THOUGHT LEADERSHIP

Can the sedimentation problem be solved?

Sedimentation is a problem for water storages, particularly in Asia. It is a major concern for all communities and industries that depend on water storages for water, food and energy security.

As sediment builds up over time, the storage capacity of a water reservoir will reduce. If we can effectively prevent, manage and reduce sedimentation, more water can be stored, which is critical for a sustainable future in a world experiencing increasing population, industry and climate pressures.

Loss of storage is certainly damaging to the performance of hydropower schemes, but it’s not the only sedimentation-related problem for hydropower developers and operators. High levels of sediment, whether in storages or flowing through run-of-river hydropower schemes, can seriously damage expensive hydro-mechanical equipment, causing significant operations and maintenance issues and costly outages.

It is clear, then, that planners, developers and operators need to consider how to minimise and mitigate sedimentation in all water storage projects, but particularly if the project is in a region that yields a large amount of sediment.

As with any problem, prevention is better than cure. Although, in some cases, prevention isn’t possible, or the problem already exists, so ‘curative’ options need to be explored.

Prevention

The best prevention of sedimentation problems begins at the source – in other words, the catchment area of the river. If we can reduce the sediment yield from the watershed, we can reduce the sedimentation issues in our reservoirs, increasing the life of the available storages. Most of the sediment that reaches a water resource project site is due to rainfall and run-off erosion and transportation of material by the river’s flow. When the sediment reaches a reservoir, it becomes trapped and builds up over time, reducing the amount of water that can be stored.

The quantity of sediment will be heavily influenced by the rate of erosion within the catchment, which depends on interactions among factors such as climate, soil, geology, topography, ground cover, land use and human activity. Some of these factors and interactions cannot be controlled, but human-related aspects affecting erosion and sediment yield can be predicted and managed through an integrated catchment management plan. Such a plan would require significant efforts to counter factors such as agriculture, mining, construction and deforestation, and include attention to revegetation and erosion prevention. Identifying and mitigating impacts in areas susceptible to the geological risk of landslides is another important part of the plan, whether this is around the reservoir rim or within the main catchment of the reservoir, as these areas of instability will affect sediment inflows.

Beyond this catchment-level reduction of sediment yield, planners and designers need to consider how to reduce the inflow of sediment into the particular reservoir, therefore maintaining the storage. Local or project-specific preventative approaches require accurately estimating the sedimentation rate during planning and design – through careful attention to measuring sediment concentration and the capacity inflow ratio – and, based on this estimate, considering building structures upstream of the main reservoir to either trap sediment or encourage sediment to bypass the reservoir. Sediment can be trapped before reaching the main reservoir with a series of weirs or ‘check dams’ upstream of the reservoir, yet these will only be effective until such time as they fill with sediment.

Intervention

The next option is to keep the sediment moving, either through the reservoir or past the reservoir, so the amount that is deposited in the reservoir is minimised. In many cases, the main transport of sediment is in flood water; thus, flood waters can be channelled past the reservoir using bypass structures, or can be allowed to pass through the dam at high velocity (known as sluicing). An important disadvantage of bypassing and sluicing of flood waters is the loss of a significant amount of water that otherwise could have been captured in the storage. As a result, it is most applicable in reservoirs with a smaller capacity in comparison to the total inflow.

Due to the very high cost of creating bypass conduits, bypassing the full length of a reservoir is rare, and would only be considered in special circumstances or where other methods are not effective. If the reservoir is located on a horseshoe bend of the river, a sediment bypass structure may be cost-effective due to its reduced length. Therefore the unique characteristics of the site need to be taken into account when developing the initial concept for the project, looking for opportunities created by the topography.

Sluicing involves discharging high flows through the dam structure during periods of high inflow to the reservoir, to allow sediment to be transported through the reservoir as rapidly as possible while minimising sedimentation. Sluicing is performed by lowering the reservoir storage prior to high-discharge sediment-laden floods. This approach requires relatively large capacity outlets to be incorporated into the dam design to enable the discharge of appropriately large flows at low reservoir levels to maintain the required velocities to transport sediment. This is achieved through low-level under-sluice gates, or tall crest gates, or both.

To understand how a reservoir will behave, appropriate investigations are needed in the planning stage to accurately determine the sediment characteristics, inflow and distribution in the reservoir. This data can then be used to model the flow and deposition of sediment within the reservoir, using sophisticated hydraulic modelling software. These models can be used to fully understand the problem, and to test solutions – helping to determine the appropriate location and capacity of low-level outlets, develop operating rules that will work for the particular reservoir, set the intake invert to avoid future problems, and develop an overall concept for the dam that works – all of which will contribute to avoiding future problems.

The sediment problem becomes particularly worrisome for run-of-river hydropower plants located in rivers which typically carry a high level of sediment, such as rivers flowing from the Himalayas. To protect against damage to equipment, desilting arrangements such as desilting tanks and chambers are generally provided, normally immediately downstream of the intake structure to the water conveyance system, whether a canal or a tunnel, along with modifications to equipment, such as coatings to better resist abrasion.

Remediation

The aim of remediation is to recover the original storage volume of the reservoir. If sediment has built up in a reservoir in the absence of, or despite, preventative measures, the options are now limited to reducing sediment levels through hydraulic methods (flushing through reservoir drawdown) or mechanical methods (excavation or dredging). The advantages of hydraulic methods are that they tend to be cheaper and easier, using water currents or flows to force sediments through gates and outlets close to the reservoir bed. Nevertheless, these methods may release large amounts of valuable water through the emptying of the reservoir, which may not be desirable during either dry conditions, or where the storage volume is very large in comparison to the annual inflow.

Mechanical methods, such as dredging and excavation, are typically very expensive and only practical on certain reservoirs, and therefore seen as a last resort. Dredging can either be via hydraulic pumps (for finer sediment) or mechanical grabs (for coarser sediment) on barges. Due to its expense, it is often only used to remove sediment from specific areas near the intake structure of a dam. If a reservoir can be completely drawn down, which is not practical for many reservoirs, accumulated sediment can be removed through scrapers or excavators and dump trucks.

Finally, there may be the potential to add new storage to the existing reservoir by raising the dam. The practicalities of this will need to be evaluated through a feasibility study process, as is normally adopted for a new dam.

The right solution depends on good information

In many cases, managing sedimentation will require a combination of strategies and technologies, such as reducing the sediment yield at the catchment level, reducing inflows of sediments into storages using appropriate structures and technologies, operating storages effectively during flood conditions, and actively managing storage levels and operating rules to allow sluicing and flushing.

Many of the sedimentation problems experienced around the world were either not predicted or significantly underestimated during design. To avoid this situation, continuous, adequate and accurate monitoring data is needed, as well as appropriate modelling and projections that take current and potential future conditions into account. Solutions that have been tested via appropriate modelling are much more likely to meet performance requirements and to avoid future risks.

The rate of sediment deposition is heavily influenced by the sediment concentration and the capacity inflow ratio, so careful estimation of these two parameters is very important in identifying the seriousness of a sedimentation problem. In existing large reservoirs, sediment management will benefit from supplementing conventional hydrographic surveys with the adoption of improved survey methods and remote-sensing techniques. The resulting data will enable more reliable estimation of sedimentation rates.

Better measurement, modelling and estimation of sediment – for existing storages as well as future reservoirs – will provide the insights we need to improve sediment planning and management. The right combination of sedimentation estimation, prevention, intervention and remediation will be critical for the long-term health of our water storages and a sustainable future.

If you would like to find out more about how Entura can help you develop a sustainable water storage solution or respond to sedimentation challenges, contact Shekhar Prince on +61 412 402 110.

About the author

Richard Herweynen is Entura’s Technical Director, Water. Richard has three decades of experience in dam and hydropower engineering, and has worked throughout the Indo-Pacific region on both dam and hydropower projects, covering all aspects including investigations, feasibility studies, detailed design, construction liaison, operation and maintenance and risk assessment for both new and existing projects. Richard has been part of a number of recent expert review panels for major water projects. He participated in the ANCOLD working group for concrete gravity dams and is the Chairman of the ICOLD technical committee on engineering activities in the planning process for water resources projects. Richard has won many engineering excellence and innovation awards (including Engineers Australia’s Professional Engineer of the Year 2012 – Tasmanian Division), and has published more than 30 technical papers on dam engineering.

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5 June, 2018