EFFECTIVENESS OF ECO-RETROFITS IN REDUCING WAVE OVERTOPPING ON SEAWALLS

characteristics at sea defences, considerable field, and laboratory studies have previously been performed (Allsop et al., 2005; Franco et al., 1994; Salauddin et. al., 2017; Van der Meer and Bruce, 2014). Recently, the revised wave overtopping manual, i.e. EurOtop (2018) has been published, incorporating formulae to predict mean overtopping rates at sea defences. Laboratory results of this study are compared with predictions from these empirical relations. Mean wave overtopping rates at vertical seawalls with sloping foreshores are estimated for non-impulsive conditions using Equation 1 and, for impulsive conditions, using Equations 2-3. where, 𝑅 𝑐 denotes the crest freeboard of the structure, 𝑔 is the gravitational acceleration, q is the average overtopping discharge per meter width of the structure, and 𝑠 𝑚−1,0 defines the incident wave steepness (= 𝐻 𝑚0 𝐿 𝑚−1,0 ). Results from recently undertaken physical model investigations on plain vertical seawalls and numerical investigations were in good agreement with the empirical predictions of overtopping for both impulsive and non-impulsive conditions. Within this study, the measured overtopping characteristics for both benchmark and artificially roughened conditions are compared with the empirical prediction formulae of EurOtop (2018).


INTRODUCTION
Terms such as 'nature-based', 'living shoreline', 'green infrastructure' and 'ecological engineering' are increasingly being used to reflect biomimicry-based engineering measures in coastal defences. Innovative interventions for nature-based sea defences have included the retrofitting of man-made water filled depressions or 'vertipools' to existing seawalls (Hall et al., 2019;Naylor et al., 2017) and the addition of artificial drill-cored rock pools to intertidal breakwaters (Evans et al., 2016). Through their capacity to retain water, such measures serve to enhance biodiversity in the built environment (Browne and Chapman, 2014). Evans et al. (2016) for example, experimentally demonstrated that the introduction of artificial rock pools to an intertidal granite breakwater enhanced the levels of species richness compared to those observed on plain surfaces of the breakwater.
Notwithstanding these biological benefits, the impetus for incorporating ecologically friendly measures to existing sea defences remains low . This situation could potentially change should it be shown that the addition of 'green' measures to sea defences could enhance wave attenuation and reduce wave overtopping as well as wave pressures on the coastal defence structures. This paper describes smallscale physical modelling investigations of seawalls and explores reductions in wave overtopping that could be realised by retrofitting sea defences with 'green' features (such as 'vertipools'). Surface protrusions of varying scale and density are used in the physical modelling to mimic 'green' features and the results from measurements of overtopping are benchmarked to reference conditions determined from tests on a plain seawall.
For the estimation of mean wave overtopping characteristics at sea defences, considerable field, and laboratory studies have previously been performed (Allsop et al., 2005;Franco et al., 1994;Van der Meer and Bruce, 2014). Recently, the revised wave overtopping manual, i.e. EurOtop (2018) has been published, incorporating formulae to predict mean overtopping rates at sea defences. Laboratory results of this study are compared with predictions from these empirical relations. Mean wave overtopping rates at vertical seawalls with sloping foreshores are estimated for non-impulsive conditions using Equation 1 and, for impulsive conditions, using Equations 2-3.
where, denotes the crest freeboard of the structure, is the gravitational acceleration, q is the average overtopping discharge per meter width of the structure, and    non-impulsive wave conditions were considered in experiments with two constant deep-water wave steepness values of 1.5% and 6%. The incident wave conditions at deep water and in front of the structure toe were determined by placing six wave probes (one set of three, see Figure 1) along the length of the flume adopting the methodology of Mansard and Funke (1980). Experiments were also carried out to measure the inshore wave conditions without the structure in place (i.e. seawall) to mitigate the uncertainties in wave measurements that may arise from the reflection generated from the structure (see Salauddin and Pearson, 2019b). Table 1 summarises the physical modelling test conditions. Measurements of wave overtopping volumes in a test sequence followed the approach of Salauddin and Pearson (2020) and used a collection tank and load-cell positioned behind the vertical structure. Additionally, an overtopping detector circuit was positioned on the crest of the seawall to identify the individual overtopping events. The calibrated load-cell, together with the overtopping detector, facilitated measurement of waveby-wave overtopping volumes by recording mass increments of water in the collection tank with respect to overtopping waves for a test run. A use of a syphon mechanism in the collection tank ensured the uninterrupted sampling of overtopping volumes for the duration of each experiment. The benchmark for the experiments with different protrusions were the experiments performed on a simple plain vertical wall (reference case) for the designed incident wave conditions. In Figure 3, the resulting average overtopping rates for the benchmark experiments, i.e., with a plain vertical seawall (reference case) are shown for both impulsive and non-impulsive conditions. The measurements are compared with the predicted values using the EurOtop (2018) empirical relations (Equations 1-3). Figure 3 shows that measured data are in good agreement with predictions for impulsive and non-impulsive conditions tested in this study.

Figure 3 -Mean overtopping rates at plain vertical walls
In Figure 4, the dimensionless mean overtopping rates for all the retrofit cases are plotted against the relative freeboard of the structure for the tested non-impulsive wave conditions. The EurOtop (2018) prediction is also shown. Uncertainty in the prediction of EurOtop (2018) is described by the inclusion of 5% upper and lower limits prediction. Figure 4 shows that in general, the overtopping rates are consistent with those predicted by EurOtop (2018), suggesting that the influence of the surface protrusions on the sea wall is not significant for nonimpulsive wave conditions. For the impulsive wave conditions that were tested, nondimensional mean overtopping rates are presented in Figure 5 with respect to the relative freeboard of the structure. The EurOtop (2018) prediction for nonimpulsive wave conditions is also included and it is clear that measured overtopping rates follow the trend of the empirical predictions. Figure 5 shows however, that mean overtopping rates were reduced significantly (by up to a factor of four) for the artificially roughened seawalls, when compared to the EurOtop (2018) empirical predictions. The degree to which overtopping is reduced is clearly related to the geometrical characteristics (length and density) of the roughness configuration -comparing Test Series 4A for example, to Test Series 4B and 4C for a given wave condition, shows reduced overtopping as the density of surface protrusions on the seawall increases.

CONCLUSIONS
This paper presents laboratory-based investigations of the influence of surface roughness on the wave overtopping characteristics at vertical seawalls, subjected to both impulsive and non-impulsive wave conditions. The role of eco-engineered measures on sea defence structures has recently been established. This research highlights a second potential benefit of such interventions in coastal zone management; that of improving the climate resilience of sea defences by reducing the flood risk from wave-induced overtopping, particularly for violent impulsive wave conditions. The research findings from this study will assist coastal engineers, ecologists, and practitioners in incorporating eco-retrofits in the resilient and environmentally sustainable design of sea defence infrastructures to address increasing wave hazards at seawalls.