WP 4: Sediment Movement

1. Overview
2. Near-field sediment flow in dam-break conditions
3. Geomorphological changes in a valley induced by dam-break flows (far field)

During the extreme flow conditions following a dike- or dam failure, there can be large movements of sediment and debris that can significantly change the valley topography and hence flood water levels. The sudden release of water from a control structure brings with it intensive scour in the flow downstream. Further away, deposition of the material can lead to changed flow channels and blocked structures (bridges etc). Debris can be transported for substantial distances whilst the heavier material is deposited or trapped once the flow velocity attenuates.

Advances in this area related to natural river floods have been made in the Third and Fourth Framework Programmes (FP3 & FP4), specifically in the Debris Flow and FRIMAR projects (Casale & Samuels, 1998) as well as ENV4960253 and ENV4960247 (Eroslope & Eroslope II). However, conditions following the failure of a major dam, for example, are much more severe than in normal flood flows in that the quantity and size of sediment will be much greater in the catastrophic flood flow.

This is an important issue for an accurate prediction of the downstream consequences and the associated impact on the flooded areas as the river bottom elevation can vary by tens of meters or the river can be diverted from its natural bed (as for the Lake Ha!Ha! dam failure, Canada, in 1997).

The problem to be solved in this theme is to improve the predictions of the motion of sediments in association with catastrophic floods. The research combines physical experiments with theoretical development on :

• flow resistance on the bottom interface and its dependence on the flow dynamics under different flow conditions (i.e. how flood flow may move bed material under extreme conditions)
• the heavy sediment transport, erosion and deposition in the near field (i.e. near the dam or defence structure)
• the bank erosion mechanism and important deposition in the far field leading to the formation of secondary dams, including the dispersion of associated pollutants (e.g. risk of secondary flooding caused by blockage from debris and menace of pollutant mobilisation as from failure of a tailings dam)

The IMPACT Partners use the extensive experimental facilities available to them in undertaking the experimental programme. An output of the research will be a set of well-documented experimental investigations of flows with transported sediment, which could serve the international research community as benchmarks for future theoretical developments beyond the scope of the current IMPACT project.

The research is divided into two work-packages that address:

• near field sediment flow in dam-break conditions
• geomorphological changes in a valley induced by dam-break flows (far field)

Typical new problems appear in the near field under sever transient flow conditions, like dam-break flow:

• the important initial resistance due to sediment inertia, requiring a refined analysis of the acting forces, both on the sediments and on the flow
• the near-field erosion processes for aggressively erosive flows caused by the failure of a water control structure, involving intense mobilisation of the granular bed, erosion of a scour hole, development of a debris front, and formation of a hydraulic jump
  
  

To achieve the objectives of the work package, research has been undertaken as well experimentally as theoretically and numerically. The experimental investigation have been undertaken in two of the IMPACT Partners' laboratories using existing experimental facilities and measurement equipment.

The results of each series of tests are documented as a validated set of benchmark data for the development and verification of mathematical models.

A first benchmark, about the case where the sediment level is the same upstream and downstream of the gate, is completed. A second one, where the sediment level is higher upstream is launched in the early 2004.

To achieve the objectives of the work package, the following problems will be faced:

• the bank failure mechanisms under rapidly changing flow conditions;
• the deposition of important amounts of sediments eroded from the banks : distribution and formation of secondary dams;
• the evolution form the near-field behaviour (debris flow front, fully laden water and sediments mixture) to the far-field highly erosive flow inducing sudden bank failures.
  

Laboratory tests have been carried out both at a reduced scale and another series will begin at a larger scale in available facilities. Accurate measurements over complete flow areas are made by recently developed digital imaging techniques. Field data are also be collected from past events.

The experimental and field data are documented as a validated set of benchmark data for the development and verification of mathematical models.

A benchmark based on reduced scale tests was launched in January 2004. Another benchmark, exploiting the large-scale facilities will start at mid-2004. Finally, a benchmark based on a real case, the Lake Ha!Ha! event in Canada, will be launched, depending on some missing data.

If you would like any further specific information relating to the IMPACT project or if you would like to participate informally with any of the model testing and comparison, please contact the project coordinator:

Project Coordinator: Mark Morris HR Wallingford Email:m.morris@hrwallingford.co.uk

back to Work Programme

back to Top