Oceanic Hazards - Natural Hazards - page 171

Amount of shoreline retreat

Shoreline
erosion

Amount of
sea level rise

Sea level after rise

Sea level before rise

Zone of
enhanced
water discharge

Seabed after rise

Nearshore
deposition

Seabed before rise

Amount of seabed
accretion

The Bruun Rule for the retreat of a shoreline with sea level rise.

Fig. 8.12

actual volume of sediment needed to raise the seabed
as sea level rises depends upon the distance from shore
and the depth from which waves can move that
material. The Bruun Rule was formulated when it was
believed that this depth was less than 20 m. Individual
waves can move sediment offshore to depths exceeding
40 m, and well-defined groups or sets can initiate
sediment movement out to the edge of the continental
shelf. Coastlines, where Bruun's hypothesis was most
successfully applied, include the east coasts of the
United States and Japan, where sea level is rising
rapidly. There are many areas of the world where sea
level is rising at slower rates, or declining, and coastal
erosion is still occurring. It would appear that sea level
change as a major, long-term cause of beach erosion
has evolved from evidence collected on, and applied
with certainty to, only a small proportion of the world's
coastline. A dichotomy also exists between measured
rates of beach retreat and the present postulated
eustatic rise in sea level. Common magnitudes of
beach retreat in the United States, and elsewhere in
the world, range from 0.5-1.0 m yr -1 . According to the
Bruun Rule, shoreline retreat at this rate would
require sea level rises three to seven times larger than
the maximum rise of 3 mm yr -1 that may presently be
occurring globally. Obviously, factors other than sea
level rise must be involved in beach erosion.
The actual rise in sea level may be irrelevant to the
application of the Bruun Rule. As shown above, sea
level oscillations at all timescales are one of the
most widely occurring aspects of sea level behavior.
More sections of the world's coastline have significant

oscillations than simply have rising sea level. The
emphasis upon the Bruun Rule has been upon rising
levels, but the rule can also predict accretion where sea
level is falling. If the Bruun Rule is to account for long-
term beach erosion in these locations, then what has to
be taken into account is the imbalance between the
amount of erosion during high sea level fluctuations
and the amount of accretion when sea level drops. It
takes a shorter time to remove material seaward during
erosional, rising sea level phases - storm events - than
it does to return sediment landward during accretional,
falling sea level periods. If eroded sediment that has
been carried seaward cannot return to the beach
before the next higher sea level phase, then the
beach will undergo permanent retreat. Bruun's
hypothesis only provides the mechanism by which
these shifts in sea level operate. The more frequent the
change in sea level, the greater the rate of erosion.
Oth er causes of erosion

(Dolan & Hayden, 1983; Bryant, 1985, 1988; Dolan et al.,
1987; Bird, 1996; Gibeaut et al., 2002)

Clearly, more than one factor can account for erosion
of the world's coastlines. Table 8.2 lists these additional
factors. Human impact on beach erosion is becoming
more important, especially with this century's techno-
logical development in coastal engineering. In some
countries such as Japan, breakwalls have been used as
a ubiquitous solution to coastal erosion, to the extent
that their use has become a major reason for continued
erosion. Wave energy reflects off breakwalls, leading to
offshore or longshore drift of sediment away from