14-Onshore-Offshore Trans

14-Onshore-Offshore Trans - Onshore-Offshore Sand Transport...

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Unformatted text preview: Onshore-Offshore Sand Transport Onsh Erosion Accretion Beach Face represents an equilibrium between onshore and offshore sand transport W a v e S w a sh B a c k w a sh B a c k w a s h r e s u l ts i n e r o s i o n - E n e r g y ? W a v e s w a s h r e s u l ts i n a c c r e ti o n - E n e r g y ? Vertical Accretion Vertical Sand deposited in thin layers by wave Builds the beach vertically Steepens the beachface swash Each layer steepens the beachface Breaking waves carry sand up the beachface Beachface Beachface Beachface Equilibrium Wave swash moving sand onshore steepening beach face energy? Back wash transporting sand offshore which flattens the beach face energy? As Beach Face Steepens, wave swash diminishes and back wash increases Equilibrium Reached when the volume moved onshore and offshore are equal Factors Affecting Beach Face Factors 1. Grain size- controls rate of percolation 2.. Sorting- how many size classes 2 comprise the sand population 3. Wave energy- how much water is delivered to the beach 4. Wave length (L) & wave period (T)4. control the frequently of waves coming onshore 5. Wave steepness (H/L)- controls erosional versus accretionary tendency of waves Grain Size Affects how much water comes (wave swash) onshore versus offshore (backwash) As a wave transports water up the beach face, some water percolates into the beach Rest of water moves back down the beachface as backwash Percolation increases with increasing grain size Percolation W a te r d r a i n s to th e s e a b e n e a th b e a c h Percolation Test Percolation tests are used to see how quickly water will drain into the ground. Usually, you saturate the hole dug into the ground and then pour a given volume of water back into the hole. The percolation rate governs how large your septic system can be, which in turn dictates how many bedrooms you can have in your new or remodeled house. Percolation Application Percolation tests are used to see how quickly your land will drain from the effluents discharged from your septic system. Porosity F l a t B e a c h , F i n e S a n d , L i ttl e P e r c o l a ti o n S te e p B e a c h , G r a v e l , H i g h P e r c o l a ti o n Porosity: defined as the amount of void space in the sediment As porosity increases the beach accepts more water from wave swash If the beach sediment is well sorted (grains all the same size) the porosity of the beach is Constant regardless of grain size! However, percolation does increase with increasing grain size! Case #1 Case Volume of a sphere: Proof: V=(4/3)πR3 Radius = 2, V = (4/3)π8 = [(32/3) π] Case #2 Radius =1 V=(4/3)π1=(4/3)π 8 balls 8 x (4/3)π = [(32/3)π] balls Therefore porosity doesn’t change but larger grain size allow water to percolate more easily Case #1 R=2 Case #2 R=1 Grain Sorting Measure of grain size variability Well-sorted one grain size population We Poorly sorted many grain size populations As sorting increases beaches will be a bit steeping (all other factors being equal) Steeper, well-sorted High porosity Flatter, poorly-sorted Low porosity Wave Energy Wave W a v e h eig h t Large waves bring in more water than smaller waves Beaches can only accept a finite volume of water and then the beach becomes saturated! Example: 200 gallons 2-foot wave 2100 gallons 100 gallons leaves as back swash 4-foot wave 400 gallons 100 gallons 300 gallons Larger waves means that a greater proportion of water leaves as backwash This means that large waves cause the beach to erode (flattens) Wave Length and Period Wave This factor tells us how much time the beach have to rid itself of water before the next wave comes ashore Wave length (L) is a function of wave period (T) As wave length (L) and wave period (T) increase the beach gets steeper! Why? L = 1.56 T2 Waves Steepness Waves * Remember Wave Steepness is a measure of the height to length of the incoming waves Length Storm waves Height STEEP Large Wave Steepness Swell waves FLAT Length Low Wave Steepness H Waves Steepness Waves H/L, Height increases Beach Flattens (beach becomes saturated H/L, Wave Length decreases Beach Flattens (less time for beach to expunge water) Thus, as H/L increases, beach flattens Isn’t this exactly what happens during a storm! Summary Summary Beachface slope is an equilibrium between sand brought onshore by wave swash and the sand transported offshore by backwash Beachface slope is dependent on the ratio between wave swash versus backwash. As more water is lost to percolation, less water leaves as backwash and thus, the beachface steepens Increasing grain size, grain sorting, wave period, wave length causes the beach to steepen. Increasing wave height and wave steepness cause the beach to flatten (erode). ...
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