Abstract
Following three types are generally found on rocky coasts (Fig. 1): Type A is a platform that slopes seaward without significant topographic break extending to the nearshore bottom; Type B is a platform that has a nearly horizontal surface with marked topographic break, a scarp, at the seaward margin; and Type C is a plunging-cliff coast with no platforms. Differences among these types should be attributed to the relative intensity of assailing force of waves and resisting force of rocks. This study attempts to investigate a quantitative relationship between these coastal types and the lithological condition (resisting force of rocks) selecting an area where regional differences of both input wave energy and tidal conditions are small. The study area is the eastern coast of Chiba Prefecture, facing the Pacific Ocean (Fig. 2).
To obtain the physical and mechanical properties of coastal rocks, compressive strength, tensile strength, shear strength, abrasion hardness, and the velocity of longitudinal waves were tested in a laboratory. The longitudinal wave velocity was also measured in situ. Results (Tables 2 and 3) show large regional variations depending upon locations. Since these properties are closely related and not independent of one another (Fig. 6), compressive strength was used as representative of other rock properties. To quantitatively express the effect of cracks in a rock body in the fields a nondimensional parameter, Vpf/Vp, was employed, where Vpf=longitudinal wave velocity in situ and Vp=longitudinal wave velocity measured in a laboratory for specimens without visible cracks. The quantity (Vpf/Vp) Sc was employed as an index for the resisting force of rocks against waves:
fr_??_(Vpf/Vp)Sc (1)
where fr=rock resisting force and Sc=compressive strength of rocks. The three coastal types are clearly distinguished by this index (Fig. 7): Type A landform is formed on a coast with smaller value of (Vpf/Vp) Sc, i. e., 8_??_20kg/cm2; Type C is formed in a place where (Vpf/Vp) Sc is of larger value, i, e., 260_??_510kg/cm2; and Type B is produced in an area where this parameter shows intermediate value, i. e., 50_??_100kg/cm2.
