128-20 Disconnected by Design? Modeling Hydrodynamic Effects of Navigation Spoil Mounds in the Lower Apalachicola River Floodplain, Florida, USA

Abstract:

Dredging – or sand mining – is a common occurrence globally to make large alluvial rivers navigable for barges. Despite it being a common occurrence, the effects of the system are still relatively unknown. From the early 1900s to 2005, the Apalachicola River in Florida, USA, was regularly dredged to allow for navigation of commercial barges. The excess material from the river channel was subsequently placed in mounds on the adjacent floodplain, potentially reducing floodplain connectivity to the main channel. 

To test the impact these spoil mounds may have on floodplain connectivity, we employed a two-dimensional, surface-water hydrodynamic model using Hydrologic Engineering Center’s River Analysis System (HEC-RAS). The model utilized LiDAR-derived elevation data along a 3.1-km reach of the floodplain and mainstem below Wewahitchka, Florida, where spoil mounds are abundant. As a comparison, the LiDAR elevation data was smoothed to approximate a floodplain without anthropogenic spoil mounds while retaining natural levees. The hydrodynamic model was calibrated using landcover-specific roughness values (Manning’s n) and stage-discharge data from the U.S. Geological Survey stream gauge near Wewahitchka, FL. 

Preliminary model results indicate that spoil mounds inhibit floodplain inundation relative to the smoothed floodplain, whereas natural levees alone do not produce the same degree of connectivity loss. These results indicate that navigational spoil mounds alter the timing and extent of floodplain inundation, of particular importance in a river system with a highly regulated flow regime. Modelling experiments will test the hypothesis that the reduction of connectivity is most pronounced at intermediate river stages, when water can overtop natural levees but remains impounded by spoil mounds, delaying or preventing flow into adjacent floodplain channels and subsequent return to the mainstem.  Although preliminary, these findings suggest that the placement of dredged material on the floodplain has the potential to reduce floodplain connectivity and alter the hydrologic processes that support habitat connectivity, nutrient transport and storage, and overall floodplain ecosystem function. Additionally, our findings can inform on best practices for spoil mound placement in systems with ongoing dredging. 

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