Baseline Processes Understanding P70

Lavernock Point to St Ann’s Head SMP2 Appendix C: Baseline Process Understanding Figure C.25: Mumbles Head to Worms Head: large scale and local scale boundaries. C1-70 Lavernock Point to St Ann’s Head SMP2 Appendix C: Baseline Process Understanding LOCAL SCALE: MUMBLES HEAD TO SOUTHGATE Interactions: This coastline is cliffed and composed of early Carboniferous limestones with north-west to south- east trending embayments, controlled by faults with the same orientation (Halcrow, 2002). Generally the cliffs are fronted by a narrow rock platform with intermittent sand and gravel pocket beaches located within individual embayments, and sand embayments at the mouth of stream valleys. Mumbles Head is composed of two limestone islands connected at low water. Bracelet Bay, Limeslade Bay, Rotherslade Bay and Langland Bay comprise limestone rock platforms backed by sand and shingle beaches. Caswell Bay is a wide flat sandy beach with a shingle strewn rock platform; Pwlldu Bay is characterised by a small gravel beach at the head of the embayment. These beaches act as local sinks for medium-sized sands due to the shelter created by the surrounding headlands (Posford Duvivier and ABP Research, 2000). West of Pwlldu Head, raised beaches may be found along the frontage at about 8m above present sea level. Sections of shore between Mumbles Head, Caswell Bay, Pwlldu Head and Pennard Burrows are directly exposed to south-westerly swell waves, resulting in an eastward transport of sediment. Around Mumbles Head and to the west of Oxwich Point there are strong tidal currents adjacent to the shore. There are, however, few contemporary sources of sediment due to the seabed being sediment-poor and the updrift coastline containing limited sediment. Sediments have tended to accumulate in the more sheltered embayments created between headlands. The indented nature of these pocket beaches means that there is little connectivity between beaches, with sediments only able to move outside of the bays when sediments move offshore during storm events. Complex tidal currents operate within Swansea Bay, with residual flows creating a weak anti- clockwise gyre. This results in a strong offshore residual flow running southwards from Mumbles Head, the interaction of which with waves has led to the development of a small sand bank which dries at low tide, known as Mixon Shoals (Shoreline Management Partnership, 2001). There may be potential sediment exchange between Mumbles Head and Mixon Shoals (Shoreline Management Partnership, 2001), but there is no data on direction or volume of any sediment flux. Posford Duvivier and ABP Research (2000) reported variable sand transport through the White Oyster Ledge and Mixon Shoals area, with westward transport occurring as a result of tidal influence, reversing under storm conditions, during which this region is part of the main transport route for sediment into Swansea Bay. Movement: These resistant cliffs have historically retreated very slowly with any erosion only affecting limited areas, sediment either being deposited on adjacent areas of accumulated sediment or being transported eastwards (coarse materials), westwards (fine sediments) or offshore. Futurecoast (Halcrow, 2002) suggested that the cliffs along this section typically exhibit ‘very low’ (less than 0.1m/year) rates of erosion, but that there is the potential for single cliff failure events to result in 10m to 50m erosion. Generally the beaches have remained stable due to their indented nature enabling sediments to be retained within the bays, although the dominant cross-shore transport of sediments during storms does cause short-term volatility. C1-71 Lavernock Point to St Ann’s Head SMP2 Appendix C: Baseline Process Understanding Coastal defences have been built within Bracelet Bay, Limeslade Bay, Rothers Bay, Langland Bay, Caswell Bay, Oxwich Bay and Port-Eynon Bay. These provide small-scale protection to the shoreline and their impacts on the wider coastline are limited. At Pwlldu Bay a shingle barrier beach provides the main defence to a couple of isolated properties. The barrier contains a number of sand and shingle beach ridges, indicating beach position during the period of progradation. The oldest (and thus most landward) ridge is aligned in a southwest to northeast orientation, with each successive ridge turning towards the current orientation nearer to a direct west to east alignment (May, 2003d). Existing predictions of shoreline evolution: Under an ‘unconstrained’ scenario, Futurecoast (2002) predicted that low rates of erosion would continue, with ‘negligible/ no change’ (less than 10m erosion) predicted over the next century. The study concluded that beaches would remain stable, although with sea level rise there could be a landward movement of the water line resulting in beach narrowing as the cliffs will continue to prevent shoreline retreat. As only a small proportion of the frontage is defended, the Futurecoast (2002) prediction for ‘with present management’ is similar, although beach steepening and lowering was predicted in those bays where defences are present, but this was not predicted to affect adjacent sections of coastline. LOCAL SCALE: OXWICH BAY (SOUTHGATE TO OXWICH POINT) Interactions: Oxwich Bay is a wide sandy embayment that formed as a result of differential erosion, over geological timescales, of the underlying geology, due to both different cliff resistances and the presence of faults. The Bay can be split into three parts: Three Cliffs Bay, Nicholaston Burrows and Oxwich Burrows. Three Cliffs Bay is a small pocket beach lying between Great Tor and the resistant Three Cliffs outcrop, controlled by south-west to north-east trending faults and surrounded by limestone rock platforms which are partially covered with shingle. Climbing and cliff top dunes are present at the back of the Bay, known as Pennard Burrows. Sand for these dunes originated from the sandy beach, although this was historic and erosion of the fringing dunes means there is little current source of sediment (Pye et al. , 2007). Pennard Pill, a small river, discharges across the beach. This river meanders across the beach and foreshore before reaching the sea. At the back of the Bay there is a narrow shingle beach. The meandering nature of Pennard Pill has meant that an area of low-lying land is enclosed at the top of the beach. Nicholaston Burrows is characterised by a fringing dune system backed by the western cliff of Great Tor. Nicholaston Burrows and Oxwich Burrows are separated by a river, Nicholaston Pill, which drains the marshland behind and dissects the dunes before discharges across the sandy beach. These dunes are generally natural although the stream has been partly trained. Oxwich Burrows is characterised by a sand dune barrier which encloses low-lying marshland at the mouth of a small valley, cut into less resistant mudstones. It is bordered to the west by Oxwich Point, a limestone cliff, and a raised-beach platform is exposed around the base of Oxwich Point. C1-72 Lavernock Point to St Ann’s Head SMP2 Appendix C: Baseline Process Understanding The shoreline is protected from direct exposure to south-westerly swell waves by the substantial limestone headland of Oxwich Point, but the coast is exposed to locally-generated waves from the south and south-east (Halcrow, 2002). Net littoral drift is weak as wave diffraction and refraction caused by the prominent headlands results in local drift reversals. There are limited contemporary sources of sediment, and therefore the beaches within the bays are thought to be predominantly relict (Halcrow, 2002). There is, however, believed to be a sediment link, albeit fairly weak and involving small volumes of sand and fine sediment, between East Helwick area (eastern part of Helwick Bank) and the adjacent beaches, such that sediment can be transported between the Bank and nearby beaches (NAW, 2006). Movement: It is thought that the main beach ridge within Oxwich Bay developed around 2,500 years ago, enclosing a brackish lagoon (Ratcliffe, 1977). Similar to the system within Swansea Bay, it is likely that low dunes gradually built up due to reworking of glacial deposits laid down on the seabed. During the 12 th and 17 th centuries, there was a period of increased storminess resulting in active dunes and sand sheets migrating inland and this probably also occurred at this site. Since the end of the 18 th century, there has, however, been little further change in the dune position (Pye, et al. , 2007). The beach at Oxwich steepened during the early part of the 20 th Century, becoming more stable since 1970; although Halcrow (2002) note that there may be cyclical variations in local sediment movements. The dunes at Oxwich were also affected during World War 2 when US troops used the Gower to practice for the D-day landings. This resulted in the dunes becoming very bare due to trampling, and thus subject to reshaping and erosion. However, a policy of no grazing followed, allowing vegetation to become re-established and the dunes to recover (CCW, date unknown). The marsh behind Oxwich Burrows was originally saltmarsh and open to the sea, but in 1790 a seawall was built, to enable construction of a serpentine ornamental lake and water meadows for grazing, over the former saltmarsh area (CCW, date unknown). Until the mid-1960s, the Penmaen Burrows sand dune system extended across the western side of Three Cliffs Bay diverting the stream to the east across the beach. These dunes have eroded recently, but the stream still outflows on the eastern side, adjacent to the limestone outcrop to the east that form the three cliffs after which the bay is named (Bridges, 1997). It has been suggested that erosion at the western end of Oxwich Bay may be due to recreational pressure leading to instability in the dunes, rather than any change in the sediment budget (May, 2003a). The cliff outcrops along this frontage are very resistant.

Load more