Neath Port Talbot County Borough Council Crymlyn Burrows & Baglan
Total Page：16
File Type：pdf, Size：1020Kb
Ke Neath Port Talbot County Borough Council Crymlyn Burrows & Baglan Burrows: Geomorphological Assessment Kenneth Pye ScD PhD MA FGS CGeol and Simon J. Blott PhD MRes BSc FGS KPAL Report No: 160894 6 February 2014 Kenneth Pye Associates Ltd. Scientific Research, Consultancy and Investigations Neath Port Talbot County Borough Council Crymlyn Burrows & Baglan Burrows: Geomorphological Assessment Kenneth Pye ScD PhD MA CGeol FGS and Simon J. Blott PhD MRes BSc FGS KPAL Report No. 160894 Report history Version 1.0: 6 February 2014 25 Kenneth Pye Associates Ltd Research, Consultancy and Investigations Blythe Valley Innovation Centre Central Boulevard Blythe Valley Park SOLIHULL B90 8AJ United Kingdom Telephone: + 44 (0)121 506 9067 E-mail: [email protected] website: www.kpal.co.uk 2 Contents page 1.0 Report scope and purpose 4 2.0 Environmental settling, geomorphological character and development of the Crymlyn Burrows and Baglan Burrows dune systems 4 2.1 Setting and environmental history 4 2.2 Bathymetry and coastal morphology of Swansea Bay 5 2.3 Tidal regime, current sand waves 5 2.4 Aeolian processes and dune morphology 7 2.5 Human interventions and management 8 2.6 Sedimentary character 10 2.7 Nature conservation aspects 10 3.0 Conclusions and recommendations for dune restoration / creation 11 4.0 References 13 Tables 16 Figures 20 Appendix 1: Field photographs 62 3 1.0 Report scope and purpose This report summarises the results of a geomorphological assessment of Baglan Burrows and Crymlyn Burrows which was commissioned by the Biodiversity Team at Neath Port Talbot County Borough Council. The main purpose of the report is to inform decision making relating to the ecological and recreational management of the dune systems. The report is based principally on the following: (1) a review of background environmental data (temperature, precipitation, wind, waves, tides) (2) examination and analysis of modern and historical maps, aerial photographs and LiDAR data (3) a site survey on 16-17 December 2013 (4) data synthesis and interpretation 2.0 Environmental setting, geomorphological character and evolution of the Crymlyn Burrows and Baglan Burrows dune systems 2.1 Setting and environmental history Baglan Burrows are Crymlyn Burrows form two halves of an estuarine dune complex at the mouth of the River Neath in north-eastern Swansea Bay (Figures 1 & 2). These dunes form residual parts of a much larger area of windblown sand which once formed an extensive fringe around Swansea Bay between Sker Point in the southeast and Black Pill in the northwest. Much of the blown sand area has been levelled and built on for industrial and residential purposes since the late 18th century, and today only isolated areas of dunes remain, notably at Kenfig, Baglan and Crymlyn Burrows. Swansea Bay occupies a shallow bedrock basin which was partially infilled by glacial, fluvioglacial and lacustrine sediments during the Pleistocene period (Price & Brooks, 1980). During the last glacial period the Vale of Neath was occupied by a major glacier which discharged meltwater and sediment into the Bay. As sea level rose in the early to mid postglacial (Flandrian) period, these sediments were partially reworked and covered by marine sediments (Culver, 1977). Sand was reworked landwards by waves and currents to form a series of barrier islands and spits around the margins of the Bay. Behind the barriers a system of salt- and brackish marshes developed, forming a 2 to 3 km wide coastal plain. Peat 4 formed in lower lying and more poorly drained areas, including Crymlyn Bog (Carr & Blackley, 1977). 2.2 Bathymetry and coastal morphology of Swansea Bay Swansea Bay is a relatively shallow embayment which is conventionally defined by Mumbles Head in the west and Sker Point in the east. Across the entrance to the Bay the sea bed lies at approximately 20 m below Chart Datum (CD) near Mumbles Head, but is generally at -10 to -15 m CD further towards east (Figure 1). Chart Datum equates to the level of the Lowest Astronomical Tide and is approximately 5 m below Ordnance Datum Newlyn (ODN) (Table 1). As a result of the large tidal range, an extensive intertidal area is exposed at low tide. The chart by William Morris dated 1801 shows that the Rivers Neath and Afan then had wide-mouthed estuaries. The Tawe also once had a wide estuary mouth, but by 1801 the western side had been embanked and land-claimed for industrial purposes (Figure 3). In the mid-1820s, when the first surveys were made for the First Edition One-Inch Ordnance Survey maps, many of the coastal barrier features were still evident, especially on the eastern shore of Swansea Bay. The Neath estuary still had a wide entrance to the River Neath estuary, and an extensive intertidal delta with a complex system of anastomosing intertidal channels existed (Figure 3). Smaller estuaries remained at the mouths of the Afan and Kenfig rivers, but the entrance to the Tawe at Swansea had been narrowed by land claim on both sides by this time. Since the 1820s there has very large scale sediment accretion either side of the Neath River entrance, and the former Baglan Bay has been entirely claimed from the sea (Figures 4 to 13). Today, the shoreline between Mumbles Head and the western end of Crymlyn Burrows is backed by hard coastal defences. The Crymlyn Burrows and Baglan Burrows frontage is undefended, although the training walls of the Neath Estuary act as ‘hard points’ which provide some stability and encourage sediment accumulation on the adjoining shores (Figure 2). Between Sandfields and Port Talbot Outer Harbour the shore is also backed by hard defences. In places, notably between Black Pill and Swansea Docks, and in the small embayment just to the north of Port Talbot Harbour, sand dunes have developed in front of the concrete sea wall and promenade. Elsewhere, the defences are fronted by a sand or muddy sand beach / tidal flat of varying width. Between Port Talbot Harbour and the Kenfig River most of the backshore area is protected by rubble revetments, rock armour and tipped blast furnace slag. The Kenfig Dunes SSSI frontage has no artificial protection but much of the dune toe is protected by a natural gravel berm, 2.3 Tidal regime, currents and waves Swansea Bay, and the wider Bristol Channel experience a hyper-tidal regime. In the north- eastern part of the Bay the mean spring and neap tidal ranges are 8.6 m and 4.1 m, 5 respectively. At the entrance to the Neath estuary the level of the highest astronomical tide (HAT), which approximates the position of the dune toe, is 5.8 m OD, but elsewhere it is closer to 5.5 m OD (Table 1). The Bay experiences dominant semi-diurnal tides and subsidiary diurnal components. Superimposed on these are shorter period fluctuations which are due to the development of standing oscillations within the Bay, and longer-period fluctuations due to storm surges. The standing wave oscillations, which are up to 45 cm in amplitude, are created by the natural resonance of the Bay and are multi-nodal (Collins & Banner, 1980). The flood tidal stream enters Swansea Bay from the southwest near Mumbles Head and flows into the centre of the Bay where it diverges. Part of the flow continues into the entrance to the Neath estuary, part turns north-westwards to form an anticlockwise eddy in the north-western part of the Bay, and part turns south-eastwards to flow parallel to the shore past Port Talbot and Kenfig. On the ebb the flow reverses and tidal currents flow in SE to NW direction past the Neath estuary entrance to former a larger anticlockwise eddy within northern Swansea Bay. The highest ebb tidal current velocities occur where the tidal stream exit the Bay off Mumbles Head (Banner & Collins, 1980). Current velocities are significantly lower in the northern part of the Bay and generally decrease shorewards, although locally higher velocities occur in the low water channels of the Rivers Neath and Tawe. Significant amounts of freshwater are discharged into northern Swansea Bay by the Neath, Tawe and Afan Rivers, especially during winter, and this results in reduced near surface water salinities and water temperatures. However, there is only weak density stratification and a relatively small effect on residual currents within the Bay. Wind-driven current circulations are relatively more important at times of strong south-westerly winds. Short term sea levels can be raised or lowered significantly by atmospheric pressure variations and storm surges generated by the passage of Atlantic low pressure systems. The may cause positive or negative differences in predicted sea levels in this area of up to 2.0 m. The largest positive surge residual recorded at the Mumbles tide gauge since 1988 (1.97 m) occurred on 4th January 1998 (Table 2). Fortunately, large surges only rarely coincide with high spring tides; the highest resultant tide recorded at Mumbles (5.73 m) occurred on 3rd January 2014 when a surge of approximately 0.5 m occurred in association with a high, but not exceptional, spring tide (Table 3). This level is only 0.23 above HAT at Mumbles but significant erosion of the dunes around the northern part of the Bay occurred due to the fact that strong onshore winds and waves occurred around the time of high water. Waves recorded at Scarweather Sands, approximately 6 km west of Porthcawl, approach the coast mainly from a direction just south of west (see inset in Figure 1). Waves approaching Kenfig Sands and Margam Sands undergo some refraction as they pass over the Kenfig Patches and North Kenfig Patches, resulting in local wave focusing and variations in alongshore breaker height.