COLD FRONT GENERATED HIGH FREQUENCY WAVES AS A MECHANISM FOR THE EROSION OF BACKBARRIER MARSH

Short-term wave induced erosion along backbarrier beaches in the northern Gulf of Mexico has been linked to the post-frontal phase of cold front passages. Not until recently has consideration been given to the importance of wave erosion on marshes fringing large bays during the entire cold front event. Two WAVCIS (Wave-Current-Surge Information System) stations were established on both the north and south sides of a small marsh island in Terrebonne Bay, Louisiana to elucidate the hydrodynamic response to these events. WAVCIS data, which includes wind speed and direction, air temperature, significant wave height and water level, were collected between June 1999 and January 2002. These data were coupled to measured shoreline change obtained from a series of north/south repetitious profiles along the length of the island. Four high-resolution topographic surveys were conducted between April and July of 1999 and a fifth in December 1999. These were complimented by annual surveys conducted in the spring from 2000 on.

Typically during a cold front, pre-frontal conditions include strong winds blowing from the southern quadrant, elevated air temperatures, and significant wave heights of equal magnitude along the north and south shorelines of the study site. Post-frontal conditions include a near 180 degree shift in wind direction where strong winds blow from the north quadrant, accompanied by a drop in air temperature. Significant wave heights of about 0.1 m along the north shoreline compared to 0.025 m on the sheltered south side shoreline are typical. Waves during both phases of frontal activity are always in the high frequency band (F> 0.2 Hz). Erosion on both sides of the island averages 2.5 m/yr and can be attributed to pre and post-frontal wave events over the time scale in question. A model is presented in which the importance of these high frequency waves in coastal marsh erosion is explained.