“ “Bilingual Course”Bilingual Course”ZHENG ZHENG JinhaiJinhai April 2009 April 2009Coastal HydrodynamicsCoastal HydrodynamicsHOHAI UNIVERSITYHOHAI UNIVERSITY 1. 1. Shoreline configuration Shoreline configuration 6.3 6.3 Coastal ChangesCoastal Changes2 2. . Coastal changes _ case studies Coastal changes _ case studies 3 3. Mechanism of c. Mechanism of coastal changesoastal changes4 4. Methods to analyze co. Methods to analyze coastal changesastal changesChapter 6The 1ongshore sediment transport in the surfThe 1ongshore sediment transport in the surfzone and the subsequent deposition of sediment zone and the subsequent deposition of sedimentin regions sheltered by islets or reefs may cause in regions sheltered by islets or reefs may causethe formation of reefs, spits or bar chains along the formation of reefs, spits or bar chains alongthe coast, such as the the coast, such as the JimudaoJimudao spit at spit at LongkouLongkou. . Chapter 6When a spit links the mainland to a offshoreWhen a spit links the mainland to a offshoreisland it becomes a island it becomes a tombolotombolo. A . A tombolotombolo may mayalso develop from a cusp-shaped body growing also develop from a cusp-shaped body growingin the lee and eventually connecting with an in the lee and eventually connecting with anisland. island. Chapter 6A growing spit oftenA growing spit oftendeflects the mouthdeflects the mouthof a river or theof a river or theentrance to a bayentrance to a bayprolonging it inprolonging it inthe direction ofthe direction oflongshore longshore sediment sediment drift. drift. Chapter 6For a given littoral compartment, suppose theFor a given littoral compartment, suppose therate of sediment drift into the compartment israte of sediment drift into the compartment isdenoted as denoted as Q Qinin, and , and Q Qoutout is the rate drift out. is the rate drift out. 1. 1. Shoreline configurationShoreline configurationChapter 6If If Q Qinin Q Qoutout, which denotes deposition within the, which denotes deposition within thecompartment, and the coastline approaches compartment, and the coastline approaches seaward: beach deposition will occur.seaward: beach deposition will occur.If If Q Qinin Q Qouout t, which denotes a net deficit within, which denotes a net deficit withinthe compartment, and the coastline retreatsthe compartment, and the coastline retreatslandward, that is to say, there will be beachlandward, that is to say, there will be beacherosion. erosion. Chapter 6When When Q Qinin is equal to is equal to Q Qoutout, which indicates that, which indicates thatthere is neither erosion nor deposition withinthere is neither erosion nor deposition withinthe compartment, therefore the coast is stable.the compartment, therefore the coast is stable.The lack of either beach erosion or depositionThe lack of either beach erosion or depositionindicates that a state of equilibrium existsindicates that a state of equilibrium existsbetween the sources and losses. between the sources and losses. Chapter 6An equilibrium beach is a beach whose curvatureAn equilibrium beach is a beach whose curvaturein plan view and profile are adjusted in such ain plan view and profile are adjusted in such away that the waves impinging on the shoreway that the waves impinging on the shoreprovide precisely the energy required to transportprovide precisely the energy required to transportthe load of sediments supplied to the beach.the load of sediments supplied to the beach.The equilibrium condition is one in which theThe equilibrium condition is one in which theshoreline is everywhere parallel to the crests ofshoreline is everywhere parallel to the crests ofthe incoming waves. the incoming waves. Chapter 6This condition is most closely approached by aThis condition is most closely approached by apocket beachpocket beach where there is little or no additional where there is little or no additionalsand being supplied to the beach. sand being supplied to the beach. The development of sand beaches in the HenryThe development of sand beaches in the HenryBay is an excellent example of the way in whichBay is an excellent example of the way in whichlocal beaches orient themselves parallel to thelocal beaches orient themselves parallel to therefracted wave crests and develop the same refracted wave crests and develop the same curvature. curvature. Chapter 6Wave 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 6With established sediment sources and under aWith established sediment sources and under acertain wave climate, a beach will tend towardcertain wave climate, a beach will tend towarda natural equilibrium where the waves are just a natu