Is that a leak? Corps says dam was 'designed to leak' and seepage is normal
Jun. 12—During a personal tour of the Tuttle Creek Lake dam during the summer of 2019, Manhattan resident Matt Coltharp saw water seeping from the hillside downstream of the stilling basin.
That summer, the area fought record-high water levels and U.S. Corps of Engineers officials debated whether they'd need to open the spillway gates. At the time, the seep near the southern end of the lake was surrounded by a makeshift sandbag wall with a plastic pipe wedged in between to help temporarily direct flow.
Coltharp said as a concerned resident wanting to seek more information on the matter, he found it difficult to find direct updates on the seep. If seepage forces are large enough, they can cause soil erosion around the foundation of a dam, which can in turn cause eventual failure of the structure.
"I'm not concerned to the point where I'm sitting at my house going, 'What was that noise? Was that the dam giving away?'" Coltharp said. "It was more like that when we were in the flood event. Right now the dam itself doesn't have as much pressure behind it, but (it's) a very concerning thing. Kansas is the main wheat producer. If that dam were to have any issues and it were to flood our lands, it takes out the national food chain. It affects more than just people's homes."
Constructing a completely watertight dam is difficult, lake officials said, and Brian McNulty, operations program manager for the Tuttle Creek Lake U.S. Army Corps of Engineers, said seepage is to be expected. The Corps has monitored seeps in the hill sides just south of the dam essentially since the dam's construction in 1962, he said.
"The dam itself in general is built on a sand foundation so it's designed to leak," McNulty said. "The four ditches that run through the state park through the River Pond area, that's all controlled seepage that's coming out of the foundation of the dam through a series of wells at the base of the dam. This is just another location because of the amount of water that's stored on the uphill side of the dam."
When water levels lowered enough, officials investigated this seep by excavating the area around it. They linked it to construction of a recently-installed electric line that had been placed to facilitate ongoing major construction projects at the dam.
Since then, the Corp has installed a filter and permanent monitoring feature to regularly check on flow levels, which they do monthly.
"Part of our purpose was we monitored (seepage) while the lake was up," McNulty said. "We continued to monitor that seep as the lake fell and then we correlated it to the lake levels. Engineering did some verifications both during the monitoring process and once the lake came down. We did some excavations in that area and verified it was indeed coming from that electric line that we installed a few years prior. We've done some things to continue to be able to monitor that."
Pressure pushes water under and around the dam, which McNulty said is captured in a controlled fashion and released back to the surface. For this seep in question, McNulty said they run the water through the back wall drainage system that is part of the construction project nearing completion. The water comes up to the surface through a newly-installed manhole that is located adjacent to the basin and eventually runs through a pipe back into the basin or downstream.
"That's pretty standard to put a filter system on it," McNulty said. "It is designed to move some material to filter, captures that and keeps it from increasing and carrying more material, so that's why we monitored it with the sandbags during the flood. Then we put a system in to filter and monitor it basically permanently after the flood was over."
