INTEGRATING LIDAR, GIS AND HEC-RAS MODELING TO DETERMINE THE EFFECT OF DAM REMOVAL ON FLOODING AND SEDIMENT TRANSPORT ALONG THE OTTAWA RIVER, TOLEDO OHIO

The effects of the removal of a small dam were investigated using the U.S. Army Corps of Engineers HEC-RAS modeling package. The dam, located on the Ottawa River, flows through the city of Toledo and drains the northern part of Lucas County, Ohio. The dam is a cement weir 2.5 m tall and 17 m wide. The investigation combined field work with GIS mapping and computer-based modeling to better understand the hydrologic features in this watershed. Field work was conducted adjacent to the dam and included the measurement of the amount of sediment trapped behind dam, the river slope, and provided data needed to compute Reynolds numbers. Field information was also used to resolve the fate of the trapped sediment pending removal of the dam. The GIS data collection integrated disparate data sources. High definition LIDAR data was obtained for the drainage basin and used to generate a 1 foot contour interval elevation model. Stream channels, banks and land use were interpreted from 1:6000 scale air photo of Lucas county. The Rational Method was used to compute runoff in each of the 146 sub-basins in the larger Ottawa River drainage basin. The GIS was used to prepare and analyze the data used in the HEC-RAS modeling software. Runoff and flood models were generated for 4 theoretical storms corresponding to 1-hour rainfall events of the 10, 25, 50 and 100-year storms. The flood model was prepared for all streams and ditches in the watershed and contained in excess of 2000 cross sections extrapolated from the data at a 100 meter spacing.

Results of the study demonstrate that removing the dam has a minor effect on flooding adjacent to the dam. In the area surrounding the dam, much of the water is confined to the channel and the flood extents are similar for each storm events The maximum difference in flood elevations between the model with the dam and without is less than 25 cm and is not evident at a distance of 1000 m above or below the dam location. Grain size analyses were used to model the fate of the trapped sediment. Only a relatively small amount of sediment is expected to be transported if the dam is removed. The effects of this sediment on the aquatic systems will be controlled and trapped by the cement-block riffle reach immediately downstream of the dam. The study concludes that the dam currently serves no useful flood protection purposes and can be safely removed.