DIMENSIONING OF DEBRIS FLOW DEFLECTION DYKES

Deflection dykes are often used as a practical protective device against debris flow damage. Their function is to deflect flowing debris away from sensitive areas, either in the channel or on the depositional fan. They must be built sufficiently high, to prevent overtopping by rapidly moving debris flow surge fronts. A series of small-scale laboratory experiments have been carried out to study the influence of velocity on the height of superelevation of deflected debris surges. The test material was dry sand, sliding over a smooth metallic surface. This is considered suitable for simulation of the rapid flow of liquid debris, with low basal friction resistance. Straight model dykes were oriented obliquely to the flow direction, at varying angles. Experiments have been carried out using a range of approach velocities and dyke slope angles. It was found that the Takahashi-Yoshida runup equation (Hungr et al., 1984) provides a reasonable prediction of runup heights. The same type of analysis was tested against a full-scale partial overflow of a protective dyke by a natural debris flow. The runup equation provides a reasonable basis for design of deflecting dykes.