The Propagation of Fluvial Flood Waves Through a Backwater‐Estuarine Environment

Abstract

AbstractCoastal cities and ports are located along estuaries and deltas where flooding from rivers can be as devastating as storm surges. Precise river discharge measurements are taken far inland of marine influences and backwater environments, creating large timing and magnitude uncertainties downstream at the coast. Long‐term discharge, water level, velocity, and salinity measurements in coastal Alabama were used to observe the timing and magnitude of discharge events flowing 238 km from rivers to the Gulf of Mexico as river flood (fluvial) waves. Waves were described and simplified using a momentum balance, phasing techniques, and wave theory from inland rivers. Results showed the coastal backwater environment transitioned to a drawdown (i.e., plunging water profile) at bankfull discharge and suggested the drawdown location and intensity was strongly influenced by the frictional transition of the delta from tupelo‐cypress forest to oligohaline marsh. The horizontal (velocity) and vertical (water level) components of fluvial waves were observed propagating through this dynamic deltaic‐estuarine environment transitioning from in phase diffusive waves to out of phase dynamic waves. The wave celerity increased with surface water slope and decreased with cross‐sectional area. Instead of larger events propagating faster, the geometry (i.e., levees and floodplains) and flooding significantly delayed and attenuated the magnitude of discharge reaching the gulf. This flooding downstream of discharge measurements modulated the estuarine water level, velocity, and flushing of salt. The use of fluvial wave theory will increase the precision of coastal flooding predictions for stakeholders and research now, as well as under future sea level rise.