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Tuesday, April 2, 2013

A Big Thaw, and a Big Dam Problem

But eventually springtime asserts itself, thawing temperatures and snowmelt, and the creek swells to bank full and all the little draws and gullies and seeps and swales are flowing vigorously on their own. The log bridge at Squeaky Tree washed away, and quite a bit of sediment moved downhill, as some of the gullies moved uphill. The bluff wants to fall into the Mississippi floodplain; all we can do is try to slow it down, 'cause we can't stop it.

This was one of the largest flooding events we've witnessed here – a bank-full event, maybe a 3-month or 6-month return period flood. I took these photos several hours after the peak had passed through.

The ford is one place we can usually rely upon to have a crossing point, but I had to go back upstream and use one of the large-log animal crossings, a bit precarious.

Just to the right of this photo's field of vision is the weir at the road culvert. Apparently, when it was built, the creek bed was perhaps fifteen feet lower, and what was at first a deep, cool swimming hole and wetlands eventually filled in with fallen down bluff and created the meandering creek we have now. With the outlet controlled by the wier, the creek through the farm will continue to look more U-shaped than V-shaped, except at the edges.

The gullies will require a different treatment than benign neglect, actual structures, terraces, impoundments, weirs, energy dissipation.

This is the bottom terrace in the observatory field, the big 11-ac field that comprises the southwest quarter of the farm. Like the other terraces, its drainage system failed some time ago, long enough that this willow colony has grown up and thrives on the periodic ponding.

With the drainage pipe failed, runoff ponds behind the terrace until it overtops, and erosion works back until there's not much earthwork left. We continue to cogitate about how to move big chunks of concrete – a raw material we have in ample supply on the property – to build erosion control structures. We're thinking of building a derrick or crane with a winch, powered by tractor hydraulics or pto or independently.

The pond rose dramatically in this thaw, to a level we'd not seen before but still well-below the outlet level of the primary spillway, a PVC pipe with an elbow joint into a corrugated metal pipe that passes through the dam embankment. The previous owners insisted to Alan and Donna that the pond had always been at the spillway level during their two years on the place.

You can see how much the ice sheet lifted, and how much larger the pond surface area became.

And then we noticed a sound of rushing water in the spillway pipe. The sound grew louder over the next couple days, and the pond level was dropping.

We had to conclude that the ice had damaged the outlet and we now had a big leak. 

There was nothing for it. The connection of PVC to CMP was rudimentary, ill-fitting, and the adhesives and clamps had crapped out. There was little choice but to pull it apart, and start to dewater the pond to the point where we could access the pipe and make repairs. After we pulled off the riser, water was free to gush down the CMP, and it made quite a flood at the outlet.

Here's the mysterious outlet works. Mysterious in that we can't figure out what the hell is going on here. There's a valve somewhere down there that we've not dared to turn. Below that is a crude well made of a tractor wheel rim plastered in concrete. It has an inlet, presumably from the valve, and an outlet through flexible plastic pipe.

But the well has rusted through and the outlet level is now below the drain pipe, so the water cut a channel instead. All the flow you see here is a result of the broken inlet up on the other side; we'd only ever seen seepage conditions here before.

So, all that set us a pretty good puzzle.

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