Stability of Gravel Bars & Bedload Transport in Paint Branch Creek

Streams in urbanized watersheds respond to changes in water and sediment supply; these channel changes create many problems for local communities. The increase in impervious surfaces in urban settings can cause an increase in overbank flooding and/or the erosion of stream banks. Where buildings are at risk, these problems are often addressed by channel restoration projects, particularly bank stabilization. In my research I examined an unconstrained reach of river where gravel bars are forming. The study site is a gravel-bar complex within the PBS channel downstream of Cherry Hill road. In the areas where the gravel bars are being formed, the channel widths are 2-3 times wider than adjacent reaches. There is concern that if the channel widening is not mitigated by erosion control measures, that the channel will continue to erode. In my research, I tested the hypothesis that gravel bar formation actually results in channel stabilization by increasing channel width, decreasing channel depth, and therefore shear stress within the braided reach. My research was based on determining 3 variables needed to analysis the dimensionless shear stress, depth, grain size and energy gradient. I found that the channel initially widens at the bar head, and becomes less wide downstream. The upstream portion of the divided reach has lower average shear stresses than the upstream reach, but the bar tail is a zone of active bed and bank scour and sediment transport. Shear stresses over the bar top are significantly lower than in the channels. Introduction and Previous Work Paint Branch stream (PBS) is the main stream in the portion of the Anacostia watershed that includes College Park Maryland. The analysis of the Paint Branch gravel bars and bedload transport have significant importance to the College Park community. Currently there are large gravel bars present in the channel of the PBS, particularly on campus and downstream, of The University of Maryland. One of these gravel bars was just removed in a costly stream stabilization project. The large gravel bars in certain regions of the PBS cause concern because the gravel bars decrease the volume of the channel, reducing the capacity of the channel to transport flood discharges. They can also divert flow and enhance bank erosion. My research focused on one section of the Paint Branch Stream in which gravel bars are present in the College Park area. The location of research will be along Little Paint Branch Creek downstream of Cherry Hill Road. The PBS is an armored, gravel-bed stream. The surface grains protect the subsurface grains from transport until a critical shear stress is achieved to move the larger grains. The surface grains act like obstacles that must be moved out of the way for the subsurface grains and total bedload transport downstream (Wiberg and Smith, 1989; Milhous, 1973 in Buffington, 1997). Bankfull stage is the largest discharge that the channel can contain without overbank flooding. In channels with wide floodplains, shear stresses increase up to the bankfull stage and the flow above bankfull stage is carried primarily by the floodplain, without a significant increase in channel shear stresses. The main point of my research was to determine whether or not PBS has stable gravel bars at bankfull stage. There have been decades of prior research on a streams ability to move the sediment of gravel-bedded streams. (Barry, 2004; Buffington et.al., 1997; Constantine et.al., 2003; Dietrich, 1898; Goodwin, 2004; Hjulstrom, 1939; Miller, 1997; Parker, 1978; Renger, 2007; Shields, 1936; Wilson, 2005) I applied prior research to determine whether or not PBS has the ability at bankfull stage to move the gravel bed sediment of the stream out of the College Park region. Channel restoration has occurred along the PBS in the College Park region. The area I researched an area that has not undergone restoration but has many gravel bars present in the middle of the stream’s channel. In gravel bed streams, bedload transport is initiated when the flow depth and gradient are sufficient to move the particles from the bed. This threshold can be expressed as a critical shear stress. By taking the ratio of the fluid shear stress to the grain resisting forces, we can also evaluate a critical dimensionless shear stress for the initiation of motion. There are a series of equations that are needed to be applied to determine the presence of stable or mobile gravel bars in the PBS system. The first equation of the series is used to calculate the boundary shear stress of the stream. The shear stress (T) is a force per area measurement that represents the downslope component force of water acting on the channel boundary.