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Problem: How to get a raster based on features to cover the floodway zone We are creating a flood hazard map in the following way: Take FEMA's flood hazard polygons and create a shapefile of the 100 year floodway zone Take the address points of residences in the county and clip to that floodway zone Take contour lines from the city and county and join them, then clip to the floodway zone Rasterize these contours (topo to raster). This is a digital elevation model of the actual terrain in the floodway zone. Take FEMA's base flood elevation polylines and rasterize based on elevation (feature to raster). This is a digital elevation model of FEMA's estimates of the elevation of the water surface during a 100 year flood event. *(This is the problem) Then simply use Multi Values to Points to add the fields for the elevation of the address and the elevation of the water surface during a flood. Create a difference field in the attribute table to be symbolized on the map (the lower the difference between the elevation of the address point and the elevation of the BFE, the greater the flood danger.
The BFE lines when rasterized do not fill the entire floodway zone like the raster based on the contours (because the contours were clipped to the floodway, then rasterized). The BFE lines are only 1 ft in elevation apart so any data cells between the two lines should be easy to estimate, but in terms of latitude and longitude, they are up to 1/8 mile apart. Simply increasing the cell size leads to unacceptable estimation. Since we cannot capture all of the addresses in the floodway, this model has limited value.
Is there a way in ArcGIS to extrapolate the rasterized BFE lines to fill the same area as the floodway? BTW, we already tried extending the BFE lines (before rasterizing) with the extend lines tool but without success since some of the polylines have their axis in the middle of the stream channel and diverge (also, extending the lines before rasterization doesn't help fill the gaps between BFEs that are quite far in distance apart). The BFE's vary in length and distance apart but do not vary in that every BFE is 1 ft in elevation different than the next BFE up or down stream. We simply need to fill in the small amount of gaps in the BFE raster that don't cover the entire floodway area.
I would not use the BFE lines to calculate the flood elevation at any point. These lines are estimates that are rounded up/down to the nearest foot. I would only use the Cross Section (S_XS) line to calculate the flood depth. You can rasterize the XS as a single elevation (using the 1% annual chance elevation), once you clip them to the floodway in your case, and then interpolate the depth between them. This will leave some room for error since there could be a large drop of elevation and there is not a XS nearby.
For the flood data that you are using, was it done recently? Do you happen to have any Non-Regulatory products as part of the final deliverables?
One other thing to look for may be to get the engineering models that were used to calculate the flood depths and use those rasters.
George: We are using FEMA 2007 BFE Data. I'm a student getting my GIS certificate interning with a county Office of Emergency Management in SW Colorado. The office is interested in this information in order to be prepared for a flood event similar to what struck Boulder, CO last September. The data I described is the only data I have been provided so I am not familiar with S_XS lines (are these the BFE lines before being rasterized? They are 1 ft. incremental cross section surveys that have length equal to the width of the floodway). I searched ArcGIS tools, did a google search, an ArcGIS help search, and did not return anything useful so I am assuming this is georeferenced surveying data. I will find out if this information is available to me. Also, unfortunately I am not familiar with Non-regulatory products (again, my searches for information were inconclusive). Since the BFEs are only 1 ft. in elevation apart and follow the floodway, it should be easy to estimate the elevations of the cells in between the BFEs (or so I thought). Once the rasters are in order, we will use the Multi Values to Points tool to associate the addresses in the floodway with the ground elevation of that point and the BFE elevation of that point. Then add a new field in the attribute table being the difference between the ground elevation and the BFE. The address points will be symbolized by this difference field, the addresses with the lower differences being in the most danger if we experience a flood event. This data can then be shared on ArcGIS online and other mediums so that it is available to emergency responders and they have a solid idea of where people may be at risk. Thanks for the response, - JB