Beach Sedimentation Cycles (1962-1985) Along a Microtidal Wave-Dominated Coast: South Shore of Rhode Island

Long-term beach profiling along the southwest shore of Rhode Island has resulted in the following data set: 4 locations measured 2 times per month since 1962; 4 locations measured 2 times per month starting between 1975 and 1977; and 2 locations measured 5 times per month beginning in 1977 and 1981. Currently, the 32 km stretch of barrier spit and headland shoreline from Watch Hill Point to Point Judith is covered by 10 profiles; all profiles are located on barrier spits and are not evenly spaced. The total number of profiles is now 3,500. Computer plotting and statistical programs have been developed which allow direct comparison of the differing data sets. Eigenfunction analyses have defined modes of variances called beach-functions. Beach-functions are named according to the geomorphic area in which they are most sensitive to change. The following beach-functions have been identified: 1) shoreface-berm; 2) backberm; 3) beachface; 4) foredune; and 5) hybrid functions which are combinations of the above beach-functions. Profile volume plots show that the beaches eroded from 1962 and 1985. Superimposed on the erosional trends are strong 10-11 year and subordinate 5-year beach-volume cycles. The importance of seasonal volume cycles varies but are always subordinate to the 10-11 year cycles and, except in one case, are subordinate to the five year cycles. Backberm and beachface temporal functions often she 2-4 year cycles the represent backberm filling and profile shortening. The 2-4 year cycles do not involve important volume changes and are though to be primarily caused by wave-climate cycles. Weekly averages of hourly water levels recorded by the Newport Rhode Island tide gauge reveals an 11-to 14-year sea level cycle with an amplitude of .15 m. Sea level highs occurred in 1972 and 1983-84, and lows occurred in 1965 and 1979. Sea level highs on the 11-to 14-year scale coincide with beach volume highs. It is hypothesized that periods of dominant southeast to east swells cause a sea level set-up on the coast. These long wave length swells, in turn, may enhance onshore sediment transport from the shoreface (about 8 m depth). Previous workers discovered a shore-parallel sand bulge at 8 m depth. It is plausible that, during periods of long wave length swells, asymmetrical wave orbital velocities cause grain-wise sand transport from around 8 m depth to the beach. Long-term (24 years) erosional trends are caused by aperiodic storms and periods of closely spaced storms. Beach erosion caused by sea level rise only becomes important on time-scales of over 25 years