Coastal Erosion: Where does the sand go?

When you are sitting enjoying a spectacular Bali sunset on a cool dry season evening, say on Kuta Beach, have you ever wondered how the sand you are sitting on ended up there?

Then a few months later you go back to the same spot and find that the beach level is a lot lower. The beach also seems narrower. The sand was swept away! Is it gone? Why, you wonder. And how?

This reminds you of the news about coastal erosion anywhere in the world: encroaching houses, roads and villages, forcing people to give up their properties along the beach. So, what happened to the sand on the beach?

Well, the beach profile is always changing. Shorelines are shifting all the time and when left undisturbed, the beach is said to be in a dynamic equilibrium. It sounds contradictory being ‘dynamic’ and in ‘equilibrium’ at the same time, doesn’t it?

In this sense we can say that, coastal erosion is a natural phenomenon because coastlines change continually, controlled by the interaction of local hydrodynamics and morphology. The changes become a problem when the equilibrium is disturbed by human intervention.

Being a relatively narrow strip of land and often located in a beautiful setting with a breathtaking view, the beach area becomes a scarce commodity with a high land value. This leads to conflicting interests among the stakeholders such as tourism, property development, infrastructure and citizens concerned about its ecosystem value.

Now, returning to the 'mystery' at hand, what happened to the sand on the beach?

On sandy beaches, the sand is constantly moved around by waves, currents and wind and continually moving within the nearshore – beach – dune system. Sand dunes undergo cycles of erosion and accretion by wind and waves. Dunes develop when there is enough supply of sand from a dry beach and the prevailing wind is strong enough to move the sand landward. When the wind reverses and blows offshore, sand in the dunes is transported back to the beach. Stormy weather erodes more dune sand and high waves move the sand further offshore and deposit it on nearshore bars.

The formation of the sandbars beyond the initial breaker line will cause waves to break further offshore from the beach, thereby reducing the storm impact on the beach. The subsequent swell will gradually bring sand back to the beach and eventually to the dunes. Under natural conditions, the changes of shoreline can be substantial due to these storm – calm cycles, but the long-term net changes may be quite small. This condition is called dynamic equilibrium.

Unfortunately, as often occurs, man with his static sense of equilibrium, intervenes this natural cycle, as stated by Souci in 1973: “once a line has been established, whether it be a shoreline or property line, man unreasonably expect it to stay put.”

Hence, a hard engineering approach is applied. Seawalls, breakwaters and groynes were built, often without proper consideration of the overall coastal processes at hand. When the construction design overlooks the local coastal processes causing the erosion, the problem might persist or even worsen. The affected beach might disappear altogether, even due to a single extreme storm event, because the hard surface of the construction reflects storm waves and displaces beach sand seaward or in the drift direction. Furthermore, seawalls prevent sediment exchange between beach and dunes, disrupting the natural sediment dynamics in the area. The sand that once laid on the beach has been transported further offshore or has drifted away along the coastline, unable to return because its ‘home’ is disturbed.

Can we keep the sand in its location so we can enjoy the sunset comfortably?

Yes we can, but that is another story.