Design Flood Hydrologic Analyses

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Prepared by Ayres Associates

​Summary
The Four Lakes Task Force (FLTF) has contracted with Ayres Associates (Ayres) to complete hydrologic
analyses to support the selection of spillway design floods for the Secord, Smallwood, Edenville, and
Sanford dams on the Tittabawassee River in Michigan. The FLTF, a delegated authority of Gladwin and
Midland Counties, MI, is in the process of redesigning and reconstructing the dams following a destructive
flood in 2020, and the selection of appropriate design floods is a cornerstone of that effort.

Design flood hydrographs developed in this study included the Probable Maximum Flood (PMF); the “half-
PMF,” which is defined by Michigan dam safety regulators as the flood resulting from half of the Probable
Maximum Precipitation (PMP); and floods generated from precipitation events having annual exceedance
probabilities (AEPs) of .01, .005, .002, .001, and .0002.

The U.S. Army Corps of Engineers HEC-HMS model, Version 4.7.1, was used to generate flood
hydrographs at various locations throughout the watershed. The HEC-HMS model was calibrated to four
observed events using precipitation time series developed by Applied Weather Associates. This resulted
in four sets of calibrated unit hydrograph and loss parameters. These were weighted for use in the PMF
and half-PMF model based on flood magnitude and calibration quality. For the exceedance probability
flood model, the calibrated parameters were weighted based on calibration quality and correspondence
between the exceedance probability of the rainfall event and the exceedance probability of the modeled
flood. The adopted model was tested against a fifth flood and accepted with no further modifications.

Table 1 summarizes the resulting peak inflows at each of the dams, assuming the spillway capacity that
existed prior to the 2020 flood event. Spillway rating curves used for this study are derived from
calculations provided by GEI Consultants prior to the May 2020 flooding (GEI, 2020). Note that the
Smallwood, Edenville, and Sanford inflows are based on the pre-failure 2020 storage-discharge
relationships at upstream dams. The spillway redesign currently underway will affect these relationships
and therefore will also affect the downstream flow releases for a given inflow hydrograph.

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