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

What is the “PMF”?

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 FERC’s mandate, the capacity of the six spillways will have to be increased to 64,000 cfs (or 28,800,000 gpm)

What needs to be done?

Spillway ComparisonCurrent configuration of spillways

Tittabawassee side adjacent to powerhouse shown; Tobacco spillway is similar

Spillway comparison

FERC-Mandated alteration of spillways

Tittabawassee side adjacent to powerhouse shown; Tobacco spillway is similar

3-Dimensional ViewsExisting spillway construction

3-Dimensional ViewsFirst concrete pour inside spillway (summer 2013)

3-Dimensional ViewsNew concrete pier extensions (summer 2014)

This work takes place “in the dry” behind a sheet pile cofferdam

3-Dimensional ViewsStoplogs are placed between piers & rest on new concrete sills

Once stoplogs are in place water levels can be returned to normal

3-Dimensional ViewsConstruction work taking place between 2015 and 2016

More concrete is

poured in the base of the

spillway

Existing gate and bridge are

removed

3-Dimensional ViewsConstruction work taking place between 2015 and 2016

Concrete rollaway slabs and most of upstream barrel arches are removed

3-Dimensional ViewsConstruction work taking place between 2015 and 2016

New concrete crest is poured

3-Dimensional ViewsConstruction work taking place between 2015 and 2016

Concrete overlay is placed over entire pier structure

3-Dimensional ViewsConstruction work taking place between 2015 and 2016

New bridge is constructed

3-Dimensional ViewsTobacco Spillway to be completed in 2016

New gate is installed and stoplogs are removed

3-Dimensional Comparison

Existing spillway Modified spillway