REPORT OF THE PORT AND HARBOUR RESEARCH INSTITUTE Vol. 14. No.4 (Dec. 1975)

1. Laboratory Investigation on the Overtopping Rate of Seawalls by Irregular Waves

Yoshimi GODA * Yasuharu KISHIRA ** Yutaka KAMIYAMA**

Synopsis

Series of measurements with irregular waves were undertaken for model seawalls of vertical walls with and without protection by concrete block mounds. Experimental data were compared and supple­mented with theoretical calculations by the combination of the author's random wave breaking model in shallow waters and the weir type overflow model. The results have been compiled as twelve diagrams for the estimation of overtopping rate for two sea bottom gradients (1/10 and 1/30), three values of equivalent deepwater significant wave steepness (0.012, 0.017, and 0.036), and two types of seawalls.

Experiments employed irregular waves with HIlS=: 15 em, and T1(3 =: 1. 7, 2.3, and 2.8 sec. Model seawalls with crest heights of 7.5 to 26.3 em were located at the water depths of 22.5 to -10 em (above the waterline) on smooth slopes of uniform gradient. The overtopping rate was obtained as the average of three measurements, each for continuous two hundreds waves. Measurements were taken for 205 cases of vertical walls and 123 cases of concrete block mounds placed in front of vertical walls. Incident wave heights were estimated with a technique of resolving incident and reflected waves from two simultaneous records of irregular wave profiles.

The results of experiments and calculations have clarified the effects of bottom gradient and wave steepness upon the overtopping rate of seawalls. Decrease of overtopping rate by concrete block mounds has been estimated quantitatively. Twelve diagrams with two supplementary figures for the effects of bottom gradient and wave steepness enable quick estimation of the overtopping rate of seawalls at any water depth from the offshore to the foreshore.

The expected rate of wave overtopping with the data of regular wave experiments was re­confirmed to almost agree with the data of irregular waves except for the neighborhood of shoreline, where the phenomenon of surf beats is predominant.

Tables of experimental data afe attached as the appendix of the present report.

• Chief of the Wave Laboratory, Marine Hydrodynamics Division •• Member of the Wave Laboratory, Marine Hydrodynamics Division