Deltas worldwide are at risk of elevation loss and drowning due to relative sea-level rise. Management strategies to restore or enhance sedimentation on delta plains, Sedimentation-Enhancing Strategies (hereafter SES), are now being pursued in many deltas but there has been limited cross-disciplinary and cross-delta review. Here we compare 21 existing and planned SES, synthesizing their physical characteristics, funding, governance arrangements, stakeholder engagement, process of implementation, environmental impact, land use change, and potential for upscaling. Strategies exist at various scales, from ~0.05 km2 - 500 km2. 79% of strategies are capable of outpacing high rates of sea-level rise. Cheaper strategies are limited to short term impacts and small spatial scales, while more expensive strategies can have longer lifetimes. Most strategies create wetlands and flood water storage. Some create opportunities for agriculture, aquaculture, housing, or recreational land use. Combinations of SES will likely be the most effective and sustainable method for maintaining elevation in river deltas.

A three-dimensional stratigraphic model was constructed of the upper 50 m of the Mississippi River Delta, southeastern USA. The model is used to understand hydrogeological connections among the Mississippi River, adjacent interdistributary bays and groundwater systems, and to understand how stratigraphic settings affect potential anthropically induced subsidence and erosion in the region. This study uses 619 geotechnical borings throughout the area along with the multiple-indicator natural neighbor (MINN) interpolation method to construct the model. Based on available data, the study focused on the Mississippi River reach from Head of Passes (river mouth) to Jesuit Bend (108 km upstream), covering an area of approximately 1,800 km2 and ranging in elevation from 3 to −46 m. The model shows typical basal coarse-grained sand bodies overlain by 10-m-thick blanket clay, which is interbedded frequently with silty and sandy sediments and occasionally with peat and organic clay. Sands are most abundant between elevations −10 and −35 m. The Mississippi River main channel incises the underlying sands, thereby providing pathways for river–groundwater exchange. Increased hydrologic loads may propagate along the flow paths potentially giving rise to high pore-water pressure and a resultant increase in land subsidence and thus local erosion of natural and local flood-control levees. This method of analysis may apply to other deltaic regions similarly subject to anthropically accelerated subsidence and erosion.

Changing climate and land use practices are bringing extended periods of high water to the lower Mississippi River. New management practices are needed to protect people, industry, and the land.

Fluvial bedforms generate a turbulent wake that can impact suspended‐sediment settling in the passing flow. This impact has implications for local suspended‐sediment transport, bedform stability, and channel evolution; however, it is typically not well‐considered in geomorphologic models. Our study uses a three‐dimensional OpenFOAM hydrodynamic and particle‐tracking model to investigate how turbulence generated from bedforms and the channel bed influences medium sand‐sized particle settling, in terms of the distribution of suspended particles within the flow field and particle‐settling velocities. The model resolved the effect of an engineered bedform, which altered the flow field in a manner similar to a natural dune. The modelling scenarios alternated bed morphology and the simulation of turbulence, using detached eddy simulation (DES), to differentiate the influence of bedform‐generated turbulence relative to that of turbulence generated from the channel bed. The bedform generated a turbulent wake that was composed of eddies with significant anisotropic properties. The eddies and, to a lesser degree, turbulence arising from velocity shear at the bed substantially reduced settling velocities relative to the settling velocities predicted in the absence of turbulence. The eddies tended to advect sediment particles in their primary direction, diffuse particles throughout the flow column, and reduced settling likely due to production of a positively skewed vertical‐velocity fluctuation distribution. Study results suggest that the bedform wake has a significant impact on particle‐settling behaviour (up to a 50% reduction in settling velocity) at a scale capable of modulating local suspended transport rates and bedform dynamics. © 2020 John Wiley & Sons, Ltd.

Excavation of sediment from alluvial channel beds are common for the purpose of navigation maintenance, flood control, civil construction, etc. The post-dredge evolution of the mining-pit as well as the continuous changing of local flow and sediment transport, however, are poorly understood. The study simulates the morphological change of a large (1.46 million m3) borrow pit mined on a lateral sandbar in the lower Mississippi River using a depth-averaged two-dimensional hydrodynamic and sediment transport model. The model is calibrated and verified against 2.5 year time-series of multibeam bathymetric surveys. Besides, the study examined the sensitivity of mining-pit evolution simulation to the variation of the non-equilibrium adaptation parameters. Study results provide an insight into appropriate selection of the adaptation parameters in order to close the non-equilibrium sediment transport formulas when modeling large anthropogenic disturbance within the geomorphic processes.

River diversions may serve as useful restoration tools along coastal deltas experiencing land loss due to high rates of relative sea-level rise and the disruption of natural sediment supply. Diversions mitigate land loss by serving as new sediment sources for land building areas in basins proximal to river channels. However, because of the paucity of active diversions, little is known about how diversion receiving-basins evacuate or retain the sediment required to build new land. This study uses observational and numerical particle tracking to investigate the behavior of riverine sand and silt as it enters and passes through the West Bay diversion receiving-basin located on the lowermost Mississippi River delta, USA. Fluorescent sediment tracer was deployed and tracked within the bed sediment over a five-month period to identify locations of sediment deposition in the receiving-basin and nearby river channel. A computational fluid dynamics model with a Lagrangian sediment transport module was employed to predict selective pathways for riverine flow and sand and silt particles through the receiving-basin. Observations of the fluorescent tracer provides snapshots of the integrated sediment response to the full range of drivers in the natural system; the numerical model results offer a continuous map of sediment advection vectors through the receiving basin in response to river-generated currents. Together, these methods provide insight into local and basin-wide values of sediment retention as influenced by grain size, transport time, and basin morphology. Results show that after two weeks of low Mississippi River discharge, basin silt retention was approximately 60% but was reduced to 4% at the conclusion of the study. Riverine sand retention was approximately near 100% at two weeks and 40% over the study period. Modeled sediment storage was predicted to be greatest at the margins of the primary basin transport pathway; this matched the observed dynamics of the silt tracer but did not match the behavior of the sand tracer. The degree to which the observational measurements deviate from the model predictions may indicate the relative influence of physical processes other than the mean riverine generated currents, such as tides, wind generated currents, and waves.

Despite being a primarily depositional landform, a crevasse splay experiences an initial evolutionary phase that is primarily erosional as sediment-laden river water spills from a main river channel and incises a new route through the river banks and levee into an interdistributary basin or floodplain. This phase sets the dimensions and the conveyance properties of the crevasse, which, in turn, influences the continued expansion or closure of the crevasse channel. However, little is known about the controlling morphodynamics or how the erosional processes transition to depositional processes during this phase. The objective of this study is to investigate these phenomena at the West Bay sediment diversion (Louisiana, USA) using coupled field observations and numerical modeling. The West Bay diversion was cut into a lower Mississippi River levee to mimic the function of a crevasse-splay, i.e., to divert river water and sediment to an adjacent receiving basin for land-building purposes. Bathymetric measurements show that the diversion channel experienced significant natural morphologic evolution during the initial decade (2004–2014). Hydrodynamic and sediment transport modeling suggests that this evolution initially increased the discharge of flow and sediment through the crevasse as the channel became wider and deeper and altered its orientation relative to the main river flow direction. After 5years, the model results predict that further evolution led to monotonically reduced diversion discharges. During this time, natural and engineered sediment deposition in the receiving basin decreased predicted basin-flow velocities and promoted a backwater effect that reduced the sediment transport capacity of the diversion channel. Observations during the final 2years show that much of the initial erosion around the diversion had abated indicating that diversion morphology may have stabilized. A modeling sensitivity analysis confirmed that the observed changes to channel geometry and orientation likely promoted flows of water and sediment through the diversion while increases in basin-bed elevation would have had a contrary effect. The morphodynamic evolution of the West Bay diversion documented in this study presents a model indicative of the erosional phase of crevasse-splay evolution in a deltaic distributary fluvial network. Study results offer an analogue on how an engineered river sediment diversion constructed for coastal restoration may function during its first years of operation and suggest that the desired land-building processes may take time to become established.

In-channel sand mining by dredge removes large quantities of bed sediment and alters channel morphodynamic processes. While the reach-scale impacts of dredging are well documented, the effects of the dredged borrow pit on the local flow and sediment transport are poorly understood. These local effects are important because they control the post-dredge evolution of the borrow pit, setting the pit lifespan and affecting reach-scale channel morphology. This study documents the observed morphological evolution of a large (146 million m(3)) borrow pit mined on a lateral sandbar in the lower Mississippi River using a time-series of multibeam bathymetric surveys. During the 25year time-series, 53% of the initial pit volume infilled with sediment, decreasing pit depth by an average of 088myr(-1). To explore the controls of the observed infilling, a morphodynamic model (Delft3D) was used to simulate flow and sediment transport within the affected river reach. The model indicated that infilling rates were primarily related to the riverine sediment supply and pit geometry. The pit depth and length influenced the predicted magnitude of the pit bed shear stress relative to its pre-dredged value, i.e.the bed-stress reduction ratio (R*), a metric that was correlated with the magnitude and spatial distribution of infilling. A one-dimensional reduced-complexity model was derived using predicted sediment supply and R* to simulate patterns of pit infilling. This simplified model of borrow-pit evolution was able to closely approximate the amount and patterns of sediment deposition during the study period. Additional model experiments indicate that, for a borrow pit of a set volume, creating deep, longitudinally-shorter borrow pits significantly increased infilling rates relative to elongated pits. Study results provide insight into the resilience of alluvial river channels after a disturbance and the sustainability of sand mining as a sediment source for coastal restoration. Copyright (c) 2015 John Wiley & Sons, Ltd.

Some of the world's largest cities are sinking faster than the oceans are rising. Humans are part of the problem, but we can also be part of the solution through monitoring and modeling.

The amount and distribution of coarse-grained sediment (e.g., sands and gravels) relative to fine-grained sediment (e.g., clays and silts) within a floodplain influences many of the floodplain geotechnical properties, including the potential for groundwater seepage. Seepage is a primary driver of levee and dam failure, and understanding its potential is of paramount concern to engineers and resource managers. This paper reports the results of a computational modeling study that simulated alluvial floodplain construction using a suite of simple geomorphic process-imitating rules.A model aggrades a floodplain cross section within an alluvial basin, creating floodplain architecture by differentiating between sediment deposited by channel processes (sand) and sediment deposited by overbank flood processes (clay). The evolution of two floodplain cross sections of the Trinity River, near Dallas, Texas is simulated using five different experimental scenarios. The study area is the site of large levee rehabilitation projects in which accurate characterization of the geologic environment has significant engineering importance. Study results predict that scenario components including the alluvial basin width, the initial topography of the floodplain base level, and the channel aggradation rate significantly affect the fraction of the floodplain width that contains channel deposits by influencing the avulsion frequency of the river during floodplain construction. Increased avulsion frequency equated to more numerous, yet smaller channel deposits. The dimensions of the channel deposits predicted by this study are similar to those typically observed in large, fully meandering river systems. The model devised for this study is relatively simple and can be run in multiple iterations to produce probabilistic outputs, such as the likely range of channel deposit widths within a floodplain cross section. This type of information is useful to engineers for a host of applications including predicting the data collection density necessary to characterize the geotechnical properties of a project site. (C) 2013 Published by Elsevier B.V.

: The amount and distribution of coarse-grained sediment relative to fine-grained sediment within a floodplain influences the floodplain's geotechnical properties, including the potential for groundwater seepage. Seepage is a primary driver of levee and dam failure, and understanding it is of paramount concern to water resource engineers and managers. This report documents the results of a computational modeling study that simulated alluvial floodplain construction by using simple geomorphic process-imitating rules. The model aggrades an alluvial floodplain, creating floodplain architecture by differentiating between sediment deposited by channel processes (coarse sediment) and sediment deposited by overbank flood processes (fine sediment). The evolution of two floodplain cross sections of the Trinity River near Dallas, Texas, is simulated under five scenarios. The study area is the site of large levee rehabilitation projects in which accurate characterization of the geologic environment has significant engineering importance. Results of the simulations predict that the average channel deposit dimensions are sensitive to the sedimentation scenario employed and are generally similar to those typically observed in fully meandering rivers. The results suggest that the channel aggradation rate influenced heavily the relative channel avulsion frequency during floodplain construction. Increased avulsion frequency equated to more numerous, yet smaller, channel deposits. Avulsion frequency and floodplain width affected the predicted fraction of the floodplain's cross-sectional width with subsurface channel deposits. The model for this study is simple and can be run in multiple iterations to produce probabilistic outputs. Such information can be used to predict the data collection density necessary to characterize the geotechnical properties of a project site.

Seven 1-D sediment transport formulae are employed to predict coarse sediment transport in a low-order, ephemeral channel with patchy bed material.Sediment transport is modeled through two different methods to characterize the sediment supply: by using a channel-averaged grain-size distribution (GSD) and by using the mean GSDs from the relatively coarse and fine textured patches that compose the channel bed.Modeling results show that the two different characterizations of the sediment supply produce significantly different values of predicted coarse sediment yield.The relative differences in the predicted yield values are dependent on the assumptions of each formula, including how the sediment supply is parameterized within the model.Modeled sediment yield values are contrasted to observed values for seven flashflood-type runoff events.The formulae accuracies typically are comparable to past analyses in fluvial systems more reminiscent of the hydraulic environments in which the models were derived (perennial flow, gravel-bed material).Study results reaffirm that the method by which sediment supply is characterized significantly impacts the performance of sediment transport formulae and that a single GSD might not adequately describe the sediment supply originating from patchy beds.