COASTAL BOULDER DEPOSITS: AN INTEGRAL PART OF UNDERSTANDING COASTAL FUTURES

Coastal boulder deposits (CBD) are supratidal accumulations of coarse clastic material. They are best known for their signature megagravel—individual clasts up to several hundred tonnes are recorded in some sites—but most deposits are dominated by less gargantuan boulder sizes, and may include abundant cobbles trapped in the interstices. Universal characteristics are that they occur on rocky coasts, above high tide, and are separated from the ocean by a bare bedrock platform. By preserving a record of high-energy marine inundation, CBD provide an opportunity to unlock long-term extreme-wave histories. But they are difficult to interpret, and difficult to date.

Debate continues as to whether individual CBD occurrences are the product of storm waves, or whether tsunami are required to emplace them. Recent advances in hydrodynamic analysis indicate that breaking or overtopping storm waves are capable of much greater force than had previously been realised, leading to an awareness that some CBD, previously interpreted as tsunami deposits, may in fact be the result of storms. Testing these ideas will take time, but is necessary to pinpoint the origins of local CBD occurrences. Equally important is the need to extract histories from CBD. The depositional ages of individual boulders are known in some cases, and those provide temporal anchor points for discrete transportation events. Regular monitoring of CBD—capturing the positions and structure of deposits—is also required to establish annual and decadal migration rates for boulder clusters and ridges. A combination of interdisciplinary approaches, as well as open-minded approaches to potential emplacement mechanisms, is the best way to attack these problems.

The value of unpacking the origins and transport histories of CBD is that ultimately they can serve as catalogues of extreme inundation events, linked also to the power of the emplacing waves: a boulder of a given size, at some known elevation and distance inland from the shoreline, requires a specific force in order to move. CBD event catalogues would therefore include specific information about flow strengths, making them unique in coastal event archives. Having these long-term records of powerful marine incursions will be valuable in long-term coastal planning. And knowing whether those inundations were due to tsunami or storm waves will permit more detailed and specific coastal hazard risk analysis.