· · · Repository Hydraulic Engineering Reports

Shock pressures of high intensity and short duration may occur during breaking of waves on coastal structures, slamming of ships, landing of seaplanes, and water entry of naval projectiles with flat nose. The phenomenon of shock pressures resulting from the impact between a solid and a liquid can better be described as a water hammer phenomenon wherein the elasticity of the solid and the compressibility of the liquid are the governing factors. The water hammer theory predicts the extreme values of shock pressures since it neglects the effect of air that might be entrapped between the solid an the liquid at the moment of impact. Analytical formulations of shock pressures as a water hammer phenomenon and as the compression of a thin layer of air entrapped between the solid and the liquid at the moment of impact are presented in this report. Tests were conducted by dropping a steel, aluminum or plastic plate whose edge was hinged at the water surface into a 3- by 3- by 6-ft steel tank that was partially filled with water. The shock pressures were measured at two locations by means of strain gage and piezoelectric type pressure cells mounted in the plate with special adapters. The ratio between the recorded and theoretical pressures when treated statistically was found to fit the Poisson distribution well. Correlation between the recorded pressures and the shape of the surface of contact between the solid and the liquid at the moment of impact indicated that although shock pressures have a great intensity, they have a short duration and occur only at some spots on the surface of the solid. Therefore (a) they should not be applied as static pressure for checking the stability of the coastal structure as a whole, (b) they may be absorbed by flexible structures, (c) they may cause cracks in rigid structures such as steel caissons filled with rock, and (d) they may affect the stability of structures that have natural frequencies within the range of duration of shock pressures. Equations and diagrams for the prediction of the magnitude and duration of shock pressures resulting from the impact between a solid and a liquid are presented herein.

This brochure presents low cost ways for the shorel ine property owner to control or slow down shoreline erosion. lt will also be of interest to community leaders, local government officials, and contractors and engineers involved in erosion control. Prepared as a public service by the U.S. Army Corps of Engineers, the brochure is part of a program to demonstrate low cost erosion control measures. The methods described here apply to all protected and inland shores of the United States where wave height does not usually exceed six feet and severe storms or hurricanes are not annual events. These sheltered areas are the only oneswhere low cost, owner-implemented protection is likely to be successful. The measures described here should not normally be used on open coastlines exposed to heavy ocean waves. The useful lifetimes of these methods vary from a year or two, for temporary structures, to over ten years, for longer-lasting installations. These numbers are approximate: unpredictable factors such as weather could shorten or prolong the expected lifetime of any shoreline structure. While all these methods have been used to reduce erosion problems, no erosion control device will ever be completely successful in all applications. The government cannot guarantee, therefore, that a particular method will be successful in your case. If you think it is likely that one or more of the measures described here could help with your erosion problem, we urge you to seek further information and assistance in designing a solution that meets your needs. In addition to this introductory brochure, the Corps of Engineers has prepared detailed reports to assist those who need further information. To obtain one of these reports, send in the postcard attached to the back cover. Additional sources of help are described in the back of the brochure under Where To Go From Here.