “dry”. If you want to calculate depth of flooding for each cell, just use the MINUS function and use fe3m.tif and HAND.tif as inputs (in that order).
Hope this works for you.
