· · · Repository Hydraulic Engineering Reports

Gedetailleerde kaart van de uitvoering van de nieuwe havenhoofden van de haven van Scheveningen.

Forces by uprush and downrush, the imporatnace of permeability, the resonance phenomenon, friction between armor blocks and between armor and sublayer, slope geotmetry and practical design principles

The purpose of the model investigation reported herein was to obtain design information for stone and dolos armor used on breakwater heads and subjected to breaking waves. More specifically, it was desired to determine the minimum weight of individual armor units (with given specific weights) required for stability as a function of sea-side slope of the structure, angle of wave attack, wave period, and wave height

The RMBFM-Project (Rubble Mound Breakwater Failure Modes) is sponsored by the Directorate General XII of the Commission of the European Communities under the Contract MAS-CT92- 0042, with the objective of contributing to the development of rational methods for the design of rubble mound breakwaters. 11 institutes from the European Union participate in the project. The paper presents an overview of the project background, the research objective, the research methodology and the research results of the project. The outcome of the project is a large amount of formulae describing important failure modes, plus development of related partial coefficients which make it possible to design according to preselected reliability levels. Due to limited space only the major activities are described.

The paper presents a design formula for Tetrapod armour on a 1:1.5 slope exposed to head-on random wave attack The formula predicts the relative number of broken Tetrapods as function of: the mass of the Tetrapods, the concrete tensile strength and the wave height in front of the structure. Thus, the formula addresses the observed problem of ensuring structural integrity of the slender types of non-reinforced armour units. The formula is based on results from small scale model tests with load-cell instrumented Tetrapods in which both the static, the quasi-static and the impact proportions of the loads were recorded.. The analysis follow the methods given in Burcharth (1993) and Burcharth et al .. (1994) ..

The slender, complex types of armour units, such as Tetrapods and Dolosse are widely used for rubble mound breakwaters. Many of the recent failures of such structures were caused by unforeseen early breakage of the units, thus revealing an inbalance between the strength (structural integrity) of the units and the hydraulic stability (resistance to displacements) of the armour layers. Breakage is caused by stresses from static, pulsating and impact loads. Impact load generated stresses are difficult to investigate due to non-linear scaling laws. The paper describes a method by which impact loads on slender armour units can be studied by load-cell technique. Moreover, the paper presents Dolos design diagrams for the prediction of both breakage and hydraulic stability.

The rational design diagram for Dolos armour should incorporate both the hydraulic stability and the structural integrity. The previous tests performed by Aalborg University made available such design diagram for the trunk of Dolos breakwater without superstructures (Burcharth et al 1992). To extend the design diagram to cover Dolos breakwaters with superstructure, 2-D model tests of Dolos breakwater with wave wall were done. Furthermore, Task IA will give the design diagram for Tetrapod breakwaters without a superstructure .. The more complete research results on Dolosse can certainly give some insight into the behaviour of Tetrapods armour layer of the breakwaters with superstructure. The main part of the experiment was on the Dolos breakwater with a high superstructure, where there was almost no overtopping" This case is believed to be the most dangerous one. The test of the Dolos breakwater with a low superstructure was also performed". The objective of the last part of the experiment is to investigate the influence of the method of placing and packing the blocks on the hydraulic stability. The Dolosse were more carefully put on the slope and the hydraulic stability of such slope was compared with that of the more randomly packed slope. The whole experiment was carried out in the period of August - November 1993 The analysis on the hydraulic stability has been finished while the stresses analysis is under way.

Research on rubble mound breakwaters when confronted with waves. The rapport covers the flow characteristics and mound stability under regular waves and under oblique wave attack. The authors find a formula for rough, permeable slopes, flow characteristics under the action of a regular wave train by a function of the type. Furthermore they conclude that the distribution of flow characteristics in sea state can be obtained on the basis of interaction curves and joint probability density function of wave heights and periods. The conclusions on the mound stability of breakwaters are: -Stability conditions of an undefined, rough, permeable slope are governed by the stability function. -The stability function depends only on Iribarren's number. -Randomness can be accounted for by using confidence bands for the stability function. -For each type of armour unit, an optimum slope of maximum stability exists. The greater the interlocking among armour units the steeper the optimum slope and the more peaked the stability maximum. -Given a rubble mound breakwater a minimum sea state exists which produces a significant failure probability. If a sea state is presented which is the same or higher than this minimum, failure of the structure is only a question of the duration of the sea state. Conclusions on the characteristics and stability of rubble mound breakwaters under oblique wave attack: -There is a dangerous lack of experimental data on the subject. -Run-up and run-down under small oblique incidence of waves (angle lower than 45 degrees) are function of Ir.cos(theta). For higher incidence angles the hypothesis is unreliable. -The stability of steep slopes under oblique wave attack is not worse than under perpendicular wave incidence. For milder slopes the opposite may be true. -The failure of probability of a rubble mound breakwater under a sea state with oblique incidence, can be calculated by taking into account the breaking limit, the interaction curve and a joint distribution of wave heights and periods.