Morphological Change in Carmarthen Bay and Adjoining Estuaries: Further Analysis

Report prepared for Halcrow Group Ltd

by

K. Pye & S.J. Blott

March 2010

K. Pye Associates Crowthorne Enterprise Centre Crowthorne Business Estate Old Wokingham Road Crowthorne Berkshire RG45 6AW UK Telephone/ Fax + 44 (0)1344 751610 E-mail: [email protected]

Report history: 1st Draft Issued for Halcrow comment (2 February, 2010)

Revised Final (16 March 2010)

Bibliographic reference: Pye, K. & Blott, S.J. (2010) Morphological Change in Carmarthen Bay and Adjoining Estuaries: Further Analysis. Report to Halcrow Group Ltd by K Pye Associates, Crowthorne. In Halcrow (2010) Swansea Bay and Carmarthen Bay, Shoreline Management Plan, Annex A2 to Appendix C: Baseline Processes Understanding. Halcrow Group Ltd, Swindon.

Contents

List of Tables...... ii

List of Figures...... iv

1 Report scope and purpose...... 1

2 Methods ...... 1

2.1 LiDAR data analysis ...... 1

2.1.1 Initial data processing...... 1

2.1.2 LiDAR data accuracy ...... 2

2.2 Calculation of tidal areas and volumes ...... 3

2.3 Determination of marsh surface elevations...... 5

2.4 Cross-sectional profiles...... 6

2.5 Hypsometric analysis and tidal volumetric changes ...... 6

2.6 Analysis of historical bathymetric and shoreline change...... 6

2.7 Quantification of net sediment volume changes ...... 6

3 Results ...... 7

3.1 Loughor estuary (Burry Inlet) ...... 7

3.1.1 Sub-divisions of the estuary...... 7

3.1.2 LiDAR DEM of the Loughor estuary...... 8

3.1.3 Historical bathymetric and coastal change ...... 9

3.1.4 Sediment volume changes ...... 12

3.1.5 Possible impacts of future sea level rise on tidal volume ...... 12

3.1.6 Hydrodynamic and sediment transport processes...... 12

3.2 The Three Rivers estuarine complex...... 13

3.2.1 General morphological character and sub-divisions...... 13

i 3.2.2 The Taf estuary...... 14

3.2.3 The Towy estuary...... 17

3.2.4 3.2.4. The Gwendraeth estuary...... 18

3.2.5 The Pembrey barrier system...... 20

3.2.6 The Three Rivers confluence area...... 21

3.3 Historical bathymetric change in Carmarthen Bay...... 22

3.4 Hydrodynamic processes and sediment transport in Carmarthen Bay ...... 22

4 Conclusions ...... 23

5 References ...... 26

ii List of Tables

Table 1 Tidal levels for ports along the coast of South Wales, in metres relative to Ordnance Datum Newlyn

Table 2 Total active area at level of MHWS, areas with no LiDAR data coverage, and assumed elevation of areas with no data

Table 3 Areas below MSL in the Three Rivers estuaries complex (from LiDAR surveys flown 2003- 2008), and estimates of water volumes below MSL assuming different average water depths

Table 4 Estimates of increases in MSL, MHW and MHWS recorded at Avonmouth between 1987 and 2008 over one lunar nodal cycle (18.6 years)

Table 5 Estimates of increases in MSL, MHW and MHWS recorded at Milford Haven between 1987 and 2008 over one lunar nodal cycle (18.6 years)

Table 6 Elevation range of active saltmarshes in the Loughor Estuary, based on LiDAR surveys 2003- 2008

Table 7 Calculations of floodable areas and tidal prisms below present tidal levels in the Loughor Estuary

Table 8 Calculations of widths of the estuary mouths: Three Rivers estuarine complex (Ginst Point to Tywyn Point) and Loughor Estuary (Pembrey Burrows to Whiteford Point)

Table 9 Lengths of the principal low water channel in the Rivers Taf, Towy, Gwendraeth and Loughor

Table 10 Sinuosity of the principal low water channel in the Rivers Taf, Towy, Gwendraeth and Loughor

Table 11 Sediment volumes and planar areas in the Loughor Estuary above selected datums, calculated from bathymetry surveys by the Admiralty in 1888, Browning and Longdin in circa 1990, and LiDAR surveys 2003-2008

Table 12 Estimates of potential errors in the calculation of sediment volumes and planar areas in the Loughor Estuary using the bathymetry information displayed on the 1888 Admiralty Chart

Table 13 Calculations of floodable areas in the Loughor Estuary below the present levels of MHWS, and future predicted levels of MHWS in 2030, 2060 and 2100 based on UKCP09 sea level rise projections

Table 14 Calculations of spring tidal prisms in the Loughor Estuary, with the present levels of MHWS and MLWS, and future predicted levels of MHWS in 2030, 2060 and 2100 based on UKCP09 sea level rise projections

Table 15 Calculations of floodable areas and tidal prisms below present tidal levels in the combined Three Rivers estuarine complex (Taf, Towy, Gwendraeth and their confluence)

Table 16 Elevation range of active saltmarshes in the Taf Estuary, based on LiDAR surveys 2003-2008 iii Table 17 Calculations of floodable areas and tidal prisms below present tidal levels in the Taf Estuary. Calculations based on LiDAR surveys 2003-2008

Table 18 Calculations of floodable areas in the Taf Estuary below the present levels of MHWS, and future predicted levels of MHWS in 2030, 2060 and 2100 based on UKCP09 sea level rise projections

Table 19 Calculations of spring tidal prisms in the Taf Estuary, with the present levels of MHWS and MLWS, and future predicted levels of MHWS in 2030, 2060 and 2100 based on UKCP09 sea level rise projections

Table 20 Elevation range of active saltmarshes in the Towy Estuary, based on LiDAR surveys 2003-2008

Table 21 Calculations of floodable areas and tidal prisms below present tidal levels in the Towy Estuary

Table 22 Calculations of floodable areas in the Towy Estuary below the present levels of MHWS, and future predicted levels of MHWS in 2030, 2060 and 2100 based on UKCP09 sea level rise projections

Table 23 Calculations of spring tidal prisms in the Towy Estuary, with the present levels of MHWS and MLWS, and future predicted levels of MHWS in 2030, 2060 and 2100 based on UKCP09 sea level rise projections

Table 24 Elevation range of active saltmarshes in the Gwendraeth Estuary, based on LiDAR surveys 2003-2008

Table 25 Calculations of floodable areas and tidal prisms below present tidal levels in the Gwendraeth Estuary

Table 26 Calculations of floodable areas in the Gwendraeth Estuary below the present levels of MHWS, and future predicted levels of MHWS in 2030, 2060 and 2100 based on UKCP09 sea level rise projections

Table 27 Calculations of spring tidal prisms in the Gwendraeth Estuary, with the present levels of MHWS and MLWS, and future predicted levels of MHWS in 2030, 2060 and 2100 based on UKCP09 sea level rise projections

Table 28 Calculations of floodable areas and tidal prisms below present tidal levels in the Three Rivers confluence

Table 29 Calculations of floodable areas in the Three Rivers Confluence below the present levels of MHWS, and future predicted levels of MHWS in 2030, 2060 and 2100 based on UKCP09 sea level rise projections

Table 30 Calculations of spring tidal prisms in the Three Rivers Confluence, with the present levels of MHWS and MLWS, and future predicted levels of MHWS in 2030, 2060 and 2100 based on UKCP09 sea level rise projections

iv Table 31 Calculations of floodable areas in the combined Three Rivers estuarine complex (Taf, Towy, Gwendraeth and their confluence) below the present levels of MHWS, and future predicted levels of MHWS in 2030, 2060 and 2100 based on UKCP09 sea level rise projections

Table 32 Calculations of spring tidal prisms in the combined Three Rivers estuarine complex (Taf, Towy, Gwendraeth and their confluence), with the present levels of MHWS and MLWS, and future predicted levels of MHWS in 2030, 2060 and 2100 based on UKCP09 sea level rise projections

List of Figures

Figure 1 General location map of Carmarthen Bay

Figure 2 Predicted differences in tidal levels in (a) Three Rivers estuarine complex (Taf, Towy and Gwendraeth), and (b) the Loughor Estuary. Contours show differences in metres from Ferryside (a) and Burry Port (b)

Figure 3 Tidal levels predicted at the Class A gauge at Milford Haven on 18th to 19th September 1997

Figure 4 Digital surface model of the Loughor Estuary, compiled from LiDAR topographic surveys 2003-2008

Figure 5 Cross-sectional profiles across the Loughor Estuary, from LiDAR data surveyed 2003-2008

Figure 6 Hypsometric analysis of tidal volumes in the present active areas of the Loughor Estuary

Figure 7 Bathymetric surveys of the Loughor Estuary, taken from charts by William Jones, John Wedge, Admiralty, Longdin and Browning Surveys Ltd. and CCW intertidal substrate survey using aerial photographs.

Figure 8 Chart of the Loughor Estuary by William Jones, 1757

Figure 9 Chart of the Loughor Estuary by John Wedge, 1808

Figure 10 Bathymetry of the Loughor Estuary, taken from Admiralty Chart 1167 (Bristol Channel: Burry or Llanelly Inlet) published in 1839

Figure 11 Bathymetry of the Loughor Estuary, taken from Admiralty Chart 1076 (St Govens Head to The Mumbles) published in 1888

Figure 12 Bathymetry of the Loughor Estuary, taken from Admiralty Chart 1076 (Linney Head to Oxwich Point) published in 1955

Figure 13 Bathymetry of the Loughor Estuary, taken from Admiralty Chart 1076 (Linney Head to Oxwich Point) published in 2001

v Figure 14 Bathymetry survey of the Loughor Estuary by Longdin and Browning Surveys Ltd. plot produced in January 1991 (reproduced for Shoreline Management Partnership, 1995)

Figure 15 Intertidal areas the Loughor Estuary, taken from the Coastal and Marine Resource Atlas, surveyed by CCW between 1996 and 2005

Figure 16 Changing position of the low water channel in the Loughor Estuary, digitised from historical Ordnance Survey maps

Figure 17 LiDAR DEM of Whiteford Burrows. Historical tide lines overlain from Ordnance Survey maps and aerial photographs

Figure 18 LiDAR DEM of Loughor Estuary between Burry Port and Llanelli. Historical tide lines overlain from Ordnance Survey maps

Figure 19 LiDAR DEM of the Inner Loughor Estuary between Machynys Peninsula and Loughor Bridge. Historical tide lines overlain from Ordnance Survey maps

Figure 20 Digital surface model of the Three Rivers Estuary (Taf, Towy and Gwendraeth), compiled from LiDAR topographic surveys 2003-2008

Figure 21 Digital surface model of the Taf Estuary, compiled from LiDAR topographic surveys 2003- 2008

Figure 22 Cross-sectional profiles across the Taf Estuary, from LiDAR data surveyed 2003-2008

Figure 23 Hypsometric analysis of tidal volumes in the present active areas of the Taf Estuary

Figure 24 Changing position of the low water channel in the Taf Estuary, digitised from historical Ordnance Survey maps

Figure 25 LiDAR DEM of Laugharne and Pendine Burrows. Historical tide lines overlain from Ordnance Survey maps

Figure 26 Digital surface model of the Towy Estuary, compiled from LiDAR topographic surveys 2003- 2008

Figure 27 Cross-sectional profiles across the Towy Estuary, from LiDAR data surveyed 2003-2008

Figure 28 Hypsometric analysis of tidal volumes in the present active areas of the Towy Estuary

Figure 29 Changing position of the low water channel in the Towy Estuary, digitised from historical Ordnance Survey maps

Figure 30 LiDAR DEM of the Towy Estuary. Historical tide lines overlain from Ordnance Survey maps

Figure 31 Digital surface model of the Gwendraeth Estuary, compiled from LiDAR topographic surveys 2003-2008

vi Figure 32 Cross-sectional profiles across the Gwendraeth Estuary, from LiDAR data surveyed 2003-2008

Figure 33 Hypsometric analysis of tidal volumes in the present active areas of the Gwendraeth Estuary

Figure 34 Changing position of the low water channel in the Gwendraeth Estuary, digitised from historical Ordnance Survey maps

Figure 35 LiDAR DEM of Pembrey Burrows and the Gwendraeth Estuary. Historical tide lines overlain from Ordnance Survey maps and LiDAR surveys

Figure 36 Hypsometric analysis of tidal volumes in the present active areas of the Three Rivers confluence

Figure 37 Hypsometric analysis of tidal volumes in the present active areas of the Three Rivers Estuary, including the confluence

Figure 38 Bathymetric surveys of the Three Rivers estuarine complex, taken from Admiralty Surveys (1830 and 1886-7), Ordnance Survey (1965) and CCW intertidal substrate survey (1996-2005)

Figure 39 Bathymetry of the Three Rivers estuarine complex, taken from Admiralty Chart 1167 (Bristol Channel: Burry or Llanelly Inlet) published in 1839

Figure 40 Bathymetry of the Three Rivers estuarine complex, taken from Admiralty Chart 1076 (St Govens Head to The Mumbles) published in 1888

Figure 41 Bathymetry of the Three Rivers estuarine complex, taken from Admiralty Chart 1076 (Linney Head to Oxwich Point) published in 1955

Figure 42 Bathymetry of the Three Rivers Estuarine complex, taken from Admiralty Chart 1076 (Linney Head to Oxwich Point) published in 2001

vii

1 Report scope and purpose

This report, which builds on an earlier review of coastal processes and shoreline behaviour in Swansea Bay and Carmarthen Bay (Pye & Blott, 2009), presents the results of further analysis undertaken to provide a better understanding of past, present and possible future shoreline changes in Carmarthen Bay and adjoining estuaries (Figure 1). The purpose of the report is to inform the evaluation of management policy options considered as part of the Lavernock Point to St. Ann’s Head Shoreline Management Plan Review (SMP2).

The additional study has involved analysis of additional LiDAR data supplied by the Environment Agency, examination and digitization of hydrographic charts, topographic survey maps and aerial photographs, evaluation of documentary sources, and field visits to selected locations. The information obtained has been used to develop an improved conceptual understanding concerning the relationships between coastal processes, open coast – estuary interaction and shoreline change. The possible implications of future climate and sea level change on the estuaries in the area have also been considered in greater detail than in Pye & Blott (2009).

For the purposes of this study, five estuarine areas have been defined within the Carmarthen Bay area and separate analysis performed on each:

• The Loughor estuary (east of a line between Whiteford Point and “The Nose” Pembrey)

• The Taf Estuary (north of a line between Ginst Point and Wharley Point)

• The Towy (Tywi) estuary (north of a line between Wharley Point and Tregoning Hill)

• The Gwendraeth estuary (east of a line between St Ishmael and Tywyn Point)

• The Three Rivers confluence area (south of a line drawn between Tywyn Point and Ginst Point).

Three sections of open coast associated with the estuaries have also been considered:

• The Pendine – Ginst Point barrier complex (including Pendine Burrows and Laugharne Burrows)

• The Tywyn Point to Pembrey Burrows barrier complex

• The Hills Tor to Whiteford Point barrier complex (including Whiteford Burrows).

2 Methods

2.1 LiDAR data analysis 2.1.1 Initial data processing Environment Agency LiDAR data were supplied as pre-processed, gridded datasets in ESRI ASCII .asc format, mainly as 2 x 2 km square tiles with data gridded at 2 m spacing resolution. Data for some areas of 1 the Loughor Estuary were supplied as 1x1 km square tiles, with data at 50 cm grid resolution. Processing was carried out using the Golden Software Surfer ® program. Data tiles were converted from ESRI .asc format to Surfer .grd format using automated visual basic macros.

A number of different LiDAR surveys cover each estuary, flown over the period 2003 to 2008. LiDAR tiles for each individual flight were combined using a mosaic routine to produce continuous data for each LiDAR survey. In order to obtain a complete digital elevation model (DEM) of each estuary, it was necessary to combine data from different flights, sometimes flown over several years, again using a mosaic routine. Where data from separate flights overlapped, the most recent data were used. Some small areas within each estuary, principally those with standing water at the time of survey, had no data; these areas were assigned a null elevation value and excluded from subsequent volume and area calculations. A correction factor was applied to take account of the incomplete data coverage (see below). The combined data were then re-saved at 2 m resolution. The composite xyz LiDAR grids were saved in Surfer .grd and ESRI ASCII .asc grid formats to allow further processing.

2.1.2 LiDAR data accuracy The vertical and horizontal accuracy of LiDAR data is largely dependent upon the precision with which the aircraft’s position and height is calculated using GPS, and the aircraft’s orientation relative to the ground (pitch, roll and yaw). LiDAR surveys are also calibrated using ground GPS base stations, the position and elevation of which must also be precisely known. LiDAR is acquired in swaths along a series of flight lines, so that data points at the edges of the swaths, or at the furthest distance from ground base stations, will be the least accurate. The height at which the aircraft performs the survey also affects the accuracy, with a compromise between a lower flight (hence greater accuracy and resolution) requiring more flight lines (hence greater time and cost). The LiDAR data used in this project were acquired by the Environment Agency using several different instruments over a period of 5 years, at varying resolution. Since it was necessary to use more than one survey to achieve complete areal coverage, the accuracy of the composite digital elevation model (DEM) is likely to exhibit some spatial variation in level of accuracy. The vertical accuracy of LiDAR data over relatively flat, un-vegetated surfaces, allowing for the variables listed above, is generally considered of the order of ±15 cm. The limited number of ground topographic data available for the estuaries considered here, and time differences between ground and LiDAR surveys, makes direct error analysis difficult.

The LiDAR data used in this study were collected in ‘first-return’ mode, whereby the distance is measured to the first object encountered by the laser beam. For this reason, elevations will be over-estimated in areas of high, dense vegetation. Although the effects of vegetation can be reduced by using filtering algorithms, the process of ‘filtering’ degrades the data and can ‘smooth’ areas where elevations change abruptly (such as at saltmarsh edges, within sand dunes, and in tidal creek networks). For this reason, unfiltered data were used in this project. Based on comparisons with the limited available ground topographic survey data, and consideration of vegetation and terrain characteristics revealed by aerial photographs, it is concluded that vertical accuracy of the resultant DEMs is likely to lie in the range +/- 10 to 20 cm over > 95% of the area considered. However, it must be borne in mind that the DEM is based on a composite of data obtained at different dates, since complete data coverage for 2008 was not provided. In areas of rapid geomorphological change, notably on the intertidal flats with active tidal channels, the detailed surface relief can change significantly between surveys, and this introduces an element of error into the composite DEM. Even within a single survey, different parts of the estuary may be flown at varying states of the tide, resulting in variable vertical coverage of the intertidal zone. In this study, the data used to construct the DEM of the Taf and 2 Towy estuaries, and the Pendine - Laugharne barrier, were all flown in a single survey on 25 January 2004. The data used to construct the Gwendraeth estuary DEM were flown in January 2003 (northern and western areas) and in March 2006 (southern and eastern areas). The data used to construct the DEM of the Three Rivers confluence area were mainly flown in January 2004 and March 2006, with a small area flown in January 2003. Consequently, a number of discontinuities are evident within the DEM mosaic of this area. The data used to construct the DEMs of the Pembrey Burrows area, Whiteford Burrows and the western half of the Loughor estuary were flown in March 2006. The data used for the northern part of the estuary between Burry Port and the southern part of the Upper Estuary were flown in January - February 2008. Data for the southern part of the Middle Estuary and the northern part of the Upper Estuary were flown in January 2003. Consequently, the mosaic DEM of the Loughor also shows a number of discontinuities, especially within the intertidal zone.

2.2 Calculation of tidal areas and volumes Present tidal areas and volumes within each defined estuary area were quantified using the composite LiDAR DEMs and available tidal level information. Levels of HAT, MHWS, MSL, MHWN, MLWN, MLWS and LAT for the Standard Port of Milford Haven were taken from 2009 Admiralty Tide Tables (UK Hydrographic Office, 2008). Additionally, the levels of MHW and MLW were calculated from 15 minute tidal level data recorded at the Milford Haven during the period 1987-2008 (obtained from the National Tidal and Sea Level Facility website).

Levels of MHWS, MHWN, MSL, MLWN and MLWS for the Secondary Ports of Ferryside, Carmarthen, Burry Port and Llanelli were obtained from the Admiralty Tide Tables using stated conversion factors relative to Milford Haven. The levels of HAT and MHW at these secondary ports were calculated by extrapolating the differences in MHWN and MHWS between the secondary ports and Milford Haven. The levels of LAT and MLW at the secondary ports were similarlycalculated by extrapolating the differences in MLWN and MLWS. All levels (Table 1) were converted to Ordnance Datum (Newlyn) using the conversion values given in the Admiralty Tide Tables.

In the Towy, MHWS and MHWN are stated in the Admiralty Tide Tables to differ by 40 cm between Ferryside and Carmarthen, while in the Loughor the height differences between MHWS and MHWN at Burry Port and Llanelli are stated to differ by 20 cm (Table 1). This equates to an average increase in tidal levels up each estuary at an average rate of approximately 3.3 cm per km.

For each estuary, a DEM was constructed which described the differences in tidal levels. For the Three Rivers Estuarine Complex, Ferryside was allocated a value of 0.0 m, Carmarthen was allocated +0.4 m, and a kriging algorithm used to predict tidal level differences, with a grid spacing of 2 m (matching the lidar data). Tidal levels were assuming to increase up the Taf and Gwendraeth estuaries at the same rate as that specified for the Towy. In the Loughor Estuary, a similar DEM of estimated tidal levels was constructed by setting values of 0.0 m at Burry Port and +0.2 m at Llanelli and applying the kriging algorithm. Figure 2 shows the estimated tidal contours and along-estuary water surface slopes superimposed on the lidar DEMs of the Three Rivers estuarine complex and the Loughor estuary. These are approximations and do not reflect the more detailed pattern of variations which undoubtedly exists between different estuary reaches; however, in the absence of further observational data they provide the best available framework for comparison.

Tidal areas and volumes were calculated for several different tidal levels including hypothetical extreme events (representing storm surge conditions) with high tidal levels reaching +1 m and +2 m above the level

3 of HAT. A digitising routine was used to prescribe the current intertidal area of each estuary, using the HAT +2 m line or, where necessary, the alignment of sea walls and embankments, and saved as ASCII xy files. For the purposes of calculation, the seaward limits of the ‘estuaries’ were defined by arbitrary straight lines, as specified above. Areas outside each active ‘estuary’ at risk of flooding on a tide reaching HAT + 2m, were also identified and digitised. These areas included:

• Taf Estuary: (1) Laugharne Marsh as far west as Pendine; and (2) Llanybri Marsh (incorporating Mwche Farm Marsh).

• Towy Estuary: no areas outside the active estuary were specified, the railway was taken to represent the eastern limit for much of the estuary’s length.

• Gwendraeth Estuary: (1) the southern marshes, to the west of the railway, around the former Pembrey Airfield, with a southern limit at OS Northing 202000; (2) the marshes east of the railway and above the tidal limit at Commissioners’ Bridge; (3) the marshes to the south and west of Kidwelly.

• Loughor Estuary: (1) the Millennium Coastal Park and the National Wetlands Centre for Wales, south of Llanelli; and (2) Pembrey Marshes, north of Pembrey Burrows, with a northern limit at OS Northing 202000.

Areas and volumes of active and reclaimed marsh areas were then computed using a combination of internal Surfer routines and Visual Basic macros. For each estuary, the lower surface level was defined by the LiDAR DEM, while the upper surface was taken as the sloping tidal surface defined using the method described above.

As shown in Table 2, a relatively small proportion of each estuary yielded no LiDAR data, mainly due to standing water which gave no LiDAR return. Elevations of the areas with no data were estimated by comparison with surrounding areas. In the Three Rivers estuaries, the water level at the time of the survey was approximately -1.0 m OD, and all areas with no data were below MSL. Therefore, the area of no data was added to all calculated areas for all tidal levels below HAT, MHWS, MHW, MHWN and MSL. In the Loughor Estuary, the water level at the time of the survey was approximately -3.8 m OD, and all areas with no data were between MLW and MLWS. Therefore, the area of no data was added to all calculated areas for tidal levels below HAT, MHWS, MHWN, MSL, MLWN, and MLW, but not calculated for areas below MLWS and LAT.

In the case of the Three Rivers estuarine complex, water volumes were calculated between the specified tidal levels (MSL MHWN, MHW, MHWS, HAT, HAT +1 m and HAT +2 m) and the DEM surfaces. Volumes below MSL were subtracted from the tidal volumes below all other tidal levels to obtain volumes between those levels and MSL. To calculate the volumes below MSL, the area below MSL was multiplied by an assumed water depth, shown in Table 3. The values for the assumed water depth were based on information obtained from Admiralty Tide Tables, aerial photographs and bathymetric charts. These volumes were then added to the volumes above MSL to produce tidal prisms for neap, mean, spring, highest astronomical and surge tides in each estuary.

4 For the Loughor Estuary, calculations were less complicated as the water level at the time of the lidar surveys was considerably lower (approximately -3.8 m OD). Tidal prisms for neap and mean tides were therefore calculated by difference between MHWN and MLWN (for neap tides) and MHW and MLW (for mean tides). For spring tides it was necessary to calculate the area below MHWS (627 ha), assume an average depth between MLWS and LAT (0.3 m), and then to calculate the additional volume between MLWS and LAT (1881 x10 3 m 3). This allowed spring and highest astronomical tidal prisms to be calculated.

Tidal areas and prisms were also calculated for a mean spring tide under a number of future sea level rise scenarios. The UKCP09 relative sea level projections were used to estimate the changes in tidal areas and tidal volumes for each estuary in 2030, 2060 and 2100 (i.e. 40, 70 and 110 years after the baseline year of 1990, or approximately 20, 50 and 100 years into the future). The UKCP09 User Interface permits predictions of relative mean sea level rise in grid squares of size 5 minutes of latitude by 5 minutes of longitude (c. 9 km x 5 km) around the coast of the UK, assuming three future emission scenarios: Low (SRES B1), Medium (SRES A1B) and High (SRES A1FI). The User Interface was used to extract the relative sea level rise for the grid square closest to the centre of each estuary (Cell 22846 for the Three Rivers and Cell 23054 for the Loughor), for each emission scenario, and for values equating to 5%, 50% and 95% of the range of climate model outputs (the projected sea level rise increments by 2030, 2060 and 2100 are summarised in Tables 13 & 18) .

Two sets of calculations were performed. In the first set of calculations the level of MHWS was assumed to rise at a rate equal to that predicted for mean sea level. However, examination of tidal records at Avonmouth and Milford Haven has demonstrated that the long-term rate of increase in MHW and MHWS has been significantly higher than the rate of increase in mean sea level (Tables 4 & 5). While the rate was a little over double at Avonmouth, MHW increased at approximately 1.4 times the rate of MSL at Milford Haven. The reasons for these differences are not well understood but may be due to in significant part to shallow water effects. In view of the observed historical trends, in this study calculations were also made assuming a rise of MHWS at a rate double that of MSL rise. The estimated rises in future MHWS level were added to the present level of MHWS in each estuary (prescribed by the sloping water surface DEMs) and tidal prisms calculated for 2030, 2060 and 2100. These calculations also assumed that the level of MLWS will remains at the present level for each time period. This assumptions may not be strictly valid, but present water volumes in the estuaries below MLWS are small (approximately 1%) compared with the mean spring tidal prism, and errors due to this assumption are likely to be small. The actual magnitude of any future difference in the rate of increase of MHWS compared with that in MSL is likely to depend on the nature of future regional tide- continental shelf interaction, whether sedimentation near the coast and in the estuaries is able to keep pace with mean sea level rise, and whether significant changes occur in the sea bed and estuarine morphology (the pattern of banks and channels). Under assumptions that sedimentation in the estuaries is able to keep pace with sea level rise, and that major changes in the estuarine morphology do not occur, it is considered most likely that MHWS will continue to increase at a rate of 1.4 to 2 times the rate of increase in MSL.

2.3 Determination of marsh surface elevations The average and range of surface elevations on areas of active and reclaimed marsh areas within each estuary, using the Grid Info function in Surfer. Depending on the size of the area and the resolution of the LiDAR data, these statistics were based on thousands to hundreds of thousands of data points. Areas were defined using a digitising routine, avoiding creeks and channels where possible, and reflecting the local morphological saltmarsh units. The mean, standard deviation and coefficient of variation, together with the

5 1, 5, 10, 25, 50, 75, 90, 95 and 99% frequency distribution percentiles of elevation values in each area, were determined.

2.4 Cross-sectional profiles Between 15 and 20 cross-sections were defined along the length of each estuary, spaced at approximately 1 km intervals. The most eaward profile in each case corresponded with the defined seaward limit of the estuary. The Surfer routine Grid Slice was used to calculate surface elevations at grid intersections along each profile (approximately every 2 m).

2.5 Hypsometric analysis and tidal volumetric changes Water volumes were also calculated at 5 cm vertical increments (above c. 0.5 m OD in the Three Rivers Estuary and above c.-3.5 m in the Loughor Estuary to +1 m above the level of HAT). The data were then plotted graphically to display the change in tidal volume as a function of tidal level (stage). In order to obtain an understanding of the rate of change in tidal volume over time during a typical high tide, these data were combined with tidal stage rise data for a predicted spring tide on 18 September 1997. Fifteen minute predicted data for this tide at Milford Haven were obtained from the NTSLF web-site and equivalent values for Ferryside and Burry Port calculated using corrections given in Admiralty Tide Tables. The three predicted tidal curves are shown for comparison in Figure 3. The rate of change in tidal volume provides an indication of possible changes in average tidal current velocities, and consequently of sediment transport potential. Graphs showing the rate of change in tidal volume were therefore plotted in order to determine at what tidal elevations the maximum velocities are likely to occur in each estuary.

2.6 Analysis of historical bathymetric and shoreline change Historical bathymetric charts, maps and aerial photographs were examined to provide qualitative information about changes in estuarine morphology and shoreline positions around Carmarthen Bay. Where possible, the positions of mean low water mark, mean high water mark, ‘back-beach limit’ (approximately equivalent to the dune toe, cliff toe, and HAT), and median line of the main estuarine low water channels were digitized using Surfer and superimposed on the LiDAR DEMs. A number of morphometric parameters were calculated, including measures of axial estuarine channel length, valley length, and relative channel sinuosity. In the case of the Loughor estuary, an attempt was made to quantify sediment volume changes over time by digitizing and comparing the Admiralty Chart of 1888, a bathymetric survey of the estuary undertaken by Browning and Longdin in circa 1990, and the 2003-08 LiDAR DEM. Additional information about changes in shoreline position, intertidal and sub-tidal morphology and human interventions was obtained from a variety of published and unpublished documentary sources.

2.7 Quantification of net sediment volume changes An attempt was made to quantify net sediment losses and gains in the Outer and Middle parts of the Loughor estuary over the past century by comparing the 1888 Admiralty Chart with the 2003-08 LiDAR DEM. No data for the Inner and Upper parts of the estuary were available from the 1888 Chart, and LiDAR coverage of the Outer Estuary was less extensive than that on the 1888 Chart. Owing to changes in shoreline position between the two surveys, some marginal parts of the Middle and Outer Estuary also had to be excluded and an area in common to the surveys was defined. The 1888 Chart was digitized using the Golden Software Didger programme. A scan of the chart was calibrated to National Grid co-ordinates using 23 control points, warped using an exponential spline function, and a total of 1008 spot heights extracted from the chart. Values were converted from fathoms and feet to metres and then gridded using a kriging algorithm in Golden 6 Software Surfer to produce a 10 m grid DEM. The DEM was then converted from 1888 Chart Datum to Ordnance Datum Newlyn (ODN) using values for MHWS quoted on the chart. An allowance for increase in MHWS between 1888 and 2008 of 2.3 mm a -1 was made based on long-term records of increases in MHW at the Newlyn tide gauge. Based on this method the increase in MHWS was estimated to be 27.6 cm over the 120 year period, leading to an estimate of MHWS in 1888 of 3.62m OD at Burry Port and 3.82 m at Llanelli. MHWS levels are stated on the 1888 to be 26 feet (7.92 m) and 22.5 feet (6.86 m), respectively, above the level of the soundings. Therefore the 1888 DEM was adjusted to ODN by subtracting 4.30 m at Burry Port and 3.04 m at Llanelli. Areas to the west of Burry Port were adjusted using the correction factor for Burry Port. Areas to the east of Llanelli were adjusted using the correction factor for Llanelli, and areas in between were adjusted by factors determined by linear interpolation.

The potential significance of error attributable to this data conversion process, or original determination of Chart Datum (level of soundings), was estimated by applying a +/- 40 cm adjustment to all the levels in the 1888 DEM and then calculating the sediment volume above -4.0 m OD.

3 Results

3.1 Loughor estuary (Burry Inlet) 3.1.1 Sub-divisions of the estuary The Loughor estuary occupies a structural depression which runs approximately east-west between the Gower Peninsula and an upland area extending from Burry Port to Pontardulais. The Burry Inlet owes its origin partly to structural control and partly to selective erosion of weaker Lower Coal Measures strata which form the southern side of the South Wales coalfield basin. The broad erosional forms of the area were probably established by weathering and fluvial erosion in Tertiary times, but the ancestral valley of the River Loughor was significantly modified by Pleistocene glaciation. Irish Sea ice moved up the Bristol Channel during the Wolstonian glaciation, but during the Devensian glaciation Welsh ice entered the area from the north and northeast (Bowen, 1995). The maximum Devensian ice extent lay NW - SE across the Gower Peninsula, passing close to Hills Tor area and across Carmarthen Bay towards Saundersfoot. A Late Glacial re-advance formed a later moraine which extends from the Whiteford Burrows area towards Ashburnham (Bowen, 1980).

Following final ice retreat, Carmarthen Bay was flooded by the sea which probably reached its maximum inland extent around 5000 – 5500 years ago. Around this time the southern side of the estuary was defined by steep slopes. A barrier beach, probably with overlying dune system, formed around this time in a position a few hundred metres seawards of the present Whiteford Burrows barrier. Freshwater peat deposits formed behind this barrier today outcrop in the sub-tidal zone, seaward of the modern barrier, and wave eroded peat blocks are commonly washed up on the shoreline. Detailed investigation of the stratigraphy and age structure of the barrier has not been carried out, but evidence suggests that a significant part of the dune system sits on a basement of glacial till which strongly influences the hydro-geology of the system (Davies et al ., 1987).

The northern side of the estuary was also defined in many places by steep coastal slopes, although in most places the maximum landward extent of the mid Holocene shoreline did not reach the position of the last interglacial shoreline (Bowen, 1990).

7 From the 17th century onwards, and especially after 1800, extensive land reclamation was undertaken along the shore between Burry Port and the Loughor Bridge. Approximately 1800 ha was reclaimed for industrial purposes on the north side of the estuary by 1850 (Plummer, 1960). Reclamation has been less extensive on the south side of the estuary, although Cwm Ivy Marsh in the lee of Whiteford Burrows was reclaimed in the 17th century (Kay & Rojanavipart, 1977). There were also significant reclamations around the head of the estuary east of Loughor and Gowerton in the 18th and 19th centuries.

For the purposes of this study the estuary has been divided into four parts:

• The Outer Estuary, lying seaward of a line drawn between Whiteford Point and the end of ‘The Nose’, west of Burry Port; this area contains a complex of banks and channels which effectively form an ebb-tidal delta , bordered by wide beaches and barrier dune systems on each side of the entrance (the Whiteford Burrows barrier on the southern side and the Pembrey Burrows barrier on the northern side)

• The Middle Estuary, defined to the west by Whiteford Point and the eastern end of The Nose and to the east by a line drawn between the slipway at Salthouse Point and the southern end of the Machynys Peninsula; this section of the estuary is relatively wide relative to its length and contains extensive intertidal tidal flats and saltmarshes, notably along the southern shore between Whiteford Burrows and Crofty

• The Inner Estuary, defined in the west by the Salthouse Point – Machynys Peninsula line and in the east by the Loughor railway bridge; the present tidally active area of the inner estuary is today considerably smaller than in the past due to land claim, mainly on the northern side around the Millennium Coastal Park, although significant areas of active saltmarsh remain on the southern shore between Pen-clawdd and Gowerton

• The Upper Estuary, extending upstream from the Loughor railway bridge to the normal tidal limit near Pontardlais; this section of the estuary is relatively narrow relative to its width and is flanked on both sides by active and reclaimed saltmarsh.

3.1.2 LiDAR DEM of the Loughor estuary A composite LiDAR DEM of the estuary is shown in Figure 4. Also shown on this figure are the positions of a number of cross-sections and defined areas within which marsh elevations were determined. The cross- sections are shown in Figure 5 and the elevations of different marsh areas are summarised in Table 6. The marsh elevations vary considerably, reflecting both differences in marsh age and a general up-estuary increase in tidal levels. The highest marshes (relative to OD) are found in the Upper Estuary upstream of the Loughor railway bridge, in the Inner Estuary just west of Gowerton, and in the Middle Estuary at

Llanrhidian. In all of these areas the marshes are of considerable age and the median marsh elevation (Z 50 ) lies above the local level of MHWS tides. The lowest marshes are mainly of recent inception in the more seaward parts of the Middle Estuary (e.g. the outer parts of Landimore marsh), where they equate approximately to the level of MHW.

The areas and volumes of the active estuary between different tidal datum levels are shown in Table 7, together with equivalent values for Pembrey Marsh adjacent to the Outer Estuary and the reclaimed areas on the north side of the Inner Estuary around the Millennium Country Park. The mean spring tidal prism is 219571 x 10 3 m 3, approximately twice that of the Three Rivers estuarine complex. The entrance to the 8 Middle Estuary is confined by the barrier dune systems at Whiteford and Pembrey – Burry Port. The minimum width at MHWS level is c. 3.0 km (Table 8) but the width of the Middle Estuary at this level increases to > 6 km between Llanrhidian and Pwll. Consequently, the Middle Estuary effectively forms a partially enclosed basin, the general shape of which might be expected to favour ebb tidal current dominance in the seaward parts of the main channel. The seaward ends of the Inner Estuary and the Upper Estuary are also constrained by topographic features which may be expected to have an influence on hydrodynamic processes.

Figure 6 provides an example of the general relationship between tidal volumes and tidal height within the estuary, using predicted 15 minute tidal height data for the highest astronomical (non-surge tide) recorded at Milford Haven over the past 30 years. The tidal volume of the present active estuary is predicted to increase exponentially as a function of tidal level up to a level approximately 0.5 m higher than MHWS, but thereafter increases almost linearly as a function of tidal height as the entire estuary area (within the defined limits) is flooded (Figure 6a). In the event of a breach or over-topping of the defences, leading to a significant increase in the tidal storage capacity, a further steepening of the curve would be expected. The maximum rate of increase in tidal volume is predicted to occur at levels between 3.05, just above MHW, and 4.35 m, just above MHWS (Figure 6b). This reflects the sudden increase in floodable area as the tide spreads over the relatively large saltmarsh area. The tidal curve used to predict the tidal volume changes, based on predicted values for Milford Haven adjusted for Burry Port using correction factors published in the Admiralty Tide Tables, is shown in Figure 6c. Figure 6d shows the change in tidal volume which would be expected for this tide as a function of time. The timing of the highest rate of increase in tidal volume in this case is governed by the tidal forcing (i.e. the highest rate of tidal stage increase mid-way through the flood), but a secondary influence imposed by the estuarine morphology is also evident (the shoulder around the level of MHWS shown in Figure 6d).

The length of the principal low water channel of the Loughor estuary was determined from the 2003-08 LiDAR data to be 25673m, compared with a distance along the central line of the estuary of 20445m (Table 9). This gives an estuarine channel sinuosity index of 1.26. This value is approximately the same as the values calculated for the Taf, Towy and Gwendraeth estuaries, and has not changed greatly over the last 130 years (Table 10).

3.1.3 Historical bathymetric and coastal change Jones’ chart of 1757 is largely schematic (Figures 7a & 8) but does show the main low water channel running approximately through the middle to northern-section of the Middle Estuary and hugging the southern shore of the Inner Estuary between Pen-clawdd and Loughor Bridge. A large intertidal embayment is shown in the area to the east of Machynys Peninsula and wide intertidal flats are shown on both sides of the Middle estuary. The chart by Wedge (1808) is less distorted but not truly proportional (Figures 7b & 9). Three separate channels are shown in the Outer Estuary, converging to a single channel in the mid part of the Middle Estuary before bifurcating again further upstream. The main low water channel in the Inner estuary is still shown as hugging the southern shore. Several significant tributary tidal channels are shown, each connecting with land drainage outlets. The Admiralty Chart published in 1839, based on a survey by Denham in 1830, shows further improved accuracy and level of detail (Figures 7c & 10). This chart shows a relatively high proportion of standing water to intertidal area in the Outer Estuary but a very low proportion of standing water at low tide in the Middle and Inner estuaries. The main low water channel is shown to be markedly sinuous, following the southern shore between Loughor and Pen-clawdd, running close to the shore off Machynys, and then through the central part of the Middle Estuary towards the northern shore off 9 Burry Port. The later Admiralty Chart published in 1888, based on surveys in 1886-87 (Figures 7d & 11), shows that by the later 19th century the main low water channel had shifted northwards, running very close to the shore between Burry Port and Pwll before dividing into two branches, one extending eastwards towards Llanelli and the other running south-eastwards across the central part of the Middle Estuary before shallowing almost to the point of disappearance. A length of training wall, construction of which began in 1883, is shown extending in a SE – NW direction away from a point near Salthouse Point. The purpose of this embankment was to divert the ebb tidal flow in the direction of the entrance to Llanelli docks in order to increase clearance depths in that area (Captain MacMullen & Associates, 1982). In the Inner estuary, by 1886-87 the main low water channel had moved away from the southern shore and ran close to the shore west of the Loughor railway bridge.

Admiralty Chart 1076, published in 1955, (Figures 7e & 12) shows a greater extent of intertidal flats and banks in the Outer Estuary, with one main and two subsidiary tidal channels. In the Middle Estuary the main low water channel still ran close to the northern shore of the estuary between Burry Port and Pwll, with a significant extension towards Llanelli. No significant low water channels are shown upstream of the Machynys, Peninsula, although a training wall spur, built in the period 1908-13 to divert the ebb flow close to the shore near Llanelli docks, is shown. The revision of this chart, based on surveys mainly in the period 1976-89 (Figures 7f & 13), shows the re-establishment of two distinct low water channels in the Outer Estuary and a southward movement of the main low water channel in the Middle Estuary, away from Llanelli Docks. This shift occurred following the development of a significant breach in the training wall south of the Machynys Peninsula, close to the junction between the original training wall and the spur constructed in 1908-13. Upstream of this point the low water channel is shown to be shallow but still located relatively close to the northern shore of the Inner Estuary.

A bathymetric survey undertaken by Browing & Longdin in circa 1990, and drawn up in 1991 (Figure 7g & 14), shows a deterioration in the southern channel in the Outer estuary and an enlargement of the northern channel. In the Middle Estuary a significant low water channel remained along the Burry Port to Pwll frontage but the main low water channel of the Loughor no longer flowed into it, having formed a new channel to the Outer Estuary c. 2 km further south, mid-way between Whiteford Point and Burry Port. A deep scour pit is shown close to the un-repaired breach in the training wall south of Machynys. Although proposals were developed in the early 1980’s for the repair of the training walls (Captain McMullen & Associates, 1982), no measures were implemented.

A further map based on aerial photograph interpretation, undertaken as part of the CCW intertidal biotope survey between 1996 and 2005 (Figures 7h & 15), provides more detail than the earlier hydrographic charts and suggests a further increase in the extent of standing water (at approximately the MLWS level) in the Outer estuary, and a possible expansion of the low water channel system in the Middle and Inner estuaries. This may, however, partly reflect the greater level of topographic detail recorded by photogrammetric analysis of aerial photographs compared with the generalised bathymetric charts based on limited numbers of depth soundings, especially in shallow water areas. An even greater level of detail and apparent channel network extent is indicated by the 2003-08 LiDAR DEM (Figure 4), which shows the main low water channel now taking an almost straight course between the training wall breach and just north of Whiteford Point. Development of this channel across the central part of the Middle Estuary in recent decades has lowered the average tidal flat levels on Llanrhidian sands, leading to increased wave energy exposure along the edge of Llanrhidian and Landimore marshes and the northern tip of Whiteford spit.

10 Changes in the position of the median line of the main low water channel, as indicated on successive editions of Ordnance Survey maps and the 2003-08 LiDAR surveys, are shown in Figure 16. The progressive southward shift in the Middle Estuary since 1946-48 is clearly evident. The onset of this process coincides with the first major breaches in the training wall to the south of the Machynys Peninsula.

Changes in the extent and morphology of the Hills Tor - Whiteford barrier complex over the same period are shown in Figure 17. The northern tip of the spit has been cut back by > 100 m since the period 1968-72, although there has been easterly extension of the secondary spit features adjacent to Burry Pill over the same period. In 1878 the dune toe along the central part of the barrier lay some 200m seaward of its present position, following apparent accretion in the period 1825/6 – 1878. Net erosion of the central part of the barrier occurred in all successive epochs after 1878. However, the seaward position of the northern end of the barrier system has not changed greatly over the period. At the southern end of the barrier system net erosion also occurred between 1825/6 and 1968-72, since when renewed accretion has occurred to the easterly drifting of sand from the Hills Tor area.

Changes in the position of the shoreline and low water channels along the northern side of the Middle Estuary (Burry Port to Llanelli), indicated by successive Ordnance Survey map editions, are shown superimposed on the 2003-08 LiDAR DEM in Figure 18. The position of the shoreline (HAT) has not changed dramatically in this area since 1825-26 but there have been major variations in the positions of MHW and MLW, reflecting movements in the banks and channels. At the time of the 1879 survey a wide beach (Cefn Padrig) lay to seaward of the railway line, with a dune capped spit extending from the small promontory east of Burry Port power station (demolished in the 1980’s). By 1905 a northward movement of the channel had completely eroded this feature and significantly reduced the extent of Cefn Padrig. At the present day a relatively deep water channel runs immediately adjacent to the defences in front of the railway embankment.

Eastward drifting of sand from ‘The Nose’, past Pembrey Old Harbour and towards the western breakwater at the entrance to Burry Port has been occurring for decades but has become increasingly problematic in recent years. Sand now by-passes the end of the breakwater and has reduced the depth of water in the entrance channel to Burry Port yacht marina. However, there has been some trimming of the dune front at the eastern extremity of ‘The Nose’. A small channel now runs from Pembrey Old Harbour parallel to the shore before turning along the western side of the Burry Port breakwater. This channel effectively limits the transfer of sand from the nearshore sand bank to the beach and dunes to landward.

Changes in the positions of the shoreline and low water channels in the Inner estuary east of Salthouse Point are shown in Figure 19. The position of HAT has remained fairly stable since 1825/6 but there have been major changes in the positions of the lower water channels, and consequently in the distribution of tidal flats and saltmarshes. The largest change involved the northward movement of the low water channel away from the Pen-clawdd frontage towards the northern shore between 1825/6 and 1879. This process allowed rapid development of saltmarsh between Pen-clawdd and Gowerton during the first half of the 20th century and led to erosional trimming of the older saltmarshes (Morfa Bacas) fronting what is now the Wildfowl and Wetland Centre and sewage works. Since 1825/6 there has been progressive expansion of saltmarsh in the embayment between what is now Machynys Golf and Country Club and the southern extremity of the Wildfowl and Wetland Centre. Low water channel movements have not significantly impacted on this area.

11 3.1.4 Sediment volume changes As described in Section 2.8, an attempt has been made to obtain a quantitative estimate of the net sediment losses and/ or gains in the Outer Estuary and the Middle Estuary over the past century by digitizing the 1888 Admiralty Chart and comparing the resultant DEM with the 2003-8 LiDAR DEM and a DEM constructed using the 1990 Longdin and Browning bathymetric survey. The results (Table 11) suggest a net increase of 34039 x10 3 m 3 of sediment in the Outer Estuary and a net loss of 19855 x 10 3 m 3 above -4.0 m OD over the 120 year period 1888 - 2008. The error analysis described in Section 2.8 indicated a potential error of 12.4% for the sediment volume of the Outer Estuary and 8.1% for the sediment volume of the Middle Estuary, assuming a potential error of 40 cm in the Chart Datum corrections (Table 12). Errors of this magnitude do not change the basic indication of net sediment gain in the Outer Estuary and net sediment loss in the Middle Estuary over the time period considered.

3.1.5 Possible impacts of future sea level rise on tidal volume In order to assess some of the possible implications of future sea level rise on the estuary, tidal areas and prisms were calculated for a number of future sea level rise scenarios based on the UKCP09 projections. As discussed in Section 2, calculations were made for the years 2030, 2060 and 2100 (i.e. 40, 70 and 110 years after the baseline year of 1990, and approximately 20, 50 and 100 years into the future), for three future emissions scenarios (Low – SRES B1, Medium – SRES A1B and High – SRES A1FI). The UKCP User Interface was used to extract relative mean sea level rise values for the grid square closest to each of the Carmarthen Bay estuaries (Cell 23054 for the Loughor), for each emissions scenario, and for values equating to 5%, 50% and 95% of the range of climate model outputs. Calculations were made using the assumptions that (a) MHWS rises at the same rate as MSL, and (b) MHWS rises at twice the rate of MSL (see Section 2 for further explanation). The results are summarised in Tables 13 & 14. Under a ‘worst case’ scenario (MHWS rising at double the rate of MSL, High Emissions scenario, 95% frequency percentile), the mean spring tidal prism over the currently active estuarine area is predicted to increase from 219571 x 10 3 m 3 to 328568 x10 3 m 3 by 2100 (an increase of almost 50%). Using the more conservative assumptions of MHWS increasing at the same rate as MSL, a medium emissions scenario and 50% frequency percentile, the projected increase in tidal prism of the currently active estuarine area is 12.8%. Addition of the currently reclaimed areas around the Millennium Country Park to the current active estuarine makes very little difference to the projected increases in tidal prism, regardless of the scenario considered.

3.1.6 Hydrodynamic and sediment transport processes Moore (1976) reported maximum tidal current velocities approximately mid way between low and high water, with residual ebb current dominance in the main channel, increasing upstream from the mouth. Telemac modelling by Robbins (2009) indicated depth-averaged ebb dominance in the main channel and general flood-dominance across most of the tidal flats. Peak flood and ebb tidal velocities of 1.6 m s-1 and 1.9 m s -1, respectively, were suggested through the estuary mouth on a mean spring tide; maximum ebb residuals were found to reach 0.5 m s -1. Field measurements in the minor channels and across the tidal flats on the southern side of the middle and inner estuary also indicated flood dominance on both spring and neap tides (Elliott & Gardiner, 1981; Carling, 1981). Simulation of the effect of increased tidal prism due to sea level rise, 20, 50 and 100 years from now suggested significantly increased tidal current velocities (up to 50%) and increased ebb residual flow on the outer salt marsh edges along the southern shore of the estuary and at Pembrey. The implications of these increases in tidal current velocities and residual currents are increased potential for saltmarsh edge erosion, widening of the estuary mouth due to increased erosion at Whiteford Point and Pembrey Burrows, and net transfer of sediment from the Middle Estuary into the Outer Estuary. 12 However, the modelling results also indicated that net sediment transport rates are likely to be highly dependent on the degree of channel stability, and in particular on the condition of the Machynys training wall. Model runs with modified bathymetry which incorporated repair of the main breach and reconstruction of the training walls to 0 m OD indicated a major reduction in tidal current velocities and ebb-directed sediment transport. This would in effect represent a return to the condition which prevailed in the estuary between the 1880’s and the mid 20th century, in the period following initial construction of the Machynys training wall. Accretion of sediment during the period 1830 – 1880 was also encouraged by the reduction in estuarine tidal prism which occurred as a result of 19th century embanking and land reclamation. No hydrodynamic model runs have so far been performed using the 1888 bathymetry, and the extent / distribution of ebb and flood dominance at that time is uncertain. However, there can be little doubt that construction of the training wall after 1882 was a major fact in allowing further build-up of sediment in the central part of the Middle Estuary, while initiating increased scour and sediment loss from the northern side of the Middle Estuary. Since breaching and abandonment of the training wall in the mid 20th century the re- establishment of a major channel through the breach has led to removal of a large part of this accumulated sediment in the central part of the Middle Estuary.

If, in the future, no measures are taken to reinstate the training wall, the effect of significant sea level rise (>30 cm) over the next century can be expected to include increased intertidal and sub-tidal erosion in the Middle estuary, widening of the estuary entrance due to erosion at Whiteford Point and ‘The Nose’, and erosion of the saltmarshes along the northern edge of Llanrhidian Marsh. However, a breach through the dune barrier south of Whiteford Point is unlikely, given the stability imparted to this area by the glacial moraine deposits beneath and seaward of the dunes. Continued eastward drift of sand eroded from the seaward end of Whiteford Burrows can be expected, with the development of sand spits and low washover dune complexes in the lee of Whiteford Point (i.e. the Berges Island area). This will provide increased shelter for Landimore Marsh. Similar eastward drift of sediment derived from The Nose is likely to lead to continued problems of siltation near the entrance to Burry Port marina. The rate of intertidal sediment drift by itself is not likely to restore the upper foreshore between Burry Port and Llanelli. However, in a situation where channel migration is unconfined by training walls, rapid changes in the size and position of any of the low water channels is possible, leading to rapid foreshore accretion or erosion. Such channel fluctuations are difficult to predict and could affect any part of the estuary.

3.2 The Three Rivers estuarine complex 3.2.1 General morphological character and sub-divisions The Three Rivers estuarine complex is a composite estuary which has a digitate form in plan view (Figure 20). The combined tidally active area of the estuarine complex (at MHWS level) is 3,561 ha and the corresponding tidal prism is 10,6368 x 10 3 m 3 (Table 15). These values represent 43% and 52% of the equivalent values for the Loughor estuary, respectively. The complex physical form of the estuary complex represents the interplay of a variety of factors, including the distribution of hard rock outcrops, the history of late Cenozoic river incision and glaciation in the region, and the pattern of coastal and estuarine sedimentation during the post-glacial period. For the purposes of the present study, the following system components have been identified within the area:

• The Taf estuary

• The Towy estuary

13 • The Gwendraeth estuary

• The Taf -Towy - Gwendraeth confluence area

• The Pendine - Laugharne barrier and back-barrier area

• The Tywyn Point - Pembrey Burrows barrier and back-barrier area

Around 5000 - 6000 years ago the shoreline lay well inland of its present position, close to the line of high ground extending between Pendine and St. John’s Hill, south of Laugharne, between St. Ishmael’s Scar and Morfa Bach, east of Kidwelly, and between Pen-y-Bont on the south side of Gwendraeth Fawr and Burry Port. The former shoreline is marked by degraded cliffs, sea caves and coarse-grained fossil beach deposits. After 5000 yr BP extensive barrier beach – dune systems, with back-barrier saltmarshes and tidal flats, developed between Pendine and Ginst Point, and between Tywyn Point and Pembrey. The stratigraphy and age structure of the barrier and back-barrier sediments is not well-established, since only very limited drilling and dating investigations have been undertaken. Cantrill (1909) described shell mounds on Laugharne Burrows which he believed had been occupied intermittently since Iron Age times. However, there is little direct evidence to support such antiquity and the majority of the associated artefacts are of medieval or post- medieval age (James, 1991). It is likely that Laugharne Burrows developed as a barrier island (Jago, 1975), possibly at a much later date than Pendine Burrows. The former sea cliffs behind Pendine Burrows are considerably more degraded than thouse behind Laugharne Burrows (Savigear, 1952; Jago, 1975; Burt, 2003; Walley, 1996), suggesting that the Pendine Burrows part of the barrier, which partly overlies a foundation of glacial deposits, may be considerably older than the Laugharne Burrows section.

The beach at Pendine and Laugharne is 1km to 1.5 km wide and relatively flat, being composed of medium to fine, well-sorted sand. It experiences a relatively high wave energy regime and easterly residual longshore currents (Jago & Hardisty, 1984). Behind the dune barrier is an extensive area of reclaimed marshland (West Marsh and East Marsh). The marshes have evolved in the later Holocene in response to the evolution of the barrier system. Tidal flooding of the eastern end of East Marsh is prevented by a low ridge of dunes and an earth embankment, seaward of which lies a narrow belt of active saltmarsh. The first sea banks around the head of the Witchett Brook and between Sir John's Hill and the eastern end of Laugharne Burrows were apparently constructed in the mid 17th century (James, 1991; Walley, 1996). Further embanking was undertaken in the early 19th century, including construction of the ‘Freething’ wall outside an existing older wall between St John’s Hill and Ginst Point.

3.2.2 The Taf estuary

(a) General character The Taf and Towy rivers occupy glacially over-deepened valleys which are incised into Old Red sandstone and are partially in-filled with glacial drift and Post-glacial alluvium. Marine reworking of the glacial deposits has formed ‘scars’ (areas of till and outwash mantled boulder lags) near the river mouths (St Ishmael's Scar, Salmon Point Scar and Pastoun Scar). Elsewhere the intertidal zone is dominated by sandy sediments of varying thickness and texture (Jago, 1980). Salt and brackish marshes occur in more sheltered areas and consist mainly of sandy silt and silt.

The Taf is tidal as far as the outskirts of St. Clears (Figure 1). Parts of the meandering tidal channel are backed by bedrock slopes while others are fringed by brackish marsh or embankments. Significant areas of 14 saltmarsh occur only downstream of the Taf - Cywyn confluence (Figure 21; Table 16). A number of cross- sections across the Taf valley are shown in Figure 22. The principal low water channel length of the Taf determined from the 2003-8 LiDAR is 14883 m, approximately two-thirds that of the Towy and more than twice that of the Gwendraeth (Table 9). The sinuosity indices of all three rivers are similar to each other and to the Loughor (Table 10).

Tide levels increase upstream from the mouth of the estuary. The median surface elevations of active mature marshes also increase in an upstream direction, reaching a maximum of over 5.0 m OD just upstream from St Clears on the Taf (Table 16). The youngest marshes near the mouth of the Taf have median surface elevations in the range 3.6 to 3.7 m OD.

The tidal volume above -0.5 m OD in the Taf estuary shows a steep rise above c. 3.5 m OD, reflecting the sudden increase in tidally flooded area as the marshes become immersed by the tide (Figure 23a & b). The calculated maximum rate of change in tidal volume for a high spring tide reaching 5.19 m OD occurs approximately 45 minutes before high water at the time when the highest marshes become flooded (Figure 23c & d). The maximum rate of tidal volume reduction occurs approximately 90 minutes after high water when water surface slopes are likely to be greatest and ebb velocities greatest.

The tidally floodable areas and tidal prisms below various tidal datum levels in the Taf are compared with those of the main reclaimed marsh areas in Table 17. The reclaimed marshes behind the Pendine – Laugharne barrier are extensive but are relatively high in the tidal frame (median surface elevation range 3.55 to 3.92 m OD). Consequently the tidal prism at MHWS level is less than might be expected.

Projected changes in tidally floodable areas and tidal prisms under the different UKCP09 scenarios are shown in Tables 18 & 19. In the worst case, with MHWS rising at twice the projected in crease in MSL and taking the 95 th model prediction percentile for the high emissions scenario, the tidal prism of the presently active estuary at MHW is projected to increase by 14520 x 10 3 m 3, or circa 70%, by 2100 compared with the present value. In the less severe case of MHWS increasing at the rate of MSL, and taking the 50th percentile of the Medium Emissions scenario, the MHWS tidal prism is projected to increase by 3436 x 10 3 m 3, or circa 17%, by 2100 (Table 19).

(b) Historical channel and shoreline movements Changes in the position of the main low water channel of the Taf, indicated by successive editions of Ordnance Survey maps and the 2003-08 LiDAR surveys, are shown in Figure 24. The total length of the main low water channel (median line) has shown only limited variation overt the period (Table 9) and there has been no significant net change in sinuosity index since 1879-87 (Table 10). However, the position of the channel within particular sections of the valley has varied considerably over time. For example, at Laugharne the low water channel lay only a short distance offshore in 1887 1949 and 2003-8, but was located towards the opposite shore at the time of the 1905 and 1965 surveys. However, with the exception of 1905, the low water channel was located close to Wharley Point at the time of all of the surveys (Figures 24 & 25).

Changes in the position of the Pendine – Ginst Point shoreline since the early 19th century are shown in Figure 25. The Ordnance Survey One-Inch map, based on a survey of 1825-86, shows two separate barrier dune systems (Great Hill Burrows and Laugharne Burrows), separated by a tidal inlet (Whitchatte Pil). A dam was constructed across the mouth of this inlet (later named Witchett Brook) some time after 1825-26 and before 1879, possibly in the late 1840’s (James, 1991). Most of the shoreline experienced progradation between 1825 and the mid 20th century, after which time the progradational trend changed to one of 15 shoreline recession. Following the construction of the dam across the entrance to Witchett Pill, the area in front became progressively in-filled with sediment between 1905 and 1969-71, by which date a foreland covered with low dunes, with a maximum width of c. 800m, had developed. In 1969-71 the dune toe (HAT) lay well to seaward of the positions shown in earlier surveys along virtually the entire shore except at Ginst Point, where there was net erosion between 1905 and 1969-71. The backshore along most of the dune frontage in 1969-71 was very narrow was very narrow and the dunes along the Pendine Burrows frontage were cliffed. Since that time most of the frontage has shown net recession of between 10 and 500 m, although there has been slight accretion in some areas near Ginst Point where coastal defence structures were put in place by the MOD during the 1970’s.

The LiDAR DEM of the Pendine-Laugharne barrier (Figure 25) shows that the dune belt at Pendine Burrows is narrower than at Laugharne Burrows, and that the Laugharne Burrows are more dissected with better- developed blowouts and partially transgressive dunes. This partly reflects the greater net sediment accumulation at the distal end of the littoral drift cell and partly the greater degree of wind exposure away from the surrounding high ground at Pendine. The seaward edge of the high transgressive dunes in both areas is marked by a distinct line which appears to slightly pre-date the 1825-6 shoreline position. Although direct dating evidence is lacking, this maximum limit of erosion and the period of large blowout dune development may have been associated with a particularly stormy period in the later Little Ice Age. The dunes provide no morphological evidence for further extremely stormy periods in the period 1826 – 1950, and it is noteworthy that renewed shoreline erosion since the late 1950’s has been accompanied by the development of a slowly retreating shore-parallel ridge, with only small, localised blowouts, rather than with the development of new large blowouts and transgressive parabolic dunes. This observation is consistent with a hypothesis that the post late 1950’s period has been less windy than certain times during the Little Ice Age.

In 1825-6 a prominent low water channel of the Taf ran approximately 500 m offshore, semi-parallel to the St. John’s Hill – Ginst Point flood embankment. After that date the seaward end of this channel moves offshore towards Wharley Point, where it has remained ever since. This has allowed the eastwards extension of Ginst Point and has encouraged saltmarsh development in the now more sheltered area in front of the St John’s Hill – Ginst Point embankment. However, after 1969-71 the low water channel south of Ginst Point again moved north-westward, leading to steepening of the foreshore and placing localised pressure on the low dunes and coastal defence structures at Ginst Point.

(c) Hydrodynamic and sediment transport regime Repeated topographic survey measurements by Jago (1975, 1980) suggested a net vertical sediment accretion rate of 0.13 m a-1 in the Taf estuary during the early 1970’s. Jago (1980) reported that the flood tide is considerably shorter than the ebb, with fast flood currents at the estuary mouth (1.8 m s -1) as the flood tide enters the estuary. Ebb currents were reported to reach 1.2 m s -1 on spring tides as the flow becomes concentrated within the low water channel, 3 to 4 hours after high water. Short-term process studies by Ishak (1997) indicated that net flood-tide directed transport of suspended sediment in the Taf is 10-30% greater than the net ebb transport, which translates into a vertical accretion rate of 1.2 to 1.6 cm a -1. Vertical sedimentation rates on active saltmarshes within the Taf were found to range from 0.4 to 1.7 cm a -1. The available evidence therefore suggests that the sediment balance of the estuary in the later 20th century has been positive, with net import of sediment into the estuary. The mineralogical and textural character of the sediments strongly suggest a principal source in Carmarthen Bay, although a significant proportion of the mud fraction is probably derived from land-based sources (Jago, 1975, 1980).

16 3.2.3 The Towy estuary

(a) General character The Towy is the longest of the Three Rivers and has a relatively large freshwater discharge compared with the Taf and Gwendraeth. The river is tidal to a point approximately 3 km upstream of Carmarthen and tidal elevations increase upstream from the estuary entrance. The MHWS level at Ferryside (4.2 m OD) is approximately 40 cm lower than at Carmarthen (Table 1).

The central part of the estuary is confined by bedrock, and by the Llanelli to Carmarthen railway line, but the upper estuary opens up with a relatively wide floodplain near Carmarthen (Figure 26). Here the tidal river is confined by flood banks backed by grazing marshes. Cross sections at different locations up the estuary (positions in Figure 20) are shown in Figure 27. Saltmarsh occurs throughout the estuary, located in small embayments, on the inside of meander bends, and behind low sandy barriers in the lower part of the estuary. The median elevations of mature marshes increase up estuary in parallel with average tidal levels, and generally lie 0.2 m to 0.5 m above the level of MHWS (Table 20). The lowest, immature marshes in the lower part of the estuary have median elevations in the range 3.68 m OD to 3.75 m OD, c. 0.25 - 0.30 m higher than the level of MHW.

The Towy has a relatively small tidally floodable area at the level of MHWS and HAT (781ha and 973 ha, respectively, Table 21) owing to the restricted extent of tidal flats and saltmarsh within the estuary. The mean spring tidal prism (22681 x 10 3 m 3) is similar to that of the Taf. The curve showing tidal volume above -0.5 m OD shows a slight break of slope at a tidal elevation of c. 4.30 m OD, slightly above the level of MHWS (Figure 28a & b). The rate of change of tidal volume is relatively constant for much of the flood and ebb tide, reflecting the limited effect of saltmarsh ‘floodplain’ storage relative to the total estuarine tidal prism (Figures 28c & d).

The potential effects of different UKCP09 scenarios on the tidally floodable areas and tidal prism of the estuary are summarised in Tables 22 & 23. In the worst case scenario, the spring tide tidal prism of the presently active estuary could increase by c. 60% by 2100. Under the less extreme situation where MHWS increases at the same rate as MSL, under the medium emissions scenario and taking the 50th model output percentile, a potential increase in spring tidal prism of c. 14% is projected (Table 23).

(b) Historical channel and shoreline change Analysis of historical maps has shown that the position of the main low water channel has varied considerably in the lower estuary but has remained more or less constant in the middle and upper parts of the estuary (Figures 29 & 30). Even in the lower estuary, the lateral shifts in channel position have been less marked than in the Taf estuary. However, such shifts have been significant in terms of erosion pressure at Ferryside and at the northern end of Morfa Uchaf. At the present time the main low water channel lies close to the shore near the slipway and pier at Ferryside. Waves driven by south-westerly winds propagate up the deep water channel of the estuary but at the margins the wave crests are refracted towards the east shore around Ferryside, facilitating net northward drift of sediment. The beach on the seaward side of the railway line has experienced net erosion in the past 40 years due to progressive eastward movement of the low water channel and increase in channel sinuosity in this length. However, considering the entire channel length between the estuary entrance and the normal tidal limit, the total channel length has increased only slightly and no significant increase in sinuosity is evident (Tables 9 & 10). A similar conclusion was reached by Bristow & Pile (2003).

17 Shoreline Management Partnership (1991) suggested that the channel and foreshore changes leading to erosion at Ferryside might be linked to variations in the balance between tidal forces and freshwater flows in the vicinity of Morfa Uchaf, with periodic periods of high freshwater flow being capable of ‘flipping’ the channel meander pattern downstream, especially if coincident with storms from the southwest which drive tide and wave energy landwards. This is a reasonable hypothesis but has not been proven by direct evidence. Estuarine channel patterns are inherently unstable and, in the absence of training walls, often display oscillation on decadal time-scales due to both to intrinsic and extrinsic factors.

(c) Hydrographic and sediment transport processes Only very limited information is available concerning hydrodynamic processes and sediment transport in the Towy estuary. Field studies were undertaken by Jones (1977) but the results have not been published. However, the available evidence suggests that the Towy, like the Taf, is flood dominated and that there has been long-term net-sediment movement into the estuary. Wave and wave-induced current transport is more important in the Towy than in the Taf on account of the greater exposure of the estuary mouth to the direction of prevailing winds and wave approach (south-westerly). Geomorphological evidence of long-term net northward longshore drift of sediment along the shores of the lower estuary is provided by spit features, capped by low dunes, at Lanstephan, Ferryside, and Morfa Urchaf.

The Towy has a length of 121 km, rising on the slopes of Crug Gynan in the Cambrian Mountains. Historically, the river has experienced significant floods following periods of heavy rain, but since the construction of the Llyn Brianne dam and reservoir, 10 km from the source, in 1972, peak flows have been subdued to some degree. Nevertheless, high river flows continue to play an important role in determining movements of the low water channel, especially in the upper and middle parts of the estuary.

3.2.4 The Gwendraeth estuary

(a) General character The Gwendraeth estuary is fed by two separate rivers, the Gwendraeth Fach and the Gwendraeth Fawr. The Gwendraeth Fach is a small river which is tidal as far as the A484 north of Kidwelly. Today the Gwendraeth Fawr is tidal only below Commissioner's Bridge, south of Kidwelly, which isolates a large area of reclaimed former marshland from tidal interaction (Figure 31). The intertidal area of the estuary was considerably larger before canalization of the upper Gwendraeth Fawr and construction of the Llanelli to Carmarthen railway line in 1851-2 (James, 1991).

The present tidally active estuary has a broadly rectangular form with an east-west long-axis. The width of the entrance has been significantly reduced over the past 60 years by the northwards and eastwards movement of Tywyn Point. At the present day the entrance is approximately 1.2 km wide between Tywyn Point and the small sedimentary foreland occupied by the Carmarthen Bay Holiday Park. The maximum width of the estuary in its middle part is approximately 2.8 km. The maximum length of the tidally active low water channel is 6661 m, a figure which has not changed significantly since 1879-87 (Table 9). The sinuosity index of the Gwendraeth estuary is low (1.31) by comparison with the Taf, Towy and Loughor (Table 10).

Sections across the Gwendraeth are shown in Figure 32. The main channel is today located close to the northern shore of the estuary where the intertidal profiles are generally steep compared with the much gentler gradients on the southern side of the estuary. The northwest corner of the Gwendraeth estuary has a

18 relatively high exposure to wave energy and the shoreline is consequently dominated by a gravel and boulder upper beach backed by low dunes, with a foreshore composed of mixed sand and gravel. Further to the north, at confluence with the Towy estuary, wave erosion of glacial deposits in the intertidal zone has formed ‘scars’, intertidal platforms composed of glacial till with a patchy boulder or gravel surface lag deposits. The back-beach in this area was formerly characterized by more extensive estuarine dune development, banked against the steep slopes behind, but most of the dune sediments have now been eroded.

Much of the estuary is shallow on account of extensive development of sandflats and saltmarshes. A large proportion of the active marsh area is comparatively recent and the surface elevations are relative low relative to the tidal frame (Table 24). The older marshes near the landward margin of the active estuary have median surface elevations which are closer to, but still below, the level of MHWS. The reclaimed marshes on the landward side of the railway and A484 have median elevations ranging from below present MHW to just below MHWS.

The area of the active estuary at the level of MHWS (839ha) is slightly larger than that of the Taf and the Towy, but the mean spring tidal prism (13674 x 10 3 m 3) is considerably smaller (Table 25).

The tidal volume of the estuary above -0.5 m OD shows a step increase at c. 3.5 m OD due to sudden increase in tidally floodable area as the tide rises above the level of the marshes (Figure 33a & b). The maximum rate of increase in tidal volume occurs midway through the flood on an extreme high tide, at the point where the tidally floodable area suddenly increases (Figure 33c & d).

Consideration of the UKCP09 sea level change projections indicates that, in the worst case scenario, the tidally floodable area at MHWS is likely to increase by 22.3% by 2100 (Table 26). The corresponding increase in tidal prism is 106.3% (Table 27). Under the less extreme medium emissions scenario, taking the 50% model output percentile and with MHWS increasing in line with MSL, the increase in tidally floodable area is projected to be 6.9% and the increase in MHWS tidal prism is projected to be 31.3% by 2100.

(b) Historical channel and shoreline change Analysis of historical Ordnance Survey maps has shown that the main low water channel of the Gwendraeth Fawr has shown major lateral changes in position since 1879, although the upstream and downstream limits have remained almost fixed and there was less change between 1965 and 2003-08 (Figure 34). This was associated with significant growth of new marshes on the south side of the estuary in the later period.

(c) Hydrodynamic and sediment transport processes Little information is available regarding hydrodynamic processes and sediment transport in the Gwendraeth estuary. Based on the morphology of the estuary and the major tidal channel features, flood flow dominance and net landward sediment transport would be expected. Historical evidence indicates long-term net sediment accretion in the estuary since the 17th century, when Kidwelly was a significant port. The results of mineral magnetic studies (Booth, 2002; Booth et al., 2005) have suggested that Carmarthen Bay has provided the main source of sediments (circq 77%) which have accumulated in the Gwendraeth estuary, but that secondary contributions from land-based sources via the Gwendraeth Fach and Gwendraeth Fawr have also been significant (13% and 10%, respectively).

19 3.2.5 The Pembrey barrier system The southern side of the Gwendraeth estuary is protected from wave action by a broad (1 km to 2 km) belt of sand dunes, parts of which are used as an MOD firing range and as a Forestry Commission plantation (Figure 35). These dunes form part of a large coastal barrier system which extends south-eastwards to Burry Port. The central part of the barrier is more than 2.5 km wide and is backed by and extensive area of reclaimed saltmarsh. Much of the northern part of the barrier falls within Pembrey Forest but the southern area, once the location of a Nobel dynamite factory and Royal Ordnance factory, now hosts the Pembrey Country Park (Ladd, 1992; Cadw, 2002-03).

The stratigraphy and age structure of the Pembrey barrier system has not been investigated in detail, but available geological and historical evidence suggests that the central part evolved as a barrier island superimposed on a thick sequence of Holocene marine sediments which extend up to 30 m below sea level (Kahn, 1968). Beach gravel deposits have been encountered at c. – 7m OD and – 12 m OD in boreholes near Burry Port, suggesting high energy shorelines in this area around 7000 years ago (Bowen, 1980, p155).

The First Edition One Inch Ordnance Survey map shows that the Pembrey Burrows barrier was much shorter than at present. The barrier had a rather blunt form at both ends, marked by Tywyn Point and ‘The Nose’. Since that time the northern limit of Tywyn Point has moved approximately 1.5 km northwards, while the tip of ‘The Nose’ has advanced approximately 3 km south-eastwards. The barrier has also shown net seawards progradation, ranging from c. 100 m in the centre to > 600 m at the northern end and >1.5 km at the southern end.

By the mid 17th century the more landward parts of the marshes behind the barrier were already embanked and reclaimed, a process which probably began in the Middle Ages (James, 1991). A major sea bank, known as ‘The Bulwarke’, was probably constructed in the early 17th century, possibly as early as 1629. Embanking of the active marshes further to the north was undertaken in the mid 18th century and in 1817-18 when another major bank, the ‘Banc–y-Lord’, was constructed (James, 1991). As in other parts of the country, it is likely that the processes of embanking and marsh enclosure resulted in additional sedimentation and marsh development on the outside of the embankments, owing to a reduction in estuarine tidal prism and tidal current velocities across the inter-tidal flats.

No radiocarbon or luminescence dates have been obtained from the Pembrey Burrows complex. Shell middens similar to those at Laugharne Burrows are present but the earliest associated artefacts appear to be pottery sherds of late 13th or 14th century date (James, 1991).

The LiDAR DEM of Pembrey Burrows displays the presence of four high dune ridges in the northern part of the complex; these merge into two main ridges in the central and southern parts of the barrier (Figure 35). The exact time of formation of the individual ridges is uncertain, and they probably overlie aeolian sands of even greater age. However, the pattern of ridges bears testimony to alternating periods of shoreline progradation and erosion, with periods of partially transgressive high dune ridge building being coincident with periods shoreline stability and/ or erosion (cf. Pye, 1990). During periods of shoreline progradation new foredune ridges and sandplains were formed in the centre of the barrier, with spits capped by low foredunes and intervening swales formed at the distal ends of the system. The morphological evidence provided by the LiDAR suggests that initial development of the complex involved the development of a low spit which was attached to the mainland just to the north of Pembrey village. The landward end of the spit system was orientated broadly WNW-ESE and may have been composed of fairly coarse grained sediments which

20 allowed only limited low dune development, while the more distal part of the spit had a more NW-SE orientation and was characterised by higher dune development. Subsequently, sediment brought onshore from sources in Carmarthen Bay was evidently drifted both to the north and south, forming new dune ridges with a broad NW-SE orientation. The greater width and greater spacing of the dune ridges in the northern part of the barrier complex testifies to greater long-term sediment accumulation in this area compared with the southern part of the complex. The development of new spit-like ridge extensions at both ends of the system between 1946-8 and 1968-69 suggests rates of longshore sand transport, in both directions away from the centre of the complex, were higher in this period than previously (or since). The late 1950s and 1960’s was a relatively cooler, stormy period which resulted in accelerated dune erosion and long-shore drifting at many British west coast localities (e.g. Pye & Blott, 2008).

A map of 1762 date shows that an area called ‘Black Marsh’, west of Pembrey village, was then not enclosed and may still have been subject to periodic tidal incursion (James, 1991). The northern limit of Black Marsh is defined on the LiDAR DEM as an arcuate zone of slightly higher ground, approximately 500 m wide (Figure 35), across which the access road from Pembrey village to Pembrey Burrows now runs. By the time of the first Ordnance Survey One Inch map this area had been cut off from the sea by the growth of a new arcuate beach and low dune ridge which had by then joined the southern end of the main Pembrey Burrows barrier complex to the narrow coastal plain near Burry Port. Although no direct dating evidence is available, it is possible that this secondary barrier system formed during a stormy period of the 18th century when erosion of the dunes further north occurred and significant quantities of sand were transported southwards and eastwards by longshore drift. This section of the shore then evidently remained fairly stable until the 1950’s, when the rate of longshore drift again increased, leading to rapid eastward movement of ‘The Nose’ and formation of the enclosed Pembrey Marsh.

3.2.6 The Three Rivers confluence area The area landward of the line between Ginst Point and Tywyn Point and the entrance to the individual estuaries is referred to in this study as the Three Rivers confluence area (see Figure 20). The areas and tidal prisms corresponding to different tidal datum levels in this area are shown in Table 28. The estimated spring tidal prism within this area (49373 x10 3 m 3) represents approximately 46% of the mean spring tidal prism volume for the combined Three Rivers estuarine complex as a whole (Table 15). Owing to the very limited development of saltmarshes on the margins of this area, the tidal volume above -0.5 m OD increases in a near-linear manner as a function of tidal level (Figure 36a) and the rate of change of tidal volume is almost constant above the level of MHWS (Figure 36b). On a high spring tide the maximum rate of tidal volume change coincides with the steepest part of the tidal stage curve, close to the level of MHWN (Figures 36c & d). Comparable graphs for the Three Rivers estuarine complex as a whole, presented in Figure 37, are very different, emphasising the importance of high intertidal flats and saltmarshes within the individual estuaries.

The potential effects of different UKCP09 sea level rise scenarios on the tidally floodable areas and tidal prisms of the confluence area, and on the estuarine complex as a whole, are shown in Tables 29 to 32. With the worst case scenario the mean spring tidal prism in the confluence area could increase by circa 35% by 2100, compared with 77% for the currently active area of the estuarine complex as a whole. The respective figures for the less extreme medium emissions scenario, 50th model projection percentile, with MHWS assumed to increase in line with MSL, are 11% and 22.4%.

21 3.3 Historical bathymetric change in Carmarthen Bay Bathymetric survey coverage of the Three Rivers estuarine complex and northern Carmarthen Bay is less comprehensive than for the Loughor estuary, mainly because of the more limited navigation interest. William Morris’s chart of 1800 is rather schematic but shows a single low water channel running from the confluence area in an approximate east - west direction along the Laugharne - Pendine frontage, creating a very wide expanse of sand (Cefn Sidan) opposite the northern end of the Pembrey Burrows system. The Admiralty Survey of 1830 shows a broadly similar pattern but with two major low water channels between Ginst Point and Tywyn Point (Figures 38a & 39). This chart also shows a significant separate low water channel from the Gwendraeth estuary crossing Cefn Sidan, and a smaller channel linking with Witchett Brook. The Admiralty survey of 1886-87 shows that two relatively wide channels seaward of the Ginst Point - Tywyn point line converged landward into a single channel, and that the separate channel across Cefn Sidan from the Gwendraeth had dried up (Figures 38b & 40). The Witchet approach channel had also reduced in size by this time and a single channel had developed in the lower Taf, running close to Wharley Point before joining a single Towy low water channel. The ebb tidal delta above chart datum level had reduced in area compared with 1830 but remained large. By 1955 the Taf - Towy confluence had moved further south and eastwards, in the direction of Tywyn Point, and the low water channel off Laugharne Burrows had moved closer to the shore. Significant ‘blind’ low water channels had also developed on Cefn Sidan sands (Figures 38c & 41). The situation shown on the 2001 edition of Chart 1076, which was only partially revised by new surveys in the 1970’s, was not very different, although there had evidently been shoaling of the eastern part of the channel off the Laugharne – Ginst Point frontage and straightening of the main low water channel along the north edge of Cefn Sidan Sands (Figure 42). In the late 1990’s the main channel remained located in the middle of the estuary entrance while the area of Cefn Sidan sands had further reduced (Figure 38d).

The available historical map and chart evidence provides some evidence for a quasi-cyclic pattern of channel shifts in the Three Rivers confluence area and the in the lower parts of the estuaries themselves. However, there is no clear evidence for a single driver of such changes. With regard to changes in the estuary confluence area, it is possible that period of strong easterly or westerly sediment drift, driven primarily by wind-generated wave action, causes extension of the intertidal banks in one or other direction until such time that the proximal parts of the flats becomes narrow or low enough to allow a new channel to break through near the 'neck', thereby causing the main low water channel to 'flip between the Cefn Sidan and Ginst Point sides of the estuary entrance. However, the paucity of long-term wind, wave and sediment transport data do not allow rigorous testing of this hypothesis at the present time. Equally, there is insufficient substantive evidence to test an alterrnative hypothesis that fluctuations is river discharge are able to bring about changes in the pattern of low water channel meanders which progressively migrate seawards.

3.4 Hydrodynamic processes and sediment transport in Carmarthen Bay Hydrodynamic modelling undertaken during the 1988’s as part of the Carmarthen Bay Study (BMT Ceemaid, 1985, 1988, 1989; Barber and Thomas, 1989) indicated that maximum tidal current velocities in the main Bay area exceed 1 m s -1 on both flood and ebb. This work also indicated a small residual ebb flow in the main channels of all three estuaries, but this conclusion is contrary to the field results obtained by Jago (1975, 1980). Based on this study, Barber & Thomas (1989) suggested that the change from accretion to erosion on the north - central part of the Cefn Sidan - Pembrey frontage after circa 1960 may have been caused by changes in the bathymetry of neighbouring parts of Carmarthen Bay – specifically an eastward movement of the main low water channel towards Tywyn Point. This suggestion is to some extent supported

22 by the historical chart and map evidence discussed earlier in this report, which indicates that changes in the number and positions of low water channels in the Three Rivers confluence area have had a major effect on the timing and pattern of shoreline accretion and erosion on both sides of the estuary entrance. However, beach and dune erosion also affected the Pendine and Ginst Point frontages in the late 1960’s and 1970’s suggesting that increased storm forcing may have been a more significant factor.

Geophysical investigations have suggested that the floor of Carmarthen Bay is covered by up to several metres of unconsolidated Quaternary sand deposits which thicken towards the northern and eastern sides of the Bay (Al-Ghadban, 1986). The modern sediments overlie glacial till in many areas and infill palaeo- valleys which extend from the mouths of the Burry Inlet and the Three Rivers Estuarine complex. The sea bed lacks major bedforms, although megaripples are present in deeper water. Large sandwaves with approximately N-S crest orientations are present outside the Bay. On the eastern side of the Bay, close to Worms Head, a is a major sub-tidal linear sand bank (Helwick Bank), almost 14 km in length and 1.2 km in width at the 15 m CD isobath. Detailed investigations have indicated a complex circulation pattern of flows and sediment transport around this bank, with limited leakage of sediment past Worms Head into Rhossili Bay and Broughton Bay (Posford Duvivier & ABPmer, 2000). Carmarthen Bay can therefore be regarded as essentially as a ‘closed system’ in terms of sediment supply.

Based on sediment grain size trend analysis, McLaren (1999) and Posford Duvivier and ABPmer (2000) concluded that there is net sediment transport out of the Taf and the Towy but net import into the Gwendraeth estuary. A complex sediment circulation pattern within Carmarthen Bay was suggested, involving radiating sediment transport pathways which emanate from three major sediment parting zones in Carmarthen Bay and three major meeting zones. It was suggested that extreme events (major storms) load the parting zones with sediment, following which regular transport processes re-distribute the sediment produce the observed patterns of transport (Posford Duvivier & ABP, 2000; Cooper & McLaren, 2007). It was also suggested that the parting and convergence zones may be related to resonance features produced by the prevailing hydrodynamics (tidal, wave and wind-driven) in the Bristol Channel as whole. It was concluded that there is little modern day input of sediment to Carmarthen Bay from rivers or from offshore, although there is apparently some supply from the Bristol Channel around Helwick Bank; the Bay therefore acts almost as a closed system, with recycling of sediments already in the bay during and following major storm events. Active sediment transport from eastern Carmarthen Bay into the Loughor estuary (Burry Inlet) was suggested, although the flux was not quantified. Cramp et al. (1995) reported almost zero net sediment flux within the Loughor estuary, but comparison of historical map data led Bristow & Pile (2003) to conclude a net reduction in area of the estuary from 8254 ha in 1876 to 7060 ha in 2000. Part of this reduction was due to embanking and land-claim, but also partly due to net sediment accretion in the tidally active part of the estuary. This finding is supported by the results of the present study. The mineralogical and micro-faunal composition of the sediments strongly suggests that the principal source of sediment lies outside the estuary in Carmarthen Bay (Carling, 1978; 1981).

4 Conclusions

A full understanding of the interactions between physical processes and coastal morphological change in Carmarthen Bay and adjacent estuaries is hampered at present by a paucity of long-term baseline data relating to bathymetric change, wind and wave regime, rainfall, river discharge, sediment sources, transport pathways and sediment fluxes. Useful contributions to understanding were made by the Carmarthen Bay

23 Study in the late 1980’s, by the Bristol Channel Marine Aggregates Study in the late 1990’s, and by a several research projects undertaken by the Universities of London, Bangor and Swansea between the 1970’s and the present, but there remain many questions relating to cause and effect which cannot be answered adequately with the available data.

The offshore areas of Carmarthen Bay and Three Rivers estuarine complex, in particular, suffer from a lack of recent hydrographic survey data, and available data for the Loughor estuary have significant short- comings. There is a general lack of geophysical and borehole data which can be used document the stratigraphy, age structure and evolutionary history of the principal coastal barriers and back-barrier sedimentary sequences, and relatively little sedimentological analysis has been undertaken. High quality tidal records are available only for Milford Haven and Swansea, and are relatively short-term, such that estimates of changes in mean sea level, tidal range and storm surge frequency are subject to major uncertainty. No systematic wave, tide or sediment transport monitoring has been constructed in Carmarthen Bay and only limited short-term field data sets have been collected in order to validate computer hydrodynamic models. With the exception of the recent study by Robins (2009), which used Telemac depth-averaged flow and sediment transport modelling to assess the present and possible future states of the Loughor estuary, the hydrodynamic and sediment transport modelling studies which have been undertaken have been relatively unsophisticated.

The increasing availability of LiDAR data over the past 10 years has provided highly useful information about the intertidal and supra-tidal morphology, but at t he present time the Loughor estuary and the entire area of the Three Rivers complex has not been covered by a single low-tide survey. While it has been possible to construct composite DEM mosaics using data from several surveys, this is not ideal from the viewpoints of creating bathymetric data sets for hydrodynamic modelling purposes, or for obtaining quantitative estimates of sediment volume change.

Despite the data limitations identified above, it is possible to arrive at a number of conclusions with a fairly high degree of confidence:

• Taking a broad-scale, long-term (centennial timescale) perspective, the geological and geomorphological evidence clearly indicates that onshore movement of large volumes of predominantly fine sandy sediment has taken place during the Holocene.

• The main source of sand has been provided by marine re-working of glacially-derived sediments on the floor of Carmarthen Bay

• The sand which has moved onshore has accumulated both as dune-capped coastal barriers (at Pendine-Laugharne, Pembrey and Whiteford Burrows) and as sedimentary wedges within the estuaries of the Loughor and the Three Rivers estuarine complex.

• The landward transfer of sediment from deeper water within Carmarthen Bay towards the nearshore zone has been accomplished by the combined action of tide and wave action, especially during major storms

• Landward transfer of sediment into the Three Rivers estuarine complex has been encouraged by net flood current dominance and by wave action

24 • Although the main low water channel of the Loughor estuary appears to display ebb dominance at the present time, the flood tidal currents are strong enough to bring sediment into the estuary and to introduce it to higher intertidal flat areas where flood dominance prevails, and where sediment accumulation can take place in the absence of wave re-suspension or longer-term tidal channel migration

• Over the course of the past 200 years the open coast barrier frontages have experienced alternating periods of shoreline progradation and shoreline retreat; at Pendine and the north-central part of Whiteford Burrows the net trend has been one of retreat, while at Laugharne Burrows, Pembrey Burrows and the southern end of Whiteford Burrows the net trend has been one of progradation

• Changes in the position and size of the low water channels has been a major driver of these accretion / erosion trends, although changes in wave regime and storm frequency/ magnitude are also likely to had a significant, though largely un-quantified, effect. Tidal channels are natural dynamic and have been responsible for alternating periods of erosion and accretion within, as well as outside, the estuaries.

• Embanking and land reclamation within the Loughor and Three Rivers estuarine complex, mainly from the 17th century onwards, was significant in reducing the tidal areas and tidal prisms within the estuaries; this may be expected to have led to a reduction in average tidal current velocities and contributed to enhanced rates of sedimentation within the estuaries.

• Construction of the Machynys training wall in the Loughor estuary after 1882 had a major impact on the estuary; effects included enhanced ebb scour past the entrance to Llanelli docks and along the Llanelli - Pwyl - Burry Port frontage, leading to foreshore lowering in this area, and reduced scour and channel instability in the central part of the Middle Estuary, leading to substantial sediment accumulation in this area between the 1880’s and 1950

• Breaching of the training wall in the mid 20th century has led to a re-direction of tidal flow through the central part of the Middle estuary, causing removal of significant quantities of sediment from this area and increasing the erosional pressure on the seaward side of Llanrhidian marshes and the northern end of Whiteford Burrows.

• There is no evidence to indicate that rise in sea level over the past 100 years has yet had any significant effect on either the open coast features of Carmarthen Bay or the estuaries. However, considerations of different future change scenarios identified by the UK Climates programme (UKCP09) has indicated that the effects on tidal prisms, and hence on tidal currents, sediment transport and the area extent of features such as saltmarshes, could be significant over the next 100 years.

• The implications of such future possible changes are likely to vary from one section of shore to another. Most at risk are the low-lying areas of reclaimed land which are currently protected from flooding and/or erosion by earth embankments with little reinforcement or protection. However, it is likely that periodic low water channel movements are likely to continue to present the most important factor contributing to flooding and erosion risk. Such movements are very difficult, if not impossible, to predict in a deterministic manner but future management planning needs to take account of the scale of potential risk involved. 25 5 References

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