Extensive research and literature is available on morphology and hydrology of reservoirs, and is therefore not discussed in this article. Material characterisation is very important for developing sediment management and removal strategies, and selection of suitable dredging equipment. Material present in reservoirs can be grouped into three categories, i.e. sediments, contaminants and aquatic plants (see Table 1).

The type and volume of sediments are determined by the source of the sediment, sediment transportation mode, prevailing climate controlling the rate of precipitation and evaporation in the area. The rate of seepage through the bed rock/soil also impacts the type and volume of sediments present in the reservoir.

The sediment management strategies can be preventive sediment management strategies by sediment control at the source (e.g. construction of embankments, sediment traps and afforestation measures) and sediment control during transportation (e.g. construction of pre-settling structures/ weirs/dams/barrages in the upstream or construction of silt traps). The type (natural or artificial) and amount of contaminants present in the reservoir needs to be assessed for defining sediment management strategies. The curative sediment management strategies include dry excavation where the reservoir is dry during the excavation process or dredging where the reservoir cannot be dried up. Surface floating mechanical or hydraulic dredgers can be used for shallow reservoirs. For deep-water reservoirs, submersible tracked or walking dredgers (Sarkar, 2007) and submersible floating dredgers (Sarkar, 2012) can be used. Specially designed submersible pumps can also be used for deep-water reservoirs.

Figure 2 shows small scale dredgers with specialised cutters for aquatic plants and contaminated sediments present in small reservoirs. A water injection dredger and a submersible walking dredger suitable for deep-water reservoirs are also shown.

Risk management

Risk can occur at any phase of the reservoir dredging life cycle. A “risk” is an uncertain event, activity or a set of circumstances that, should it occur, has a positive or negative impact on at least one of the reservoir dredging project objectives, i.e. scope, schedule, cost and quality. There can be multiple impacts and one or more stakeholders can be affected. The various phases of the reservoir dredging life cycle are: 1) pre-tendering; 2) tendering; 3) mobilisation; 4) execution; and 5) site restoration and demobilisation. The risk categories that can occur during reservoir dredging life cycle phases are: 1) technical; 2) economic; 3) commercial; 4) organisational; 5) political; and 6) social (nearby residents). Environmental risks will be generated by dredging activities. An Environmental Impact Assessment (EIA) should be performed prior to any dredging activities to provide sufficient knowledge about the associated ecosystems and flora and fauna thriving in the reservoir and surrounding areas.

Risk management for reservoir dredging is a continuous process performed throughout the various phases of the reservoir dredging life cycle. For each phase, the steps shown in Figure 3 should be applied to reduce the likelihood of an identified risk occurring and/or to reduce the potential impact of the risk on the reservoir dredging project.

To identify, categorise and analyse risks, the hazards associated with activities for each phase of the reservoir dredging life cycle are to be identified. The consequences of a hazard can be on people, equipment, environment and reputation of relevant stakeholders. Risk is calculated as:

Risk = consequence x likelihood [1]

The calculated risk needs to be evaluated with the risk tolerance of an individual organisation. Each organisation has its own unique risk profile based on assets it wants to protect, goals it wants to achieve and its ability and willingness to handle risks. Figure 4 shows the factors considered for calculating the risk tolerance of an organisation.

Risk response is the action taken to reduce the probability of a hazard arising or to reduce the impact of the hazard if it arises. Standard control and reduction measures are taken to reduce the risk rating (see Equation 1). These are referred to as “proactive responses or mitigations” and are usually funded by the project budget. If “reactive responses or mitigations” are required, then these are funded by the contingency budget. The contingency budget is dependent on the risk tolerance of the relevant organisation.

Early identification and involvement of stakeholders and decision makers are necessary in order to mitigate reservoir dredging risks. Stakeholders care about or have a vested interest in the project, while decision makers are legal authorities or groups that have accountability for management action. All decision makers are stakeholders, but not all stakeholders are decision makers. The importance of the stakeholders during various phases of the reservoir dredging life cycle are presented in Table 2.

Risks of reservoir dredging

Reservoir classification is presented in Figure 1. There are risks associated with different reservoir types, which can affect the overall risk of reservoir dredging. Major risks associated with dredging each type of reservoir are presented in Table 3.

The risks associated with the various phases of the reservoir dredging life cycle are presented next. Risk categorisation and stakeholders affected by the hazard and stakeholders who can mitigate the risks are shown in Tables 4-7.

Risks associated with demobilisation of the dredging equipment are similar to those mentioned for mobilisation activities. However, the risk to the overall project is lower as the works have been completed before demobilisation commences. The main consequences for site restoration are damage to the environment and associated ecosystems. The end users, local business groups and tourism department are affected if the site restoration steps taken are inadequate. During the site restoration phase the following checks are to be performed as a minimum:

1. Post-dredging monitoring of reservoir water quality and comparison with pre-dredging water quality;
2. Migration and extinction of any species not identified in the Environmental Impact Assessment, which is usually performed before the execution of any dredging activity;
3. Timeline for return and resettlement of species either disturbed or moved due to reservoir dredging activity (Sarkar, 2013);
4. Effect of reservoir dredging on downstream ecosystems and associated flora and fauna;
5. Dredged material in the designated disposal area is not generating any environmental damage and ecological risks to the local community and existing flora and fauna;
6. Disposal, encapsulation or further transport of contaminated sediments in not generating any additional risks; and
7. Any change in landscape of the area surrounding the reservoir to be inspected and brought back to the state as close as possible to the original.

Conclusions

A holistic approach is necessary for reservoir dredging operations by considering the associated water systems and the morphological and hydrological characteristics of the catchment and downstream areas. Identification and early involvement of stakeholders is necessary for a stakeholder driven process, which will help to identify and effectively manage and mitigate risks associated with reservoir dredging. Adequate data should be collected about the reservoir either from existing databases or suitable surveys to develop reservoir sediment management and restoration strategies and methodologies. The sedimentation pattern and material characteristics present within the reservoir is fundamental for any dredging activity. An Environmental Impact Assessment should provide sufficient knowledge about the associated ecosystems and flora and fauna present in the reservoir and surrounding areas. The volume of dredged sediments from the reservoirs can be of significant volume compared to capital dredging. Hence, identification of suitable disposal location, disposal methodology is very important. At planning stages, beneficial uses of dredged material and circular economy should be considered to reduce risks and have minimum impact from the dredged spoil. If contaminated sediments are present, handing and treatment of such material needs to be considered as early as possible.