A current client of ours asked for Mark's help with some tech support: 

Mark,

I have a dam that I want to know how much water it will store before the water goes over the dam.

I have an existing water surface elevation, I have an inlet elevation for the spillway pipe, and I have the top of dam elevation.

I want to put a surface at the elevation of the spillway and another at the top of dam and compute the volume between the 2 surfaces.

I ran a horizontal down the middle (as close as possible) of the pond and a vertical at the spillway elevation. 

When I run roadway modeler the surface crosses itself.  What would be an easier way to go about this?

- Mike   

Here is the Zen Dude's response:

Hi Mike!

If I understand your situation correctly I'll make these suggestions / comments:

1)  First, I wouldn't go the modeler route ... it's too much work. : )

2)  Have you ever used the Compute Pond Volumes command? It's in theHydraulics and Hydrology Add-in tool set. This is a pretty good tool for pond volumes when your DTM forms a natural pond. It basically creates another surface at the specified elevation(s) behind the scenes and then calc's the volumes. If the elevation that you input doesn't form a 'pond' then you don't get a volume. The usability of the resulting reporting is a little weak, but it does the trick. It can do incremental volumes as well as a single volume and is pretty intuitive. You just need to use that"Locate" button to pick a coordinate location that is inside the 'pond' limits.

3)  Sometimes the dam design alone isn't enough to get the pond volume using theCompute Pond Volume command. But in some cases you can just merge the dam design into the existing surface to form a 'finished' DTM and run the pond volume from that merged surface.

4)  Another way to approach it is the direction that you took ...build other surfaces and just run a triangle or grid volume between the surfaces. There are a couple ways to create any 'water' surfaces. One way is to just create a 3D graphic in the file at the water surface elevation and then use the Generate Slope Surface command to project a 0.00% slope from that graphic to intercept the existing ground. That will be yourExterior Boundary. Then you can run volumes between the existing and the water surface.

5)  Another way similar to the one above is to just place a huge square or shape at the water surface elevation around the pond area (going well outside of it). Then by using the "Generate Isopach" surface command, you can find the intercept between the water surface and the existing ground. The Generate SlopeSurface command above will do that automatically, but if there are little inlet and undulations in the shoreline the earlier projection might not work.

6)  Also, if you know the water surface elevation, just display contours (with that water surface elevation in mind) and use the water surface contour as your exterior boundary to create the water surface. Viewing Contours is exactly what the Generate Grade Contourcommand does ...but it has the capability to just create a single contour.

I've pretty much laid out the options in the order that I would consider them, but the existing terrain and complexity of the water surface shoreline is a big factor on which option will work best.

I hope I understood your question clearly. Let me know if anything here helps, or if you need additional explanation.

I hope this helped.

Civilly yours,  

- zen