Last winter when Prospector was small, I was writing two books and doing some web training, but I also took on a little bit of work with the local NPDES (stormwater and erosion control) review agency to stay sharp and get out of the house now and again. One of the things I worked on was helping them with some standards for infiltration requirements.
I know this doesn't work everywhere, but in coastal, sandy areas like where I live, we can have infiltration practices receive our runoff. This can take the form of dry ponds, infiltration trenches, porous pavement and underground infiltration systems. The Center for Watershed Protection has a slide show on infiltration practices. If you can get past some of the formula slides, there are images of some common examples. I also found this listing of innovative stormwater designs that you can tour in Montgomery County, PA (which is actually where I grew up!) Examples are things like porous pavement, biofiltration, naturalized wetlands and more. The listings also include pdfs with plans, pictures and narrative about each practice. Definitely worth checking out. Click on the image below to see the pdf about porous pavement at Johnson and Johnson's facility.
In Hydraflow Hydrographs, you can enter a value for exfiltration rate. (It's exfiltration here because its _leaving_ the practice, infiltration when _entering_ the soil.)
One of the issues that the regulators were running into was that there would be variable infiltration rates across the proposed practice surface. Just taking a regular average of the rates would be either too conservative or too optimistic. In the example below there are test locations ranging with very variable rates. (These rates could be min/inch, inch/hour, or however your jurisdiction would want them shown.)
I am always looking for ways to get Civil 3D to help solve these problems.
Now, of course you will have to check with your local regulators, but one idea they came up with was, at least for conceptual design and preliminary calculations, was to create zones of infiltration rate.
One way you might do this is to turn your boreholes/test pits into points with the infiltration rate as the Z value (or if you already have surface information you can use user defined properties, but I will save that for another day.)
Then doing an elevation analysis, you get something like this.
The trouble with the above image is that it doesn't cover the whole bottom of the pond. Of course, you will probably need more boreholes/test pits, but if you are doing a feasibility study or trying to work out if this is a good area to place your pond, you can try this.
Make a feature line connecting the points around the perimeter of your surface and move that feature line up to surface elevation (1). Then offset that feature line until it is well clear of the pond bottom (2).
Then add that feature line to your surface.
Add the pond bottom as a nondestructive breakline boundary and it will clean up around the area you want.
Add an elevation analysis legend table. This will give you information about how much area has what range of infiltration rates. You could then do averages of these ranges. Alternatively, you could make contours or perhaps even add labels to extrapolate infiltration rate in a certain zone. Be sure to work closely with your soil scientist, geological/geotechnical professional and regulators to find out what they are comfortable with, and how they would like to see you build your model. You would likely use this as a starting point for further testing and investigation.
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