Rivers end at the sea, or so we are taught. However, the study of fluvial hydrology commonly ends where tidal influence begins. Here, fluvial and marine forces overlap in long transitional regions (often 100s of kilometers) where coastal cities and ports face a double threat from river flooding and storm surge. Along these transitions, fluvial processes are poorly understood, leaving the dynamics of how rivers end unresolved. In a recent publication, The Propagation of Fluvial Flood Waves Through a Backwater-Estuarine Environment, historical data is used to examine the hydrodynamics of how rivers end and to understand the risk of river flooding for coastal communities.
Usually, in rivers, large flooding events move from upstream to downstream faster than small events. This study identified a different model by tracking flooding events as they moved from the river to the coastal ocean. The river delta, which is common in many natural systems, turned out to be very important for understanding when and where flooding is likely to happen.
Using years of observations (in some cases nine decades of data), this study found the Tombigbee-Alabama Delta (Gallery 1; also known as the Mobile-Tensaw Delta) delays and reduces flooding for cities along the delta and bay. This effect is caused by both the broadening coastal geometry and the vegetation that naturally occurs in the delta.
Gallery 1: Tombigbee-Alabama Delta
Most of the delta is a densely packed tupelo-bald cypress swamp, supporting the most biodiverse location in temperate North America (Picture 1). For large events, the delta swamp acts like a sponge quickly absorbing the initial floodwaters, and then slowly releases the water back to the main rivers. This gives communities more time to prepare and reduces the risk of river flooding overlapping with a storm surge during a hurricane. The slower release of water from the delta also slows the impact on the bay, delaying the initial flushing while also keeping the salinity low for a longer period of time. In contrast, smaller flooding events moved downstream faster. This occurs because smaller flooding events remain in the confines of the river channel, where they are not impacted by the swamps of the delta.
These findings indicate that the intensity of coastal flooding can be decreased and delayed by allowing inland regions of rivers to flood and/or by managing vegetation type, both of which reduce the downstream height of water. This will minimize flood impacts on coastal communities. To learn about the underlying hydrodynamics of rivers in fluvial-marine transitions check out Dykstra & Dzwonkowski (2020; https://doi.org/10.1029/2019WR025743)
By: Steven L Dykstra
PhD Candidate University of South Alabama
Steve Dykstra is a DISL/FDA Fellow (Dauphin Island Sea Lab, FDA Gulf Coast Seafood Laboratory) and a PhD Candidate (University of South Alabama) studying hydrology and physical oceanography with Dr. Brian Dzwonkowski. Steve’s current research includes climate change/variability, watershed processes, and tidal-fluvial dynamics, which also inform collaborative studies on bio-physical interactions, ecosystem dynamics, and food safety management. His previous consulting and NGO work in Asia and Africa supported hydropower, coastal engineering, and WASH programs. Steve hopes to graduate later this year and is looking for opportunities to continue answering challenging questions. Please contact him at sdykstra@disl.org.