Coastal Ecology and Sustainability Tides, Waves, Currents etc. Lecture 1

Tides, Waves, & Currents Manifestation of the Moon’s and Sun’s force of the gravity acting on the Earth hydrosphere Shallow water waves affecting the World’s oceans from top to bottom Heights (vertical excursions) reach as high as 15m (Bay of Fundy) Law of Gravitation Centrifugal force counteracting gravity Gravitational pull of Moon is greater than the centrifugal force Result is two ‘tidal bulges’ Tidal period is 12 hours and 25 minutes (takes an additional 50 minutes each day fro Earth to catch up with the Moon in its orbit) Additive effect of Sun (although at greater distance than Moon from Earth) Diurnal, Semi-Diurnal and Mixed Tides Tidal Bore Tidal Currents – flood tides and ebb tides See also: http://www.onr.navy.mil/focus/ocean/motion/tides1.htm http://www.sfgate.com/getoutside/1996/jun/tides.html

Tides, Waves, & Currents As wind passes over the water's surface, friction forces it to ripple. The strength of the wind, the distance the wind blows (fetch) and the length of the gust (duration) determine how big the ripples will become. Waves are divided into several parts. The crest is the highest point on a wave, while the trough, or valley between two waves, is the lowest point.Wavelength is the horizontal distance, either between the crests or troughs of two consecutive waves. Wave height is a vertical distance between a wave's crest and the next trough. Wave period measures the size of the wave in time. A wave period can be measured by picking a stationary point and counting the seconds it takes for two consecutive crests or troughs to pass it. In deep water, a wave is a forward motion of energy, not water. In fact, the water does not even move forward with a wave. If we followed a single drop of water during a passing wave, we would see it move in a vertical circle, returning to a point near its original position at the wave's end. These vertical circles are more obvious at the surface. As depth increases, their effects slowly decrease until completely disappearing about half a wavelength below the surface. See http://www.coastal.udel.edu/faculty/rad/wavetheory.html http://www.salemstate.edu/~lhanson/gls214/gls214_waves.html

Tides, Waves, & Currents

Tides, Waves, & Currents Longshore Current. This current is caused when waves strike the beach at an angle. The front part of the wave hits the shallow water first and slows down. The rest of the wave bends as it comes onto the shore creating a current that parallels the beach. Larger waves, which strike the beach less often at greater angles, create stronger Longshore currents. In areas where Longshore currents often occur, sandbars form. Rip currents are a potentially dangerous effect of Longshore currents. Rip currents, sometimes called rip tides, can happen when Longshore currents, which move parallel to the beach, bounce seaward because of a change in the bottom's structure. Breaks in sandbars are also optimal places for rips to happen. One type of vertical current is called a coastal upwelling. Winds blowing offshore (or toward the ocean) push water away from the shore. Deep, colder water rises to replace the water that has been blown out into the ocean. This cold water from deep off the ocean floor brings many nutrients to the surface. Downwelling is another coastal happening. Onshore winds (or winds blowing toward the shore) push water toward the coast. This drives the nearshore surface water down and away from the coast. http://www.onr.navy.mil/focus/ocean/motion/currents2.htm

Erosion and Deposition etc.. Hard and Soft Coastlines Erosion and Deposition Wave-cut cliffs Wave-cut platform Sea arches Sea stacks Beaches Spits Bay-mouth bars Tombolos Barrier islands Other erosional and depositional features e.g. through build-up of deposits produced up-current of groins and other hard structures built along the shoreline, or behind breakwaters that are built parallel to the shore

Erosion and Deposition etc.. Climate Change and Sea Level Rise/Fall The Mississippi delta region is currently sinking, and so in that area, sea level is rising rapidly, and delta marshes are being invaded by the sea The New England region, as well as Canada, is currently rising (through isostatic uplift because of the removal of the glaciers). Frequency of Storms Disasters e.g. Tsunami / Hurricanes Influence of Mankind e.g. coastal management Hydrological Catchment, Coastal Zone and the Marine Environment