RELATIONSHIPS AMONG GRAIN SIZE, INFILTRATION RATE, AND FORESHORE SLOPE ON NEW JERSEY BEACHES

The foreshore slope of a beach represents the active portion of a shoreline, where marine processes interact with the beach environment. The resulting geomorphic nature of the foreshore is indicative of the interactions between these two environments. To understand the nature of these interactions, a total of 27 locations along the New Jersey shoreline were selected for the collection of slope, grain size, and infiltration rate data. Three initial results can be stated: (1) An increase in mean grain size correlates with an increase in infiltration rates (r² = 0.86). The larger grains form larger pore openings, reducing the affect of friction caused by surface tension, resulting in greater sediment permeability. In contrast, variations in sorting, skewness, or kurtosis show no correlation with infiltration rates. (2) An increase in infiltration rate correlates with steeper foreshore slopes (r² = 0.73). This supports the hypothesis that with an increased infiltration rate, there is less backwash volume, causing more sediment to be deposited toward the top of the foreshore. This potentially creates a steeper slope. (3) An increase in mean grain size correlates with an increase in foreshore slope (r² = 0.73). These three relationships suggest a cyclical interactive process whereby each factor influences the others. One variable that is not accounted for in this interaction is the elevation of the water table. At the base of the foreshore, where the water table is close to the surface, the infiltration rate is lower, due to saturated conditions, compared to infiltration rates measured near the top of the slope. If the water table is relatively close to the surface, as can be the case with a gently-sloping foreshore, infiltration rates might be lower overall compared to a steeper beach with the same grain size. Consequently, on beaches with low foreshore slopes, a high water table might play a more significant role in maintaining that slope compared to mean grain size.