| | 1: | | Title: | | | | | Volume/Number: | 2000 | | | | Issuing Agency: | | | | | Description: | Flooding, upland soil and streambank erosion, sedimentation, and contamination of drinking water from agricultural chemicals (nutrients and pesticides/herbicides) are critical environmental problems in Illinois. Upland soil erosion causes loss of fertile soil, streambank erosion causes loss of valuable riparian lands, and both contribute large quantities of sediment (soil and rock particles) in the water flowing through streams and rivers, which causes turbidity in sensitive biological resource areas and fills water supply and recreational lakes and reservoirs. Most of these physical damages occur during severe storm and flood events. Eroded soil and sediment also carry chemicals that pollute water bodies and stream/reservoir beds. Court Creek and its 97-square-mile watershed in Knox County, Illinois, experience problems with flooding and excessive streambank erosion. Several fish kills reported in the streams of this watershed were due to agricultural pollution. Because of these problems, the Court Creek watershed was selected as one of the pilot watersheds in the Illinois multi-agency Pilot Watershed Program (PWP). The watershed is located in environmentally sensitive areas of the Illinois River basin; therefore, it is also part of the Illinois Conservation Reserve Enhancement Program (CREP). Understanding and addressing the complex watershed processes of hydrology, soil erosion, transport of sediment and contaminants, and associated problems have been a century old challenge for scientists and engineers. Mathematical computer models simulating these processes are becoming inexpensive tools to analyze these complex processes, understand the problems, and find solutions through land-use changes and best management practices (BMPs). Effects of land-use changes and BMPs are analyzed by incorporating these into the model inputs. The models help in evaluating and selecting from alternative land-use and BMP scenarios that may help reduce damaging effects of flooding, soil and streambank erosion, sedimentation (sediment deposition), and contamination to the drinking water supplies and other valuable water resources. A computer model of the Court Creek watershed is under development at the Illinois State Water Survey (ISWS) using the Dynamic Watershed Simulation Model (DWSM) to help achieve the restoration goals set in the Illinois PWP and CREP by directing restoration programs in the selection and placement of BMPs. The current study is part of this effort. The DWSM uses physically based governing equations to simulate propagation of flood waves, entrainment and transport of sediment, and commonly used agricultural chemicals for agricultural and rural watersheds. The model has three major components: (1) hydrology, (2) soil erosion and sediment transport, and (3) nutrient and pesticide transport. The hydrologic model of the Court Creek watershed was developed using the hydrologic component of the DWSM, which is the basic (foundation) component simulating rainfall-runoff on overland areas, and propagation of flood waves through an overland-stream-reservoir network of the watershed. A new routine was introduced into the model to allow simulation of spatially varying rainfall events associated mainly with moving storms and localized thunderstorms. The model was calibrated and verified using three rainfall-runoff events monitored by the ISWS. The calibration and verification runs demonstrated that the model was representative of the Court Creek watershed by simulating major hydrologic processes and generating hydrographs with characteristics similar to the observed hydrographs at the monitoring stations. Therefore, model performance was promising considering watershed size, complexities of the processes being simulated, limitations of available data for model inputs, and model limitations. The model provides an inexpensive tool for preliminary investigations of the watershed for illustrating the major hydrologic processes and their dynamic interactions within the watershed, and for solving some of the associated problems using alternative land use and BMPs, evaluated through incorporating these into the model inputs. The model was used to compare flow predictions based on spatially distributed and average rainfall inputs and no difference was found because of a fairly uniform rainfall pattern for the simulated storm. However, the routine will be useful for simulating moving storms and localized thunderstorms. A test to examine effects of different watershed subdivisions with overland and channel segments found no difference in model predictions. This was because of the dynamic routing schemes in the model where dynamic behaviors were preserved irrespective of the sizes and lengths of the divided segments. Although finer subdivision does not add accuracy to the outflows, it allows investigations of spatially distributed runoff characteristics and distinguishes these among smaller areas, which helps in prioritizing areas for proper attention and restoration. The calibrated and verified model was used to simulate four synthetic (design) storms to analyze and understand the major dynamic processes in the watershed. Detailed summaries of results from these model runs are presented. These summary results were used to rank overland segments based on unit-width peak flows, which indicated potential flow strengths that may damage the landscape, and were based on runoff volumes that indicate potential flood-causing runoff amounts. Stream channel and reservoir segments also were ranked based on peak flows and indicate potential for damages to the streams. Maps were generated showing these runoff potentials of overland areas. These results may be useful in identifying and selecting critical overland areas and stream channels for implementation of necessary BMPs to control damaging effects of runoff water. The model also was used to evaluate and quantify effects of the two major lakes in the watershed in reducing downstream flood flows and demonstrating model ability to evaluate detention basins. The model was run for one of the design storms with and without the lakes. The results showed significant reduction of peak flows and delaying of their occurrences immediately downstream. These effects become less pronounced further downstream. This report presents and discusses results from the above applications of the DWSM hydrology to the Court Creek watershed along with descriptions of the watershed, formulations of the hydrology component of the DWSM, limitations of the model and available data affecting predictions, and recommendations for future work. Efforts are currently under way at the ISWS to add subsurface and tile flow routines to the DWSM that would improve model predictions and their correspondence with observed data. It is recommended that stream cross-sectional measurements be made at representative sections of all major streams in the Court Creek watershed and that stream flow monitoring be continued or established at least at outlets of major tributaries and upper and lower Court Creek. A minimum of four equally spaced raingage stations are recommended for recording continuous rainfall. | | | | Date Created: | 9 24 2004 | | | | Agency ID: | CR-2000-04 | | | | ISL ID: | 000000000803 Original UID: 999999994080 FIRST WORD: Hydrologic | |
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