Customer: Alcan Smelting & Power UK and Halcrow Water

Service: Consultancy - physical modelling

Problem/Background: Treig dam is a12m high embankment dam with its downstream face acting as a spillway. Reports during the 1990s raised concerns about the potential for erosion damage and indicated that the Probable Maximum Flood (PMF) was much higher than had previously been appreciated.

Objectives/Approach: To investigate:

• Degree of wall overtopping during flood discharges
• Ability of the existing roller bucket at the dam toe to contain the hydraulic jump
• Likelihood of erosion problems downstream
• Uplift pressures on the concrete slabs covering the dam face

A 1:30 scale physical model was constructed and tested

Solution: The test programme involved the development and testing of modifications to the model to improve hydraulic conditions. The optimised changes included:

• Increasing training wall heights to contain flood flows
• Installation of a rockfill apron downstream of the roller bucket and extending across and along the river bed.

Benefits:

• Demonstration fo the performance of the spillway under flood discharge conditions up to and including the PMF
• Modifications developed and tested to ensure that reservoir discharges are contained and controlled and potential minimised erosion and scour damage.