IMPACT OF RECENT CHANNEL INCISION on PEAK FLOWS IN SOUTHWEST OHIO

Although historic soil erosion caused significant channel aggradation in much of the eastern United States, in recent decades that trend has been reversed and headwater channel incision is common. This incision is a result of several factors. Reduced cultivation of marginal land, improved soil conservation, and sediment trapping in impoundments have reduced sediment supply. Also, localized urbanization and perhaps climate change that could increase the frequency of high-intensity events may have increased transport capacity. In southwest Ohio, streams with drainage areas of 100-500 km² have incised as much as 6 m in the last 50 years, and in many cases the streams have incised 1-2 m into glacial sediments that underlie valley alluvium. This incision is sufficient to prevent overbank flows for all but the largest floods.

We modeled the passage of a ~2-year event through two adjacent watersheds: Four Mile Creek and Indian Creek. Four Mile Creek (430 km²) has a channel that has incised ~4+ m below its historic floodplain, while Indian Creek (250 km²) is incised 1-2 m. The greater incision of Four Mile Creek is likely a result of construction of a reservoir that has trapped sediment, reducing sediment supply downstream. HEC-HMS was used to create hydrographs for a 2-year 1-day storm on the upper portions of the two watersheds. The passage of these flows through the lower portions of the watersheds was simulated using the HEC-RAS one-dimensional unsteady state model. In Four Mile Creek the peak discharge at the outflow of the modeled reach was about 20% lower than that at the inflow, while in Indian Creek peak attenuation was about 40%. We attribute the difference in peak attenuation to the different degree of incision in the two streams. We also modeled the effects of incision in Four Mile Creek for different degrees of incision. Results indicate that incision and resulting loss of floodplain storage has caused an increase of peak discharges of up to 10%, flow depth increases of > 50%, velocity increases of 30-100%, and more than a doubling of stream power. This constitutes a strong positive feedback mechanism that contributes to rapid channel change. These data suggest that changing stream channel geometry can have significant impacts on hydrologic fluxes that should be considered in watershed management and stream restoration.