Abstract
In the context of “green” approaches to coastal engineering, the term “eco-engineering” has emerged in recent years to describe the incorporation of ecological concepts (including artificially water-filled depressions and surface textured tiles on seawalls and drilled holes in sea structures) into the conventional design process for marine infrastructures. Limited studies have evaluated the potential increase in wave energy dissipation resulting from the increased hydraulic roughness of ecologically modified sea defences which could reduce wave overtopping and consequent coastal flood risks, while increasing biodiversity. This paper presents results of small-scale laboratory investigations of wave overtopping on artificially roughened seawalls. Impulsive and non-impulsive wave conditions with two deep-water wave steepness values (=0.015 and 0.06) are evaluated to simulate both swell and storm conditions in a two-dimensional wave flume with an impermeable 1:20 foreshore slope. Measurements from a plain vertical seawall are taken as the reference case. The seawall was subsequently modified to include 10 further test configurations where hydraulic effects, reflective of “eco-engineering” interventions, were simulated by progressively increasing seawall roughness with surface protrusions across three length scales and three surface densities. Measurements at the plain vertical seawall compared favorably to empirical predictions from the EurOtop II Design Manual and served as a validation of the experimental approach. Results from physical model experiments showed that increasing the length and/or density of surface protrusions reduced overtopping on seawalls. Benchmarking of test results from experiments with modified seawalls to reference conditions showed that the mean overtopping rate was reduced by up to 100% (test case where protrusion density and length were maximum) under impulsive wave conditions. Results of this study highlight the potential for eco-engineering interventions on seawalls to mitigate extreme wave overtopping hazards by dissipating additional wave energy through increased surface roughness on the structure.
Highlights
As our climate continues to change, coastal defences will be subjected to increasing pressures from rising sea levels and a higher frequency of storm surges from extreme climatic events
The use of “hard” engineered coastal defence structures has, for the most part, remained the standard for the management of risks posed by sea and wave hazards, with seawalls and breakwaters featuring prominently in many flood alleviation schemes
Notwithstanding the body of evidence that highlights differences in the abundance and species richness of benthic organisms supported by artificial structures compared to natural systems (Chapman, 2003), the implementation of “hard” structures has, until recently, remained largely unchallenged
Summary
As our climate continues to change, coastal defences will be subjected to increasing pressures from rising sea levels and a higher frequency of storm surges from extreme climatic events. Damage to sea defences and significant flooding of coastal areas from wave overtopping will present risks to lives and properties and this will need to be managed (IPCC, 2014, 2018; Vitousek et al, 2017). Approaches for managing flood risks in coastal zones have traditionally relied on “hard” engineered sea defence solutions which are costly to both install and maintain, and can be visually unattractive. The negative impacts of “hard” engineered solutions include losses of biodiversity in adjacent coastal zones (Barbier et al, 2011; Browne and Chapman, 2014) and the lack of dynamic response to climate change (Borsje et al, 2011; Temmerman et al, 2013; Pontee et al, 2016; Vuik et al, 2016; Morris et al, 2018). Eco-retrofitting of engineered infrastructures can contribute to climate adaptation by integrating natural systems into existing structures and incorporating building integrated living systems (BILS), as advocated by Birkeland (2003, 2009), can produce positive ecological benefits
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