Dam Question

The Dam is located in the City of Lake Mills just west of S Main Street and north of Veterans Lane. The dam is on Rock Creek and impounds Rock Lake. Rock Lake is a natural drainage lake which has a few inlets and one outlet. In 1865, a mill dam was constructed for the purposes of hydraulic power production. This mill ceased operation in 1933 and the mill property was sold to the City of Lake Mills in 1935. In 1940, the location of the dam was moved to the outlet of the millpond where it remains today.

The dam consists of a four-foot-diameter corrugated metal pipe outfall regulated by timber stoplogs. A concrete intake structure with a trash rack that is located upstream from the outfall. There are concrete wingwalls on either side of the intake structure.

The stoplogs consists of 5 boards that are each about 4 feet long and 6 inches wide. The bottom board is not taken out because it is too hard to extract due to the force of the water. That bottom board sits in a concrete base so that about 3 inches of the board is in the water path. Therefore, there are 27 inches of restricted flow by the boards in the dam. Besides the dam, the other water flow confinement is the culvert that is just downstream from the dam. This culvert is 4 feet in diameter.

Debris that collects in front on the trash rack can significantly hinder the flow of water through the dam. City staff is responsible for removing trash every day.

The dam has a very small capacity compared to the size of the lake. Therefore, the City has to maintain the water level at the median of the allowable range.

The lake is about 1,371 acres in size. One inch of lake water is about 5 million cubic feet. The dam, if fully open, can pass about 45 cubic feet per second. Therefore it takes about 72 hours with all boards out of the dam to lower the lake one inch. This calculation does not take into account precipitation, evaporation, runoff from the watershed, or groundwater flow. Therefore, draining off one inch from the lake after a one inch rainfall will take longer than 72 hours.

There is a grate, or trash rack, located just upstream of the dam that blocks debris from flowing through the dam structure. The debris is scheduled to be cleaned out every week day by the City of Lake Mills. Debris in front of the dam has been shown to significantly hinder the water flow through the dam.

The water level regime of the lake set by the DNR generally mimics what the levels would be if there was no dam: lows in the winter, rising levels in the spring, diminishing levels in the fall. However, there are a couple distinct differences - the natural levels would not be as high without the dam; and the natural levels would have more variation and from time to time would have much more drastic highs and lows.

The first option the Council may want to consider is removing the dam; the DNR can't make anyone to keep their dam. They can require repairs and the dam owners’ response can be to repair or remove the dam. A dam owner can apply to remove the dam without any need for dam repairs. In the process of removing a dam, others wishing to own the dam with the financial capability to do so, can come forward to make a deal. A municipality can only transfer ownership to another governmental entity with taxing authority like a town, county, or lake district.

If the Council wants to keep the dam, it should consider management type improvements to the dam. A lake level monitor that can be checked anywhere online and can email notices of high or low water levels. An automatic trash cleaner can save many man hours and overtime costs. The type of gate used can make adjusting the level easier. Modifying the outfall from a corrugated metal pipe to a channel can make this repair last much longer. Combining the removal of the old shoe factory and restoring the ditch will significantly reduce the hazard level rating and make the Highway “89” project easier to design.