RAPID: In-Channel Alluvial Benches and Their Environmental Controls

University Of Alabama Tuscaloosa, Tuscaloosa AL

Investigators

Abstract

In-channel alluvial benches are shelf-like deposits of river sediment that are located above the channel bed but below the main floodplain surface. Their positions and morphologies may complicate routine determinations of bankfull stage, so vitally important to stream restoration plans. Their presence can impact the quality of hydraulic and riparian habitat, while their mass budgets are important indicators of fluvial dynamics. This research focuses on measuring modifications of old in-channel benches documented by Kilpatrick and Barnes in 1964 in the Southern Piedmont of the southeastern United States, and of younger benches that may have originated during drought conditions occurring in the Piedmont over the last decade. Researchers elsewhere have found that benches may be produced during drought and destroyed during wetter, flood dominated regimes. The return to higher rainfall in 2009 relative to most of the last decade thus has the potential to initiate rapid transformation of benches, affecting sediment budgets and riparian habitat throughout the region. Alternatively, land use history may override flood regimes as the dominant control on bench formation and destruction. This project seeks to differentiate controls on bench mass balance to elucidate bench dynamics in Piedmont streams using surveying, landform dating techniques, flood data analysis and land use change assessment. This project is time sensitive because year 2009 rainfall to date has been much greater than that occurring annually for most of the last decade in the southern Piedmont. This recent meteorological transition from drought to wetter conditions presents an unparalleled opportunity to document in-channel benches that may have been created during decadal drought and to observe the effects of a transition to higher stream-flow regime at its beginning. This research contributes to better understanding about sediment storage in rivers, as not much is known about the formation and persistence of in-channel alluvial benches as sediment storage features. What is known about alluvial benches comes from studies conducted in locations with climates significantly different from those of the temperate Eastern United States. Thus, this research will address whether our current understanding of benches from previous research is generally applicable to systems in diverse climatic settings. This research project examines the occurrence, formation, and persistence of a specific type of river sediment storage feature. As such, it will result in a better understanding of causes, processes, and conditions leading to in-channel sediment storage, which has considerable potential to help make sediment budgets more precise and meaningful in terms of understanding sediment dynamics in rivers. From an applied fluvial geomorphology perspective, this research will provide new insight into the field determination of bankfull stage, by examining geomorphic surfaces located below the floodplain or main valley flat that are hydrologically significant. Finally, this research will provide new insight into the coupling of landscape and atmospheric phenomena by examining the potential impact of drought versus more commonly cited instigators of change in river sediment dynamics, such as historic alteration of land use /cover and surface hydrology. Research results will be widely disseminated in conference presentations and research journals.