Physical factors which may contribute to saltmarsh erosion

Sea level rise • Increase in tidal range • Tidal asymmetry • Increase in storminess • Channel migration

Sea level rise

Numerous studies have defined sea level rise as a major cause of saltmarsh erosion. Relative sea level change affects wave height, tidal currents, position of the water table and the position of high water. During the last glaciation, the coastline was located further seaward than it is today, but sea level rise since the early Holocene has led to the landward migration of the shoreline.

Over shorter timescales, of the order of several decades, changes in sea level can affect both saltmarsh morphology and vegetation community type. Limited sediment supply and an increase in sea level rise can result in erosion of saltmarsh through creek lengthening, landward migration through an increase in wave height, water depth and water turbulence.

Although tide gauge records clearly indicate that global average sea level has risen in the past century (Woodworth, 1987), according to some scientists there is as yet no firm proof that the changes have had any significant effect on UK saltmarshes (Pye, 2000). This may change over the course of the next century if predictions of future accelerated sea level rise turn out to be correct (see also Coastal squeeze).

Increase in tidal range

All saltmarshes are within the intertidal zone and are, therefore, exposed to tidal immersion. The once daily (diurnal) or, more commonly, twice daily (semi-diurnal) flooding of saltmarsh and the associated impact of tidal waters is, therefore, an important influence on marsh development.

There is evidence from tide gauge records that tidal range has increased at some stations around the UK coast (Woodworth et al., 1991) and that the frequency of extreme high tidal levels, in particular, has increased. Other things being constant, this should bring about acceleration in vertical marsh accretion rates. However, bigger tides result in bigger tidal currents, with the likelihood of increased scour both on the flood and ebb; although, to date, no such link between marsh creek widening and increasing frequency of high tides has been scientifically established.

Tidal asymmetry

Sediment transport in shallow coastal and estuarine environments is influenced to a significant extent by the nature of tidal asymmetry. The deep water tidal wave becomes distorted as it enters shallow coastal waters and estuaries, the nature of the distortion being dependent on non-linear interaction between the offshore tide and nearshore shelf and estuarine morphology (Pye, 2000). The nature and causes of tidal asymmetry in estuaries has recently received much attention, but its significance in terms of saltmarsh accretion and erosion has so far received only limited consideration.

Increases in mean sea level and tidal range will result in changes in the way in which tidal waves propagate within estuaries, and this will depend primarily on the length, depth and plan morphology of the estuary. The resultant steepening of the tidal curve may increase current speeds and hence reduce deposition and enhance erosional potential.

Increase in storminess

The stability of saltmarshes can be affected by changes in three aspects of the wind/wave climate: mean wave height; wave directional frequency; and storm surge frequency (linking both extreme waves with extreme tidal levels). Changes in wave energy conditions can have important influences at the mudflat/saltmarsh interface. An increase in significant wave height or a reduction in the recurrence interval will result in more mudflat/saltmarsh erosion due to the reduced time period for recovery. Changes in the direction of waves can also be as important, as changes in longshore sediment transport can occur through slight changes in fetch. Furthermore, since inshore wave height is strongly dependent on water depth, and wave energy is a power function of wave height, changes in the frequency of storm surges may have a considerable impact on saltmarsh evolution. Storm surges frequently result in the breaching of sea defences and may trigger a progressive phase of erosion if repairs to the defence are not made.

Channel migration

Estuarine channels are naturally dynamic and low water channels commonly meander over time. Marshes adjacent to the outside bends of meanders are frequently ‘cliffed’ and experience slow erosion, while marshes adjacent to the opposing slip-off slopes tend to accrete. This pattern may reverse on a timescale of several decades. Good examples have been documented in the Mersey and Humber estuaries, but the process is almost universal. Where natural meander migrations are confined by engineering works, there is often a large-scale, long-term reduction in estuary capacity (O'Connor, 1987; Pye, 1996; van der Wal et al., 2002).