资源环境科技发展态势分析平台

Coastal 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.

Plain Language Summary Most coastal cities and ports are located along rivers near the sea and are increasingly vulnerable to flooding from both sources due to intense development and sea level rise. River discharge is measured far inland where deltas, estuaries, and marine forces do not interfere, but these same influences make it difficult to observe when and how much flooding occurs near the coast. Usually in rivers, large flooding events move from upstream to downstream faster than small events. A study was conducted to track flooding events through a coastal environment using long-term observations in Alabama. In contrast to flooding behavior in rivers, the results showed small flooding events moved downstream faster than large events. This occurred because large events caused water to flow out of the river channel, spilling over the forested floodplains where it traveled more slowly. This flooding of the delta also decreased the impact of large events and demonstrates large river discharge events can be tracked in coastal areas. These findings indicate stakeholders can decrease the intensity of coastal flooding and provide more time to prepare by allowing inland regions of rivers to flood and/or by managing vegetation type, both of which reduce the downstream height of water.