Decadal-scale impacts of a segmented, shore-parallel breakwater system T.J. Dolphin a, b, ⁎, C.E. Vincent a , J.C. Bacon a , E. Dumont a, c , A. Terentjeva d a School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, UK b Centre for Environment, Fisheries and Aquaculture Science, Lowestoft, NR33 0HT, UK c Scottish Association for Marine Sciences, Dunstaffnage Marine Laboratory, Oban, PA37 1QA, UK d Waves Engineering Laboratory, Institute of Cybernetics, Tallinn University of Technology, EE-12618, Tallinn, Estonia abstract article info Article history: Received 22 October 2009 Received in revised form 21 January 2012 Accepted 5 March 2012 Available online 21 April 2012 Keywords: Shore-parallel breakwaters Littoral drift Shoreline retreat Coastal erosion Beach recharge 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 3.5 km 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 asso- ciated 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 enter- ing 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 1.3×10 6 m 3 sediment recharge designed to remedy severe gap erosion, lead to the growth of salients into tidal tombolos (effectively changing the X/h ratio (distance off- shore/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 260 m 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 fur- ther recharge will be needed unless the littoral drift supply can be restored. The evidence (nearshore morphol- ogy, 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 break- waters and downdrift beaches, but may return to shore ~2.5 km downdrift. As a direct result of recharge, Sea Pal- ling 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 po- tential to block littoral drift and starve the local beaches — the opposite effect of that intended by both techniques. © 2012 Elsevier B.V. All rights reserved. 1. Introduction The nine segmented shore-parallel breakwaters at Sea Palling, on the East Anglian coast of the southern North Sea, were built in 1994–1997 to provide coastal communities in a low-lying region with protection from inundation by the sea (Fig. 1). Historically this region has regularly suffered flooding following the breaching of the narrow sand-dunes that provide the primary defence against storm- waves and surges, the two most recent sea floods being in 1938 and 1953. After each event the dunes were artificially rebuilt and, subse- quent to the 1953 storm surge when 7 people were drowned at Sea Palling, a 14 km-long sea-wall was constructed to provide further protection. The beaches in front of the dune and sea-wall are fed from eroding cliffs to the north-west (Fig. 1). Sediment from these cliffs is transported to the west and to the south-east; the sediment divide is around Cromer. The littoral transport ‘cell’ extends from Cromer, past Sea Palling, as far as Great Yarmouth. Sediment is driven along the beaches by waves. The largest waves, often associated with surges, occur during storms in the northern North Sea and Norwegian Sea, and transport sand to the south-east (maximum 361,000 m 3 y -1 ; Wang and Reeve, under review). Smaller waves, generated by easterly and south-easterly winds, also drive sand back towards the north-west (max- imum 72,000 m 3 y -1 ; Wang and Reeve, under review). The average annual (net) sand transport past Sea Palling, based on wind and wave data, has been variously estimated as 150,000 m 3 y -1 (Vincent, 1979), 154,000 m 3 y -1 (Wang and Reeve, under review), 230,000 m 3 y -1 (Onyett, 1982), 400,000 m 3 y -1 and 90,000 m 3 y -1 (Halcrow, 2001), and >300,000 m 3 y -1 (SNSSTS, 2002), all towards to south-east. The beaches are predominantly moderately well-sorted sand (Leeder, 1982; McCave, 1978) with modal size 265 μm and some small gravel. Coastal Engineering 66 (2012) 24–34 ⁎ Corresponding author at: School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, UK. Tel.: +44 1502 524268; fax: +44 1502 513865. E-mail address: tony.dolphin@cefas.co.uk (T.J. Dolphin). 0378-3839/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.coastaleng.2012.03.004 Contents lists available at SciVerse ScienceDirect Coastal Engineering journal homepage: www.elsevier.com/locate/coastaleng