6. Towards a Solution
6.1 Restoration of Sediments
The Saskatchewan River Delta is one area that has been significantly modified by the construction of dams upstream (Liaghat et al., 2017). The geomorphology and hydrology of the delta have been substantially impacted by these dams. According to future projections, it suggests that the channel erosion accelerated process caused by sediment starvation (Smith et al., 2016) will eventually lead to the widening of a single channel’s way through a former delta, leading to the alteration to the delta ecosystem and the identity of the people of the Cumberland House.
There is a need by the communities to have a better understanding of the capacity of sediment restoration in the improvement of the ecosystem’s health. To assess the biological relevance of sediment starvation and to evaluate whether sediment restoration would be a viable idea for the management of this ecosystem, the following needs to be done. There is a need to test the deposited sediment for toxic chemicals and nutrients (Rovira & Ibàñez, 2007). There is also a need for sediment transport modeling and a planning process based in the community.
To restore deltaic wetlands, the basic scheme would be to reverse the main river channel’s artificial channelization (Delta Stewardship Committee, 2018). This can be done by controlled openings that divert water, and more importantly, sediments onto drowned wetlands. In the restoration of the delta, the idea is to allow the new wetlands to be created by natural processes, though the process could be accelerated using dredge sediment to help in the building of the platform (Wohl et al., 2015). It is required that to allow the delta-building process to reconstruct associated wetlands and delta lobes, a substantial part of the sediment supply to the wetlands should be restored.
6.2 Sediment Transport in Main River Channels
Another natural process that is important for the restoration of the delta is sediment delivery from upstream, although in many cases, this process is altered by human activities like the construction of dams (Wohl et al., 2015). According to recent measurements from the Wax Lake Delta, 50-70% of the deposits building the land are composed of sand, although in the lower Mississippi River load, sediments of the same size do not even add up to 10% (Paola et al., 2011). The implication of this is that we need sediment diversion structures to restore sand into the channel downstream. In transport, there is no even distribution of sand throughout the river water column, but there are high concentrations that are lopsided towards the bottom of the river (Paola et al., 2011).
6.3 Reintroduction of Dredged Material
Tactics to reintroduce recovered or excavated material back into the downstream channel are done at Rhine River Iffezhenim, Germany, and Danube River downstream of Vienna, Austria (Schmutz-Moong, 2018). Most of the dams have a capacity of the mean annual ratio of 0.2 to 3 (storage divided by mean annual discharge) and a lifespan of 50-2000 years considering their sedimentation. Determining the mean annual ratio and lifespan of a dam is helpful to find out which management practice would be suitable for a given dam. There is a need to carry out flushing or sediment sluicing during floods and through outlets with large bottoms when the capacity to mean annual flow ratio is below 0.03. The flushing should preferably be carried out with free-flow conditions. Flushing as an operation is sustainable, and it can be used to reach a long-term equilibrium storage capacity. For regions with less variable hydrology, seasonal flushing for two months per year could be used (Kondolf et al., 2014). Such areas should at least have a capacity to mean annual flow ratio of up to 0.2. In cases where the capacity to mean annual flow ratio is more than 0.2, there is no excess water for flushing, and the operating model is typically for storage (Kondolf et al., 2014).
To recover the lost storage capacity, we can carry out dredging or density current venting. Flushing of the reservoir is an effective mitigation for sediment remobilization and the restoration of natural dynamics of sediments and the formation of type-specific habitats (Batalla & Vericat, 2009). The levels of the reservoir are reduced to pre-impounding levels to enable the river to erode deposited sediments. At least twofold the mean annual flow is required (Batalla & Vericat, 2009). However, there are also immediate adverse effects associated with reservoir flushing on the physiochemical conditions. For instance, the increased drift, gill and skin injuries, fish kills, stress, and indirect impact like a reduced supply of food as a result of the loss of benthic invertebrates and increased drift, lost habitats due to sedimentation, and reduced growth are some of the flushing impacts (Greig, Sear & Carling, 2015).
6.4 Sediment Management
The main goal of sediment management should be to make the dams to be as permeable to sediments as possible. Some of the options of management include sediment bypass, sediment flushing, and reservoir downstream sediment augmentation. Basson and Rooseboom (1999) developed some general guidelines relating the sediment inflows and water inflows, the size, and applicable mitigation measures. They identified a relationship between the mean annual water, sediment inflows, and capacity of the reservoirs, as well as the appropriate mitigation measures (Rovira & Ibàñez, 2007).
6.5 Sediment Sluicing
The aim of this mitigation measure is to maintain the suspension of sediment and move it through the impoundment before it can be deposited. It typically involves the reduction in water level in the impoundment through opening gates in the event of increased sediment concentrations. There is a similarity between turbidity venting and sediment sluicing, but in turbidity venting, low-level gates are used to enable sediment-laden water to flow along the reservoir bottom to the dam toe.
6.6 Bypass Structures
These could be constructed canals, tunnels, or existing river channels. They can be used in passing water bearing high levels of sediment and bedload around an impoundment.
6.7 Next Step
Although, as discussed above, there exist various ways to limit the sediment loss in the downstream of a dam, many factors need to be considered to decide which management practice would be ideal for a given dam. These factors include the cost budget available for sediment restoration, current geomorphological conditions such as state of the river bed, and erosion rate. Ecological conditions should also be taken into account while choosing best sediment restoration practice. Sediment management is an expensive and challenging process, regardless of which technique is used. There is no perfect sustainable solution for restoration but we can make efforts to optimize the result. Methods like sluicing and bypass structures are feasible for regions where sediment load occurs more often and in a predictable manner. These methods result in preventing sediment deposit in the impoundment of the dam. While others like dredging take into account removal of sediment deposits. Thus, reclaiming the lost storage. With aging of EB Campbell Dam, additional attention should be centered on the maintenance of sediments in the delta before geological phenomenon renders the river useless.