Bruun rule

The Bruun rule gives a linear relationship between sea level rise and shoreline recession based on equilibrium profile theory, which asserts that shore face profile maintains an equilibrium shape, and as sea level rises the increasing accommodation space forces this equilibrium profile landward and upward to preserve its shape relative to the new sea level. As such, the Bruun rule analysis assumes that the upper beach is eroded as the shore profile moves landward, and that the volume of eroded material is deposited offshore, resulting in a rise of the nearshore bottom which maintains a constant water depth. The Bruun rule predicts coastal recession to be as much as 10 to 50 times sea level rise, depending on the slope of the beach. The mathematical notation of the Bruun rule is: R = \frac{SL}{h+B} = \frac{S}{\tan\beta}), in metres • B is the dune height above sea level, in metres • β is the average slope of the active profile == History ==

In 1954, Per Moller Bruun published a paper describing beach profiles and cross shore equilibrium. Per Bruun has since published works to clarify the limitations and practicality of his model, including "Review of Conditions for Uses of the Bruun Rule of Erosion" in 1983, bThe Bruun rule has been most often modified to account for longshore transport, overwash, and Aeolian sediment transport.'' have modified the rule to account for onshore transport, and cross shore movement over the supposed depth of closure. and Hinkel et al. in 2013 used the rule as part of a wider methodology to describe the effects on sea level rise on and around tidal inlets. The biggest challenge that remains seems to be isolating the effect of sea level rise on beach morphology from the coupled effects of wave energy, tidal currents, wind action, sediment supplies, sediment types and grain size, among others. Korean Peninsula, Shuidong Bay, Norfolk, Rhode Island, Florida, Accra, and Hawaii. == Criticisms ==

The effect of climate change on beaches is challenging to accurately model, as it is an interdisciplinary subject that involves ocean, earth, and atmospheric science as well as civil engineering and policy. Reliable coastal climate change impact assessments are needed to underpin effective strategies of adaptation in order to prepare growing coastal communities and high value coastal assets. As a result, models for estimating coastal erosion as a result of sea level rise - including the Bruun rule and models based on the Bruun rule - are constantly being reviewed and updated.'' Some of the rule's most criticised assumptions include the nonexistence of gradients in longshore sediment transport, the existence of a depth of closure, a closed sediment budget, and the availability of sufficient sand sources. Cooper and Pilkey describe the use of the Bruun rule as "a "one model fits all" approach" Bruun did not present a rigorous mathematical derivation for his rule, which has caused confusion in the research community. For example, Rosen in 1978, Allison and Schwartz in 1981, Dean and Maurmeyer in 1983, and Zhang, Douglas and Leatherman in 2004 have all mathematically derived the Bruun rule differently, with disagreement on the assumptions and limitations of the Bruun rule unique to their own derivations. However, the latter revision by Zhang et al. presents an alternative derivation showing that "even though very simple, the Bruun model has considerable generality". Some field and laboratory tests have supported the Bruun rule, although claimed experimental flaws in these publications have been criticised. Amongst others, Ranasinghe and Stive in 2009, and later Andersen et al. in 2015, have concluded that "no study has produced comprehensive, well-accepted verification of the Brunn model". However, there is a near consensus that the basic qualitative model of shoreline recession is valid, despite quantitative data gleaned from the Bruun rule being dubbed as "very coarse approximations" or "broadly indicative estimates". Despite these criticisms, the Bruun rule is credited for its simplicity, and there remains "no simple, viable alternative". ==See also==