Morphological Change in Carmarthen Bay and - South Wales SMP SMP2... · 2012-09-03 ·...

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.

i

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

ii

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

iii

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

iv

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


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


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


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


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

v

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

vi

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


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


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

vii

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


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

1

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

2

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

3

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

4

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.

5

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 x103 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

6

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

7

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).

8

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 (Z50)

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 103 m

3, approximately twice that of the Three Rivers estuarine complex. The entrance to the

9

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

10

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.

11

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.

12

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 x103 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 103 m

3 to

328568 x103 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.

13

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 103 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

14

• 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

15

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 95th 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 103 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 103 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

16

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).

17

3.2.3 The Towy estuary


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 103 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).

18

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


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

19

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 103 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).

20

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

21

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 x103 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%.

22

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

23

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

24

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

25

• 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.

26

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