Mississippi River Hydrodynamic and Delta Management Study : Delta Management modeling : AdH/SEDLIB multi-dimensional model validation and scenario analysis report

Abstract

This report details the process of developing and validating a multidimensional hydrodynamic, salinity, sediment transport, and coastal wetland morphology model of the Lower Mississippi River Delta. This model has been developed to run various sediment diversion scenarios. The results of these scenario analyses are documented in this report.The morphologic modeling results for the diversion scenario analyses show net land gain in the near vicinity of the diversion outlets and net land loss farther away from the outlets. The areas of land gain roughly correspond with the zones of sand deposition whereas the areas of largest land loss correspond with areas where there is diversion-induced inundation but not significant deposition of sediment from the diversion. The modeling results indicate that diversion-induced inundation results in a reduction in plant productivity, which induces an acceleration of land loss.Significant uncertainty exists with respect to the response of the existing wetland vegetation to diversion-induced inundation. The magnitude of this uncertainty can only be narrowed with further consensus building within wetland science.With respect to salinity, the receiving waters tend to freshen significantly during diversion operations. However, when operations cease, the recovery of salinity is almost entirely determined by prevailing offshore and/or riverine conditions.

Impacts of Muddy Bed Aggregates on Sediment Transport and Management in the Tidal James River, VA

“…In contrast to fines, bed aggregates were more frequently transported by bedload, rather than suspension. Bedload transport of fine sediment is currently not included in commonly used sediment transport models (e.g., AdH, SEDZLJ, Delft3D, ROMS), and therefore are not represented in their outputs (Brown et al 2019;Thanh et al 2008;Lesser et al 2004;Warner et al 2008). These aggregated bed clasts also had larger periods of immobility when compared with the predicted transport processes of silt-sized particles, increasing their residence time within the estuary.…”

“…Numerical models are commonly utilized as tools to predict sediment movement. However, at present many of these numerical models (e.g., AdH, SEDZLJ, Delft3D, ROMS) either weakly describe or do not include aggregate properties and transport processes (Brown et al 2019;Thanh et al 2008;Lesser et al 2004;Warner et al 2008). In many applications, such as the placement of dredged material in channel-adjacent areas, the lack of aggregate transport process descriptions could result in very poor and misleading estimates of project performance and impact (Perkey and Smith 2019).…”

Net land gain or loss for two Mississippi River diversions: Caernarvon and Davis Pond

“…The changes in the West Bay diversion (birdfoot delta) that are used to validate their model are for an area that has open water areas overlying mineral soils, and the model uses sediment deposition, not land area, as the metric. The far‐field inundation of wetlands, therefore, is not significant in the West Bay diversion, unlike for the two proposed diversions located northward and halfway to New Orleans (Brown et al ). These latter two diversions go into shallower water with wetlands.…”

“…The interactive effect of hurricanes and diversions is not included, and land gain or loss in wetland soils beyond the initial outfall area are not used to calibrate either model. The Brown et al () model validates the area of delta formation in the outfall area of the Caernarvon diversion, but not in the far‐field. The changes in the West Bay diversion (birdfoot delta) that are used to validate their model are for an area that has open water areas overlying mineral soils, and the model uses sediment deposition, not land area, as the metric.…”

Physical factors that influence muddy bed aggregate production, size, and durability

“…At present, many numerical models used in sediment management projects (e.g. AdH, SEDZLJ, Delft3D, ROMS) do not include aggregate properties and transport processes in their simulations, and limit the transport of fine sediment to suspended load (Brown et al 2019;Thanh et al 2008;Lesser et al 2004;Delft Hydraulics 2007;Warner et al 2008Warner et al , 2010. Because these bed aggregates can significantly alter the transport and fate of fine sediment, guidance is needed to inform the scientific and engineering community on the physical properties that result in bed aggregate production, and when it is appropriate to incorporate them into numerical sediment transport simulations.…”