Modeling the Hydrology and Hydraulics of the Cache River System

The Cache River is located in the extreme southern part of Illinois, just north of the confluence of the Ohio and Mississippi Rivers. Major floods in both of the two rivers have significant impact on the hydrology and hydraulics of the Cache River. In 1915, a cutoff to the Ohio River was constructed east of Karnak that resulted in sub-dividing the Cache River watershed into the Upper and Lower Cache River watersheds. The Upper Cache River watershed consists of the eastern part of the watershed draining directly to the Ohio River through the Post Creek Cutoff. The Lower Cache River watershed consists of the western part of the watershed draining to the Mississippi River through a diversion channel at the outlet. However, part of the Lower Cache River can at times reverse flow, draining to the Upper Cache River and Post Creek Cutoff. Hydrologic and hydraulic models have been developed to simulate the hydrology and hydraulics of the Cache River system to evaluate flow and water level conditions along the river under different alternative scenarios that can be implemented as part of a restoration effort for the Cache River. The hydrologic model that simulates the rainfall-runoff process for tributary watersheds is based on the HEC-HMS model. The model is used to compute runoff from tributary watersheds for selected storm events. Outputs from the HEC-HMS model are then used as inputs to the hydraulic model, UNET. The UNET model, a one-dimensional unsteady-flow dynamic wave routing model, is capable of modeling the complex hydraulics of the Cache River System that experiences flow reversals during flood events in the Lower Cache River.