Gravel Beaches

Introduction

Gravel beaches are widespread around the world, including the USA, Canada, Japan, Argentina, New Zealand and the wave dominated coastlines of Northern Europe . In the UK, about one third of the coastline is protected by such beaches .

In coastal defence schemes, considerable use is made of coarse-grained (gravel) sediment to replenish eroding beaches, often in conjunction with structures such as rock or wooden groynes or offshore breakwaters. This is because such beaches are known to be an efficient form of natural coastal defence . Two examples from the UK are those at Sidmouth, Devon and Elmer, West Sussex.

Research into the particular characteristics of gravel beaches and their engineering analysis is relatively new, with much of the progress being made recently. In the 1990’s small scale experimental measurements and early field studies lead to the subsequent development of parametric models for profile prediction, longshore transport and the stability of barrier beaches. Since 2002, two major large scale experimental studies and some new comprehensive field studies have been undertaken. Since that time, good progress has also been made in process based numerical modelling systems for long and cross shore transport.

The emphasis of the article is to present in a clear and concise manner the most relevant concepts, basic understanding and insights into the behaviour of gravel beaches, to provide a summary of the available engineering analysis techniques and modelling approaches and to provide guidance to further reading and a list of the most relevant references to work carried out since 1990.

Beach Origins

The current world’s coastlines were formed as a result of the last ice age, which ended about 10,000 years ago, see the beach management manual . At that time large ice sheets covered more of the world’s land masses than they do at present. As they melted there was a rapid rise of sea level (about 120m between 20,000 and 6000 years ago). Vast quantities of sediment were carried by rivers to the sea during this period, eventually forming the pre-cursor to our present coastlines as the rate of sea level rise rapidly reduced about 6000 years ago. Much of our beaches today are composed of the remnants of these sediments, composed predominantly of sand and gravel. These sources of beach material have subsequently been supplemented by coastal erosion of soft cliffs and the reduced but continuing supply of sediments from rivers. Material has also been derived from offshore banks left behind by relatively rapid rises of sea level after cold episodes in the earth’s climate.

Sediment classification and properties

Traditionally sand and gravel sizes have been classified according to the Wentworth scale. This defines sand as being of diameter between 0.0625mm to 2mm. Material sizes larger than this are classified as gravel, subdivided into granular (2mm to 4 mm), pebble (4mm to 64 mm), cobble (64mm to 256 mm) and boulder (>256 mm). Rounded gravel, typical of a significant number of UK beaches, is referred to as shingle.

There are several physical properties of sand and gravel beaches which are important in the study of coastal sediment transport. The first is the sediment density (ρs), typically 2650 kg/m3 for quartz. The rest are required in recognition of the fact that a beach comprises a mixture of the beach material, interspersed with voids which may be filled with air or water. Thus the bulk density (ρb) is defined as the in situ mass of the mixture/volume of the mixture, the porosity (ps) as the volume of air or water/volume of the mixture, typically between 0.25 to 0.4 for a gravel beach, the voids ratio (e) as the volume of air or water/volume of the grains, typically between 0.33 to 0.66 for a gravel beach, and finally the angle of repose (Φ), which is the limiting slope angle at which the grains begin to roll, typically about 35o in air. In water this reduces to about 30o.

The material sizes on any particular beach will normally comprise a range of grain sizes, thus it is standard practice to measure the grain size distribution by a sieve analysis from which the percentage by weight of material passing through a range of sieve sizes is plotted against particle size. The median size is denoted by D50, representing the diameter for which 50% of the grains by mass are finer. The spread of sizes is often indicated by the values of D84 and D16 and their ratio is used to measure the degree of sorting. A well sorted sample is one in which there is a small range of sizes (D85/D16<2), whereas a well mixed sample has a large range of sizes (D85/D16>16).