PERMEABILITY CHARACTERISTICS OF RUBBLE MATERIAL - NEW FORMULAE

Abstract

Rubble mould structures have been used extensively to protect coastal areas of human interest. These include breakwaters, seawalls and related structures. To be successful, a rubble mould structure should absorb most of the energy from the incident waves and be able to withstand the pore pressures generated during the process. These are determined by the geometry of the structure and the hydraulic properties of rubble. Advances in computer resources enable the interaction between waves and rubble structures to be simulated numerically (Ref 1, 2 and 7). The general approach is to describe the porous medium as a continuum, having properties of dimension, porosity and permeability. The flow of water into and through such a porous continuum may then be described, depending upon the velocities and pore pressures induced. In physical hydraulic modelling of coastal structures various researchers (Ref 5) have considered the importance of permeability characteristics on the scaling of porous rubble core material. Similitude is usually achieved by selecting a model material, of prototype porosity, which yields a comparable hydraulic gradient to the prototype when subjected to an equivalent Froude scaled flow velocity. Both the numerical and physical methods of modelling, however, require a good description of the energy dissipation process which is related to the permeability of the structure. At present, most modellers use formulae which were originally developed for flows in sand. That proposed by Engelund (Ref 4) has been most widely used.