Beach erosion management: the polyglot on the beach

What is a polyglot you ask? The dictionary defines a polyglot as a multilingual person or one with the ability to master multiple languages. So, yes, this piece is about “one” with multiple language ability, but no, it is not about humans like you and I, happily strolling on the beach on a beautiful day. It is about sand!

Sand is a polyglot? Well, it was Raymond Siever, a renowned geologist from America, who coined the term. In his book: ‘Sand’, when discussing the origin of sand, he stated that ‘Sand grains are polyglot: they speak to us in terms of mineral and chemical composition, grain texture, size, geologic age, and other properties too numerous to describe’. As languages can be expressed in both verbal and non-verbal ways, so indeed, the many characteristics of sand can be used to trace its origins and its movement history. Identifying the sand sources and the transport pathways to the beach are among the important factors in assessing beach erosion and accretion.

The source and the history of the sand and its movement can often be deduced from its petrographic or geochemical characteristics. Using petrographic methods, i.e. the science of describing rocks, we can deduce the textural and mineralogical characteristics of sand. The mineralogy and geochemistry of a sand sample indicate the composition of the parent rock material, while the textures and sand size characteristics reflect the environmental conditions impacting it.

Sand is defined as non-cohesive sediment with grain diameters in the range of 0.063 to 2mm. It can be composed of rock fragments, the so called lithic sand, and mineral grains, which originate from the weathering of parent rock material or from biogenic sources.

The easiest way to distinguish the source of sand grains is from their colours. Obviously, this has to be supported by mineralogical examination, but at least we can make an initial guess. When we look around the world, we find white and black sand on beaches, and any hue of colour in between them: green, reddish, pink...you name it. Black sand is often of volcanic origins or contains black coloured minerals, for example pyroxene and tourmaline. White sand originates from various sources. Pure white sand might originate from limestone, contain mineral hydromagnesite, or contain quartz without any impurities. Biogenic sand is mostly whitish with possible contributions from corals, foraminifera, molluscs, sea urchins, algae and sponge spicules. Some foraminifera species give a pink hue to beach sand such as seen on the Bermuda Islands and Flores. Green colour in sand is contributed by the mineral glauconite and can be found in, for example, Papakolea Beach, Hawaii. Hematite, rutile, spinel and garnet are among the many minerals giving a reddish to orange and brown hue to sand grains.

Using the colours, supported by mineralogical and geochemical examination, we can identify various sources of beach sediments, ranging from river, sand dunes, cliffs and rocky shores, offshore origins, or artificial nourishment. These sources deliver various sizes, shapes and colours of sand depending on its grain composition.

The particle size characteristics and the statistical properties of sand grains such as the degree of sorting, skewness and kurtosis, can be used to infer the geomorphic setting and the sediment transport mechanism. Sand grain size is determined by the energy of the transport medium. Given similar current velocity, the finer and lighter sand grains can be transported further from their sources.

Sand grain texture, such as the roundness and angularity, gives information on the length of time and/or distance of transport route, while the distribution of grain sizes provides indications of which environment these grains come from. Sorting is mostly affected by how the sand was transported to its present location: a high velocity transport medium will result in poorly sorted sediment, while sediment moved by wind is usually well sorted. Hence river sand is usually poorly sorted and beach sand tends to be rounded. Dune sand, which is transported by wind, is generally rounded and well sorted.

Kurtosis and skewness are important statistical indicators of the bimodality of a distribution and the sedimentary origin. Non-normal distributions indicate that the sediment is composed of two or more superimposed modal fractions. High skewness and kurtosis values indicate that the sediment underwent high energy reworking in and/or adjacent to the depositional area. Extreme kurtosis values indicate that the sediment experienced more sorting in the previous environment before being deposited in the present environment.

So what can we learn from what this polyglot sand grain tells us? Well, beach erosion or accretion depends on the availability of sand and the assurance that it can be timely delivered to the beach. Therefore, if we can identify where the sand comes from and the routes it traveled, we can figure out how to manage beach erosion by keeping the sources sustainable and the pathways unobstructed. It is of the utmost importance to carefully investigate the hydraulic processes, both inland and in the nearshore, before considering any development on or near the beach.