The Cache River basin located in southern Illinois has characteristics that are unique in the State of Illinois and the nation, with its diverse physical, chemical, and biological features that produced a great diversity of natural communities. Because of these unique characteristics, the Cache River basin contains some high quality bottomland hardwood forests and wetlands that have been recognized nationally and internationally. However, changes in land-use practices and hydraulic modifications during the last century have significantly threatened the ecological integrity of some of these valuable habitats and wetlands. To sustain their value and importance, these habitats need restoration and protection. One of the key goals of resource managers working in the area is to restore the Cache River’s natural hydrology to a level that can sustain a viable ecology throughout the river corridor. To evaluate the results of different restoration measures, the Cache River Joint Venture Partnership needed reliable hydrologic and hydraulic models. The Illinois State Water Survey developed calibrated hydrologic and hydraulic models and evaluated the hydrology under current conditions and under various restoration scenarios. Results then were compared to the reference/base condition. The reference/base condition refers to the condition when the hydrology of the Lower Cache River was controlled on the east end by Karnak Levee with two 48-inch gated culverts that prevented flow from Post Creek Cutoff into the Lower Cache River and by in-channel weirs at Route 37 and “Diehl Dam” located west of Long Reach Road. The top elevation for “Diehl Dam” was set at 328.4 feet above mean sea level. After analyzing all the scenarios considered with different combinations of flooding conditions, structural changes, and boundary conditions, the study conclusions can be summarized as follows: 1)The current condition exposes the Lower Cache River corridor, especially the eastern portion, including the community of Karnak, to more flooding during major floods, such as 100-year or greater floods from the Upper Cache and Ohio Rivers. However, the current condition improves flood drainage for some parts of the area during more frequent 1-, 2-, and 5-year floods. 2)Installing the East Outlet Structure with stop logs and three or more 72-inch culverts will lower flood elevations from the reference/base condition for the portion of the river east of Karnak Road Bridge, including the community of Karnak, because of increased outlet capacity of the larger culverts. 3)Moving “Diehl Dam” 2,800 feet from its current location under current conditions will increase the area flooded by the 100-year flood by only 8 acres. The additional acres flooded are distributed in small increments throughout the Lower Cache River floodplain. Water levels in the stream channel between current and proposed locations will be higher than the current condition during low- and moderate-flow conditions. 4)Partially reconnecting the Lower Cache River with the Upper Cache River by diverting some flow from the Upper Cache to the Lower Cache River will not increase flood elevations from the reference/base condition during major floods such as a 100-year flood but will raise flood elevations during more frequent 1- and 2-year floods. During low- and moderate-flow conditions, reconnection will create slow-moving westerly flow in the Lower Cache River and will not cause flooding.
|Original language||English (US)|
|State||Published - 2008|
|Name||ISWS Contract Report 2008-01|