Preliminary field observations on beach cusp formation and characteristics on tidally and morphodynamically distinct beaches on the Nigerian coast

Abstract

Between November, 1984 and December, 1986 observations of cusps were made on some tidally and morphodynamically distinct beaches on the Nigerian coast with a view to assess their origin, conditions for formation, distribution, dimensional and textural characteristics. No conclusive evidence can be forwarded at present with respect to their origin. Nevertheless, after formation, subsequent development and destruction are effected by swash action. Field examination of most of the commonly cited cusp-dependent parameters indicates cusp existence and extinction to be primarily controlled by foreshore slope. Two levels of cusps were typical of the coarse to medium-grained, reflective and microtidal beaches of Badagry and Victoria. The lower-level cusps were consistently absent on the above beaches when mid-tide level slope exceeded 13° and 15°, respectively. On the contrary, the fine-grained, mesotidal and largely dissipative beach at Ibeno depicted only a single set of cusps which were consistently extinct on slopes less than 3°. Cusps were most prevalent during calm coastal condition (November–January) and on foreshore segments depicting reflective beach-state. The regularity of cusp spacing was noted to improve with increase in beach reflectivity. The coarser beaches showed higher cusp relief while cusp length was longer on the fine-grained beach. In general, the spacing, slope, grain-size statistics and permeability of cusps noted in this study conform with those of earlier investigators on other beaches. Temporal changes in dimensional characteristics of cusps are considered essential for evaluating the potentials of cusps as signatures of beach and surf-zone morphodynamics. The above defines the direction of further studies.