Storm-sequence control of beach evolution and overtopping at vertical promenade defences

• Mid-scale flume shows sandbar growth equalises nearshore wave energy. • Toe scour or accretion drives wall-specific overtopping volumes and discharges. • Ignoring profile change mispredicts design overtopping. A mid-scale laboratory campaign was conducted in the CIEM wave flume (UPC-Barcelona, 1:10 scale) to quantify the coupled evolution of a sandy beach and wave overtopping at a vertical promenade with an emerged toe. Two storm sequences were reproduced: a rising cluster in which the deep-water significant wave height H m 0 increased stepwise from 0.47 to 0.57 m (C2_sub), and a constant-peak cluster that maintained H m 0 = 0.57 m throughout (C3). Surface elevation was measured at 38 cross-shore locations, overtopping volumes were captured in load-cell tanks, and bed profiles were surveyed after every 300-wave segment using a mobile laser-mechanical profiler and LIDAR system. The constant-energy cluster injected 21 % more incident wave energy than the rising sequence, yet both produced similar post-storm profiles: once wave breaking became depth-limited over a rapidly forming sandbar ( x ≈ 42-50 m), sea-swell heights and mean periods converged between clusters, whereas infragravity energy remained slightly higher in C3. The findings demonstrate that (i) sandbar growth equalises nearshore wave energy across different storm histories, but (ii) transient toe evolution governs overtopping and sediment overwash. Accurate design of urban seawalls with shallow foreshores therefore demands time-resolved morphodynamic representation rather than reliance on peak-condition parameters alone.