Laboratory design-studies of the effect of waves on a proposed island site for a combined nuclear power and desalting plant

Abstract

There were four major objectives to this investigation: 1) the determination of the degree of stability of the island face when constructed of armor units of various weights; 2) the run-up for a two-dimensional wave system impinging on the island face; 3) the run-up envelope on the four sides of the island in a three-dimensional model; and 4) the wave patterns caused by the effect of the island on its wave environment. Models having three different length scales were tested in the wave tank (1:50, 1:45, and 1:40) and these models are referred to as the two-dimensional models. One model was tested in the wave basin at an undistorted scale of 1:150 and it is referred to in this report as the three-dimensional model. The first two-dimensional model was built to a scale of 1:50 and essentially corresponded to the original design proposed by Omar Lillevang, Consulting Engineer to the Bechtel Corporation. The prototype tribar weight, equivalent to the model tribar used, was 18.9 tons. This structure was stable; however, it was overtopped by waves. With an increase in the crest elevation from +30 ft. to +40 ft. some overtopping was still experienced. The second model was built at an increased scale, 1:40. At the same time the composite slope which existed in the original design was changed so that the island face had a continuous slope of 3 horizontal to 1 vertical with the crest of the defense at elevation +40 ft. This particular model scale was chosen so that, according to the literature, the tribars would be at a condition of incipient failure for high waves. Since the same armor units were used in this model as were used in the 1:50 scale model, the increase in model scale reduced the equivalent weight of the tribars to 9.7 tons and the maximum weight of the armor rock "B" from 10 tons to 5.1 tons. The prototype structure which corresponds to this model was found to be unstable, as expected. It was observed in testing that a critical feature of the construction which contributes to the stability of the structure is the degree to which the cap-rock section is interlocked with the tribar section. The modification made to the slope of the island face and the increased crest elevation eliminated the problem of overtopping, and the maximum run-up for a 14-sec. wave was to elevation +38 ft. Since the model having a 1:40 length scale was unstable and that with a scale of 1:50 was stable, a third model was constructed with a model scale between these two values, a scale of 1:45. The equivalent prototype tribar weight and the maximum weight of the "B" rock for this third model, still using the same model armor units, were increased to 13.8 tons and 7.3 tons respectively by this change. The slope of the wave defense and the crest elevation were the same for this structure as they were in the 1:40 scale model, i. e., a continuous slope of the island face of 3 horizontal to 1 vertical and a crest elevation of +40 ft. This model was satisfactory both with respect to stability and to run-up. Run-up measurements were made for waves of various heights at wave periods of 16 sec., 14 sec., and 12 sec. The maximum run-up was to elevations +39 ft., +35 ft., and +31 ft. respectively for these three wave periods. The three-dimensional model of the ocean bottom and the island was built to an undistorted scale of 1:150 with the island constructed the same as the 1:45 scale two-dimensional model. In these tests in the large wave basin the wave direction was varied as well as the wave period and wave height. The run-up envelopes obtained showed that, for comparable wave heights, the worst condition of run-up was for normally incident waves impinging on the seaward face of the island. The run-up measured for the normally incident direction was usually approximately 10% less than the run-up in the two-dimensional model for the same wave periods and wave heights. For the case of oblique wave incidence the maximum run-up was at the island corner first attacked by the wave with the run-up decreasing with distance from this corner, and this run-up was comparable to the maximum run-up experienced at normal wave incidence. However, the maximum average run up for the oblique case was significantly less than that experienced in the case of normal wave incidence. The run-up on the shoreward face of the island for all wave directions was of the order of 1/10th of that experienced on the seaward face. Detailed observations of the wave pattern in the lee of the island indicated that there were regions near the beach where the currents were in a direction opposite to the observed general current. From overhead photographs it was found that generally this occurred in regions where the waves which diffract from around the sides of the island intersect. Measurements were made of the maximum elevation of the water surface in the region of the causeway for the case of oblique wave incidence.

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