Environmental Engineering Reference
In-Depth Information
careous marshes and fens, including several rare species. A rare thalloid liverwort Petalophyllum ralfsii ,
which occurs in shallow calcareous dune slacks and habitats containing damp turf, is restricted to few-
er than three dune slacks at Whiteford. The diverse vegetation of these damp areas also supports a wide
variety of invertebrates including spiders, beetles and wasps. There are around 150 slacks at Whiteford,
which vary greatly in size from around 70 square metres to 40,000 square metres, the latter slack being
aptlyknownasthe'GreatPlain'.Althoughtheymayappeartobescatteredrandomlyacrosstheareathere
are in fact three rows of slacks running parallel with each other down the long axis of the spit.
Theslacksinthewesternrowaremedium-sizedandelongatedinthedirectionoftheprevailingsouth-
westerly winds. Located within the fixed dunes, they are deeply scoured and separated from adjacent
slacks by tall dune ridges. In contrast the slacks in the eastern row are large and their orientation is not so
pronounced. They are mainly surrounded by low fixed dunes or undulating dune grassland and grade into
atransitionzonewiththesaltmarsh.Themostsoutherlyslacksofthewesternrowandthemostnortherly
slacks of the eastern row are quite different in character to the others because of their location on high
dunes and they are small and often very dry. In contrast the central row of slacks is highly variable, re-
flecting the varied structure of the dunes along the centre of the spit.
The process of slack formation appears to have begun in the early medieval period when, as noted
previously, the intensity of the onshore winds increased dramatically. The high winds scoured out deep
hollows and piled the sand up in high ridges, aligned in parallel with the wind direction. Once formed,
theseridgesshelteredtheeasterndunesfromtheworsteffectsofthewindandgenerallytherewouldhave
been smaller quantities of sand deposited on the eastern side of the spit. These smaller dunes would have
been shaped by the calmer inland winds. Such winds move the sand as a series of low waves perpendic-
ular to the wind. This tendency of moderate winds to produce long, closely packed ridges can be seen at
Whiteford today along the main accretion zone.
The dunes at Whiteford overlie an impermeable layer of glacial debris, but although considerable
amounts of water seep out from the edges of the system it is not sufficient to prevent the water table from
rising in most winters to flood many of the slacks. The slacks are therefore in effect temporary ponds, the
wettest slack holding water up to half a metre or more in depth in some months of the year (Fig. 84). In
contrast there are slacks where the water table never rises to more than a metre below the slack surface.
In between these two extremes there is a complete range of water movement. It is this widely fluctuating
water table that is largely responsible for the rich and diverse range of plants found in the slacks. The
hydrology of coastal sand dunes is well understood and a typical cross-section from the sea to a drainage
lineatthelandwardedgeshowsthefreshwatertablerisinginadomeshape,beinghighestabovesealevel
along the mid-line. Fresh water constantly escapes to the sea, while sea water penetrates below the layer
of fresh water on the seaward side.
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