Postprint version. Published in Journal of Waterways, Harbor, and Coastal Engineering, Volume 101, February 1, 1975, pages 59-71.
Publisher website: http://www.asce.org
NOTE: At the time of publication, the author William Durgin was not yet affiliated with Cal Poly.
The prediction of forces, or more precisely, the pressure distributions, experienced by submerged structures due to the passage of gravity waves has become important with the advent of large offshore structures. If the size of a structure is small compared to the wavelength then the forces can be evaluated by the assignment of suitable drag and inertia coefficients using an approach similar to that of Morison (6) in which case it is necessary to determine the coefficients experimentally for any given geometry (2). If the size of the structure is a significant fraction of the wavelength, the preceding method is invalid and a more complicated analysis such as the diffraction theory of Garrison and Rao (1) is needed. Unfortunately, the application of diffraction theory for a given geometry is exceedingly difficult unless one assumes that the structure size is small compared to the wavelength, which may not be realistic.
In order to circumvent the evaluation problems of diffraction theory an approximate analysis based on thin airfoil theory was developed. The analysis to be presented is developed for a thin horizontal flat plate, although extension to other objects is not difficult. One applicable physical situation is that of a protective cap over a vertical water intake. Typically such a cap might be 50 ft (15 m) wide submerged approximately mid-depth in water 25 ft (8 m) deep. Also typically, wavelengths of the order of 200 ft (61 m) would be experienced making the plate width of the order of one-fourth the wavelength. The question of wave induced loadings on such structures associated with nuclear power stations prompted the investigation presented herein.