  Articles and gems - Protecting sandy coasts - Rippels, runnels and waves
This is one of the sandy coast pages (first page).
Ripples and waves are everywhere. You say "yes" and produce audible waves. You may heat your food with waves,
in a microwave oven. You enjoy the sunshine: infrared and ultraviolet waves, and visible waves.
You let strong winds blow across an ice surface and you produce ripples in the ice: differential sublimation
(ice going into vapor). You throw a pebble into a pond: waves, ripples. You let the wind blow across the surface
of the water and the friction will create waves.
You let the moon's gravity pull at the earth's water and you create waves: tides.
Waves in water often create ripples in sediment: symmetric ripples. The friction of flowing
water across unconsolidated sediment also creates ripples: assymmetrical ripples.
Left: ripples on Shell Key, Florida. Right: ripples on Norderney, Germany (picture by Suzanne Hulscher).
Click on the pictures for a larger version.
The above Norderney ripples look fairly symmetrical and run parallel to the shore.
They were probably caused by waves: a fairly symmetrical oscillating movement of
water particles, perpendicular to the shore.
We also saw undulating, scalloped flow ripples in a dry runnel on Norderney. Runnels are
filled at flood tide and run dry at ebb tide. They tend to be dominated by currents, not
by waves. Runnels are separated from the sea by ridges.
Runnels may also be affected by waves, for instance at higher-than-average tides (storm
surges). In that case, existing ebb current ripples may be modified by waves.
Ladderback ripples are the result.
Ripples caused by waves tend to run parallel to the shore, while current ripples in runnels tend to be
perpendicular to the shore. That's because the general current direction is parallel to the shore.
By the way, geologists call the direction in which the ripples run "strike". These
ripples on the Norderney beach in the picture above strike E-W.
Current ripples tend to be asymmetrical: no oscillating water movement but water
predominantly moving in one direction only. All ripples usually form perpendicular to the water movement.
The following pictures were taken at the Zandvoort beach on February 16, 2001. Let's make some
observations there. If we look to the south (left),
we notice a filled runnel. If we look to the north, we find an empty runnel.
If you can click on the pictures below, you will end up on a larger version and will clearly see that the
southern runnel is filled with water.
 
The water in the runnel on the left flows from left to right in the above picture.
Notice the grain size gradient from courser to finer material from bottom to top in the picture on the right? (Above.)
The pictures above on the left show a filled runnel on the foreshore at Zandvoort,
The Netherlands. You can actually
watch ripples migrate, there. Amazing. You can see the sediment grains whirl in the water.
You can see large grains, such as shells, being dropped in the ripple valleys. The water
depth is greater in the valley. So the flow surface increases and the velocity decreases.
Q remains the same. The shell drops. But look: the shell becomes rapidly covered
with sand. It's wonderful to watch.
At great current velocities (above 1.2 to 1.9 ms-1), antidunes develop
instead of ripples. Antidunes migrate upstream.
In the pictures below, you will notice a beautiful set of asymmetrical ripples.
Click on the picture to see the larger version.
 
See how that beautiful parallel set of current ripples transforms into scallops?
That is more or less what we also saw on Norderney? The velocity of the flow and
the degree of uniformity of the flow velocity (or the lack of it) determine the shape of
ripples, and the material of the ripples, and, to some degree, water depth.
Notice how the filled runnel empties into a small bay? Let's walk to that bay.
Left: looking to the south. Right: looking to the north.
We have a combination of waves here. Some waves are caused by the strong local wind. Other
waves were generated by winds at a distant location.
The larger waves break at the edge of the shore and lose most of their energy there.
The tide appears to be rising. Judging from the fact that the southern runnel
was filled and the northern runnel was not, you'd think that the tides progress
from south to north at this location.
Notice the "ladderback" waves in the picture above on the right? Ladderback ripples
look like that.
Notice the wave diffraction patterns?
In a bay with a gradually rising bottom, wave energy is spread. Wave fronts diverge. At
a cape with a gentle slope, wave energy is concentrated. Wave fronts converge.
Notice the sediment ripples? Sort of hummocky in some places? Like above?
And sort of regular and nice in others? Like below? Undulating and merging flow ripples
(asymmetrical).
Waves in water consist of circular water movements. As waves move into shallower water, the
circular path of the moving water particles becomes flattened and energy is dispersed. It is
this energy that moves sediment. Then, finally, waves break.
There are four types of breaking
waves (see page 30 of the Open University book by Bearman (ed.), 1994):
- spilling breakers
(a steep wave running onto a shallow beach slope and creating foam and bubbles)
- plunging breakers
(a less steep wave running onto a shallow to intermediate beach slope)
- collapsing breakers
(an intermediate wave running onto an intermediate to steep beach slope)
- surging breakers
(a low wave with a long period that runs onto a steep beach slope)
Wave steepness is wave height divided by wavelength (H/L). To ships, and to people aboard
those ships, steepness matters more than wave height.
Spilling and plunging breakers are associated with gentle slopes. Plunging breakers on beaches
with gentle slopes are usually caused by storms at some distance. It takes steeper beaches to
get plunging breakers in a local storm.
A frequently used factor to characterize a state of sea is significant wave height.
Significant wave height increases with wind speed. It's the average height of
the highest one third of the
waves in a particular time slot. Significant wave height is denoted by
H1/3 or Hs.
Wave speed is indicated by c, which stands for celerity of propagation. There are several
formulae for wave speed, mainly according to water depth (friction).
Wave energy depends on wave height and density. The density of seawater is determined by
salinity and temperature. Waves in an estuary have less energy than waves at sea, because in
an estuary, the salinity is lower. Salinity means: the amount of salt that is dissolved in
the water.
Beaches generally show different profiles in summer and winter due to the difference
in wave climate.
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Most recent changes to this page:
November 24, 2007
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