Waves waves Waves are defined by the following: Wave height H Wavelength L Period T Velocity V Breaking Waves

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1 Waves The winds not only drive surface currents, it also causes waves. Waves appear on the surface as a series of crests and troughs, moving in the direction of the wind. Waves are defined by the following: Wave height H is the vertical distance between the top of a wave, or crest, and the bottom of the preceding wave, or troughs. Wavelength L is the distance between successive crests or troughs. Period T is the time it takes for one wave to pass a given point. Velocity V is the speed at which a crest, or other specified point, travels. Velocity (V) = wavelength (L)/Period (T) Although waves look like they are moving along, this is only an illusion.(وهم) A floating object such as a boat or a bird does not move forward in a wave train but moves up and down with each passing wave. This is because, when under a wave crest, the water moves up and forward; while under the troughs, it moves down and back; thus, on the whole, water particles don't go anywhere at all as the wave passes, but move in circles or orbits. One could imagine wave action is like the snapping of a rope (The above right Figure). When you snap a rope, the rope itself does not move forward. The movement of your hand produces mechanical energy that is transferred in waves along the length of the rope. Similarly, a wave starts with the energy of the wind pushing on the water. Mechanical energy is transferred to each successive wave. When waves are symmetrical, water particles move in orbits. The diameters of these orbits decrease with increasing water depth and become insignificant at depths greater than ½ L. Breaking Waves What causes a wave to crash, or break, on the beach? As a wave approaches the shore, it enters shallow waters. As the bottom of the wave makes contact with the seafloor, the wave slows (due to friction), which decreases its wavelength, too. This occurs 60

2 when the water depth is about one-half the wave s wavelength. When the water depth is less than one-half the wavelength, the top of the wave which moves faster than its bottom is effectively pushed upward and the wave height increases. These effects are due to the insufficient space available for complete orbits to occur. As a result, the faster top pitches (ينحدر) forward and crash. This action produces a type of wave known as a breaker. Longshore currents and rip currents Longshore Current A current located in the surf zone and running parallel to the shore as a result of waves breaking at an angle on the shore. It also called littoral current. Longshore currents form because waves are continuous and, in most cases, approach the shore at an angle. When a wave enters shallow water it is slowed by the rising sandy or rocky bottom and eventually breaks. As one wave meets the shore and breaks, another wave is right behind it, preventing the broken wave from flowing backward. This causes a build-up of water at the shoreline. This build-up of water is then forced to form a current that flows parallel to the shore close to the water s edge. Longshore currents affect shorelines by redistributing sand and sediment along their path. This redistribution is known as littoral drift and responsible for extensive erosion and transport of beach sands along outer coast beaches. Rip currents Occasionally longshore currents suddenly run offshore in a dangerous, jet-like flow of water that typically extends from near the shoreline out past the line of breaking waves called rip currents. Clues for identifying a rip current Difference in water color (Suspended sediments may be transported back to sea in the rip current.) Line of foam, seaweed, or debris moving out to sea. An area of confusing waves Formation of rip current 1. The orientation of the coastline 2. The angle of incoming waves 3. The presence of man-made coastal structures 4. The flow through channels in sandbars 61

3 Shaping the coastline of Palestine The coastline of Palestine forms a small section of a larger concave system (a littoral cell ) that extend from Alexandria, Egypt to the Bay of Haifa, North of Palestine. This littoral cell forms the southern east corner of the Levantine ش رقي Basin (the next picture). The Nile River, especially its sediment yield originating from Africa s mountains, has shaped this entire coastline, including the coastline of Palestine, over the last 15,000 years. The Nile sand transported by northern east (NE) directed wave driven longshore currents along the entire concave coastline in an anti-clockwise direction. Since the building of the Aswan dams the sand supplied to Palestine's coastal system is derived mainly from erosion of the Nile Delta and from sands offshore Egypt. The sands are transported along the coasts of northern Sinai and Palestine. Their volume gradually declines northward with distance from their Nile source. The longshore transport terminates in Haifa Bay where some sand is trapped, and the rest escapes to deeper water by bottom currents and through submarine canyons. Mediterranean Akko Haifa Long-Shore currents and the local erosion problem: New structures along the coastline, like breakwaters, and commercial ports causes blocking the along shore sand transport and causes an erosive effect on the coast downstream (the northern coast). An example is the recently constructed Gaza fishing harbor (the right picture), that has locally disturbed the coastal erosion and sedimentation pattern, resulting in local coastal sand erosion problems. The planned Gaza Sea Port will even more increase this problem. 62

4 Tsunami Tsunami (seismic sea waves) is long, fast waves produced by earthquakes and other seismic disturbances of the sea floor. Tsunami is a Japanese word meaning means harbor ( 津, tsu) waves ( 波, nami). They were once called tidal waves, but they have nothing to do with the tides. They are produced instead by earthquakes and other seismic disturbances of the sea floor, so they are more properly called seismic sea waves. A tsunami watch is issued whenever there is an earthquake stronger than 6.75 on the Richter scale. When such a disturbance occurs, it can produce very long, fast-moving waves. Tsunamis may have wavelengths of 240 km and can travel at over 800 km/hr as fast as a jet airplane. In the open ocean, tsunamis are not very high, usually less than 1 m. Most of the time ships at sea don t even notice the passing of a tsunami. Tsunamis usually get higher when they approach shore and may reach as much as 20 to 30 meters high. A few tsunamis occur almost every year, but most are not very damaging. Occasionally, however, the waves grow huge and cause great death and destruction. Fig. Diagram illustrating how Tsunami s form Since tsunamis are unexpected, especially in developing countries, it can be so destructive. On Sunday, 26 December 2004, the greatest earthquake in 40 years about 150 kilometers off the west coast of northern Sumatra Island in Indonesia. The earthquake generated a disastrous tsunami that caused destruction in 11 countries. The resulting tsunami devastated the shores of Indonesia, Sri Lanka, South India, Thailand and other countries with waves up to 30 m (100 feet) high. It caused serious damage and deaths as far as the east coast of Africa, with the furthest recorded death due to the tsunami occuring at Port Elizabeth in South Africa, 8000 km (5000 miles) away from the epicentre. Anywhere from 228,000 to 310,000 people are thought to have died as a result of the tsunami. 63

5 Tides According to Newton s Law of Gravitation; the force of gravity acting between any two bodies is proportional to the product of the masses of the two bodies and inversely proportional to the square of the distance between them. Both the sun and the moon exert significant gravitational attraction on the ocean, but the moon exerts twice the gravitational attraction and tide-generating force as the sun because it is closer to the earth (384,400 km instead of 149,600,000 km). Tidal motion can be measured throughout the ocean, but it is especially noticeable at the shoreline in the form of tidal currents and vertical motion. The extent of the tide is largely determined by the difference in gravitational attraction on either side of the earth. On the side closer to the moon the gravitational attraction pulls water toward the moon. On the opposite side of the earth, a minimum of gravitational attraction combines with the earth's spin to produce a net excess of centrifugal force, creating a tidal bulge away from the earth. Corresponding depressions (low tide) will exist on parts of the earth between the bulges, where there is no net excess of gravitational pull relative to centrifugal force. Because the moon "passes over" any point on the earth's surface every 24 hours, 50 minutes, or once each tidal day, ideally there should be two low and two high tides per day. Because the moon's position relative to the earth's equator shifts from 28.5 N to 28.5 S, the relative heights of high and low water differ geographically owing to changing vectors of gravitational attraction. Types of Tides Spring tides الع الي الم د occur when Earth, Moon and Sun are aligned in a straight line, thus, the gravitational force exerted by the sun amplifies that of the moon, and maximal tidal range is achieved producing very high, high tides and very low, low tides. This occurs at the full and new moons. Neap tides المعت دل الم د Occur when sun, earth, and moon form a right angle, which happens when the moon at the first and third quarters, the gravitational effects tend to cancel each other out, and neap tide occurs, with the minimum vertical range. Two spring tides and two neap tides occur each lunar month (approximately 29.5 days). 64

6 Tides in the Real World Given the daily rotation of the earth, one might expect two equal high tides each day, separated by 12 hours. However, tides in the real world behave somewhat differently. The presence of irregularly shaped basins, the tilt of the earth's axis, and the Coriolis effect tend to cause significant deviations from this idealized expectation. We can recognize three major patterns to tidal cycles: Diurnal tides المد والجزر اليومي have one high tide and one low tide each lunar day. Semidiurnal tides ي ومى النص ف والج زر الم د have two high tides and two low tides each lunar day, and each tide is the same height as the previous one. Mixed tides المرآ ب والج زر الم د also have two high tides and two low tides each lunar day, but each tide is a different height than the previous one. In Gaza Strip, so far, no systematic records are available of tides. However, at Ashdod, some 40 km North of Gaza City, the following tidal levels are given as below: Mean high water springs (MHWS) 0.6 m Mean high water neaps (MHWN) 0.4 m Mean low water neaps (MLWN) 0.1 m Mean low water springs (MLWS) 0.0 m Effect of Tides on the Life Cycles of Marine Organisms The incoming tide signals the final chapter in the life cycle of many marine organisms as their remains are washed up on the shore. But the rising tide also heralds the beginning of life for other life-forms. For the grunion (Leuresthes tenuis), a small fish (15 cm), life begins at high tide. Grunion run Fertilization 65

7 During the spring and summer, thousands of these silvery fish swim up onto the sandy beaches, carried in by the high tide. This so-called grunion run occurs at night during the new moon and full moon when the tide is highest. The female grunions wiggle into the sand and lay thousands of eggs as the males deposit sperm around them. Afterward, the fish are swept back into the sea by the water. The spawning is timed so exactly that it occurs only on the second, third, and fourth days that follow a new or full moon. After the grunion eggs are fertilized, they incubate in the sand for two weeks until the next new or full moon occurs. At that time, the waters of the high spring tides will reach the eggs and wash them out of the sand. The eggs then begin hatching into tiny grunions as they are carried seaward by the outgoing tide. El Niño-Southern oscillation (ENSO). Condition in which warm surface water moves into the eastern pacific, collapsing upwelling and increasing surface water temperatures and precipitation along the west coast of North and South America. Normal Conditions Normally, strong trade winds push warm equatorial waters across the Pacific, resulting in storms and high rainfall on the west side of the Pacific, and cold upwelling on the east side. El Niño Conditions In an El Niño year, the trade winds diminish and reverse. This allows the warm equatorial water to flow back east, resulting in more storms and rain in the eastern Pacific, and reduced amounts of upwelling. 66