Lindorm, Inc.
Lindorm, Inc.
"Sedimeter sensors designed and
built by Lindorm are a fantastic
tool for measuring water column
turbidity and siltation." (Gravity)

Waves Shape the Beach

The water in waves moves in closed orbitals. This makes waves a form of energy transfer without mass transfer. Since there is no mass transfer involved, waves are not affected by the Coriolis effect the way winds are. Therefore, they go straight ahead on deep water.

The velocity of the waves is a function of the depth, when the depth is less than half the wavelength. This is called shallow water, and it is relative to the wavelength. The shallower it gets, the slower the waves move. If waves come in obliquely towards a coast, the landward end of a weavecrest slows down the most. The imaginary wave rays bend towards the shore. This is called refraction. When the waves reach the shore obliquely, they create an alongshore current.

Since the period can not change when waves get in on shallow water, the wavelength decreases. This means that the same energy must be compressed over a smaller area. As shown here to the right the energy is related to the square of the wave height. Although some energy is lost by friction against the bottom, the net result is typically that the waves get higher shortly before reaching land.

Waves can not get steeper than 1:7 without breaking. They typically break when the depth is about 75% to 80% of the wave height. When they break they dissipate most of their energy. It is lost as turbulence, and if the bottom consists of sand, the turbulence can stir up large quantities of sand, which will remain for a number of seconds in suspension. This sediment is carried along by the longshore current, which typically reches from the shoreline to just off the breakers.

If the beach is steep the waves can reach the shoreface without breaking. The waves go up as swash on the beach, and return as backwash. The swash is in the direction of wave advance, while the backwash is in the downslope direction. If the waves come in obliquely this creates a zig-zag transportation pattern for sand on the beach. The net result also here is sand transport in the direction of the waves.

Natural Erosion

Beach erosion occurs when the incoming sediment transport to a stretch of coast is less than the outgoing. This can happen if the sediment transport capacity increases over the stretch of coast. It can also happen if the incoming capacity was not utilized, which would be the case if there was no sand available for transport. The latter is a common effect of human interference in the beach system.

Erosion can, however, also be natural. The sea level changes eustatically, and the land level changes isostatically and tectonically. This brings changes in the base level, which affects the beach equilibrium profile. As the geomorphologic development advances, the supply of sand from erosion of coastal bluffs can increase of decrease. The world is in a continous evolution, always on the way towards a quasi-equilibrium, but seldom reaching it.

Erosion and sedimention, also known as siltation, can be measured directly using a SediMeter instrument, either for research or for real-time monitoring. The latter may be necessary in e.g. navigation channels while dredging or construction is going on nearby.

Energy content

The wave energy is a function of the square of the wave height. The energy in joule per square meter is given by:


r = water density
g = acceleration of gravity
hRMS = root mean square of the wave amplitude

Tombolo in Costa Rica
Tombolo in Costa Rica. The general transport direction on a coast depends on the prevalent wave direction. The local wave direction is influenced by the bottom topography (also known as hypsography), through the process of wave refraction. A tombolo is an extreme example of the effect of wave refraction behind a nearshore island. On both sides of the island the waves are refracted towards the island. The effect is that the sand transport on the mainland beach is directed to a point behind the island, from both directions. The end result of the process is a tombolo such as this one.
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