Entsminger, L. .
1978.
Beach Pads & Beach Cusps, St. Joseph Peninsula..
Geology. Tallahassee, Florida State University.
This study is an investigation of beach pads and beach cusps on St. Joseph Spit, Franklin County, Florida. A beach pad is defined as a roughly triangular shoreline feature with its base parallel to shore and the next longest side being updrift. An associated bar extends from the most seaward point of the pad, in a downdrift direction, nearly parallel to the updrift side. A beach cusp is defined as a cuspate feature which usually occurs in groups along the foreshore as a series of alternating horns (pointing seaward) and embayments. Coarser sediments are associated with the horns. Both features were examined by field investigation of associated wave parameters, form and internal structure, as well as water and grain motion occurring around both beach pads and cusps. Laboratory procedures included study of landsat imagery and air photos for the occurrence and measurement of physical parameters of beach pads and cusps. Laminae samples from both types of features, taken in the field, were subjected to granulometric evaluation by sieving and subsequent statistical treatment. Beach cusps in the study area were found to have two mean spacings: 12 meters and 38 meters. The 12 meter beach cusps were often associated with beach pads and occurred along the active foreshore. The 38 meter cusps were found higher on the beach after storms and are referred to as "storm cusps". Both sizes of beach cusps were commonly found to be asymmetric, pointing into the direction of wave attack. Granulometric analysis showed that sediments collecting on the horns were coarser than those in the embayments. Beach cusps were observed forming in the presence of intersecting waves of different wavelengths. The sequence of wave and cusp interaction was documented. It was concluded that beach cusp asymmetry is the result of oblique wave attack and that maintainence of these cusps was due to a regular variation in swash energy along the foreshore. Both intersecting waves and langmuir circulation were postulated as possible causes of this variation in swash energy. Beach pads were found along the shoreline of St.Joseph Spit. They were observed in groups alternating with embayments (which often cut into the dune-ridge). Beach pad length varied from 100 meters to over 400 meters. Beach pads were observed t~ migrate downdrift under oblique wave attack. Migration rates varied with size and ranged from 0.6 to 1.2 meters per day. Granulometric analysis showed a systematic change in mean grain size and sorting along the beach pad.The presence of beach pads affected wave height and breaker angle which varied along the pad. Grain motion studies showed that sand could be added to the pad, travel through the pad along the foreshore or travel along the bar to the offshore. It was concluded that because of their size, their ability to migrate and the movement of sand through each pad that they play an important role in sand transport. Because they are built and migrate under oblique wave attack, beach pads are an excellent indication of a (momentarily) unidirectional littoral drift system. Beach pads migrate due to their inability to completely adjust to a wide variety of breaker angles.