Day 2 @stcNCED @saflumn – check your levels

Yesterday’s trial produced one overwash lobe; today’s produced two. Getting there.

I spent much of today off by an eighth of an inch. I misaligned the blocks on the screed we’re using to level the planar surfaces, reminding myself of the classic construction adage to “measure twice, cut once.” After a lot of frowning and head-scratching, I found my error – and it was mine. (Note that Kyle, the undergraduate helper majoring in aerospace engineering, did not misalign his block.) Realigned the blocks, re-screeded the berm and plain, rechecked and re-rechecked that all was level. Scanned the initial topography. Turned on the water. Waited anxiously beside the tank, peered into the tank, clambered around on the catwalk behind the tank, for thirty minutes as the reservoir filled.

The site of the initial breach was nearly a horse race. The left? The right? Will it be the middle? (No, it would not be the middle.) From my vantage looking downstream, the left side crested first, followed a few seconds later by the right. The right then dominated the remainder of the run, churning itself down to a stable grade and stranding the left channel high in the berm profile. Meanwhile, all quiet on the middle front.

Reset.

At the far end of the flume, where the pond liner folds over a weir and drapes into a six-inch PVC drainpipe, a few tiny lobes had formed. Unintentional, but heartening. (I pointed them out to anyone who stopped by – “That’s what we’re going for!” I’d say. They’d say, “What are you pointing at?”)

Today I added a rough barrier at the downstream end of the “back-barrier” flat.

Reservoir, from the right.

Reservoir, berm, and plain, from the left.

Scanning the initial topography.

A few cm shy of the crest – lots of groundwater seeping from the downstream toe.

Water atop the berm…

End result? Two lobes.

Meanwhile, at the outfall, this had happened. (Lobes!)

Initial topography – fisheye and corrected.

Final topography – fisheye and corrected.

What today confirmed is that I started with a berm that was far too high. But this is what getting acquainted with a tank involves. Much of yesterday’s work focused on where to put the existing sand in the flume. Pile it here? Spread it out there? How big should the reservoir be? Utilitarian concerns had set the berm height – but it took me until this afternoon to remember that.

Last moves of the day were back on the screed. We cut down the berm by 5 cm (from 15 to 10 cm), then five more again. First thing tomorrow morning, I’ll turn on the water and see what this latest berm, set 5 cm above back-barrier platform, has to show us.

– EDL