topic Re: Flood Hazard Mapping Question in Water Resources Questions

Flood Hazard Mapping Question

JeromeBernard — Wed, 02 Apr 2014 18:53:03 GMT

We are mapping flood hazard zones and I am wondering if anyone could help me with the following:
We have an address points shapefile located within a FEMA flood zone- addresses only, not Lat/Long coords
We have 5 ft. interval contour lines (we may be able to get lidar based 1 ft contours)
We have the FEMA polylines which give base flood elevation estimates for a 100 year event
We can obtain a DEM of the area if needed
The final product of this project is to create a query or model which will add three new fields to the address points attribute table: 1. Elevation of the address point based on the contours (or DEM if necessary), 2. the FEMA flood elevation associated with that address point and 3. the
difference between the two. The lower the difference between the flood elevation and the address elevation, the more at risk that address is (which can then be symbolized).
The County Office of Emergency Management will use this model to assess which homes are at the greatest risk during flooding (and show FEMA the grant $ is being used properly).
Any tips, pointers or suggestions which tools will be needed would be greatly appreciated. We will soon have access to the complete ArcInfo package and associated tool extensions.

Re: Flood Hazard Mapping Question

MattMead — Thu, 03 Apr 2014 15:20:02 GMT

Good news and bad(maybe) news.

The good news is that what you have outlined there is really pretty easy to set up.

you will need:
Your structure point file
FEMA's BFE lines
DEM/raster of the terrain data
Spatial Analyst license

Step 1. FEMA BFE lines need to be interpolated into a raster.
Step 2. Use this tool: Spatial Analyst Tools ->Extraction ->Extract Multi Values to Points: Point file is your structures file, load in both your terrain DEM and the raster of your water surface elevations.

This creates fields in your existing structures point file with the name of each raster and attributes it with the raster value at each point's location.

Step 3. Create a field in your structures file that you can then use field calculator to calculate the difference in.

So that is all you need to do to create the file you are looking for.

The bad news though is that based on what you say it is to be used for, its possibly not going to accurately give you the information you want. The reason I say this is that FEMA is either going to care about first floor elevations or the lowest adjacent grade which is probably not where the point was placed. If you have access to a structures footprint file there is a way that you can punch out holes in the terrain raster and then calculate the minimum value for each...Spatial Analyst->Zonal->Zonal Statistics(input feature is the structure file, raster is your terrain, stats type is minimum). Doing that and using its output instead of the terrain data in Step 2 should give you something a bit closer to what you are looking to do, but again it requires the structure outline which can be a royal pain to develop if its not available. Sometimes assessors or other county entities(planning dept maybe?) have first floor elevations. If you can get them in a table you could do a join based on address or parcel ID or something.

Re: Flood Hazard Mapping Question

JeromeBernard — Fri, 04 Apr 2014 16:42:06 GMT

Thank you very much. I will start downloading a DEM and try this technique!