Dredging of Reservoirs

Abstract

The main focus of this research is determining when dredging of reservoirs is feasible. First the main issues associated with the sedimentation of reservoirs are investigated. Most important are the loss of reservoir volume, sediment shortages downstream of the dam and possible blockage of water intakes.
The sedimentation issues occur mostly in sediment laden rivers. Reservoirs close to the origin of the river, for instance in mountainous terrain usually do not suffer from severe sedimentation issues.

A survey is held under authorities responsible for different reservoirs. Results indicate that not all reservoirs suffer sedimentation issues, but part of the worldwide dams and reservoirs does.

The most important conclusion is that sedimentation at this moment will only be dealt with if it is clearly affecting the function of a reservoir or if the function of a reservoir is threatened in the near future. The matter has to be urgent enough that owners are willing to spend funds on mitigating sedimentation issues. If added value can be gained through removing sediment this improves the business case of dredging. In the survey this is verified by the respondents.

Methods of dealing with reservoir sediment are discussed. Different dredging methods for the dredging of reservoirs are presented and investigated. The main advantages of dredging are found to be precise removal of sediment, avoiding large sediment flushes and no excessive loss of water.
Using a 1-dimensional Sobek model, it is shown that part of the sediment entering the reservoir can be used for downstream river restoration.

Different factors that influence the feasibility of dredging are identified. The function of the reservoir is important. For the functions of irrigation, storage of hydro power, water supply and flood control the available volume is for example important. Sediment control is therefore important.

By modelling different shapes for reservoirs using Delft3D, it is found that the type of reservoir only partly determines the location of sediment in the reservoir. Other factors influencing the deposit location are sediment concentration and the type of sediment. The maximum reservoir depth is found to be linked to the dam height.

To determine if the dredging of reservoirs is feasible it has to be clear that the reservoir function or intake is threatened by sediment. Based on the function of the reservoir a business case can be made comparing costs and benefits. Benefits of extra reservoir volume are estimated. Possible added values are taken into account, such as the possible sale of dredged material. A dredging model is created to estimate the costs of reservoir dredging.

This model takes different parameters such as sediment composition, dredging depth, reservoir type and transport distance into account. Using this model for different scenarios it is shown that dredging prices can start as low as 1.01 euro.
If the sediment is made up of soft and loose material this reduces the price of dredging. Furthermore the transport distance of the dredged material to shore has a large influence on the dredging costs. The transport distance for the dredged material must be kept small and the volume of material dredged must be over 25,000 $m^3$ to find a stable price. For all reservoirs these parameters are different, leading to a reservoir specific dredging price estimate.

Low dredging prices are possible under favourable conditions. At these prices, dredging can compete with other sediment management techniques.
On top of that, the added benefits of dredging have to be taken into account: downstream river restoration, the possible sale of dredged sediment and increased downstream soil fertility. These benefits increase the feasibility of dredging a reservoir further. The dredging of certain reservoirs is seemingly beneficial. Reference reservoir dredging projects have to be performed to confirm that this

Further research into the deposition of sediment in different types of reservoirs is recommended. Also more investigations into the downstream return of sediment are recommended.