Dam Committee Report, July 11, 2007

In mid June with the help of Joe Hejnal and Kevin Seelbach, we extended the leak pipe to obtain flow measurements on the leak in the dam. With the help of Clarue and Marty Holland on June 25 we measured the flow rate of the leak. For multiple tests it took 2.5 to 3.0 sec to fill a 12.5 gal container. This puts the leak flow rate at 250-300 GPM. This compares to a reported leak rate in 1988 of 50-60 GPM. So the flow rate today is about five times what it was in 1988.

So how much drop in the level of the lake is due to the leak? Lake Tish is a 120 acre lake, so a leak rate of 250-300 GPM will lower the lake about 1 inch per week or 4 inches per month.

On Friday July 6 dam engineer Don Eskridge was brought in to make observations on our dam and discuss what options we have with regard to the leak. Don knows much of the history of our dam and was involved with the installation of the seepage collector in 1988.

Here is a short summary of what he said:

There are two problems associated with the increase in the leak flow rate from 50 GPM in 1988 to 300 GPM in 2007.

The first is that we may be approaching the limit of the seepage collector. If the capacity of the seepage collector is exceeded, water will start to seep out of the back of the dam, rather than be carried harmlessly to the toe of the dam as it is now. Water seeping out of the back of the dam would be of immediate concern and require immediate action.

The second problem with such a high flow rate is water conservation. At the current leak rate, the lake level will drop one inch in 7.5 days due to the leak. But even if we completely stop the leak the lake level will still drop. Additional water is lost thru evaporation and other minor leaks thru the back of the dam and the spillway. For 7 hot, rainless days in July the lake level dropped 2.25 inches. Based on measurements the leak is only responsible for 40% of this drop.

Don concluded by saying that as long as there is not a dramatic increase in leak rate or the leak water becomes muddy, we should have one or two years to build a reserve fund to repair the dam. But we have to do something in the next few years since the leak will not fix itself and will only get worse.

A report of this meeting will be prepared soon.

-- Rich Hirsch for the Dam Committee