Investigation on the failure of a block ramp

The block ramp at the Grand Eau River, near the village La Tigne (VD), was destroyed for the most part during the floods in 1999 (10^th -16^th May). The destroyed part had a length of 70 m, an average slope of 17 % and a width of 12 m.

The ramp had a curved course in plan view as well as a channel bed with transverse slope. Recordings of the gauge showed a maximal discharge of 52 m³/s, which correspond approximately to a flood with a recurrence period of 5 years. The design flow was dimensioned to a discharge of 90 m³/s by applying an empiric criteria for block ramp stability.

Due to the investigation the failure is caused by two essential factors: The reduced resistance and the increased local stress on the other hand. Observations of the situation after the floods in 1999 and the history of the block ramp assumed that its structure wasn’t homogenous and that weak spots existed.
The analysis of the hydraulic conditions show, that the flow field on the ramp wasn’t one-dimensional due to the complex geometry. This shows figure 2, which is the result of a two-dimensional numeric simulation. The stress is increased locally caused by the curve and the transverse slope.

The experiences of the failure at the Grande Eau River show, that the application of one-dimensional criteria for the block ramp stability can result in an overestimation of stability due to complex boundary conditions. The design of such ramps has to occur either in consideration of a high excess charge or in an application of physical or numerical models.