Wave impacts on storm walls, a study on scale effects

Abstract

Low-lying countries such as Belgium and the Netherlands are naturally susceptible to overtopping wave attacks. Coastal regions are therefore facing the threat of such events causing damage to dike-positioned structures and buildings. Research in this filed is often conducted using scaled-down 2D hydraulic physical models, which introduces so called scale effects that can bias the results with regard to measured wave-structure impact forces. This thesis therefore attempts to determine the effects of scaling for post-overtopping wave impacts. To achieve this, large scale physical modelling tests were conducted at Deltares’ Delta flume and compared with a small scale model of similar configuration. A second thesis aim stems from the application of measurement devices in an innovative manner while conducting the large scale tests, and their applicability is assessed here.
The model comparison shows that in general the small scale test performs very similarly to its large scale counterpart, and scaling effects did not appear to bias the impact events considerably. This was also observed for the pre-impact flow evolution, where the wave interference mechanisms and resulting bore characteristics were also largely similar. The force-time signal for the largest impacts also is related to the pre-impact mechanisms observed. Classification of the extreme bore impacts according to their impact peak ratio did outline a discrepancy between the two models, which was also linked to the difference in entrapped air pocket impacts.
The four measurement instruments assessed are a Terrestrial Laser Scanner (TLS), waterproof action camera(s), optical phase detection probe (OPDP) and a high speed camera mounted behind an opening in the impacted wall. Data from the former two was also used in this analysis and their performance was determined to be good.