Author Posting. © Annual Reviews, 2007. This is the author's version of the work. It is posted here by permission of Annual Reviews for personal use, not for redistribution. The definitive version was published in Annual Review of Earth and Planetary Sciences 36 (2008): 601-647, doi:10.1146/annurev.earth.35.031306.140139.

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Coastal Impacts Due to Sea-Level Rise

In the face of a global rise in sea level, understanding the response of the shoreline to changes in sea level is a critical scientific goal to inform policy makers and managers. A body of scientific information exists that illustrates both the complexity of the linkages between sea-level rise and shoreline response, and the comparative lack of understanding of these linkages. In spite of the lack of understanding, many appraisals have been undertaken that employ a concept known as the “Bruun Rule”. This is a simple two-dimensional model of shoreline response to rising sea level. The model has seen near global application since its original formulation in 1954. The concept provided an advance in understanding of the coastal system at the time of its first publication. It has, however, been superseded by numerous subsequent findings and is now invalid. Several assumptions behind the Bruun Rule are known to be false and nowhere has the Bruun Rule been adequately proven; on the contrary several studies disprove it in the field. No universally applicable model of shoreline retreat under sea-level rise has yet been developed. Despite this, the Bruun Rule is in widespread contemporary use at a global scale both as a management tool and as a scientific concept. The persistence of this concept beyond its original assumption base is attributed to the following factors: 1. Appeal of a simple, easy to use analytical model that is in widespread use. 2. Difficulty of determining the relative validity of ‘proofs’ and ‘disproofs’. 3. Ease of application. 4. Positive advocacy by some scientists. 5. Application by other scientists without critical appraisal. 6. The simple numerical expression of the model. 7. Lack of easy alternatives. The Bruun Rule has no power for predicting shoreline behaviour under rising sea level and should be abandoned. It is a concept whose time has passed. The belief by policy makers that it offers a prediction of future shoreline position may well have stifled much-needed research into the coastal response to sea-level rise.

Sea-level rise and shoreline retreat: time to abandon the Bruun Rule

Controls on coastal dune morphology, shoreline erosion and barrier island response to extreme storms

Sandy beaches occupy more than one-third of the global coastline1 and have high socioeconomic value related to recreation, tourism and ecosystem services2. Beaches are the interface between land and ocean, providing coastal protection from marine storms and cyclones3. However the presence of sandy beaches cannot be taken for granted, as they are under constant change, driven by meteorological4,5, geological6 and anthropogenic factors1,7. A substantial proportion of the world’s sandy coastline is already eroding1,7, a situation that could be exacerbated by climate change8,9. Here, we show that ambient trends in shoreline dynamics, combined with coastal recession driven by sea level rise, could result in the near extinction of almost half of the world’s sandy beaches by the end of the century. Moderate GHG emission mitigation could prevent 40% of shoreline retreat. Projected shoreline dynamics are dominated by sea level rise for the majority of sandy beaches, but in certain regions the erosive trend is counteracted by accretive ambient shoreline changes; for example, in the Amazon, East and Southeast Asia and the north tropical Pacific. A substantial proportion of the threatened sandy shorelines are in densely populated areas, underlining the need for the design and implementation of effective adaptive measures.

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Sandy coastlines under threat of erosion

Influence of inherited geologic framework on barrier shoreface morphology and dynamics

The concept of shoreface profile of equilibrium is the basis for most models used to quantitatively describe and predict profile response on beaches. We question the validity of the concept as used in coastal engineering. The equilibrium shoreface profile concept is based on the following assumptions: (1) underfying geology does not play a role in determining the profile shape; (2) shoreface sediment is moved only by the interaction of wave orbitals with the sea floor, unidirectional current flow is not accounted for; (3) there is no significant net movement of sediment seaward of a so called "closure depth." The equilibrium shoreface profile equation implies: (1) offshore bars do not play an important role in shoreface sediment transport; (2) grain size is the only variable determining shoreface profile shape variability; (3) the shoreface transport system is two-dimentional; and (4) all shorefaces in the world can be described by a single equation with sediment grain size as the only variable. To varying degrees, all of these assumptions fail to be met in real world situations in light of well documented oceanographic and geologic phenomena. A fundamental reexamination of the engineering methods of determining nearshore shoreface evolution is needed. As currently practiced such methods are based on poor oceanographic assumptions.

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The Concept of Shoreface Profile of Equilibrium: A Critical Review

A field investigation was carried out on the prototype beach to determine the properties of windblown sand and parameters governing the aerial transport. The results suggest that the sand separates into two strata but the dynamics involved in this stratification process are still uncertain.

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A Field Investigation Study to Determine the Properties of Windblown Beach Sand

Reply to: Dubois, R., 1993. The concept of shoreface profile of equilibrium: A critical review. Journal of Coastal Research, 9(1), 255-278.

The coastal engineer who works largely full time in the field and on prototype conditions, keeps up to date, by practicing in two domains concurrently. The first is carried out in the field and consists of observing, collecting data, collating it and researching the results. The second domain consists of his on-going studies, basically continually reading the literature and where possible attending suitable coastal conferences. The first domain has hardly changed at all in the last 30 years, apart from the deployment of much more sophisticated and often much less reliable sensors, but in only the last 10 years, the second (or study) domain has changed nearly beyond recognition.

The Coastal Engineering Literature and the Field Engineer

In the 1990 Winter edition of the Journal of Coastal Research (Vol 6, No1) there are two very important items dealing with beach nourishment The main paper by Leonard, Clayton and Pilkey (pp 15-36) considers an analysis of beach design parameters and the second of probably greater importance, is the additional comments by Pilkey alone titled: "A time to look back at Beach Replenishment" (pp iii-vii) Both items it must be noted, are based only upon works carried out on the Eastern seaboard of the United States of America

Discussion of: Pilkey, O. H., 1990. A time to look back at beach replenishment (editorial), Journal of Coastal Research, 6(1), iii-vii. And, Leonard, L.; Clayton, T., and Pilkey, O.H., 1990.

A. W. Sam Smith

Papers

The Concept of Shoreface Profile of Equilibrium: A Critical Review

A Field Investigation Study to Determine the Properties of Windblown Beach Sand

Reply to: Dubois, R., 1993. The concept of shoreface profile of equilibrium: A critical review. Journal of Coastal Research, 9(1), 255-278.

The Coastal Engineering Literature and the Field Engineer

Discussion of: Pilkey, O. H., 1990. A time to look back at beach replenishment (editorial), Journal of Coastal Research, 6(1), iii-vii. And, Leonard, L.; Clayton, T., and Pilkey, O.H., 1990.