Predict Flood Waters - How To?

[PaulFrey]

Does anybody know how to step by step predict where the flood waters will go given a certain rise in the water body's height?  I have the water body, a DEM, just no idea how to combine all this information.  I can't figure out a way to tell the program the river will be rising. 

Any help would be greatly appreciated.

-Paul

[DonovanCameron]

Hey Paul,

What version of ArcGIS and level of licensing are you using?

Do you have spatial and/or 3d analyst extensions?

Also, what is this for? Personal, business, non-profit, educational?

[PaulFrey]

Donovan,

I am using ArcGIS 10, and have the 3D Analyst and Spatial Analyst extensions.  I have been messing around spatial analyst, but with nothing to show for it.  It's for business, I work for Cattaraugus County in SW New York, the Allegheny River is in the process of flooding, and we would like to predict where the rising waters are going to go.  I downloaded a VB script from the ESRI website that says it does this, but the results it outputs don't seem right at all.

Paul

[DonovanCameron]

I downloaded a VB script from the ESRI website

Which script in particular? I would like to see and examine their methodology.

It sounds like you are doing some floodplain mapping. Have you any site specific data? Like GPS points of the last flood high mark?

These sample points can be used for some interpolation to compare with the DEM you have that can give you a gradient flood zone, instead of a static one that you would get if you were to simply trace a contour at a given height, or you may be left with determining the CFS (cubic feet/sec) runoff from various sources for a given scenario (whether based on the real world or a likely scenario) and that would determine your water level rise amount.


Here is a link to the HEC-GeoRAS GUI for ArcGIS.

[JeffLee]

if you have river cross-sections and you know the rate at which water is rising, a crude way would be at each time step, to assign elevations to each of your cross section, Build a TIN from your cross section elevations, convert the tin to raster and subtract the cross-section elevations from the DEM.  The positive values from the subtraction should give you the extent of your floodplain.