AutoCAD Civil 3D

[#1]

Glad to see so many people thinking about my question.  The reason I asked was because I was hoping for an easy way to determine the tidal prism for a marsh.  In this case the tidal prism is the volume between the MLLW and the MHHW.

I have the surface for the marsh topography.  I was going to create two more surfaces as planes, one for the MLLW elevation and the other for the MHHW elevation.

I wanted to "clip" the marsh topo with each of these surfaces to create two new surfaces.  One representing the site at low tide, the other at high tide.  From there I would do a comparison surface to get the volume difference.

I just couldn't figure out how to do the "clip" surfaces easily.  Yesterday I wasn't thinking of it in terms of clipping, but now that I am perhaps someone will come up with a really easy way to do what I am asking.

In the end I ended up creating two volume surfaces, one for the volume between the MLLW and the topo and another for the MHHW and the topo.  I then took the difference of the two fill values to get the tidal prism. 

Erik

[#2]

From each of your plane surfaces, turn the border display on, then extract the border from the surface. You can then use this to clip a copy of the marsh surface to create your two surfaces for volume calculation.

[#3]

Easier solution then.

a) use bounded volumes

b) make a flat boxed feature line bigger than (or = to) your area of interest with elevation=LW, then HW, creating a flat surface by adding the feature line as a beakline to an empty surface.

Compute a volume between your marsh surface and your LW, and then again your HW. Ignore the cut volumes, using only the fill (vol between lower marsh surface and higher LW/HW surface). The cut volume is where the marsh is above your water levels and should be insignificant. Break out a calculator and take the difference.