Working on my research paper this week, I learned a lot about shoreline dynamics and thought I would share some interesting facts.
We all know that there are high tides and low tides, two of each daily but did you know that as the tide goes from low tide to high tide, it is called the flood tide? Once high tide is reached, it followed by the ebb tide, the tide from high to low. This is called a tidal cycle. Why a tidal cycle? Many people believe that tides rise because the Moon pulls Earth’s oceans toward it. But if that were the whole story, there would be a bulge only on the side of the Earth that is facing the Moon and there would only one high tide each day. The correct explanation for tides must account for why Earth has tidal bulges. Only one works: Earth must be stretching from its center in both directions (toward and away from the Moon). That’s right, our tides are determined by the position of the Moon. In addition, the Sun also has some gravitational effect. Every two weeks, when the Sun and Moon are in a straight line (new and full Moon), the tides are higher. These higher tides are known as spring tides. When the Moon is in the first quarter or third quarter phase, the Sun’s gravitational force cancels out some of the Moon’s force and we get lower than average tides known as neap tides. During the spring equinox around March 21, we have the highest of all spring tides; likewise, during the fall equinox around September 22, we have the lowest neap tides. Tides are important to shorelines because wave attack is constantly shifting onshore and off.
Waves are very important to geologic dynamics of the shoreline because it provide a means of sediment transport and deposition as well as erosion. How are waves generated you might ask? Well, wind generates waves. The harder and longer the wind blows, the larger the waves. This is especially problematic during large storms at sea. As the wave moves across the water surface, the water is moving in circular orbits but does not affect the water below. Once the wave enters shallow water, the orbital motion is disrupted and the wave speeds up and increases in height. Incoming waves create both longshore currents and rip currents. Longshore currents are currents resulting from the bending of waves. Longshore currents make the wave head into the shore at an angle. This is why you can see a wave start breaking either from left to right or right to left. The importance of the longshore current is sediment transporting and then depositing sediment in the nearshore zone of the beach. As the water recedes back towards the ocean, it creates another current known as a rip current. Rip currents are fed by the longshore current. They are narrow and nearly perpendicular from the shoreline. Rip currents only transport fine-grained sediment (silt and clay) offshore through the breaker zone.
So where does the sediment come from? The sediment is transported by streams and rivers to the coast and is then distributed along beaches by longshore currents or waves.
The beach is considered the zone of active sand movement and as long as there is sediment or beach sand supply, one factor of the dynamic equilibrium is in balance. Thus stems the first problem. Man has built dams along rivers which have cut off sediment supply. Beach erosion is actually man-made.