COMPARISON OF FLOOD PEAK ATTENUATION BY CHANNEL-POND SYSTEMS AND DISTRIBUTARY CHANNEL SYSTEMS IN A FORESTED COASTAL PLAIN FLOODPLAIN

Coastal Plain watersheds in Southeastern United States often contain wide floodplains that can significantly attenuate flood peaks and store sediment prior to discharge to coastal ecosystems. Stream morphology can vary within these floodplains. We examined the relationship of channel morphology to flood hydrology in two types of channel-floodplain systems: a) sites with several wide, rectangular channels that overflow into floodplain ponds and b) distributary channel systems of 11-15 channels that convey water and sediment across much of the floodplain width. We monitored gauge height at sites throughout a 188 km² watershed to evaluate flood wave propagation. We measured discharge and channel morphology at gauge sites and in all channels at the two floodplain sites. Although overbank flooding frequency is similar between these floodplain reaches, overbank flooding duration and channel hydraulics are quite different. In the channel-pond sites, at-a-station hydraulic geometry analysis indicates that increases in discharge are accommodated by velocity and depth, with little change in channel width; this maintains high flood wave celerity in this portion of the channel-floodplain system. At floodplain sites with 11-15 distributary channels, the number of active distributary channels increases with discharge, thus the effective at-a-station hydraulic geometry (calculated by summing the channel discharges and widths) indicates most of the increase in discharge is accommodated by width with small increases in velocity and significantly lower maximum velocities. Channel velocities are heterogeneous in the distributary channels at moderate to low flows, providing reaches for suspended sediment deposition. Lower maximum velocities in the distributary channels generates a decrease in flood celerity and the increase in flood duration observed within these floodplain reaches with distributary channels.