Quantitative Predictions for the Position and Height of Berms

Abstract

Using experimental results on berms created in a small-scale wave flume, the position and height of berms in an equilibrium state were examined. The berm position, X, which is defined as the horizontal length from the shoreline to the berm crest, was found to be expressed by: [formula omitted] and 0.305 for collapsing related berms and for surging related berms, respectively, where is the breaker height, T is the wave period, g is the gravitational acceleration, and ϕ is the reduction factor due to the roughness and permeability of the beach. The reduction factor was given by [formula omitted], where D* the dimensionless grain diameter of sediment, D* = [g(ρs/ρ − 1)/v2]1/3D, D is the sediment diameter, ρs is the sediment density, ρ is the fluid density, and v is the kinematic viscosity of fluid. The berm height, Bh, was given by: Bh/(gT2)5/8Hb1/8 D1/4 ϕ= 0.117 and 0.067 for the collapsing related berms and for surging related berms, respectively. Analyses of the existing prototype-scale experiment data indicated that no scale effect is involved in these laboratory relations as far as collapsing related berms are concerned. Time-series data of the beach profile obtained at Ajiga-ura Beach, Ibaraki, Japan, were used for the examination of applicability of these relations to the field situation, where collapsing related berms developed. The height of the mean high waters was used as a reference level in measuring the berm position and height. The breaker height, H̄b, and wave period, T̄, of mean waves averaged over the period of berm development were substituted respectively for the wave characteristics in the laboratory-based equations. The berm position, X′, was described by [formula omitted] and the berm height, B′h, was expressed by [formula omitted]. © 1994, The Association of Japanese Geographers. All rights reserved.