How can we deal with sediments to keep hydropower sustainable?

16 November 2016 - by Robert Boes

Hydropower is the most important source of renewable electric energy worldwide, but it’s not uncommon that it has to face a serious challenge: sediments that get deposited in storage lakes or increase the erosion of hydraulic turbines. Many countermeasures can be taken to minimize these negative effects, but they need to be optimized site-specifically. To keep hydropower plants operational for as long as possible, researchers are developing new methods of monitoring sediments in real time, while also expanding their knowledge on how sediments are transported, deposited, and removed and how machinery is subject to sediment-induced wear.

Many storage lakes and stretches of river upstream of run-of-river hydropower plants are filling up with sediments transported by the inflowing rivers. Without adequate countermeasures, the storage volume continuously diminishes and eventually completely vanishes, having negative effects on hydropower production. A storage power plant would turn into a run-of-river plant, losing its main asset of supplying peak energy when demand is high. Further negative effects of sediments are an increasing wear of hydraulic machinery such as turbines and pumps, as well as of auxiliary infrastructure like weirs, desanding facilities and flushing channels, resulting in increased operation and maintenance costs. The hydro-abrasive wear of turbines adversely affects their geometries, which leads to reduced efficiencies and the loss of power production.

Fully silted Koorawatha reservoir in Australia (Source: H. Chanson, University of Queensland)

Fully silted reservoir

Hydro-abrasive wear of Pelton turbine runner (dull main splitter and wavy bucket; turbine on display at Emosson dam, VS; Source: VAW, ETH Zurich)

Hydro-abrasive wear

Erosion of the sediment bypass tunnel invert in Palagnedra (TI) by several meters (Source: C. Auel, formerly VAW, ETH Zurich)

Erosion sediment bypass tunnel

Sediment deposits in the Räterichsbodensee reservoir (BE) during water-level drawdown for construction works (Source: J. Stamm, Kraftwerke Oberhasli AG)

Sediment deposits


Need for countermeasures

There are a multitude of countermeasures to cope with sediments. They all have in common that they require either high investments or an appreciable amount of water that can no longer be used for energy production, thereby reducing revenues. Nevertheless, to keep hydropower facilities economical, environmentally friendly and socially acceptable over the long term, there is no alternative to finding sustainable solutions to cope with sediments. Moreover, water protection acts in many countries today require sediment continuity to be re-established in river systems where sediments are partly or fully trapped upstream of weirs, dams or other engineering structures across a river.

Solutions to the challenges

Interesting solutions that have already proven to be technically feasible and environmentally favorable include the following:

  • Constructing sediment bypass tunnels around dams
  • Venting turbidity currents
  • Regularly flushing through low-level dam outlets
  • Removing sediments using hydro-suction

In my view, another approach that deserves further research and development is to continuously pass fine sediments to downstream stretches of river via the power waterways and turbines. This method keeps the sediment flux similar to what it was before the construction of the hydropower plants. Despite increasing the wear on hydraulic machinery, this method seems promising for a number of mainly Alpine reservoirs where alternative solutions are either cost-prohibitive or unfeasible due to legal restrictions. To counter turbine wear, special coatings may significantly reduce hydro-abrasion. In addition, real-time and continuous monitoring of suspended sediment characteristics such as concentration and particle size allows for short-term decision-making by the operators. One option is to “switch off” power production during floods when certain threshold values are exceeded and the hydraulic machinery would be eroded disproportionately.

As part of our research in the SCCER-SoE, we are focusing on the erosion of turbines, on sediment bypass tunnels and flushing channels, including the development of prediction models and measuring techniques for the real-time monitoring of bed load and suspended load, applying both experimental and numerical methods. Therefore, improved knowledge of the relevant processes and measuring techniques serves as a basis to develop sediment-management solutions that are tailored to the specific sites.

With ageing reservoirs and ongoing glacier retreat, sediment management at hydropower schemes is becoming even more important. However, glacier retreat also offers important new possibilities for further hydropower development, in line with the Swiss Energy Strategy 2050.

Discharge from the the Solis sediment bypass tunnel outlet into the Albula river (GR; Source: C. Oertli, EWZ Kraftwerke Mittelbünden and C. Auel, formerly VAW, ETH Zurich)

Downstream view of sediment-water mixture discharging from Solis sediment bypass tunnel into the Albula river (GR). The bed load concentration in the right half of the jet is higher due to secondary current effects caused by a right-hand tunnel bend.


You can also find this blog post in the Zukunftsblog of ETH Zurich.

Author

Robert Boes is the Director of the Laboratory of Hydraulics, Hydrology and Glaciology (VAW), Professor for Hydraulic Structures at ETH Zurich, and Principal Investigator at the SCCER-SoE.