Floods and Cold Front Passages: Impacts on Coastal Marshes in a River Diversion Setting (Wax Lake Delta Area, Louisiana)

Abstract

Roberts, H.H.; DeLaune, R.D.; White, J.R.; Li, C.; Sasser, C.E.; Braud, D.; Weeks, E., and Khalil, S., 2015. Floods and cold front passages: Impacts on coastal marshes in a river diversion setting (Wax Lake delta area, Louisiana). Journal of Coastal Research, 31(5), 1057-1068. Coconut Creek (Florida), ISSN 0749-0208.Diversion of the Mississippi River is considered the most effective way of offsetting Louisiana's coastal plain land loss. The Wax Lake delta, a man-made diversion that represents mostly sand-rich distributary mouth bar deposits, is cited as a model for land building from a large river diversion. The fine-grained suspended load is mostly deposited outside of ''the delta.'' Numerical simulations of diversion delta building focus primarily on sand deposition. The fine-grained suspended load is only secondarily considered. Research results reported here suggest that natural processes associated with the synergistic relationship between floods and cold front passages can effectively distribute suspended sediments to maintain and rebuild wetlands outside the sand-rich delta. Measurements (fixed acoustic Doppler current profilers) of current speed, current direction, and water level in a mainland marsh channel, north of Atchafalaya Bay, indicate that as cold fronts approach, local water levels can be elevated by as much as 1 m, and normal discharge into the bay is reversed so that flow is into the coastal plain marshes. LIDAR bank elevations indicate that overbank flow is initiated by these events. Streamside sediment accretion (2-y monitoring period) using feldspar plots indicates up to 6.8 cm. Longer term Cs-137 data reveal an average sediment accretion rate of .1 cm y À1 over 50þ years. Marsh flooding results in denitrification of nitrate in river water plus nutrient uptake resulting in increased plant productivity, considerably greater than for similar coastal plain marshes free of riverine sediment input. Fine-grained suspended sediments are important for rebuilding Louisiana's coastal plain, which is naturally constructed mostly of riverine silt and clay plus organic matter.

Factors controlling sediment trapping in two freshwater tidal wetlands in the Biesbosch area, The Netherlands

“…Low rates of accumulation of 0.6 to 1.2 mm year −1 have among others been found by Craft and Casey (2000), while high rates of accumulation in the order of 20-180 mm year −1 have been found by Darke and Megonigal (2003), Mitsch et al (2014), Auerbach et al (2015), and Roberts et al (2015). Negative sediment budgets, such as found for the Zuiderklip area, have been reported by among others (Sheehan and Ellison 2015) and Van der Wal and Pye (2004).…”

“…Low trapping efficiencies of 0.10-0.20 have been reported by Syvitski et al (2009) for small, steep-gradient rivers and efficiencies of 0.50-0.60 for larger deltas with multiple distributary channels. Multiple studies on sediment retention have reported sediment trapping efficiencies between 0.27 and nearly 1.0 for Mississippi and Atchafalaya diversion systems (Xu et al 2016), where the trapping efficiency was higher further from the coast (Roberts et al 2015;Xu et al 2016).…”

“…Paola et al (2011) defined river delta restoration as "diverting sediment and water from major channels into adjoining drowned areas, where the sediment can build new land and provide a platform for regenerating wetland ecosystems." This type of river delta restoration is currently considered or implemented in the Tidal River Management project in Bangladesh (Khadim et al 2013), the diversion projects in the Mississippi deltaic plain (DeLaune et al 2003;Day et al 2007;Paola et al 2011) or in the Atchafalaya subdelta (Roberts et al 2015;DeLaune et al 2016), and the Plan Integrale de Protección del Delta Ebro for the Ebro Delta (Calvo-Cubero et al 2013).…”

LaVegMod v2: Modeling Coastal Vegetation Dynamics in Response to Proposed Coastal Restoration and Protection Projects in Louisiana, USA

“…The predicted landscape at year 50 ( Figure 8, Panel A) shows that the marshes are maintained and even grow in those areas affected by water from the Atchafalaya River in the central part of the Louisiana coast. It has been shown that natural processes associated with the synergistic relationship between floods and cold front passages can effectively distribute suspended sediments to maintain and rebuild wetlands outside the sand-rich delta of the Atchafalaya River [32]. Although only 20% of the ecosystem restoration budget in the 2017 LCMP is used for sediment diversion (Table 3), the land gained over the FWOA by ecosystem restoration (Figure 8, Panel D) is largely due to the sediment diversions.…”

Optimizing Sediment Diversion Operations: Working Group Recommendations for Integrating Complex Ecological and Social Landscape Interactions

“…To maximize sediment resuspension and transfer to the wetland surface, winter operations would take advantage of cold front passage (most prevalent from November to March) prior to the consolidation of the material on the bay bottoms. Operating a diversion in the winter and/or early spring enhances the chances of landward redistribution as approaching cold fronts bring onshore winds that generate waves and currents that increase coastal set-up and drive sediment landward and onto marsh surfaces [61].…”