A lot has been said and written about the proposed draw down of Wixom Lake as a necessary component of the FERC-mandated PMF construction. The drawdown is required during the phase of construction in which the sheet pile cofferdam is installed while work on concrete pier extensions takes place.
The pier extensions provide the ability for “stoplogs” to be installed in front of the spillways and for the sheet pile cofferdam to be removed. All of this makes it possible for the rest of the construction on the spillway to be done “in the dry.”
While the cofferdam is in place, half of the dam’s ability to pass flood waters is out of service. That is why the drawdown is necessary, and why it takes place in summer. The drawdown is a precautionary measure.which provides a “buffer” in the reservoir in case of an unexpected flood, so that the one remaining spillway structure has a chance to pass enough flood water to avoid dam failure. While rare, floods have been known to happen in summer.
The FERC-mandated construction schedule calls for this to take place on the Tobacco dam in the summer of 2014 and on the Tittabawassee side (the powerhouse) in summer of 2017.