Decadal-scale impacts of a segmented, shore-parallel breakwater system

Between 1994 and 1997 nine segmented, shore-parallel, rock-mound breakwaters, were constructed at the meso-tidal beach of Sea Palling, on the North Sea coast of the UK, to provide protection for a low lying hinterland vulnerable to storm surge inundation and a 3.5km section of sea wall that was in danger of collapse due to low beach levels. In this paper we assess the effectiveness of these breakwaters, and of the associated beach recharge events, in stabilising the beaches both within and around the breakwater system, and we examine their impact on the wider littoral drift system. We discuss a ‘trapping’ mechanism whereby sand entering the system as littoral drift is effectively retained at both ends of the system. At the northern (updrift) end, the trapping of littoral drift sediments, aided by a 1.3×106m3 sediment recharge designed to remedy severe gap erosion, lead to the growth of salients into tidal tombolos (effectively changing the X/h ratio (distance offshore/depth) on Pope and Dean's (1986) morphology-existence diagram). The largest tombolo (Tombolo 5) is at the updrift end, is only inundated during large storm surges, and forms a 260m wide barrier to littoral drift and the alongshore supply of sediment to breakwater beaches. In response to a large reduction in littoral drift, shorelines in the centre of the system are steadily retreating, with some embayment shorelines being closer to the seawall in 2005 than they were pre-construction. These shorelines have not reached a steady-state and further recharge will be needed unless the littoral drift supply can be restored. The evidence (nearshore morphology, sediment starved beaches, down-drift recharge, shoreline change patterns and a simple sediment budget) indicates that an estimated 80% of littoral drift sediments are directed offshore at Tombolo 5, bypass the breakwaters and downdrift beaches, but may return to shore ~2.5km downdrift. As a direct result of recharge, Sea Palling makes a less-than-ideal case for assessing the applicability of micro-tidal shoreline response equations to breakwaters in tidal settings. The Sea Palling experience also demonstrates that the use of beach recharge in combination with breakwaters in settings of high littoral drift requires very careful consideration as it has the potential to block littoral drift and starve the local beaches — the opposite effect of that intended by both techniques. ► Shoreline response to breakwaters and subsequent beach recharge has be