The paper shows the results of stability tests of block ramps. Block ramps or rock chutes are short and steep longitudinal structures where the bed of the channel is armored with rocks. In order to ensure the ramp functionality it is important to know the failure mechanisms and the critical discharge values. Tests were performed in two channels where rock chutes of different slopes were modeled. The structures were investigated both in a base configuration and with the reinforcement of protruding boulders. Boulders produce an increase in stability. Different arrangements of boulders (rows, random, arc and reinforced arc) were investigated. Results allowed to evaluate the failure mechanisms and also to describe the morphological evolution of the ramp. Three steps of failure were considered: the initial movement, the local failure and the global failure. Measurements of the discharge at the moment of failure and an evaluation of the mean water depth allowed to determine the critical conditions in terms of the particle densimetric Froude number. Beside a relationship of the critical water discharges in terms of the geometric characteristics of the rock chute is given. It is to be kept in mind that due to short length of the ramp uniform flow conditions were not guaranteed during the tests.
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