Abstract
Introduction and background Vegetation in both fresh and sea waters is not only ubiquitous in natural habitats but also instrumental for a variety of reasons. It provides the foundation for many food chains [4], contributes to the thriving of fish and corals [6], plays a role in reducing coastal erosion [1] and drastically improves the water quality by producing oxygen [3]. Furthermore, many engineering applications rely upon and would benefit from a better understanding of the flow physics characterising these problems. Despite the numerous reviews [2, 5, 6] that have attempted to capture different aspects of canopy flows over flexible vegetation, a satisfactory understanding of this topic is still elusive. For this reason, a simple controlled experiment aimed at comparing wall-bounded flows over rigid and flexible roughness was designed and carried out. Experimental facility and details Three different surfaces are considered in this work: a smooth wall and two rough-wall cases. The first rough surface is characterised by rigid roughness (i.e. conventional rough wall), while in the second case the flow develops over flexible roughness elements (i.e. aquatic vegetation). Experiments were designed to compare the statistical properties of flexiblerough beds as opposed to their rigid counterpart when the roughness height under wind loading, heff , is matched. The tests were carried out in the Donald Campbell wind tunnel at Imperial College London (freestream turbulence Tu < 0:5%U1). The tunnel working section measures 2:98 m in length, with a 1:37 m x 1:12 m cross section. The conditions were set to represent a nominally zero-pressure gradient at a freestream velocity of 12 ms