Salt marsh grass for reducing overtopped flow momentum: Experimental results and XBeach calibration

Salt marsh grass on the crest of coastal structures has the potential to reduce overtopped flow momentum and therefore, mitigate wave overtopping hazards. However, implementation is hampered by the lack of quantitative data from large-scale live vegetation experiments and validated design tools. To address this, experiments were conducted with live salt marsh grass exposed to post-overtopping flow at full geometric scale. The flow, created by a laboratory dam-break flume, replicates hydraulic parameters of overtopped flows on the crest of sloped coastal structures following non-impulsive wave breaking. The deflection of the vegetation was categorised into emerged, transition and submerged regimes. For each canopy length, these regimes were predicted using maximum momentum flux and the total momentum of the incident flow. The numerical model XBeach-Veg accurately simulated the emerged regime, with an average scatter index $SCI=0.19$. For the submerged regime, the downstream front speed was under-predicted (bias = -0.82 m/s and $SCI=1.04$). Disabling the vegetation module and raising the impermeable bed level to replicate the submerged vegetation geometry accurately simulated the downstream front speed (bias = 0.01 m/s and $SCI=0.15$). Emerged salt marsh grass is effective at reducing downstream flow momentum and can mitigate nuisance overtopping hazards to pedestrians. For high incident flow momentum, the salt marsh grass deflects until submerged, does not reduce downstream flow momentum and is therefore not effective at mitigating hazards.