Friday, October 28, 2011

Permeability and the Emriver Stream Table

Got the students to do some groundwater stuff in lab today. One of the steps was to estimate the permeability of a column of sand. I used the falling-head permeameter rig that I had videotaped and demo'd earlier.

But instead of a third column of sand, I tossed some of the plastic media that comes with the Emriver setup. If you've seen these stream tables in action, you know that groundwater flow is pretty important in determining bank stability and, ultimately, what the open channel is doing.

Here's the rig:

Here's a scatter plot of all the measurements made by me and the students. For the most part, my measurements are the series that don't vary too much, while the student measurements are all over the place (well, within about 20% anyway)

This ins't a really good way to display univariate data - it's hard to determine any kind of tendency or something like that. So if I collect all the calculated permeability values and display them in a box plot, you can see that the permeability of the plastic media is about 0.3 cm/sec.

So the permeability of this material is really pretty high. You could decrease permeability a little bit more by compacting the media, but there's typically a lot of water that's flowing through the media as "groundwater." So if you're sketching up a plan for Emriver experiments, accounting for groundwater flow is probably a helpful step - especially if you're trying to produce steep gradients in a stream bed. The extra groundwater flow will tend to encourage movement of the plastic media in directions you may not expect at first.

1 comment:

  1. Very interesting, thanks! Yes, the permeability of the Em2's plastic media is high. It could be reduced by adding more fine material. Which would then clog the pump/filter and require the dreaded *maintenance*. That's the sort of trade off we agonize over.