OCNG 251 Oceanography Tuesday Nov 18 2008 We have to continually be jumping off cliffs and developing our wings on the way down Kurt Vonnegut Beware of the man who works hard to learn something learns it and finds himself no wiser than before He is full of murderous resentment of people who are ignorant without having come by their ignorance the hard way Kurt Vonnegut Cat s Cradle Review a Introduction Sediments are produced by the weathering chemical and mechanical breaking down of rocks into particles that are then moved by air water and ice Sediments can also be formed from the accumulation of shells or micro and macro debris of organisms Sediments can therefore consist of Mineral particles Fossil particles b Classification Sediments can be subdivided on the basis of The size of the particles grain grain size size Their mode of formation origin origin Terrigenous sediments Biogenous sediments Authigenic sediments Volcanic sediments Cosmogenous sediments The surface of the Ocean 1 Waves 1 Review 2 Waves a Anatomy of a wave b Wave motion speed and energy c Deep Shallow water waves c Review Factors that control sedimentation Relationship between average grain size and energy of bottom currents Erosion Transport and Deposition sedimentation depend on velocity of current and grain size Settling rate of suspended particles varies with diameter Stokes Law d Sedimentation in the Oceans Two major areas of sediment deposition on the basis of water depth i Shelf sedimentation sedimentation Shallow close to terrigenous source theoretical equilibrium ii Deep Sea Sedimentation Two main sources External terrigenous mud and sand Internal biogenic particles authigenic particles Three categories Bulk emplacement Pelagic sediments Authigenic sediments Sea level a Anatomy of a wave A wave can be regarded initially as an ideal a sinusoidal wave though the ideal regular waves bears little resemblance to real waves observed at sea irregular in form and period Concept of level Geoid level surface on which the potential energy is everywhere the same Sea level change Oscillation due to geological changes in the hydrological cycle A Characteristics H Height 2 x A L Wavelength from crest to crest T Period time required for two successive crests to pass a point h Depth of water column below mean surface level Sea level change Sea level change Oscillation due to geological changes in the hydrological cycle Adapted from Pinet 2000 a Anatomy of a wave Ideal waves waves If some water is lifted up and allowed to fall back under the action of gravity its inertia will cause it to overshoot the equilibrium position Pressure forces will then push it back up and oscillation will ensue Energy can be transmitted at the interface boundary between two fluids of different densities resulting in progressive orbital waves Adapted from Garrison 2002 a Anatomy of a wave All waves can be regarded as Progressive waves waves in that energy is travelling through or across the surface of the material i e water Standing waves i e plucked guitar strings can be considered as the sum of two progressive waves of equal dimensions but traveling in opposite direction later later a Anatomy of a wave A few waves A case of temporal and spatial scales Waves can result from periodic and non periodic disturbance of the water surface In the case of non periodic disturbances the water particles are displaced from an equilibrium position restoring force acts to bring back equilibrium to the system flat flat water level Gravitational force exerted by Earth Surface tension tendency of water molecules to stick together and present the smallest possible surface to the air Water waves are affected by both of these forces When L 1 7 cm capillary waves When L 1 7 cm gravity waves main interest of oceanography a Anatomy of a wave Wind waves waves As the wind blows formation of ripples capillary waves As the wind increases in speed waves become larger gravity waves Variety and size of wind generated waves are controlled by four principal factors Wind velocity Wind duration Unobstructed distance over which wind blows fetch Original sea state Waves are classified according to their wave period a Anatomy a Anatomy of a wave The ideal relationship among wave properties If one characteristic can be measured the other two can be calculated a Anatomy of a wave a Anatomy of a wave Causing factors The relationship is a direct one one Wind Speed km h 20 Fetch km Duration h Wave Height m Wave Length m Wave Period s 25 2 8 0 33 10 6 3 2 30 77 7 0 88 22 2 4 6 60 660 27 5 5 1 89 2 9 9 90 2446 65 13 9 201 6 13 9 Although the relationship is a direct one one One condition will always be limiting as to the maximum development of wave height and period e g Wind Speed 30 kn Fetch 200 nm Time 40 hrs H 7m T 12 sec b Wave motion Speed and energy There are mathematical relationships linking the characteristics of wavelength L wave period T and wave height H to wave speed and to wave energy Wave speed celerity speed distance time C L T time one wavelength takes to pass a certain point Wave speed can also be represented by the equation gL 2 h tanh 2 L Where g acceleration due to gravity L Wavelength tanh hyperbolic tangent h water depth c Wave motion Speed Approximations 1 For h L 2 L 2h Deep water waves If x is larger than then tanh x 1 tanh 2 2 h L 1 within 0 5 gL 2 h tanh 2 L gL or 1 25 L 2 Wavelength or period is the only variable affecting the wave speed b Wave motion Speed Approximations 1 For h L 2 Deep water waves c Wave motion Speed Approximations 2 For h L 20 L 20h Shallow water waves If x is small then tanh x x tanh 2 2 h L 2 2 h L within 3 gL 2 h tanh 2 L gh Circular motion of particles Exponential decrease of path with depth The change in pressure size size of the orbit is undetectable at a water depth L 2 the wave base base b Wave motion Speed Approximations 2 For h L 20 Shallow water waves Elliptical motion of particles Exponential decrease of vertical path with depth Not of horizontal path or 3 1 h Or C L T gh gh 1 2 L 3 13 h h 1 2 T The water depth is the only variable affecting the wave speed c Wave motion Speed Approximations 1 2 3 For h L 2 Deep water waves L 2h For h L 20 Shallow water waves L 20h For L 20 h L 2 2h L 20h c Wave motion Speed Approximations Deep water waves that have the longest wavelength and greatest period will arrive first in regions distant from the storm which generated them The separation of waves by virtue of their differing rates of