# Increasing Flood Plain

I am doing a project where we are attempting to determine how many more buildings would flood if the FEMA flood plain increased by 1',2' and 5'. I have the elev of the flood plain rasters from lidar data, but I am having trouble figuring out how much more area will be inundated by the rises I described earlier. I'm using raster calc and I have a buffer for the flood plain that also has elevation data. Any thoughts?

Flood plain in the county with elevation data (raster)

• Arcmap 10.2, should have specified – Warner Oct 19 '17 at 14:01

You could create 1' contour lines, using your LiDAR data. Then use those contour lines to calculate your 1', 2', 5' increase. Using a basic approach in determining the increase potential and create polygons over those to display.

In addition, you could look into Esri's Floodplain delineation from lidar points, provides a general overview of options of using LiDAR within the ArcMap environment.

Being able to accurately estimate extents of different magnitude floods is a critical application in water resources. This information is needed to assess risk for insurance, emergency management, and planning. Lidar is quickly becoming the predominant source of topographic data for floodplain delineation. This is because it's both accurate and cost-effective.

Flood extents are typically estimated by comparing models of water surfaces and ground surfaces. Water surface profiles for a given flood event (for example, 50-year flood) are created by a hydraulic engineer or hydrologist using tools like HEC-GeoRAS. Each profile represents the height of water across the river at a given location. The length of each profile is long enough to capture the largest possible flood extent going out from either side of the river.

Profiles are generated at reasonable intervals along the river where the flooding is being modeled.

Run the Surface Difference geoprocessing tool using the water surface TIN and a bare earth terrain made from lidar points as inputs.

The primary output from the Surface Difference tool is a polygon feature class.

The Surface Difference tool provides an option to output a difference raster.

• Thank you for your detailed response, but I don't have the water surface profiles. If could generate those from my raster elevations then I would easily be able to increase the height of my flood plain. I have stream center lines, and the raster image that I am going to add to my question, as well as lidar data for the whole county. – Warner Oct 19 '17 at 14:21
• I would look into the first part I mention about creating 1' contour lines and using a basic approach in determining the increase potential and create polygons over those to display. – whyzar Oct 19 '17 at 14:25

Workflow I am using:

1. Split flood outline by points placed at regular interval:
2. Interpolate bits into 3D polylines, using elevation model and compute average elevation (Z) values for them.
3. Identify "impact" areas using either Eucledian Allocation or Watersheds of the bits:
4. Transfer increased levels from the bits to relevant polygons and compute new flood extent using conversion to raster and raster calculator:

Both methods will produce similar results in well defined valleys, although my personal preference is hydrological assignment.