A STATIONARY SAND WAVE ON MAGILLIGAN
STRAND, COUNTY LONDONDERRY
R. W. G. CARTER
School of Biological and Environmental Studies
The New University of Ulster
Coleraine, Co. Londonderry
Northern Ireland BT52 ISA
ABSTRACT
On Magilligan Strand the formation of an unusual form of rhythmic
shoreline - termed a stationary sand wave - has had a profound effect on
erosion rates of the dune cliffs. The sand wave has expanded laterally
to c. 1200 m since its formation in 1968/69, but its centre has remained
in one place. Examination of maps, charts and photographs suggests
that similar features have formed in the past, but reasons for their
eventual decay can only be speculated upon.
Introduction
This paper describes an unusual type of rhythmic shoreline at Magilligan,
Co. Londonderry, which has exercised considerable control over recent
coastal erosion.
Rhythmic beach topography is now recognised as a common form of
response to energy and momentum fluxes in the nearshore zone (LonguetHiggins, 1972; Komar, 1976, p. 274). Often, these wave processes manifest
themselves as sand waves (or shoreline meanders) (Bakker, 1968; Dolan et ah,
1974) which are known to migrate alongshore, although the pattern of movement may be either continuous and linked to longshore currents (Dolan,
1971) or intermittent as a response to occasional storm activity (Phillips,
1964). Almost all authors note the important relationship between sand
wave formation and rates of coastal recession, plus development of secondary
features such as washover fans. In the light of his observations on sand
waves Dolan (1971, p. 178) has described sand beaches as 'non-stationary
sinuous forms'. However the example described here does not easily fit this
description.
Study area
Magilligan Strand is 14 km in length and forms the seaward side of a
cuspate foreland composed largely of relict glacial sands. Recent coastal
changes have largely been confined to the recurve of the foreland, although
several longer-term and more general periods of shoreline advance and retreat
have been recorded (Carter, 1975). Since the mid-1950s the north-east facing
sea coast has shown a general tendency to retreat. Ground surveys together
with regular air photography have allowed coastal changes to be closely
monitored.
126
MAGILLIGAN STRAND
127
Formation and persistence of sand waves at Magilligun
Extensive sand wave fields are rarely formed on the northern coast of
Ireland. Precise reasons for their development are obscure, although they
appear to coincide with high energy waves and temporary elevations of mean
sea-level, and thus may be related to the focussing of wave energy on the
beach via refraction across offshore sand banks and/or the reflection of wave
run-up from the dune cliffs. At Magilligan a series of sand waves formed in
the area 7 to 10 km from the proximal point in the winter of 1968/69, and
again, but on a smaller scale in February 1974. In both cases the sinuous
beach plan rapidly reverted (2-4 months) to the more normal straight outline.
There was one exception to this; from the earlier period a single wave persisted, and in fact increased both in length and depth (Plate 5*). Initially
(1969) the wave was 320 m in length and around 12 m in depth. By 1974 it
had extended to 1200 m in length and 40 m in depth, although the centre
remained stationary. This indicates a constant length: depth ratio of around
30:1. Since 1974 the entile feature has changed little. Geomorphologically
the edges of the wave are characterised by a wide accretion zone, often
occupied by a berm and shell pavement. These accretion areas are not the
same as those described in north-western Florida by Sonu (1973) nor those
in Lake Michigan noted by Tanner (1975) and called 'beach pads'. One
major difference is that they have no intimate connection with the nearshore
topography. At the centre of the wave the backshore zones are absent, and
the foreshore tends to be both narrow and steep (30-45 m and 4 ° - 6 ° at
L.W.O.S.T. compated to 65-90 m and 1°- 2° elsewhere). Offshore a single
continuous break-point bar parallels the sand wave (Carter and Kitcher, 1979,
Fig. 4), and no evidence of rhythmic topography has been found, although
crescentic bars may exist briefly at times when such topography is forming.
A Centre
B Margin
0
10
20
CO
•9
30 H
SAND WAVE
FORMATION
i.0
»
II
\
50
60
1950
1960
1970
YEARS
Figure i. Average annual rates of cliff erosion. Site A is at
the centre of the present sand wave, site B at the
N.W. edge. Based upon air photographs.
' The plates follow p. 210.
128
CARTER
Dune cliff erosion
Before 1968 average rate of dune cliff erosion at Magilligan was 2.5 m
year - 1 , but since the formation of the sand wave, erosion has ranged from
9.0m year-1 at the centre of the wave to less than 0.2m year-1 at its margins
(Fig. 1). Between mid-1974 and 1976 the maximum rate decreased to 5.0 m
year-1, and since January 1976 no further erosion has taken place. Sand
removed from the cliff at the central portion of the wave appears to accumulate
at the edges, and these areas serve to protect the landward cliff from damage.
Almost all major episodes of cliff erosion and sediment redistribution occur
in very short (1-3 hour) intervals when wave and tide conditions are favourable. Prolonged periods of erosion may eventually lead to the development
of major dune blow-outs. A possible active example of this was noted at
Narin, Co. Donegal by Johnson (1975).
SAHD WAVE LOCATIONS
A
LOUGH
FOYLE
CO. LONDONDERRY
5 Km
Figure 2. Location and dates of previous sand waves.
Examination of charts, maps and air photographs for Magilligan (for list
see Carter, 1975, Table 1) shows the occasional presence of similar, large
sand waves (Fig. 2). It is likely that they are a major long-term influence on
the stability of the coast, by controlling and locally magnifying dune erosion.
Sand wave decay
Three reasons are put forward to account for the eventual decay of sand
waves:
MAGILLIGAN STRAND
129
(i) the length: depth ratio increases, either by sand infilling the central
part or by removal of the marginal accumulations through accelerated
longshore drift,
(ii) an abnormal erosion phase eliminates the feature during widespread
modification of the entire shoreline.
(iii) cliff erosion causes a major breach" in the duneline, allowing excess
sediment to move inland rather than along the shore. In this way the
breach would provide a pathway for wind borne material (as described
in Carter, 1977).
As yet the Magilligan sand wave has shown no tendency to decay, although
at present the expansion has stopped. From the previous examples noted in
Fig. 2, it would: seem alternatives (i) and (iii) are the most likely to have
occurred.
. . .. . . . : . . . : •': . . ./:
Conclusions
With the type of stationary sand wave described here, erosional activity
is concentrated into a restricted zone at the wave centre, and it would appear
that a major phase of shoreline disequilibrium could be initiated on an otherwise stable coastline. In the more commonly described moving variety,
effects are, in time, evenly distributed longshore.
REFERENCES
W. T. BARKER
R. W. G. CARTER
R. W. G. CARTER
R. W. G. CARTER and
K. J. KITCHER
R. DOLAN
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and its application on the Dutch Isle of
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1977 'Rate and pattern of sediment interchange between beach and dune': 3-34 in
W. F. Tanner (ed), Coastal sedimentology,
Florida State University, Tallahassee.
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R. DOLAN, L. VINCENT and
B. HAYDEN
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in West Donegal, unpublished B.Sc.
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M. S. LONGUET-HIGGINS
A. W. PHILLIPS
C. J. SONU
W. F. TANNER
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currents', 203-248 in R. Meyer (ed),
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Dock. Harb. Auth., 45, 19-21.
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. Michigan', J. Great Lakes Res., 1, 171-178.
ACKNOWLEDGMENTS
I would like to thank John Shaw for drawing the figures and Pauline Galbraith
for typing.