The Probable Maximum Flood (PMF) is an event calculated by FERC to occur once every 10,000 years By comparison, the flood of September, 1986 was a 100-year flood event The FERC requires all high hazard dams to be able to safely pass the 10,000-year flood without failing (overtopping) FERC considers Edenville to be a high hazard dam because its failure would likely result in loss of life and property at Sanford Lake and downstream

Six spillways will be reconstructed to double their capacity to pass flood water There are 3 spillways at the Tobacco River side of the dam and 3 spillways adjacent to the Edenville powerhouse on the Tittabawassee River side The total capacity of the six spillways is currently 32,000 cubic feet per second (cfs) (or 14,400,000 gallons per minute) To comply with FERCs mandate, the capacity of the six spillways will have to be increased to 64,000 cfs (or 28,800,000 gpm)

Current configuration of spillways Tittabawassee side adjacent to powerhouse shown; Tobacco spillway is similar

FERC-Mandated alteration of spillways Tittabawassee side adjacent to powerhouse shown; Tobacco spillway is similar

Existing spillway construction

First concrete pour inside spillway (summer 2013)

New concrete pier extensions (summer 2014) This work takes place in the dry behind a sheet pile cofferdam

Stoplogs are placed between piers & rest on new concrete sills Once stoplogs are in place water levels can be returned to normal

Construction work taking place between 2015 and 2016 More concrete is poured in the base of the spillway Existing gate and bridge are removed

Construction work taking place between 2015 and 2016 Concrete rollaway slabs and most of upstream barrel arches are removed

Construction work taking place between 2015 and 2016 New concrete crest is poured

Construction work taking place between 2015 and 2016 Concrete overlay is placed over entire pier structure

Construction work taking place between 2015 and 2016 New bridge is constructed

Tobacco Spillway to be completed in 2016 New gate is installed and stoplogs are removed

Existing spillway Modified spillway