In ArcGIS how can I show the area that will be flooded if I know the height, length and width of a dam?
Mapping Center Answer:
First, you will need a DEM, and you will need to know:
- The location of the dam
- The elevation at the base of the dam
- The elevation at the top of the dam
First, determine the watershed from the location of the dam (we will call this the "pour point"). To do that:
- Use the Flow Direction tool with the DEM as the input raster.
- Then use the Watershed tool with the flow direction grid as the input raster.
Second, reclassify the DEM so that you can find the elevations that are between the base of the dam and the top of the dam. Because the water elevations behind the dam will vary by season (lower water in drier seasons), you can also set the classes to show the variations in water elevation behind the dam. But be sure to set one class break at the elevation of the top of the dam so you can see what the maximum storage capacity will be.
For example, in my test, I used 10m increments to set 20 classes, starting at the approximate elevation of the dam. The elevation of my hypothetical dam was 1389 feet, so I used 1390, 1400, 1410, etc… as the class breaks. There were 20 total classes with all values over 1580 in the last class. Keep note of what each class means as the output classes will not reflect the range – mine were simply "1", "2", etc… up to "20".
Now you can determine the area within the watershed behind the dam by using the Combine tool to combine the watershed raster with the reclassed elevation raster so that a unique output value is assigned to each unique combination of the input values. The result I got looked like this:
From this, you can determine the location of the water behind the dam. In my test, anything that is blue is what could potentially be behind a 200m high dam at the location of the pour point.
To help, I zipped up my data set and attached it here so you can take a look at it.
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