CHANNEL MORPHOLOGY EVOLUTION AND OVERBANK FLOW IN THE GEORGIA PIEDMONT¹

Abstract

Historic changes in stream channel morphology were investigated in the Georgia Piedmont to better understand the hydrologic processes and functioning of the region's riverine systems. USGS gaging station data and channel geomorphology data were collected from thirty study sites in the Upper Oconee River Basin for flood frequency analysis. Historic and modern (i.e., present‐day) channel capacity discharge (i.e., overbank flow) was calculated using Manning's equation and historic channel cross‐section records. The recurrence interval for overbank flow was estimated for each site from flood frequency data. Results indicate that channel expansion has occurred throughout the basin, especially in upper reaches. Recurrence intervals for modern overbank events were variable and generally high ranging from < 2 to > 500 years for first to third order streams. They were less variable and lower for fourth and fifth order streams, ranging from < 2 to 3 years. Potential depositional thresholds were identified that exemplify the complex response of sediment distribution patterns throughout the basin. Results indicate overbank flows occur less frequently now than they once did due to historic accelerated sedimentation and subsequent channel expansion. One application of these findings is that these basin processes are likely applicable across the region and may impact the hydrologic functioning of associated Piedmont riverine wetlands that depend on flooding regimes.

A Paleoecological Perspective on Wetland Restoration

“…Many riparian and floodplain areas in the eastern United States also became filled with sediment as a result of high sediment erosion rates associated with land clearing and stream impoundment. This resulted in the pre-settlement floodplain surface becoming entombed in the deposited sediments (Ruhlman and Nutter 1999;Walter and Merritts 2008;de Wet et al 2011). Wood buried beneath these sediments may then be an important source of information for guiding restoration designs and establishing pre-disturbance reference conditions.…”

Anthropocene streams and base-level controls from historic dams in the unglaciated mid-Atlantic region, USA

“…The geometry of single-channel meandering streams generally is viewed as the result of self-adjusting hydraulic variables in response to changing sediment load and discharge or to base-level lowering [23,24]. The cause of a change in channel conditions is commonly determined by searching for contemporary perturbations in upland sediment load or runoff, downstream base-level controls or the crossing of a threshold [8,9,18,[24][25][26][27].…”

“…Yet, many streams in the mid-Atlantic Piedmont continue to be unstable and degrading, with both bed scour and bank erosion observed as widespread phenomena [1,7,25,26,[39][40][41].…”

Morphodynamics of a 1000‐year flood in the Kamp River, Austria, and impacts on floodplain morphology

“…The impacts of floods on river morphodynamics are strongly related to the river type (confined, braided, meandering), sediment supply/boundary conditions, flood flow patterns/valley orientation (Miller, 1990(Miller, , 1995Cenderelli and Wohl, 2003), temporal sequence of past floods (Beven, 1981;Cenderelli and Wohl, 2003), duration of the flood peak (Costa and O´Connor, 1995) and the recurrence interval for the overbank flow (Q ov ). Ruhlman and Nutter (1999) described the recurrence interval of morphological changes, analysed from flood frequency data, as generally high. The interval ranges from <2 to 500 years for first-to third-order streams and is less variable and lower for fourthand fifth-order streams, ranging from <2 to 3 years.…”