- •Preface
- •1 A Voyage of Discovery
- •1.2 Goals
- •1.3 Organization
- •1.4 The Big Picture
- •1.5 Further Reading
- •2 The Historical Setting
- •2.2 Eras of Oceanographic Exploration
- •2.3 Milestones in the Understanding of the Ocean
- •2.4 Evolution of some Theoretical Ideas
- •2.5 The Role of Observations in Oceanography
- •2.6 Important Concepts
- •3 The Physical Setting
- •3.1 Ocean and Seas
- •3.2 Dimensions of the ocean
- •3.3 Sea-Floor Features
- •3.4 Measuring the Depth of the Ocean
- •3.5 Sea Floor Charts and Data Sets
- •3.6 Sound in the Ocean
- •3.7 Important Concepts
- •4.1 The Earth in Space
- •4.2 Atmospheric Wind Systems
- •4.3 The Planetary Boundary Layer
- •4.4 Measurement of Wind
- •4.5 Calculations of Wind
- •4.6 Wind Stress
- •4.7 Important Concepts
- •5 The Oceanic Heat Budget
- •5.1 The Oceanic Heat Budget
- •5.2 Heat-Budget Terms
- •5.3 Direct Calculation of Fluxes
- •5.4 Indirect Calculation of Fluxes: Bulk Formulas
- •5.5 Global Data Sets for Fluxes
- •5.6 Geographic Distribution of Terms
- •5.7 Meridional Heat Transport
- •5.8 Variations in Solar Constant
- •5.9 Important Concepts
- •6.2 Definition of Temperature
- •6.4 The Oceanic Mixed Layer and Thermocline
- •6.5 Density
- •6.6 Measurement of Temperature
- •6.7 Measurement of Conductivity or Salinity
- •6.8 Measurement of Pressure
- •6.10 Light in the Ocean and Absorption of Light
- •6.11 Important Concepts
- •7.1 Dominant Forces for Ocean Dynamics
- •7.2 Coordinate System
- •7.3 Types of Flow in the ocean
- •7.4 Conservation of Mass and Salt
- •7.5 The Total Derivative (D/Dt)
- •7.6 Momentum Equation
- •7.7 Conservation of Mass: The Continuity Equation
- •7.8 Solutions to the Equations of Motion
- •7.9 Important Concepts
- •8.2 Turbulence
- •8.3 Calculation of Reynolds Stress:
- •8.4 Mixing in the Ocean
- •8.5 Stability
- •8.6 Important Concepts
- •9 Response of the Upper Ocean to Winds
- •9.1 Inertial Motion
- •9.2 Ekman Layer at the Sea Surface
- •9.3 Ekman Mass Transport
- •9.4 Application of Ekman Theory
- •9.5 Langmuir Circulation
- •9.6 Important Concepts
- •10 Geostrophic Currents
- •10.1 Hydrostatic Equilibrium
- •10.2 Geostrophic Equations
- •10.3 Surface Geostrophic Currents From Altimetry
- •10.4 Geostrophic Currents From Hydrography
- •10.5 An Example Using Hydrographic Data
- •10.6 Comments on Geostrophic Currents
- •10.7 Currents From Hydrographic Sections
- •10.8 Lagrangian Measurements of Currents
- •10.9 Eulerian Measurements
- •10.10 Important Concepts
- •11.2 Western Boundary Currents
- •11.4 Observed Surface Circulation in the Atlantic
- •11.5 Important Concepts
- •12 Vorticity in the Ocean
- •12.2 Conservation of Vorticity
- •12.4 Vorticity and Ekman Pumping
- •12.5 Important Concepts
- •13.2 Importance of the Deep Circulation
- •13.3 Theory for the Deep Circulation
- •13.4 Observations of the Deep Circulation
- •13.5 Antarctic Circumpolar Current
- •13.6 Important Concepts
- •14 Equatorial Processes
- •14.1 Equatorial Processes
- •14.6 Important Concepts
- •15 Numerical Models
- •15.2 Numerical Models in Oceanography
- •15.3 Global Ocean Models
- •15.4 Coastal Models
- •15.5 Assimilation Models
- •15.6 Coupled Ocean and Atmosphere Models
- •15.7 Important Concepts
- •16 Ocean Waves
- •16.1 Linear Theory of Ocean Surface Waves
- •16.2 Nonlinear waves
- •16.3 Waves and the Concept of a Wave Spectrum
- •16.5 Wave Forecasting
- •16.6 Measurement of Waves
- •16.7 Important Concepts
- •17 Coastal Processes and Tides
- •17.1 Shoaling Waves and Coastal Processes
- •17.2 Tsunamis
- •17.3 Storm Surges
- •17.4 Theory of Ocean Tides
- •17.5 Tidal Prediction
- •17.6 Important Concepts
- •References
Chapter 17
Coastal Processes and Tides
In the last chapter I described waves on the sea surface. Now we can consider several special and important cases: the transformation of waves as they come ashore and break; the currents and edge waves generated by the interaction of waves with coasts; tsunamis; storm surges; and tides, especially tides along coasts and in the deep ocean.
17.1Shoaling Waves and Coastal Processes
Wave phase and group velocities are a function of depth when the depth is less than about one-quarter wavelength in deep water. Wave period and frequency are invariant (don’t change as the wave comes ashore); and this is used to compute the properties of shoaling waves. Wave height increases as wave group velocity slows. Wave length decreases. Waves change direction due to refraction. Finally, waves break if the water is su ciently shallow; and broken waves pour water into the surf zone, creating long-shore and rip currents.
Shoaling Waves The dispersion relation (16.3) is used to calculate wave properties as the waves propagate from deep water o shore to shallow water just outside the surf zone. Because ω is constant, (16.3) leads to:
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CHAPTER 17. COASTAL PROCESSES AND TIDES |
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Figure 17.1 Properties of waves in intermediate depths between deep and shallow water. d = depth, L = wavelength, C = phase velocity, Cg = group velocity, α = angle of crests relative to contours of constant depth, H = wave height. Subscript 0 refers to properties in deep water. From values in Wiegel (1964: A1).
using an iterative technique, or from figure 17.1, or from Wiegel (1964: A1). Because wave velocity is a function of depth in shallow water, variations
in o shore water depth can focus or defocus wave energy reaching the shore. Consider the simple case of waves with deep-water crests almost parallel to a
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Figure 17.2 sub-sea features, such as submarine canyons and ridges, o shore of coasts can greatly influence the height of breakers inshore of the features. After Thurman (1985: 229).
17.1. SHOALING WAVES AND COASTAL PROCESSES |
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Figure 17.3 Left: Classification of breaking waves as a function of beach steepness and wave steepness o shore. Right: Sketch of types of breaking waves. After Horikawa (1988: 79, 81).
straight beach with two ridges each extending seaward from a headland (figure 17.2). Wave group velocity is faster in the deeper water between the ridges, and the wave crests become progressively deformed as the wave propagates toward the beach. Wave energy, which propagates perpendicular to wave crests, is refracted out of the region between the headland. As a result, wave energy is focused into the headlands, and breakers there are much larger than breakers in the bay. The di erence in wave height can be surprisingly large. On a calm day, breakers can be knee high shoreward of a submarine canyon at La Jolla Shores, California, just south of the Scripps Institution of Oceanography. At the same time, waves just north of the canyon can be high enough to attract surfers.
Breaking Waves As waves move into shallow water, the group velocity becomes small, wave energy per square meter of sea surface increases, and nonlinear terms in the wave equations become important. These processes cause waves to steepen, with short steep crests and broad shallow troughs. When wave slope at the crest becomes su ciently steep, the wave breaks (figure 17.3 Right). The shape of the breaking wave depends on the slope of the bottom, and the steepness of waves o shore (figure 17.3 Left).
1.Steep waves tend to lose energy slowly as the waves moves into shallower water through water spilling down the front of the wave. These are spilling breakers.
2.Less steep waves on steep beaches tend to steepen so quickly that the crest of the wave moves much faster than the trough, and the crest, racing ahead of the trough, plunges into the trough (figure 17.4).
3.If the beach is su ciently steep, the wave can surge up the face of the beach without breaking in the sense that white water is formed. Or if it is formed, it is at the leading edge of the water as it surges up the beach.
An extreme example would be a wave incident on a vertical breakwater.
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CHAPTER 17. COASTAL PROCESSES AND TIDES |
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Figure 17.4 Steep, plunging breakers are the archetypical breaker. The edge of such breakers are ideal for surfing. From photo by Je Devine.
Wave-Driven Currents Waves break in a narrow band of shallow water along the beach, the surf zone. After breaking, waves continues as a near-vertical wall of turbulent water called a bore which carries water to the beach. At first, the bore surges up the beach, then retreats. The water carried by the bore is left in the shallow waters inside the breaker zone.
Water dumped inside the breaker zone must return o shore. It does this by first moving parallel to the beach as an along-shore current . Then it turns and flows o shore perpendicular to the beach in a narrow, swift rip current . The rips are usually spaced hundreds of meters apart (figure 17.5). Usually there is a band of deeper water between the breaker zone and the beach, and the longshore current runs in this channel. The strength of a rip current depends on the
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Figure 17.5 Sketch of rip currents generated by water carried to the beach by breaking waves.