Sediment Exchange Between the Created Saltmarshes of Living Shorelines and Adjacent Submersed Aquatic Vegetation in the Chesapeake Bay

Rising sea levels and the increased frequency of extreme events put coastal communities at serious risk. In response, shoreline armoring for stabilization has been widespread. However, this solution does not take the ecological aspects of the coasts into account. The “living shoreline” technique includes coastal ecology by incorporating natural habitat features, such as saltmarshes, into shoreline stabilization. However, the impacts of living shorelines on adjacent benthic communities, such as submersed aquatic vegetation (SAV), are not yet clear. In particular, while both marshes and SAV trap the sediment necessary for their resilience to environmental change, the synergies between the communities are not well-understood. To help quantify the ecological and protective (shoreline stabilization) aspects of living shorelines, we presented modeling results using the Delft3D-SWAN system on sediment transport between the created saltmarshes of the living shorelines and adjacent SAV in a subestuary of Chesapeake Bay. We used a double numerical approach to primarily validate deposition measurements made in the field and to further quantify the sediment balance between the two vegetation communities using an idealized model. This model used the same numerical domain with different wave heights, periods, and basin slopes and includes the presence of rip-rap, which is often used together with marsh plantings in living shorelines, to look at the influences of artificial structures on the sediment exchange between the plant communities. The results of this study indicated lower shear stress, lower erosion rates, and higher deposition rates within the SAV bed compared with the scenario with the marsh only, which helped stabilize bottom sediments by making the sediment balance positive in case of moderate wave climate (deposition within the two vegetations higher than the sediment loss). The presence of rip-rap resulted in a positive sediment balance, especially in the case of extreme events, where sediment balance was magnified. Overall, this study concluded that SAV helps stabilize bed level and shoreline, and rip-rap works better with extreme conditions, demonstrating how the right combination of natural and built solutions can work well in terms of ecology and coastal protection.

Sediment Balance Estimation of the ‘Cuvette Centrale’ of the Congo River Basin Using the SWAT Hydrological Model

In this study, the SWAT hydrological model was used to estimate the sediment yields in the principal drainage basins of the Congo River Basin. The model was run for the 2000–2012 period and calibrated using measured values obtained at the basins principal gauging station that controls 98% of the basin area. Sediment yield rates of 4.01, 5.91, 7.88 and 8.68 t km−2 yr−1 were estimated for the areas upstream of the Ubangi at Bangui, Sangha at Ouesso, Lualaba at Kisangani, and Kasai at Kuto-Moke, respectively—the first three of which supply the Cuvette Centrale. The loads contributed into the Cuvette Centrale by eight tributaries were estimated to be worth 0.04, 0.07, 0.09, 0.18, 0.94, 1.50, 1.60, and 26.98 × 106 t yr−1 from the Likouala Mossaka at Makoua, Likouala aux Herbes at Botouali, Kouyou at Linnegue, Alima at Tchikapika, Sangha at Ouesso, Ubangi at Mongoumba, Ruki at Bokuma and Congo at Mbandaka, respectively. The upper Congo supplies up to 85% of the fluxes in the Cuvette Centrale, with the Ubangi and the Ruki contributing approximately 5% each. The Cuvette Centrale acts like a big sink trapping up to 23 megatons of sediment produced upstream (75%) annually.

Classification of Management Alternatives to Combat Reservoir Sedimentation

Sedimentation is steadily depleting reservoir capacity worldwide, threatening the reliability of water supplies, flood control, hydropower energy and other benefits that form the basis of today’s water-intensive society. The strategies available to combat reservoir sedimentation may be classed into four broad categories. Three proactive categories seek to improve the sediment balance across reservoirs by: (a) reducing sediment yield from the watershed, (b) routing sediment-laden flows around or through the storage pool, and (c) removing deposited sediment following deposition. The fourth category (d) consists of strategies that adapt to capacity loss, without addressing the sediment balance. Successful management will typically combine multiple strategies. This paper presents a comprehensive classification of both proactive and adaptive strategies, consistent with current international practice. Functional descriptions and examples are given for each strategy, and criteria are provided to differentiate between them when there is potential for ambiguity. The classification categories can be used as a checklist of strategies to consider in evaluating sediment management alternatives for new designs as well as remedial work at existing sediment-challenged reservoirs. This will also help practitioners to more clearly describe and communicate the nature of their management activities. Widespread application of both active and adaptive strategies is required to bring sedimentation under control to sustain benefits of water storage for today’s and future generations.

Storm-induced deformations of barred beach slope

It is shown that behavior of the multiple-bar system during a storm can be described in frameworks of the model assuming that bars move toward the sea with almost the same mean speed, while the over-bar depths increase in accordance with the local bed slope. Analysis of suspended-sediment balance over the bar crest leads to expression for the bar movement speed, which demonstrates very strong dependence on ratio of wave height to the over-bar depth. Using empirical criterion of bar activity the model suggested is capable assess the bar displacement during a storm action and predict the post-storm bed profile. Comparison with the published field data leads to conclusion that the computed and observed storm-induced deformations of barred beach slopes are in satisfactorily agreement.

Multidecadal Sediment Balance Modelling of a Cascade of Alpine Reservoirs and Perspectives Based on Climate Warming

This paper presents a comprehensive methodology to model and determine the annual sediment balance of a complex system of interconnected reservoirs, based on the detailed interpretation of a multi-decadal data series of reservoir management and modelling of sediment fluxes. This methodology is applied to the reservoirs of Oberaar, Grimsel, Räterichsboden, and Trift, which are located in the Swiss Alps. Additionally, the effects of climate warming on the annual sediment yield are investigated. Modelling results show that at present, the hydropower cascade formed by Oberaar, Grimsel, and Räterichsboden retains about 92% of the annual sediment yield, of which only the finest fraction leaves the system and enters the river network. Very fine sediments (d < 10 μm) account for 28% of the total sedimentation rate and in the case of Oberaar, it can reach up to 46% of the total sedimentation rate. Under a climate warming scenario, both sediment yield and runoff are expected to increase in terms of the annual average throughout the XXIst century, which will likely lead to greater annual inputs of sediments to the reservoirs. This, in turn, will lead to a higher sedimentation rate and suspended sediment concentration in the reservoirs, unless active management of the sediment fluxes is implemented.