The influence of structural permeability on armour layer stability of rubble mound breakwaters

The influence of structural permeability on armour layer stability of rubble mound breakwaters

De Heij, J.

Civil Engineering and Geosciences

Hydraulic Engineering

2001-02-01

The first one who studied the problem of armour layer stability was Iribarren (1938). Iribarren derived stability criteria that were based on the acting physical processes on the slope of a structure. In the stability criteria of Iribarren only the influence of a few structural and hydraulic parameters were taken into account. In 1988 Van der Meer derived a stability relation in which the stability of armour layers was depending on several hydraulic and structural parameters. Van der Meer concluded that the stability of armour layers is strongly influenced by the composition of a structure. He implemented the influence of the composition of the structure in a permeability coefficient, P. This permeability coefficient is defined for four different structures. Van der Meer is a good tool for the design of armour layers, it has little physical background. The empirical character of the stability relation is found back in the definition of the permeability coefficient. If a different structure is designed than tested by Van der Meer, an estimation of this permeability coefficient has to be made. Since the permeability has large influence on the stability, new methods should be developed to calculate armour layer stability. Due to the irregularity of armour stones it would take a lot of effort to obtain a full analytical solution for the problem of armour layer stability. However, a good attempt into the direction of an analytical solution can be made, since good models are available that describe the water motion on and in coastal structures. An example of such a model is the model ODIFLOCS. ODIFLOCS stands for Qne dimensional flow Qn and in foastal structures. With ODIFLOCS it is possible to calculate the velocities on a coastal structure for certain hydraulic and structural parameters. With these velocities the hydrodynamic forces on the stones in the armour layer can be calculated. Iribarren (1938) proposed a model for armour layer stability. He assumed a hydrodynamic drag force that act on a stone, which is caused by the run-up or run-down, parallel to the slope of the structure. Two stability criteria for the stability of rock on a slope can be distinguished, which are upward and downward stability respectively. The drag force can be written as a function of the velocity on the slope of a breakwater. Iribarren was not able to calculate these velocities, since no models were available to calculate them, and estimated them by wave celerity in shallow water. Van den Berk (1999) was able to proceed the approach of Iribarren and calculated the velocities with the numerical model ODIFLOCS. He modelled homogeneous structures and calculated the armour layer stability for homogeneous structures for several hydraulic and structural conditions. He found that his results and the results ofVan der Meer were strongly correlated. In this research a next step will be made to see whether physical background can be given to the influence of structural permeability on armour layer stability. Structures with different permeabilities will be modelled and the velocities on the slope will be calculated for different hydraulic conditions. With the use of these velocities, which will be calculated with the numerical model ODIFLOCS, the stability can be determined. The test showed that the strongest increase in stability was found for gentle slopes in combination with low and high wave steepness. The test with steep slopes gave significant lower influence of structural permeability, compared with gentle slopes, on armour layer stability. This is contradicting with the findings of Van der Meer, who found a strong influence ofpenneability on the stability for high values of the surf similarity parameter. This shows that the penneabilities that are modelled in this research do not correspond to those tested by Van der Meer.

numerical model
breakwater
stability

http://resolver.tudelft.nl/uuid:ab1b820e-ed76-4328-80d7-1eeafb6ac09f

Student theses

master thesis

(c) 2001 J. de Heij