Dam Leak Questions/Answers, August 14, 2007

Q: How much water are we losing due to the leak?
A: We are losing approximately 300 GPM due to the leak. Losing this much water will drop the lake 1 inch in 7.5 days or about 4 inches per month. In the 1990s the leak flow rate was measured at 50 to 75 GPM, so the leak is about 6 times worse today than it was 10 years ago. 

Q: Is the lake level dropping only because of the leak?
A: No. For the four week period July 4 to August 1 the lake level dropped 10 inches. Four inches of that drop was due to the leak; Six inches was due to evaporation and other seepage. So in July the leak was responsible for 40 percent of the drop in lake level and other sources (mainly evaporation) were responsible for 60 percent of the drop in lake level. In the winter when evaporation is less important, you'd expect the leak to be the primary cause of a drop in lake level. As of Monday August 13 the lake is down 23 inches.

Q: Why does the dam leak?
A: Missouri is a cave state and limestone forms the base of the lake and the dam. The limestone along the north side of the dam contains fractures that permit water to escape from the cove nearest the dam. Over time the water has eroded the limestone and as a result the leak has gotten worse.

Q: How do the engineers recommend that the leak be repaired?
A: Both Dave Taylor and Don Eskridge recommended drilling thru the top of the dam into the limestone and grouting with a plastic material that will swell and seal the cavity.

Q: How much will it cost?
A: Eskridge estimated the cost to be in the $50,000 range; Taylor estimated $60,000 to $100,000.

Q: When was the leak last repaired?
A: In 1965 the leak was reduced to 50 GPM.

Q: What happens if we do nothing?
A: There are two problems:
•	At some point the capacity of the seepage collector will be exceeded and water will start coming out of the ground at the back of the dam. This would require immediate action. Eskridge has calculated the capacity of the collector to be 1350 GPM, so the current flow rate of 300 GPM is not taxing the collector at this time.
•	The lake level is dropping 4 inches a month due to the leak. And it won't get any better without our intervention. As long as the leak water remains clear and the flow rate doesn’t increase dramatically, the dam is apparently not in danger.

Q: What is the next step?
A: We know what to do and who should do it. We don't know when it will be done or how much it will cost. Future work will involve contacting Dave Taylor and getting him more involved to try to get a better idea as to when he could start work and how much it will cost.

[The following questions were answered by Don Eskridge -- RHH]
Q: What if we built a standpipe straight up until the water stops traveling upwards?
A: Well, that hasn’t reduced the pressure any, so the water will find a way to flow out somewhere else. All the standpipe does is show you how much pressure the water is under based on the height of the water.

Q: Is there any equipment to help find the leak?
A: Yes, most likely equipment is acoustic emissions. It’s an extremely sensitive listening device that is lowering down a hole and measures the noise. Where the noise is the loudest is closest to the zone of the leak. You do that in a triangular pattern so you can triangulate in to where the problem is. There is a possibility that with the flow at 300 GPM that there would be so much noise that the device would be overloaded and not work. Unfortunately it ain’t cheap.

Q: What about pumping the leak water back into the lake?
A: Nothing wrong with that. You have power in the area so it would not be diffi cult getting a drop. To pump 300 GPM, 100 feet of head, you need a 10 Hp pump. If power costs 7.2 cents per Kw, we’d need $0.72 per hr to run that pump = $518.40 per month. But it doesn’t fix the leak.

Q: Someone suggested that the current leak is a pressure relief valve and that if we plug this leak by drilling thru the top of the dam and grouting, the dam will just leak somewhere else.
A: By drilling and grouting you are creating a small bathtub wall. And if there’s a hole beyond the edge of where we grouted, that the water can eat into and work its way around, it will. The pressure relief valve analogy is not 100 percent accurate.

-- Rich Hirsch for the Dam Committee