海岸动力学英文PPT课件Coastal Hydrodynamics_6.2.ppt

Coastal Hydrodynamics,Shoreline configuration,§6.3 Coastal Changes,2. Coastal changes _ case studies,3. Mechanism of coastal changes,4. Methods to analyze coastal changes,Chapter 6,The 1ongshore sediment transport in the surf zone and the subsequent deposition of sediment in regions sheltered by islets or reefs may cause the formation of reefs, spits or bar chains along the coast, such as the Jimudao spit at Longkou.,Chapter 6,When a spit links the mainland to a offshore island it becomes a tombolo. A tombolo may also develop from a cusp-shaped body growing in the lee and eventually connecting with an island.,Chapter 6,A growing spit often deflects the mouth of a river or the entrance to a bay prolonging it in the direction of longshore sediment drift.,Chapter 6,For a given littoral compartment, suppose the rate of sediment drift into the compartment is denoted as Qin, and Qout is the rate drift out.,1. Shoreline configuration,Chapter 6,If Qin Qout, which denotes deposition within the compartment, and the coastline approaches seaward: beach deposition will occur. If Qin Qout, which denotes a net deficit within the compartment, and the coastline retreats landward, that is to say, there will be beach erosion.,Chapter 6,When Qin is equal to Qout, which indicates that there is neither erosion nor deposition within the compartment, therefore the coast is stable. The lack of either beach erosion or deposition indicates that a state of equilibrium exists between the sources and losses.,Chapter 6,An equilibrium beach is a beach whose curvature in plan view and profile are adjusted in such a way that the waves impinging on the shore provide precisely the energy required to transport the load of sediments supplied to the beach. The equilibrium condition is one in which the shoreline is everywhere parallel to the crests of the incoming waves.,Chapter 6,This condition is most closely approached by a pocket beach where there is little or no additional sand being supplied to the beach. The development of sand beaches in the Henry Bay is an excellent example of the way in which local beaches orient themselves parallel to the refracted wave crests and develop the same curvature.,Chapter 6,Wave refraction diagram of Henry Bay for a 14s southwesterly swell. Every fifth wave crest is shown as a broken line, and sand beaches are represented by thickened lines.,Chapter 6,With established sediment sources and under a certain wave climate, a beach will tend toward a natural equilibrium where the waves are just capable of redistributing the sands supplied from the sources.When jetties, breakwaters, or other structures are constructed in the coastal zone, the natural equilibrium will be upset, sometimes will disastrous consequences.,2. Coastal changes_ case studies,Chapter 6,Jetties and breakwaters act as partial or total dams to the littoral drift, blocking the natural sediment movement along the shoreline. That is to say, coastal engineering structures upset the natural equilibrium between the sources of beach sediment and the littoral drift pattern. In response, the shoreline must change its configuration in an attempt to reach a new equilibrium.,Chapter 6,Interfering with the longshore sand transport,Redirecting wave energy,Chapter 6,Case 1:Madras Harbor, India, showing accretion on the updrift side of barbor and erosion on the downdrift side.,Chapter 6,The initial construction of the harbor for the port of Madras took place in 1875, the breakwater extending outward about 1000m from the original shoreline. this coast is characterized by a strong littoral sand transport from south to north, so that following the breakwater construction, sand accumulated to the south side of the harbor, the shoeline progressively advancing. In the following 36 years (1876-1912), more than 1.8 million square meters of new land formed.,Chapter 6,Waves to the north of the harbor continued to transport sand northward; this sand was not replaced, however, because the sand that would have normally been deposited there was trapped behind the breakwater. This resulted in rapid erosion to the shoreline north of the breakwater for a length of 5km along the shore, and it became necessary to install seawalls to check the destruction.,Chapter 6,Subsequently, to prevent harbor shoaling, the breakwater was extended seaward and a suction dredge installed to pump sand past the harbor. It is seen that the work done in bypassing the harbor by dredging has replaced the natural transport system due to wave action.,Chapter 6,Case 2:Sand deposition in the protected lee of Santa Monica breakwater, California,Chapter 6,At Santa Monica in 1934 a 600-meter-long detached breakwater was constructed parallel to the shoreline and about 600m offshore. Immediately following its construction, sand began to deposit in its protected lee. Up-coast from the breakwater the shoreline advanced, while on the down-drift side the shoreline eroded. Only dredging prevented attachment of the shoreline to the breakwater with a complete closure of the harbor.,Chapter 6,Case 3: Deposition_erosion pattern around the Santa Barbara breakwater,Chapter 6,Originally the breakwater at Santa Barbara constructed in 1927-28 was detached, but in 1930 it was extended and connected to the shoreline to prevent harbor shoaling. The predominant waves are from a westerly direction, causing a large littoral transport to the northeast, computed to average about 215000 cubic meters per year. The breakwater interrupted this littoral drift and caused deposition on its updrift side.,Chapter 6,Sand accumulated on the west side of the breakwater until the entire area was filled, the sand then moving along the breakwater arm, swinging around its tip, and depositing in the quiet waters of the harbor as a tongue or spit of sand. Without dredging, the spit would have eventually grown across the entire harbor mouth, attaching to the opposite shoreline and closing off the harbor.,Chapter 6,To prevent this closure of the harbor, dredging of the spit was initiated and presently operates continuously. This dredged sand is dumped on the beach to the immediate northwest of the breakwater in order to replenish the sand lost by the blockage of the littoral drift and prevent further erosion which took place following the breakwater construction.,Chapter 6,Case 4: Historical changes around the Ocean City Inlet,Jetties constructed shortly after a hurricane open at the Ocean City Inlet in 1933 interfered with the southerly longshore sand drift. The southern end of F. Is. Then underwent accretion and prograded seaward. The northern end of A. Is. became sediment-starved and rapidly eroded.,Chapter 6,Case 5: Pattern of deposition-erosion around the Tillamook jetty.,Under certain circumstances jetties constructed on coasts with a zero net littoral drift have still caused coastal erosion. One example of this is the beach erosion and sand spit destruction at the entrance to Tillamook Bay, Oregon.,Chapter 6,A north jetty was constructed at the mouth of the bay in 1914-17 and was later (1933) rebuilt and extended. Bayocean Spit to the south of the entrance, separating Tillamook Bay from the ocean, suffered progressive erosion following construction of the jetty, especially in its narrow midportion and southern half. Homes and other buildings of the resort village on the spit were progressively destroyed.,Chapter 6,At the same time that spit erosion was occurring along most of the length of the Bayocean Spit, deposition of sand and shoreline advancement was taking place in the immediate vicinity of the jetty. This is especially apparent to the north of the jetty, where the shoreline advanced by some 1000m between 1917 and 1935. In addition, however, the shoreline to the immediate south of the jetty also advanced by some 150m. Even more important, a large shoal developed at the mouth of the harbor, making the entrance nearly impassable even to small craft.,Chapter 6,Because of the shoal, a new south jetty was constructed in 1974. A somewhat different pattern of deposition-erosion around the jetty thereby emerged. Near the jetty itself deposition of sand occurred, while at greater distance from the jetty, both to the north and south, erosion took place. The reason for this distribution is that the jetty provides small embayments together with the shoreline that existed prior to jetty construction. Sand moves alongshore to fill these embayments until the shoreline is straight and again in equilibrium with the waves.,Chapter 6,The jetty may also provide areas to either side which are partially protected from the waves. Sand is transported into these areas and accumulates, the weak, diffracted waves being unable to remove the sand. Therefore, the long- term result is an accumulation of sand near the jetty within the “embayments” and sheltered areas created by the jetty construction. To supply this sand to the deposition zone, erosion occurs at greater distances from the jetty.,Chapter 6,The alongshore change in beach topography is caused mainly by the local balance of the longshore sediment transport. The nonuniformity of the longshore transport rate along the shoreline is the principal mechanism of erosion and deposition of beach sediment.,3. Mechanism,Chapter 6,Iwagaki formulated the following equation,where B is the width of the littoral zone, Qx the lonshore transport rate, h the mean water depth in the littoral zone, hi the water depth which determines the offshore limit of the littoral zone,the bottom sediment porosity, and the x-aixs is taken in the longshore direction.,Chapter 6,The above equation indicates that the coastal change has two contributions: the local change of longshore transport, and the time variation of hi which is determined by the time history of the incoming wave characteristics. Even on a coast where the local variation of longshore transport is zero, beach erosion can occur with increasing wave height. When the time variation of hi is zero, erosion or deposition will completely depend on the sign of the local change of longshore transport.,Chapter 6,Based on these considerations, we can say the long-term variation in coastal topography is in general generated by the local variation in the longshore transport rate. Through use of the above rules, if the rate of sediment transport along a beach can be evaluated, equilibrium, erosional, or depositional regions along the coast can be determined as demonstrated in this figure. The above procedure is applicable to the prediction of the shoreline change due to the placement of coastal structures.,Chapter 6,Long-term coastal changes,deposition,erosion,equilibrium,erosion,equilibrium,Chapter 6,If the budget of littoral sediments is known, the art of the approach is in evaluating the sources and losses such that their balance agrees with the beach erosion or deposition. An assessment of the long term state of a beach can be determined by considering the credits and debits of sand that pass through a coastal system.,4. Methods,Chapter 6,INPUTS (credits) : + Longshore transport into beach + River supply + Cliff erosion + Onshore transport OUTPUTS (debits) : - Longshore transport out of beach - Wind transportation into dunes - Offshore transport BALANCE: Accretion/ Erosion/ Steady state,Chapter 6,Hypothetical sand budget,INPUTS: V+ +50,000 m3/yr O+ +5,000 m3/yr C +5,000 m3/yr OUTPUTS: V- -55,000 m3/yr O- -15,000 m3/yr BALANCE: -10,000 m3/yr (net erosion),Chapter 6,By using some assumptions, we can obtain a simplest analytical solution. The analytical approach attempts to obtain mathematical solution for the shape of shoreline in the longshore direction. It is to solve a relationship which relates the sand transport to the wave parameters, together with a continuity equation for sediment movement in the longshore direction.,Chapter 6,The numerical simulating is an approach to investigating shoreline configuration which is more versatile than seeking analytical solution.,THANK YOU,“Coastal Hydrodynamics” chapter 6,