Beach Foreshore Sedimentology and Morphology in the Apostle Islands of Northern Wisconsin

Abstract

ABSTRACT Foreshore sediments and slopes of 39 beaches in the Apostle Islands of northern Wisconsin were found to be related to various measures of wave energy, wave steepness and offshore slope. Stepwise regression analysis was used to link the response variables with the process variables. Wave parameters were hindcast using the S-M-B wave hindcasting procedure and were also inferred from foreshore morphology. Mean grain size of the foreshore sediments appears to be very marginally related to the process variables and is primarily a function of local source materials. Sorting of the sediments shows a strong positive relationship with the percentage of gravel in the sample and a weak positive relationship with angle of wave approach and wave height indicating that coarse sediment, poor sorting, strong longshore movement, and high energy are linked to some degree. Weakly negatively skewed sediments are associated with high energy and/or strong longshore movement which flatten and coarsen the foreshore, serving to weaken the winnowing effect of backwash and increasing the degree of trapping of fines, respectively. Low energy situations with a minimum of longshore drift operate to add finer mat rial to the foreshore and thereby steepen it which facilitates the winnowing process, producing strongly negatively skewed sediments. More platykurtic grain-size distributions are associated with the relatively coarse-textured, less negatively skewed sediments comprising flatter foreshores while the more leptokurtic grain-size distributions typify the relatively fine-textured, more negatively skewed sediments characteristic of steeper foreshores. Foreshore slopes are related to mean grain size, wave energy and wave steepness. Steep foreshore slopes are associated with fine-textured sediments, suggesting the effects of constructive wave activity. Foreshore slopes are also generally inversely related to both wave steepness and wave energy.