Modeling bed material transport through colonial-age mill dam impoundments, northern Delaware
Abstract
Two hundred -year old colonial mill dam impoundments of the White Clay and Red Clay Creeks in northern Delaware are barely 20% 'filled' with sediment. Field evidence suggests that gravel-sized bed material supplied from upstream is able to pass through these impoundments, implying that the impoundment morphology has reached an equilibrium morphology controlled by the prevailing hydraulic regime. We assess this hypothesis using backwater modeling to compute local boundary shear stresses and the Wilcock-Crowe sediment transport equation to determine bed material transport rates in a representative impoundment with a ~ 2 m high dam and a 1.5 km-long impoundment. While previous conceptual models suggest that cobbles could only be transported through impoundments during catastrophic storm events or after impoundments had completely filled, our analysis demonstrates that transport rates of cobbles during the 2 year flow are significant. Even smaller discharges can be effective: the 0.5 year discharge moves coarse-grained bed material (11.2 - 32 mm) at moderate rates outside the impoundment and at low rates within the impoundment, suggesting net accumulation. Larger flows (5-25 year) transport material at similar rates both outside and within the impoundment. Averaged over time, this regime would keep the bed of the stream in quasi-equilibrium, with lower flows filling the impoundment and higher flows removing the accumulated sediment. This behavior is reminiscent of the response and recovery cycle described for quasi-equilibrium stream channels, suggesting that these impounded channels behave as graded streams with a reduced slope rather than as stagnant quasi-lacustrine systems incapable of transporting the sediment supplied from upstream. The current morphology (only 20% filled by sand and gravel) may not have existed in the past. Cores from floodplains adjacent to the impoundments reveal deposits of laminated mud suggesting a former fine-grained impoundment fill that has subsequently been eroded. Changes in land use and spillway operation have likely altered the discharge regime, sediment supply, and hydraulics of the impoundment, creating a different impoundment morphology. Our study suggests that the morphology of impoundment streams adjusts through time to achieve a quasi-equilibrium morphology controlled by the prevailing sediment supply and hydraulic regime.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFMEP51A0696P
- Keywords:
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- 1825 HYDROLOGY Geomorphology: fluvial;
- 1808 HYDROLOGY Dams;
- 1862 HYDROLOGY Sediment transport;
- 1849 HYDROLOGY Numerical approximations and analysis