Coastlines

COASTLINES

Coastlines are the interface between water and land. Waves in water will affect the land in many ways, especially along coastlines. Let's look briefly at why we get waves.

Waves on the surface of the water are from wind energy transferred to water due to the friction between the water and blowing air. Water moves in waves that have crests or peaks and troughs or lows.

The distance between a crest or trough is called the wavelength, while the height of the wave or wave height is from the crest down to the trough. The wave amplitude is not the same as the wave height. It is defined as half the wave height and is measured from the middle point to the crest. The wave period is the time it takes to get one crest to pass and another to arrive at any given point. The wave speed or velocity is the forward momentum or energy of the wave in distance divided by time. The wave base is the spot below which the water is undisturbed by wave activity. The fetch is the distance involved in the blowing of any wind. A long fetch means the wind energy was strong.

Figure 60 shows wave anatomy in detail:

Figure 60.

The inside of a wave will be a circular orbit of water. This works great until the orbit gets too elliptical. Then it just breaks and crests as a breaking wave. Waves near shore will break because the ground beneath the ellipse of water interrupts the ellipse action. The wave then just breaks at that point and rolls to shore.

Waves in the ocean away from shore are choppy. This is because waves are coming from all over and are cancelling themselves out. Near the shore, you get breaking waves that are mostly of the same height and frequency.

Tsunamis are not caused by wind but are from energy you get off the sea floor after an earthquake, eruption, or landslide. Water lifts and bulges upward to create a high energy wave that can travel far without loss of much energy. The wavelength of these waves is so long, you wouldn't necessarily notice them in deeper waters. The wave heights of these waves are low so it takes shallow waters to see them and feel their tremendous

energy.

Coastlines around earth are complicated. Not all are sandy beaches with gentle wave action. Coastlines include all inland and ocean to land interfaces. All interfaces like this are called littoral zones. These are divided into several parts, including these:

• Offshore zone – this is the part always below water. The area is affected by turbidity and currents.

• Nearshore zone – this area is where the water depth is less than half of a wavelength of an onshore wave. The width of this depends on how shallow the area is.

• Shoreface zone – this is where sand is deposited and continually disturbed. There are two parts of this; the upper and lower parts. Only the upper part is disturbed all the time. The lower part only gets disturbed when there are storm waves.

• Surf zone – this is where waves break and surfers surf.

• Foreshore – this is the wet or dry sandy areas where sand might be sorted on a beach. Waves wash up this part and backwash rolls back into the ocean.

• The berm is the ridge you'll see where people put their dry beach towels. There are often two berms. The summer berm is closer to the water because wave action

is not as steep. Winter berm is a sand ridge built up when winter wave energy is greater and waves pile up sand further inland.

• The backshore – this is the area above sea level all the time, but is where sand dunes can build up due to blowing sand.

Waves rarely hit the land at a perpendicular angle or dead-on. They refract off the coastline at some other angle, even though it doesn't appear that way. This is because waves bend near shore and come on looking straight. There is actually a longshore current that drags sand down the coast in what's called longshore drift. In North America, most longshore drift is from north to south. Wherever the water is quieter, sand will tend to be deposited.

Rip currents happen when a wave train comes straight onto the shore at a 90 degree angle and has no place to go after that because of an inlet of some kind. The rip current is the effort by this energetic water to get back out to sea. It usually slips out through a narrow but intense current that can carry swimmers with the current straight out into the ocean. The only way back is to swim parallel to the beach, get out of the current, and ride the waves back to shore.

Undertow is different from a rip current in that it is under the wave. It is strong near the surf zone area and represents the water slipping back to the ocean to make up for the wave above it pushing water toward the shoreline.

Cliffs, exposed areas of bedrock, rocky shoreline, stacks, arches, and other features represent emergent coasts where the sea level has fallen somewhat relative to the land. A submergent coast is the opposite – where the sea level rises and buries some of the land near the coast. Sometimes, tectonic activity leads to a sudden submergent coast as the earth sinks. This is how you get lagoons, estuaries, tidal flats, fjords, and barrier islands. A fjord is a glacial valley that has become flooded by sea level after an ice age.

Tidal flats are mudflats that change with the tides. The three major sections of tidal flats are the barren zones, the salt pans, and the marshes. Barren zones are linked to stronger water that has coarse sediment. You might see ripple marks in this region. Marshes are wetter and vegetated with both sand and mud. Salt pans or salt flats are not submerged as much as the other areas, so they are full of fine-grained silt and/or mud.