Protection of outfall structures

Abstract

Stability of rock on horizontal bottoms and steep slopes subjected to wave attack has been a subject of investigation in the past. But the stability of rock on gentle slopes has not been investigated systematically. The objective of this research is to investigate the relations between the different variables involved. A possible application is the protection of outfall structures. Due to the lack of information on this subject, relations derived for stability of rock on horizontal bottoms subjected to wave attack were applied. Application of these theories imply a destabilization of the rock by orbital velocities causing shear stresses at the bottom. Orbital velocities were calculated along the profile of the structure with the linear wave theory and substituted in the stability relations for horizontal bottoms according to the theories of Ranee & Warren and Jonsson / Sleath, respectively. The results of the calculations were expressed in the stability variable H/ADn 5 0 versus the relative waterdepth, h/H. This was done for regular as well as for irregular waves. The calculations showed an increase in the stability for increasing values of the wave steepness. Also, the deeper the water the higher the stability values. Experiments were conducted in the large wave flume of the Laboratory of Fluid Mechanics at the Delft University of Technology. The model consisted of an impermeable 1:25 slope, on which several materials were tested. Regular and irregular waves were applied and for various conditions the wave heights and bottom velocities along the test slope were measured. The experimental results were compared with the calculations. For regular waves it appeared that for h/H values larger than one the calculations describe the stability of the rock quite well. For h/H values smaller than one the calculations are not adequate to describe the stability of the rock. The location of maximum attack was around h/H = 1. For irregular waves the location of attack was not that clear. The damage was not as concentrated and more spread out. The location of maximum attack was around h/Hs = 1. For both regular as irregular waves the general tendency could be described by the calculations but the 'plunging' effect o f the more curl-shaped waves with lower values of the wave steepness resulted into a more severe attack on the structure. For irregular waves more experiments have to be conducted for the slope section where waves are not yet broken. This to confirm or reject the theories derived applied to horizontal bottoms for the stability of rock on gentle slopes attacked by irregular waves. To investigate whether the computer simulation ODIFLOCS, can be used to simulate wave motion on gentle slopes, a comparison was made with the measurements in the experimental model. ODIFLOCS proved not to be suitable for simulation of wave motion on gentle slopes, mainly due to short comings of the numerical scheme used by ODIFLOCS which was developed for 'short' steep slopes.