The Great Mississippi River Flood of 1927: Morphodynamic analysis of the Caernarvon crevasse event

Wetlands in deltas across the globe have been severely affected by climate change-induced sea level rise. One strategy to mitigate these impacts is to engineer large river sediment diversions. In this study, we use a morphodynamic model to simulate and quantify the depositional footprint of the 80-day Caernarvon crevasse event that occurred during the Great Mississippi Flood of 1927 in Breton Sound Basin, Louisiana, USA, as an analog to engineered diversions. We examine the effects of river sediment load by modeling the same crevasse splay formation under the past, current-day, and future projected sediment load decline of the Lower Mississippi River. To assess the model's performance, we compared crevasse formation to field-measured deposition thickness and mass per area. The model shows that under current sediment loads, a flood discharge comparable to the 1927 event would build 70% less land was it to occur today. Further, silt falling velocity and clay flocculation percentage are the two key factors controlling splay footprint and deposition. The model also demonstrates the effects of sediment supply decline on the land-building potential of engineered sediment diversions. These sediment diversions, designed to mimic natural crevasse splay formation, are being pursued as a coastal restoration strategy in the Mississippi Delta. The analysis presented here emphasizes key attributes of engineered diversions and their relevance to the successful implementation of these restoration strategies, including flow capacity, ability to distribute sediment in the receiving area, and the extent to which they would induce marsh inundation. • Silt falling velocity and clay flocculation percentage are the two key factors controlling splay footprint and deposition • Lower sediment load would result in a 70% reduction in crevasse splay if the 1927 event were to happen today • Both the diversion size and its ability to distribute sediment distally are critical when planning a restoration project Relative sea level rise, the combination of rising sea level and land subsidence, is rapidly decimating coastal wetlands across the globe, including the marshes that form the Mississippi River Delta. Sediment river diversion projects, which mimic natural flooding events to deposit sediment and build land, are a promising restoration strategy. This study uses a morphodynamic model to simulate the Great Mississippi Flood of 1927 and quantify its land-building potential under past, present, and projected future sediment loads. We find that despite comparable flood discharges, current and projected declines in river sediment significantly limit land creation compared to historical conditions. A river’s flow capacity and sediment management are both critical to the success of engineered diversions for building wetlands. To predict landscape changes in the coming decades, a more thorough understanding is needed of the composition of the clay and mud that comprise the sediment in this region and how those factors affect sediment deposition. This knowledge will improve the efficacy of river diversions and protect these critical ecosystems against the effects of relative sea level rise.