Geography Reference
In-Depth Information
so-called 'Bruun rule' (Bruun 1962; Dean 1991)
has proved to be a flawed but remarkably persistent
concept. It begins from the premise that there is
an equilibrium depth of water offshore. With sea
level rise, water depth increases and sea bed
deposition must take place to restore the
equilibrium depth; this is achieved by shore
erosion and shoreline retreat (Figure 8.1). These
parameters can be easily cast into simple
mathematical form and thus the degree of
shoreline shift calculated from estimates of sea level
rise. Despite the difficulty of establishing both
upper and, particularly, offshore points of closure
for studied profiles; of accepting sudden rather
than progressive changes in sea level; and in the
absence of allowing for longshore processes in
controlling profile characteristics, (and see Pilkey
et al
. 1993, for example, for additional problems)
the Bruun rule has been widely quoted, albeit
usually with reference to a Great Lakes study,
where the local geology, topography, bathymetry
and lake climate are peculiar and not easily
transferable to open coasts.
More sophisticated analyses have attempted to
establish what associations exist between beach
states and breaking wave characteristics; what the
range of these states might be; and how the rate
and direction of change between states might be
predicted. Largely through the efforts of the
'Australian School', six characteristic domains have
been identified (Figure 8.2). The term 'reflective'
describes one end-member: steep, narrow, coarse-
grained beaches, often typified by rhythmical
longshore topography, where wave energy is
THE NATURE OF THE PROBLEM
Beaches and their adjacent nearshore zones are
highly effective buffers to incoming wave energy,
with offshore bathymetry reducing incident wave
energy by 95-99 per cent (Carter and Woodroffe
1994). As the ratio of force to resistance is high,
and changing inputs result in demonstrable
morphological change, the application of a
process-based approach to coastal studies (largely
promulgated through oceanography, coastal
engineering and sedimentology rather than
geomorphology) has been highly successful,
developing from the studies of the Beach Erosion
Board in the United States in the immediate post-
Second World War period and reaching its fullest
expression in the encyclopaedic Shoreline
Protection Manual (CERC 1984). Thus the
forecasting of deep-water wave conditions from
meteorological and other vari-ables, the
modification of deep-water waves by nearshore
bathymetry, the nature of breaking waves, and
water and sediment movements within the
breaker zone are now well known (e.g. Komar
1976).
The changing state of intertidal and shallow
subtidal nearshore profiles has long been
recognised, with the use of such terms as 'winter'
and 'summer' beaches (e.g. Shepard's (1950) classic
studies in southern California) or 'storm' and
'swell' profile types (Komar 1976). The idea of
shifts between equilibrium beach states has proved
particularly attractive when extended to the
possible effects of sea level rise. In particular, the
Figure 8.1
The 'Bruun Rule'. On
sea level rise (sea level 1 to 2)
sediment is transferred from the
shoreface sediment store (V
1
) to
the nearshore zone (V
2
) to re-
establish an equilibrium water
depth. The coastline will retreat
(R) until stability is re-
established. Point D is the
'closure depth', the outer limit of
profile adjustment.
Source: After Bird 1993.
