# How to create a reservoir for a dam site?

I have created the watershed for a proposed dam site point using SRTM 90m DEM. Now my task is to create a reservoir or pondage where water would be collected from the watershed.

Above a watershed can be seen with a red line on which Dam can be created. A dam also has a reservoir where water is collected and it is always inside the watershed, now problem is to delineate that reservoir.

Practically, reservoir should be in lowest elevation (flat area), so can we create such using Dem data.

• Construct, like going in the field and building it?
– Iris
Dec 1, 2015 at 8:15
• oh..I meant create using arcmap. Dec 1, 2015 at 8:20
• If I am not wrong it should be the lowest elevation surface from the point of dam site to the inside of watershed. Dec 1, 2015 at 8:23
• I can try to be much more clear, there is no need to give my question a negative vote. Dec 1, 2015 at 9:09
• I still don't understand your question? What is a dam site point? And how do you want to create a dam. Do you want to change your SRTM that it has a dam like feature? Do you want to model a 3d-feature? Do you want to create a point feature where you would build a dam in realtion to the length of a river?
– Iris
Dec 1, 2015 at 9:13

I'm not an expert in defining reservoir limits and capacities so my suggestion may be too crude or even flawed?

To pond up behind the dam, you have to know the height of the dam, that seems fundamental to me. For arguments sake lets say its 50m.

2. Identify the height of the cell where your dam will be placed in this clipped elevation, for arguments sake lets say it was 200m.
3. 200 + 50 = 250m which is the maximum elevation will pond up behind.
4. Extract cells that are 250m or less.
5. The boundary of this would be the limits of the reservoir.

Below is an example of the steps I describe above. A clipped out grid, extracting and coding cells below a set height. Note the areas identified by the arrows these are areas outside my crudely clipped catchment which fulfil the requirements. If you have created a watershed using the hydrological tools then this should not occur.

I would suggest using the Contour with Barriers tool. It's available with the Spatial Analyst extension at all license levels.

Basically, you create a polyline/polygon vector layer ahead of time that represents your dam. Then, you designate a starting elevation and it will build contours from there based on your DEM, and then increment at whatever intervals you decide. This allows you to test multiple dam heights at once. To keep things clean and to potentially improve performance, you may want to clip the DEM to your watershed boundary before running the tool, as Hornbydd suggests.

After you have chosen the contour line you like, convert it to a polygon and you have your reservoir surface.

• Seems much easier but polygons aren't formed in arcmap 10.1. Dec 2, 2015 at 9:27
• They aren't formed in 10.3 either. That's why I said convert it to a polygon in my last sentence. There are tools and methods available to convert polylines to polygons that don't require an Advanced license if that is an issue. The real benefit to this tool is that you can experiment with different dam heights all at once to see how it affects reservoir surfaces. Dec 2, 2015 at 15:25