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Compendium on methods and tools to evaluate impacts of, and vulnerability and adaptation to, climate
change
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Bruun Rule Description
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Description
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The first and best known model relating shoreline retreat to an increase in local sea level is that
proposed by Per Bruun (1962). The IPCC reports that 1 cm rise in sea level erodes beaches about 1 m
horizontally. This becomes a large issue for developed beaches that are less than 5 m from the ocean
(IPCC, 1998).
The Bruun rule states that a typical concave-upward beach profile erodes sand from the beach face and
deposits it offshore to maintain constant water depth. The Bruun rule can be applied to correlate
sea-level rise with eroding beaches. The Bruun rule estimates the response of the shoreline profile
to sea-level rise. This simple model states that the beach profile is a parabolic function whose
parameters are entirely determined by the mean water level and the sand grain size. The analysis by
Bruun assumes that with a rise in sea level, the equilibrium profile of the beach and shallow
offshore moves upward and landward.
The analysis is two-dimensional and assumes that,
- The upper beach is eroded due to the landward translation of the profile;
- The material eroded from the upper beach is transported immediately into the offshore and
deposited, such that the volume eroded is equal to the volume deposited;
- The rise in the nearshore bottom as a result of deposition is equal to the rise in sea level,
thus maintaining a constant water depth in the offshore (SCOR, 1991).
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Appropriate Use
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The Bruun rule is only applicable for small scale local sites.
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Scope
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Over long stretches of coast, the Bruun rule and associated cross-shore transport models become
complex. There has been a number of critiques e.g. Cooper and Pilkey (2004).
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Key Output
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Shoreline recession (in metres relative to sea-level rise).
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Key Input
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An increase in sea level, (S), cross shore distance (L) to the water depth (h) taken by Bruun as the
depth to which nearshore sediments exist (depth of closure), and B is the height of the dune.
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Ease of Use
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Easy to use with numerous assumptions.
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Training Required
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Familiarity with the coastal zone being investigated.
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Training Available
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None
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Computer Requirements
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None, unless it is incorporated into a model.
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Documentation
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Originally proposed by Per Bruun in 1962.
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International Studies
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Bruun (1962, 1988)
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Contacts for Framework, Documentation, Technical Assistance
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See applications
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Cost
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No cost to use the Bruun rule.
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Validity
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Bruun rule has been applied but caution needs to be exercised where other factors influence sediment
budget or control profile.
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