Role of Hydraulic Geometry in Flood Peak Attenuation in Coastal Plain Streams

Many Coastal Plain streams have wetland or multiple-channel reaches that distribute flow across the floodplain, thus influencing channel velocities and flood celerities. The contribution of channel morphology to flood peak attenuation can be investigated through channel hydraulic geometry relationships. These relationships describe changes in width (w), average depth (d), and average velocity (v) with increases in discharge (Q). We examined the effects of multiple-channel systems in Zekiah Swamp Run watershed, a forested stream system in southern Maryland. Field measurements of channel width, depth, velocity, gradient, and grain size were made at multiple sites across a range of flow events (low flow to bankfull flow). Stream gauges were installed on a subset of sites ranging from upstream single channel tributaries to a complex multiple-channel site.

At-a-station hydraulic geometry relationships (w = aQ^b; d = cQ^f; v = kQ^m) show heterogeneity among the individual channels across all sites. At the downstream multiple-channel location, a summation HG relationship was developed for the total flow. An increase in total discharge does not uniformly increase flow in all channels, but is accommodated by an increase in the number of active channels. The summation HG exponents indicate that width (b=0.46) and average velocity (m=0.43) showed the largest increases with increasing discharge (depth exponent, f=0.11). The behavior of individual channels at this complex multi-channel location is explored through the development of "distributary" hydraulic geometry relationships, constructed for single flow events using HG data for the individual channels. The distributary HG results show that as flow increases, the relative proportion of discharge accommodated by velocity decreases (m=0.62 at low flow to m=0.31 at bankfull flow), while the width exponent increases (b=0.06 to b=0.37), and the depth exponent stays the same (f=0.3). The downstream hydraulic geometry for bankfull discharge in the Zekiah Swamp Run system shows flow width significantly increases while average velocity decreases as the flow moves into the multiple channel system. This distribution of flow into multiple channels is one mechanism for the flood peak attenuation that we have previously observed in Atlantic Slope Rivers.