Stability of riprap side slopes in open channels

Abstract

The present design techniques for the sizing of rock in riprap side slopes give results that differ appreciably. In previous researches both deterministic and probabilistic methods have been applied, but the assumptions made have been based on inadequate experimental and field data.In this investigation, experiments were conducted in 10m long channels having a 1¹/₂ : 1 side slope protected with a rock layer of 1¹/₂ diameter equivalent thickness. Both an outdoor flume and an indoor tilting flume were utilised and, in all, six models were tested, including two bed widths (0.4 and 0.5m), two filter types, and a uniform and a graded rock.In the case of the first three models, increasing the discharges up to the maximum pumping capacity did not produce any more than minor rock displacement, although failure was predicted by all of the recognised methods.In the last three models, where the bed slope was increased, failure did occur. The mode of failure was assessed and, in one case, a more detailed examination was made with the aid of coloured rock particles.It was found that the riprap was more stable than predicted by any of the recognised approaches. Consequently, with the aim of obtaining a more fundamental appreciation of the incipient motion and the forces acting, a special force measuring apparatus was devised whereby the lift and drag forces acting on a spherical particle placed in the riprap slope could be measured simultaneously. Preliminary experiments were conducted to establish the appropriate sphere diameter and its location at the level of maximum wall shear.Advantage was taken of the data obtained in the force measurements to devise modified deterministic and probabilistic techniques affording greater conformity with the experimental results.The effect of particle shape and orientation was investigated experimentally by measuring the forces acting on four different shaped non-spherical particles as well as on the spherical particle.Recommendations for further research on this important topic have been made.

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