An experimental study of breaking-wave pressures
Garcia, William J.
Research ReportAbstract: Tests were conducted to gain more information concerning the shock pressures created by water waves breaking against vertical barriers. These wave pressures were studied using small-scale oscillatory waves in a flume fitted with a beach slope and test wall. The variation of pressure with both time and position on the wall was determined for several wave heights, wave periods, water depths, and beach slopes. Great scatter in the magnitude of the shock pressure was observed for each of the wave conditions tested. This variation in the value of the shock pressure is believed to be caused by slight variations in the shape of the incident breaking wave. Therefore, many tests were made using the same wave conditions in order to more accurately determine the magnitude of the shock pressure. The variation of pressure with time was found to be similar to that reported by previous investigators. The pressure-time variation can be divided into two parts; namely, initial shock pressure which occurs as the wave strikes the wall and a secondary pressure which is associated with the runup. The shock pressure is characterized by a very intense pressure peak of short duration and is followed by the much less intense but longer duration secondary pressure. The maximum shock pressure that occurred for each wave condition was localized over a small region of the test wall between the still-water level at the wall and the elevation of the crest of the wave striking the wall. Above the region of maximum shock pressure, the magnitude of pressure decreases to zero. Below the region of maximum pressure, the shock pressure also decreases but to a value of approximately one-tenth the magnitude of the shock pressure and it then remains fairly constant to the bottom of the test wall. This type of distribution of shock pressures on the wall was observed for all tests. Upon analysis of the maximum shock pressures observed for each of the wave conditions tested, it was found that the shock pressure increased with both wave height and wavelength. It was found through dimensional analysis that pressure is proportional to the cube root of the wave energy. Upon comparison of the data collected in this experimental program with the above relation between pressure and wave energy, only fair conformity was noted due to the small range of test data. Therefore, the range of data was expanded by the inclusion of the shock pressure data of other investigators from both model and prototype studies. Very good agreement was noted over this larger range of data. As opposed to the shock pressure, little scatter was noted in the magnitude of the secondary pressure. It was also noted that the secondary pressure varies regularly along the wall from a maximum at the bottom to zero at the point of maximum runup. This regular distribution is expected since the secondary pressure is caused by the runup of the wave rather than its impact on the wall. The secondary pressure was compared with the pressure caused by the same size wave forming a clapotis on the wall. The clapotis pressure was almost identical with the observed pressure. The characteristics of the wave at the point of breaking were also studied in order to make a comparison between waves breaking on an unobstructed beach and on a beach obstructed by a wall. Although it might be expected that a barrier on the beach would have a great effect on the breaking waves, the data showed the effect to be negligible. The depth of water in which the wave would break on an unobstructed beach is slightly greater than the depth of water at the wall which would cause the same wave to break and produce maximum shock pressures. The wave height at breaking for both the obstructed and the unobstructed beach was found to be the same. NOTE: This file is large. Allow your browser several minutes to download the file.
Hydraulics Laboratory (U.S.)Engineer Research and Development Center (U.S.)
Barriers; Shock pressures; Water wave breaking; Water wave forces; Water wave experiments; Seawalls; Ocean waves; Breakwaters; Coastal structures; Beach shores; Shock waves
Research report (U.S. Army Engineer Waterways Experiment Station) ; H-68-1.
Approved for public release; distribution is unlimited.