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

Abstract

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 It has, however, been superseded by numerous subsequent findings and is now invalid.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:

• Appeal of a simple, easy to use analytical model that is in widespread use.

• Difficulty of determining the relative validity of ‘proofs’ and ‘disproofs’.

• Ease of application.

• Positive advocacy by some scientists.

• Application by other scientists without critical appraisal.

• The simple numerical expression of the model.

• 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.

Introduction

The global rise of sea level during the Holocene (Pirazolli, 1991) and during the historical period have prompted studies of the short-term response of shorelines to changes in water level (Hands, 1979, Hands, 1980, Hands, 1983, Brambati et al., 1998, Ciavola and Corbau, 2002), geomorphological analysis of shoreline behaviour during transgression (Rosen, 1978, Carter and Orford, 1993, List et al., 1994, List et al., 1997) and investigations of transgressive coastal stratigraphic sequences (Kraft, 1978, Thom, 1983, Cowell et al., 1995, Cattaneo and Steel, 2003). Factors that cause changes in the morphology of coasts are numerous and include sediment supply, variations in wave energy, tidal currents, wind action, sediment type, tidal inlet dynamics, morphological feedback, etc. Isolating the influence of sea-level rise from these other factors is perhaps the biggest challenge in discerning its impact.

Prediction of future sea-level rise and the resulting shoreline retreat are, however, among the most important tasks facing coastal and global change scientists, particularly given the population concentration in coastal zones. Cohen et al. (1997) estimated that over 2 billion people (37% of the global population) live within 100 km of a coastline. Much of this concentration is in the tropics, but dramatic increases have been noted in the temperate regions of, particularly, the Mediterranean and the USA ocean coasts. In the Mediterranean, the coastal population was estimated at 146 million in 1990 and the urban coastal population alone is projected to rise to 176 million by 2025 with an additional 350 million tourists (Hinrichsen, 1998). In the United States, 55–60% of the population live in the 772 coastal counties of the Atlantic and Pacific coastlines. Coastal population density in the United States rose from 275 to 400 people per km² between 1960 and 1990 (Hinrichsen, 1998). Considerable effort has been expended on the prediction of sea-level rise (e.g. IPCC, 2001) although much uncertainty remains. The effort to predict shoreline behaviour related to such sea level changes has, however, received less attention.

Both identification and characterization of the critical parameters that control shoreline behaviour is difficult. While the prospect of future shoreline erosion related to sea-level rise is of global concern (Bird, 1985) it is increasingly apparent that the patterns of shoreline change during transgression are non-uniform and highly site-specific (Cattaneo and Steel, 2003). Thus, it might be expected that predictions of future shoreline erosion rates for given sea-level rise must be based, in significant part, on local geomorphological and sedimentological characteristics including the geological framework, sediment supply and dispersal rates, sediment type, existing geomorphology, vegetation, lithification rates, abrasion, contemporary dynamics, human influences, etc.

In spite of the scientific acknowledgement of significant local level control in shoreline response to sea-level rise, we show in this paper that the most widely used contemporary method of quantifying shoreline change is the 50-year-old Bruun Rule of erosion (Bruun, 1954, Bruun, 1962, Bruun, 1988). The “Bruun Rule”, so named by Schwartz (1967), is a simple generic geometric model of nearshore profile evolution under rising sea level that is often assumed to work on all sandy shorelines.

In this paper we illustrate the extensive contemporary use of the Bruun Rule as a predictor of shoreline retreat and demonstrate that it is a significant contributor in shaping societal response to rising sea level. We critically review the Bruun Rule shortcomings, and discuss the reasons for the persistence of the approach in spite of its lack of scientific credibility. From this analysis, several generic conclusions are presented regarding the application of this and similar earth surface models for environmental management and policy formulation.