Wave forces on vertical breakwaters: Reliability of design formula

Wave forces on vertical breakwaters: Reliability of design formula

Bruining, J.W.

D' Angremond, K. (mentor)
Van der Meer, J.W. (mentor)
Schiereck, G.J. (mentor)
Fontijn, H.L. (mentor)

Civil Engineering and Geosciences

Hydraulic Engineering

1994-08-01

Through the increasing draught of large vessels the need for breakwaters at greater depths becomes more and more important. In this respect vertical breakwaters may be the best alternative because of the considerable savings in construction time as well as the lower construction and maintenance costs. Therefore a lot of research is performed nowadays in order to obtain better understanding of the dynamic processes involved. Until then the design of vertical breakwaters is based on the quasi-static approach. The formula of Goda is one of the latest design methods based on the quasi-static approach. Goda's formula is used world-wide, it is however not known in what way it describes reality. Therefore the empirical character of Goda's formula is investigated. From the comparison of measured values and values calculated with Goda's formula it is clear that Goda overestimates the wave forces and overturning moments. Especially for inclined superstructures Goda overestimates the uplift and horizontal force. It is tried to find an explanation for the overestimation but conclusions could not be drawn, partly caused by the scatter in the measurements. It is however clear that the height and the shape of the superstructure, the wave period, breaking (or non-breaking) of the waves on the foreshore and the presence of a rubble-mound foundation have large influence on the measured wave forces. For the horizontal forces Van der Meer et al., (1993) proposed a modification of Goda's formula. This modification implies that the crest height is determined at the transition from vertical to inclined, and not at the top of the structure. Good results are also obtained when this modification is used for the calculation of the overturning moments, caused by the horizontal force. As far as the uplift force and the moment caused by this force is concerned it can be said that Goda is only valid for breakwaters with a vertical superstructure. In order to determine the reliability of Goda's formula the bias and the scatter in the measurements are described. Therefore the exceedance curves of the forces and moments were investigated. A two parameter Weibull distribution was fitted through the 5%..0.1% exceedance probabilities. The shape parameter was found to be close to 2, implying the Rayleigh distribution. This Rayleigh distribution is rewritten into an expression for the exceedance probability of all forces and moments depending only on the modified Goda formula. With this expression probabilistic calculations have been performed. It is found that the horizontal and uplift force, calculated with the modified Goda formula, are inside the 90% confidence bound obtained from the easurements. The same thing holds for the overturning moments caused by these forces. Due to the high scatter in the measurements it is advised to rely on physical model tests rather than on the formula of Goda. (This report is also published as Delft Hydraulics report H1903).

vertical breakwaters
caisson breakwaters
Goda formula

http://resolver.tudelft.nl/uuid:9a8d906c-9357-4f46-b2dd-0953a9a307df

Student theses

master thesis

(c) 1994 Bruining, J.W.