Crevasse Splays Versus Avulsions: A Recipe for Land Building With Levee Breaches

• Published in: Geophysical research letters 2018-05, Vol.45 (9), p.4058-4067
• Main Authors: Nienhuis, Jaap H., Törnqvist, Torbjörn E., Esposito, Christopher R.
• Format: Article
• Language: English
• Subjects: Accretion; Aggradation; Avulsion; Bypasses; Computer simulation; Consolidation; crevasse splay; Delft3D; Earth; Evolution; Flood basins; Floodplains; Fluvial sediments; Geomorphology; Land area; Levees; Mathematical models; Numerical models; Replication; Restoration; river avulsion; River sediments; Rivers; Sea level; Sea level rise; Sediment; Sediment discharge; sediment diversion; Sediment traps; Sedimentation; Sediments; Silt; Soil; soil consolidation; Soil investigations; Soil strength; Substrates; Vegetation; Vegetation type; Water discharge; Water surface slope; Wetlands
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2018GL077933

Natural‐levee breaches can not only initiate an avulsion but also, under the right circumstances, lead to crevasse splay formation and overbank sedimentation. The formative conditions for crevasse splays are not well understood, yet such river sediment diversions form an integral part of billion‐dollar coastal restoration projects. Here we use Delft3D to investigate the influence of vegetation and soil consolidation on the evolution of a natural‐levee breach. Model simulations show that crevasse splays heal because floodplain aggradation reduces the water surface slope, decreasing water discharge into the flood basin. Easily erodible and unvegetated floodplains increase the likelihood for channel avulsions. Denser vegetation and less potential for soil consolidation result in small crevasse splays that are not only efficient sediment traps but also short‐lived. Successful crevasse splays that generate the largest land area gain for the imported sediment require a delicate balance between water and sediment discharge, vegetation root strength, and soil consolidation. Plain Language Summary Man‐made sediment diversions from the Mississippi River into adjacent wetlands are an ambitious and novel concept to form new land, replicating how large portions of the Mississippi River Delta have kept pace with sea level rise over the last thousands of years. However, the geomorphic evolution of diversions remains uncertain. How much new land will be formed, and is that dependent on vegetation type or soil consolidation rates? Will its channels silt in and close the diversion? Here we show with a numerical model that successful diversions generating the largest land area gain require a delicate balance between water and sediment discharge, vegetation root strength, and substrate erodibility. Diversions enhance local subsidence rates, but this can, counterintuitively, enhance land‐building by allowing more sediment to flow into the diversion. Based on our model experiments, we have also found that diversions into easily erodible substrates can initiate a river avulsion and form a new river delta lobe, shedding light on the origin of river delta shapes we find on Earth today. Key Points Levee‐breach model reveals a continuum from nonchannelized overbank deposition, to crevasse splays and to river avulsions We find a significant geomorphic effect of floodplain vegetation and soil consolidation on crevasse splays and avulsions Successful sediment diversions require a delic