2.2 Short-term dune evolution processes
The long-term patterns of dune evolution described above, although producing underlying trends of change, are often difficult to detect because of substantial and rapid changes in dunes in the short-term (i.e. over days or weeks). It is usually severe short-term erosion events, occurring either singly or cumulatively over a few years, that cause concern and lead to attempts to influence the natural processes.
Plate 5 Beach-dune systems undergo cyclical changes in appearance due to broadly seasonal weather patterns. Under fairweather conditions, sand collects on the upper beach and dune face. When storms occur this is removed, giving the dunes an eroded appearance even though there may not be any long-term erosional trend.
Dunes are dynamic landforms that respond to changing conditions along the coast, usually brought about by the varying weather. Coastal dunes accumulate sand blown inland from the beaches in front of them by onshore winds (accretion). They lose sand by a variety of mechanisms, divided here into two main classes:
- deflation caused by winds
- marine erosion caused mainly by waves.
The main concerns about dunes usually arise when it is perceived that the erosion processes have removed more sand than the accretion processes can replace over time scales of months or a few years and this erosion threatens developed areas, infrastructure, commercial interests or important natural assets. Problems can also arise when accretion dominates, because the excess of sand blows inland affecting agriculture, blocking roads or even inundating built up areas. Natural erosion or accretion of undeveloped dune systems is not normally considered to be a problem, but rather an integral part of geomorphological evolution. A brief description of the accretion and erosion processes is set out below.
2.2.1 Accretion of dunes
The basic requirements for the formation of coastal dunes are:
- a supply of sand over a wide drying foreshore
- a backshore area of low relief
- predominant onshore winds for at least part of the year.
Dunes usually begin to form at the crest of a beach, with wind-blown sand accumulating around small objects such as clumps of seaweed, driftwood or other debris cast up along the strand line. Once formed, the low hills of loose sand are colonised by salt-tolerant, pioneer plants that both increase the resistance of the surface layer of sand to wind erosion and reduce the wind speeds over the surface (Plate 6). The embryo dunes or foredunes will continue growing, unless they are destroyed by wave action at high tide levels.
Plate 6 Sand accretion on backshore fixed by sea-lyme and marram grass to form embryo dunes.
In Scotland the main pioneering colonisers of loose sand include sand couchgrass (Elymus farctus) and lyme grass (Leymus arenarius). They are able to withstand short periods of immersion by seawater and have long roots, rhizomes and runners, which help to bind the surface grains and extend the vegetative cover laterally. As the foredunes grow vertically above the level of wave run up, they are colonised by marram grass (Ammophila arenaria), which thrives on continual burial by the blown sand deposits. The marram dominated dunes remain unstable and are known as yellow dunes due to the exposure of sand between the clumps of vegetation. If new foredunes develop to windward of the yellow dunes, the marram ceases to be supplied with fresh sand deposits, and other species colonise and stabilise the dune surface. Lichens typically carpet the surface between other species, often giving the dunes a grey appearance and hence the name grey dunes. The composition of the ultimate vegetation cover will vary depending on the soil chemistry, with the calcium rich sands of some parts of Scotland giving rise to species-rich grasslands. Where strong winds and gales regularly influence the dune system sand may be blown some distance inshore, forming the machairs typical of the Western Isles.
The rate of dune vegetation growth depends on the climatic conditions, with growth in some regions being inhibited by low rainfall and extreme temperatures. Around Scotlands coastline, however, the wet and windy weather generally allows rapid and vigorous growth of dune vegetation.
Ideal conditions for the transport of sand from a beach to the dunes occur after constructive (i.e. low height, long period) waves have deposited sand on the upper beach and inter-tidal foreshore. At low tide the sand dries and onshore winds can carry substantial volumes of sand onto the dunes. The transport of sand is diminished if the sand is wet due to rain or ground water drainage and can be prevented entirely if winds are too weak to mobilise the grains or are blowing offshore.
The vertical growth of dunes can be curtailed by one of three factors, namely:
- The dune reaches a height where the vegetation is no longer able to protect the dune surface from wind erosion. In some parts of the world, coastal dunes can reach over 50m above sea level. In Scotland, the winter wind speeds this high above the sea surface can be very strong, and dunes rarely reach more than about 25m in height.
- New foredunes intercept a large proportion of the wind-blown sand, preventing the further growth of yellow or grey dunes. This situation occurs on prograding coasts, and may result in the formation of many dune ridges over the years. A good example is found along the coastline north of Aberdeen at Balmedie where the dunes extend over 250m inland of the present beach crest.
- Damage is caused to the dune vegetation by human influences (trampling, groundwater extraction, farming, construction, etc.) or by animal grazing/burrowing, hence diminishing the capacity of vegetation to trap and retain further sand.
The presence of a wide belt of dunes means that there has been a period of accretion in the past. However, this does not mean that the accretion is continuing. Lines of dunes set back from the beach may have originally formed many centuries ago, but there are sites in Scotland where these ancient dune systems are being eroded, re-distributing the sand in them to other parts of the coastline. Examples of these changes can be seen at Morrich More near Tain and the Eden estuary side of the golf links at St Andrews.
2.2.2 Dune erosion
There are two main types of dune erosion, namely:
- Surface erosion by wind action (deflation)
- Marine erosion of the toe and seaward face of the dunes.
The former can and does occur naturally, following disease or damage to vegetation caused, for example, by drought, lack of nutrients or over-grazing by rabbits. However, the onset and subsequent problems of deflation are often worsened by human activities. Typical causes include excessive trampling, driving vehicles over dunes, the removal of sand for building materials or agricultural use, excessive extraction of ground water, and allowing excessive grazing by farm animals. If the vegetation is seriously damaged, it no longer acts to stabilise the sand and wind action will rapidly remove the exposed loose sand, forming a blow out (Plate 7). Once initiated, a blow out can spread rapidly and large areas of dunes can be affected, to the extent that blown sand becomes a major nuisance further inland. There are numerous manuals and handbooks explaining how to repair and prevent further damage caused by deflation (for example BTCV, 1996). The methods include fencing, installing boardwalks, protection of the eroded dune surface by thatching and transplanting dune stabilising grasses.
Plate 7 Dune blowout, Harris. As the name implies such features are formed by wind scour rather than wave attack.
This Guide concentrates on erosion of dunes caused by marine processes (Plate 8). A typical sequence of events leading to erosion of the front face of dunes is as follows:
- Beach profiles in front of the dunes become flatter as a result of destructive wave action (large waves generated by local onshore winds).
- The upper beach levels fall, allowing the high tide mark to reach the toe of dunes.
- Waves impact directly onto the dune toe, causing sand to be removed and under-cutting the dune face.
- The front face of the dune collapses onto the beach, and the sand is carried down the beach.
Plate 8 Active marine erosion of dunes at Cruden Bay.
The consequence of such events is that the front face of the dune retreats, but the beach profile receives an additional volume of sand. Hence the dunes can act as a temporary reservoir of sand, accumulating it during mild weather, typically during the summer, and surrendering it back to the beach during storms. The erosion of a dune during a storm event, or several such events, may provide enough sand for the beach profile to recover to pre-storm levels and thus prevent further erosion in the following days and weeks. However, if the sand eroded from the dunes is swept along the shoreline by longshore drift, and away from the eroded section of dunes, then the next storm event will cause further erosion and retreat of the dune face (Plate 9). If this sand loss is replaced by an influx of sand from offshore or updrift deposits, there may be no nett loss and the dunes should recover over time. Where significant dune erosion persists for several years, it is likely that the sediment balance has been upset resulting in a nett removal of sand from the affected area.
Plate 9 Debris of look-out tower undermined by marine erosion, Eden Estuary.
Although the sediment balance can be disturbed by natural causes (depletion of the source, a change in the mouth of a river or estuary, gradual shift in wave climate), it is often the result of human activities along an adjacent frontage or nearshore area. These may include dredging, construction of harbour breakwaters, protection of eroding cliffs or construction of beach control structures such as groynes or breakwaters. Erosion in these cases is not natural. An environmentally conscious shoreline manager is faced with the dilemma of accepting erosion of a dune system as a natural process, or implementing management operations that may be destructive to the very habitat and landscape that he seeks to protect.
Regardless of cause, the process of marine erosion of dunes is much more difficult to manage than the deflation described above. It is not generally possible to do anything about the causes of the erosion, namely high tidal levels, winds, rain and wave action, and often not possible to influence the human activities that may exacerbate the situation. The only natural defences that dunes have against the impacts of waves are a wide, high beach in front of them and the binding strength of well developed vegetation. If the beach is undergoing long-term erosion, then it is difficult to remedy this, short of expensive large-scale engineering works such as beach nourishment or the construction of groynes or breakwaters.
In many cases, however, the lowering of beach levels and subsequent rapid erosion of dunes is a short-term problem, that after a few years would naturally heal (Plate 10). Indeed the ability of dunes to recover and repair themselves following marine erosion is a major advantage from the viewpoint of long-term shoreline management. Replacing dunes with artificial hard defences such as seawalls may lead to the beach, dunes or structures deteriorating, with potential further expense in repairs and replacement, and should therefore be avoided if at all possible.
The main theme of this Guide, therefore, is to assist in the strengthening and re-building of dunes to reduce the adverse effects of marine erosion, while minimising the risks to people and property behind them. This is largely achieved by palliative measures, designed to reduce the adverse consequences of dune erosion, rather than to try to prevent such erosion entirely.
Plate 10 Recovery of beach following episode of erosion and construction of rock revetment, Elie.