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.
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.
Maitland, NSW, Australia
Windows 7 Enterprise 64bit
Intel core i7 2600 @ 3.40 GHz with 16GB Ram
Civil 3d 2013 64bit
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.
Win 7 Pro SP1 64 bit
Intel i7 3770 @ 3.40 GHz
16 GB RAM
Civil 3D 2014 (IDSP)