Beach ridge plains and sea level change

Beach ridge systems of the low-to-moderate energy swash-built type can be used to determine ambient near-shore wave energy values, as well as sea level changes up to 4--5 m. Wave energy values can be obtained from individual samples or mean values of individual samples, the grain-size kurtosis (inverted) is useful, as is the sixth moment measure. Sea level change information comes from sequences of beach ridge samples, spanning one or more abrupt changes in smoothed kurtosis. Other procedures are also available for each of these tasks. Settling-lag ridges (horizontal bedding) may be useful also. Dune ridges and storm-surge ridges are not appropriate for this kind of work. There are also limitations on nature and general size of the sediment in the ridge (preferably quartz sand or coarse silt; 50-gram samples), and on map geometry of the system (no sharp curvature, no splaying; parallelism or nearly so). Sampling must meet strict standards as to location, depth and thickness of the sampled layer: lab work requires 30-minute sieving on quarter-phi screens. Each beach ridge represents a sea level rise and then fall, perhaps 5 to 30 cm, over a few years or decades, but not a storm. The longest presently-known sequence ofmore » beach ridge is located in an area of slow glacio-isostatic rebound, and spans some 12,000 years, but ridge systems in stable areas cover only about 3,200 years or less. Each long system typically shows many changes of sea level, in the range of one to 4--5 meters, but there does not appear to be any clearly-marked periodicity that persists throughout the entire record, other than the time interval between the ridges themselves. This interval commonly averages 35--50 years, but in a few settings is only about four years, and in others 10--12. In a few instances, the interval may be 60 years or more.« less