Suspended Sediment Dynamics and Flux in the Macrotidal Taf Estuary, South Wales. Abdul Kadir bin Ishak 1997

As with the research by Walley, I hesitate to go so deep as to risk becoming lost. What follows is my digest of the essence of Ishak’s thesis as applicable to our particular enquiry into the benefit the community receives from the saltmarshes in the Taf Estuary.

The hydrodynamics of the River Taf:

The River Taf is a macrotidal estuary, with a mean spring tidal range of 7.5m.
Due the accumulation of sediment at the entrance of the estuary, the tide must rise 2metres before it can enter it. This means that when it does it enters at a high velocity and rises rapidly. Correspondingly, it is held back upon the ebb and therefore has a slower fall. This is the principle of an asymmetric tide where flood rise is shorter than the ebb (previously referenced in the discussion of the study by Stuart Walley).
Peaks in flow correspond to the point at which the tide overflows the sand flats at the entrance and when it overflows the saltings.
Ishak makes a case for the “Coriolis Effect” for tidal incursion and fresh water flow, which is that the rising tide will be drawn to the left side of the estuary whilst the downflow of fresh water will be drawn to the right. This will induce an anti clockwise horizontal mixing of salt and fresh water. Arguably, local features such as the dune barrier, equally well influence the circulation of tidal flow.
An effect of an asymmetric tide is that more sediment is brought into the estuary on the flood than is taken out on the ebb.

Sedimentation:

Although this is not consistent, annual sedimentation rates can be as much as 6.2cm/yr. However, this can be very dynamic and therefore should not be considered indicative. A more realistic average for the accretion of fine particles within the estuary is 1.8cm/yr.
Saltmarsh accretion: two historic horizon indicators for saltmarsh accretion are the fallout from atmospheric nuclear tests in the 1950’s-1960’s which put radio nuclide into the atmosphere, and which became measurable within saltmarsh sediments from 1963-1964. The other event is the fallout from Chernobyl in 1983, although the effects of this event are much more localized and therefore of varying intensity.
The average accretion rate for the saltmarshes on the Taf, measured at station points on Delacorse Marsh (Boathouse Marsh) Malt House Marsh and Black Scar Marsh is 1.01cm/yr. It is regrettable but understandable that Laugharne Town Marsh was not measured since it is here that the most tangible threat of overtopping would be felt, but also where there is the greatest amount of human intervention and therefore the most unreliable source for data.
According to Ishak, the estimate for sea level rise is 1.5-2.00mm/year, which is well within the the limits of what current levels of accretion will cope with.

Taken together, these studies by Walley and Ishak encapsulate both the coastal dynamics of Carmarthen Bay and the sedimentary system Taf Estuary and how they link directly to a cycle of human response, from exploiting change to eventually adapting to it.

Thomas van Veelen, technical commentary:

With particular emphasis upon Laugharne, the question of landscape change in the Taf Estuary is fundamental to this research. Given the rapid development of new salt marsh at Laugharne South1 through the 20th century, it is crucial to this historical analysis to identify the period over which the Laugharne Shore changed from beach to salt marsh, whether for example the conditions during the catastrophic flood event of 1607 were at all comparable to today, how flood risk has changed over time and to what degree changes in the fortunes of the town mirror the evolution of the estuary?

A salt marsh needs (at least) three conditions to establish: sediment supply, seed supply and non-erosive calm hydrodynamic conditions when seeds settle (Poppema et al., 2019). When these conditions are satisfied, a marsh may develop as a mutual relationship between flow and vegetation. Salt-resistant plants opportunistically colonise in small outcrops that focus flow, which in turn promotes scour as a precondition for creek expansion (Temmerman et al 2005). This creates slack conditions in areas peripheral to the creek network, which promote further propagation and create a dynamic for vegetation and the creek network to respond reciprocally to each other

Sediment and seed supply have always been plentiful. We know from bin Ishak that the Taf is essentially a flood dominant estuary importing sediment on the flood tide and carrying seed stock down on the ebb from salt marsh species upstream (bin Ishak 1997). Concerning a calm hydrodynamic environment: early photographic evidence shows a sandy gravelly, shelly shore, which normally would be associated with relatively high wave and tidal energy conditions, however the configuration of the current marsh suggests calm conditions typically consistent with the slack water zone of an embayment. Given the context of an easterly propagating barrier spit, the further it extends, the more shelter it provides to the lower estuary as a whole. This in turn provides suitable conditions for the transformation of the foreshore from gravelly beach to mudflat to pioneer marsh and eventually to generate the evolved salt marsh as we see it now.

As saltmarsh matures, so it reflects the fluvial characteristics of any other floodplain watercourse. Recently, at a point where the bends in the channel system of the Town Marsh have become increasingly tortuous, a pronounced meander has broken through. This might be due to a strong flow event that could not be contained, such as when the river Corran is running in spate. Whatever the cause, sediment transport patterns are affected, the bed gradient locally increased, leading to higher velocities in the new channel and increased scour, whilst the relic of the old channel, may slowly fill. Sediment transported out on the ebb will stabilise pioneer marsh downstream, potentially promoting further expansion of the system. However, since this is the only major event that has happened in the marsh since 1946, it is safe to assume that it is the result of normal fluvial behaviour and consistent with a dynamic system.

Having discussed the past and current changes within the estuary with reference to Walley and bin Ishak, it is worth while reflecting upon those implications, which are most applicable to coastal planning in the future. From their respective studies, the key elements for estuarine evolution are the spit extension and the stability of the salt marshes.

Although the barrier spit continues to propagate eastwards and there is, at present, little evidence of this slowing down, it may not continue: as the spit extends eastwards, the mouth of the Taf narrows causing an increase in tidal velocities, which would erode the cross section of the mouth, thus hampering further propagation to the extent that there may be a point when an equilibrium is reached (Roos et al., 2013). Yet as long as the spit extends northward, it provides increasing shelter against rough conditions within Carmarthen Bay (Pye & Blott, 2009, from SMP2, Appendix C), which benefit marsh accretion. The continuous propagation is further evidenced by its extension beyond the stabilisation works by MOD as referenced by Walley.

Where sea level rise is widely acknowledged to be the dominant threat for UK salt marshes – all marshes in the UK have at least 80% likelihood of retreat by 2100 due to sea level rise –, scour would become a specific threat for the marshes in the Taf estuary. At present the rate of accretion of the marsh is more than keeping pace with current estimates of sea level rise; However, the main estuarine channel is very mobile (Bristow & Pile, 2003) and currently scours marshes both at Laugharne South and along the frontage of the Town Marsh, as was shown in a field campaign by Dr. Tom Fairchild in August 2017. However, in the absence of sufficient supporting evidence and a specific study of channel movement, there is not enough information against which the future correlation between the tidal channel and the loss of saltmarsh due to scour may be predicted.

Apart from their obvious ecological value plus the contribution to landscape experience they provide for the local community, stable salt marshes perform a vital in coastal protection. Salt marsh vegetation is increasingly appreciated for its ability to attenuate wave energy (Temmerman, 2013). Furthermore, the canopy contributes to accretion by capturing sediment, (Bouma et al., 2014), which is subsequently stabilized within the root structure (Fagherazzi et al., 2012; Möller et al, 2014). This generates a shallow habitat, which provides sufficient friction to dampen and decelerate tidal and wave energy.

We have traced a gradual transition from an open coast to a sheltered estuary filled with marshland. At present, the spit provides shelter against the most extreme waves whilst the marshes provide a buffer zone between extreme wave action within the estuary and the settlement. Arguably, a storm similar to that of 1607 is less likely to threaten the village today. However, a combination of climate change, with the effect of sea level rise and increased frequency and intensity of storm events (IPCC, 2018), taken in the context of contemporary flood risk management standards, is likely to press our existing saltmarsh complexes beyond their ability to cope. For Laugharne, the most recent Shoreline Management Plan (SMP2, 2012) recommended the construction of a flood wall in front of the village. However, this proposal was rejected by the villagers, who, for the time being, prefer to mitigate risk through the use of demountable flood boards by individual householders.