THE EVOLUTION OF CONVEX-BANK FLOODPLAINS IN A MID-ATLANTIC PIEDMONT STREAM

Convex-bank floodplains accumulate as lateral migration occurs, and have a strong influence on the cross-sectional geometry of alluvial rivers, especially in sinuous reaches. For example, in sinuous reaches, the balance between concave bank erosion and convex-bank deposition determines the width of the channel. In single thread, mixed-load, gravel-bed streams, convex-bank floodplains begin as gravel bars. These irregularities in the channel bed reduce the velocity of the flow above them, inducing further deposition of sand and gravel. As these deposits reach some critical thickness, the deposit accretes laterally as a result of the deposition of bed material along the channel proximal edge, while it accretes vertically through over-bank sedimentation. Eventually a natural levee or "floodplain ridge" develops, separating the region of over-bank sedimentation from the channel-proximal region where both bed load and over-bank deposition continues to occur. The presence of grassy vegetation on convex-bank floodplains increases the rate of sedimentation by increasing the bank angle, trapping previously deposited sediment, and attracting new sediment from suspension. Data from convex-bank floodplains along a gravel-bedded stream in northeastern Delaware suggests that these features evolve from bank adjacent bars. Historical maps show that the modern meandering reach was less sinuous during the 1860s. Aerial photographs from as far back as 1937 show the rates of lateral accretion of these features over the past 70 years. Lithological cross-sections of these features show basal gravel topped by finer material. The floodplain ridges contain mostly very-fine grained sand (finer than 0.25 mm) typical of a levee deposit. The channel-proximal region contains inter-bedded sand and mud, typical of a point bar deposit. The area between the floodplain ridge and the valley flat consists predominantly of mud, presumably material deposited from suspension. Maps of these floodplains reveal differences in topography, and spatial distributions of surface grain size and vegetation height. Vegetation density appears to have a strong effect on the volume of sediment in a single location on the floodplain.