Impacts of Boundary Condition on Unsteady Flow Modeling in Using the UNET Model

Abstract

Since 1995, the Illinois State Water Survey (ISWS) has conducted a series of studies on unsteady flow modeling of the lower Illinois River using the one-dimensional unsteady state flow (UNET) model with a primary focus on the management options of Levee and Drainage Districts (LDDs) for flood peak reduction should overtopping or levee failure occur. The same model also is being used as a tool for evaluating impacts of proposed or ongoing restoration efforts in the Illinois River basin. The Illinois River is one of the major tributaries of the Mississippi River. Most of its floodplains from Peoria to Grafton have been leveed for agricultural purposes through the establishment of LDDs. A total of 24 LDDs were established between 1879 and 1916. The UNET model uses either stage or flow hydrographs as boundary conditions. The Hydraulic Engineering Center’s storage database (HEC-DSS) is used in the UNET model for input and output hydrographs and hydrograph ordinates, which enables easily adapting the model for large river systems and various scenarios. For the lower Illinois River, the UNET model was used to evaluate the impact of using a single LDD or combinations of selected LDDs to reduce peak stages. Spillways of certain sizes were assumed to be placed for floods through the levees. The UNET model also is being used to evaluate the possible reduction of flood stage fluctuations during the growing season by opening selected LDDs or by removing the levees to open a portion or all floodplain areas to the river. It is generally assumed that a well-calibrated and verified hydraulic model will be sufficient for evaluating management and restoration alternatives that involve possible modifications of model geometries. Application of the UNET model for the lower Illinois River showed that the stage and flow relations can be changed drastically by modifying the channel geometries, however. Boundary conditions for the UNET model have to be adjusted accordingly to maintain mass balance for the system. This study presents comparisons of stage and flow relations altered by possible restoration efforts under assumed scenarios, and the problems caused when appropriate boundary conditions were not used in the UNET model for the Illinois River system. Copyright ASCE 2004 World Water Congress 2004