Modeling of
597
Part C | 27
27. Modeling of Coastal Waves and Hydrodynamics
Patrick J. Lynett, James M. Kaihatu
This chapter presents an overview of the available methods for modeling coastal waves. First,
an overview of the relevant coastal processes,
from shoaling to turbulent mixing, is provided to
establish a basis to compare the various modeling approaches. The bulk of the discussion
centers on modeling wind waves and includes
a brief overview of the linear and analytical
theory available to quantify coastal transformation, and then follows with a summary of
spectral and phase-resolving approaches. Modeling long waves is discussed next, with a focus
on tsunami simulation. Finally, the chapter
summarizes techniques to couple the various
models and reviews recent advances in the
topic.
27.1 Wind Wave Modeling ........................... 598
27.1.1 Linear, Analytical
and Semi-Empirical Approaches . 598
27.1.2 Spectral Modeling:
Phase-Averaged
and Phase-Resolving Approaches 600
27.1.3 Depth-Integrated
and Boussinesq-Type
Approaches............................... 602
27.1.4 Navier–Stokes Equation-Based
Approaches............................... 603
27.2 Modeling Long Waves .......................... 604
27.3 Coupled and Nested Techniques ........... 605
27.4 Summary of Model Properties .............. 606
27.5 Conclusions ......................................... 608
27.6 Nomenclature...................................... 608
References................................................... 608
As an ocean surface wave enters the coastal zone,
broadly defined here as the water landward of the continental shelf or some other depth transition, interaction
with the seafloor and currents causes significant transformation. Waves will first start to feel the bottom when
the depth (h) to wavelength (L) ratio is less than 1=2,
and waves are considered to be in shallow water, where
the effects of the bottom are strong, when this ratio dips below approximately 1=25. Two-dimensional
(2-D) variations in the seafloor can lead to wave focusing due to the effects of refraction. Wave shoaling
causes the wave amplitude to grow and the wavelength
to shorten, steepening the wave front until the wave
breaks.
Depending on the properties of the incoming wave,
nonlinearities in the wave behavior are important for
some distance before breaking and through the surf
zone. Shallow water nonlinearity is governed by the
wave height to water depth ratio; as the ratio gets larger,
it is expected that nonlinearity becomes increasingly
important. From this definition, it is clear that nonlinearity must be important near breaking, as this is the
point when the wave height is similar to the depth. Nonlinearity in the coastal zone can change the speed at
which a wave travels, where it breaks, and how it interacts with currents.
Nonlinear waves in the coastal zone also induce
a net mass transport in the direction of the wave. This
net mass transport is due to wave radiation stress,
which is name given to the net positive momentum
flux in the direction of wave propagation, and leads
to wave-induced currents. Currents moving parallel to
the shoreline are termed alongshore currents and are
primarily responsible for alongshore sediment movement. These currents can generate instabilities, often
driven by alongshore bathymetry variation, which can
manifest as strong rip currents and shallow turbulent coherent structures. Such features induce high horizontal
shear on the flow, and lead to significant horizontal and
vertical mixing.
When designing coastal structures, engineers are often most concerned with extreme event loads. Here,
extreme events may be storm surges and the accompanying storm waves or tsunamis, for example. It is
a present challenge for coastal engineers to determine
these extreme event loads, as there is currently no
accepted design code for extreme wave loads. Furthermore, the sea level rise associated with climate change
597
Part C | 27
27. Modeling of Coastal Waves and Hydrodynamics
Patrick J. Lynett, James M. Kaihatu
This chapter presents an overview of the available methods for modeling coastal waves. First,
an overview of the relevant coastal processes,
from shoaling to turbulent mixing, is provided to
establish a basis to compare the various modeling approaches. The bulk of the discussion
centers on modeling wind waves and includes
a brief overview of the linear and analytical
theory available to quantify coastal transformation, and then follows with a summary of
spectral and phase-resolving approaches. Modeling long waves is discussed next, with a focus
on tsunami simulation. Finally, the chapter
summarizes techniques to couple the various
models and reviews recent advances in the
topic.
27.1 Wind Wave Modeling ........................... 598
27.1.1 Linear, Analytical
and Semi-Empirical Approaches . 598
27.1.2 Spectral Modeling:
Phase-Averaged
and Phase-Resolving Approaches 600
27.1.3 Depth-Integrated
and Boussinesq-Type
Approaches............................... 602
27.1.4 Navier–Stokes Equation-Based
Approaches............................... 603
27.2 Modeling Long Waves .......................... 604
27.3 Coupled and Nested Techniques ........... 605
27.4 Summary of Model Properties .............. 606
27.5 Conclusions ......................................... 608
27.6 Nomenclature...................................... 608
References................................................... 608
As an ocean surface wave enters the coastal zone,
broadly defined here as the water landward of the continental shelf or some other depth transition, interaction
with the seafloor and currents causes significant transformation. Waves will first start to feel the bottom when
the depth (h) to wavelength (L) ratio is less than 1=2,
and waves are considered to be in shallow water, where
the effects of the bottom are strong, when this ratio dips below approximately 1=25. Two-dimensional
(2-D) variations in the seafloor can lead to wave focusing due to the effects of refraction. Wave shoaling
causes the wave amplitude to grow and the wavelength
to shorten, steepening the wave front until the wave
breaks.
Depending on the properties of the incoming wave,
nonlinearities in the wave behavior are important for
some distance before breaking and through the surf
zone. Shallow water nonlinearity is governed by the
wave height to water depth ratio; as the ratio gets larger,
it is expected that nonlinearity becomes increasingly
important. From this definition, it is clear that nonlinearity must be important near breaking, as this is the
point when the wave height is similar to the depth. Nonlinearity in the coastal zone can change the speed at
which a wave travels, where it breaks, and how it interacts with currents.
Nonlinear waves in the coastal zone also induce
a net mass transport in the direction of the wave. This
net mass transport is due to wave radiation stress,
which is name given to the net positive momentum
flux in the direction of wave propagation, and leads
to wave-induced currents. Currents moving parallel to
the shoreline are termed alongshore currents and are
primarily responsible for alongshore sediment movement. These currents can generate instabilities, often
driven by alongshore bathymetry variation, which can
manifest as strong rip currents and shallow turbulent coherent structures. Such features induce high horizontal
shear on the flow, and lead to significant horizontal and
vertical mixing.
When designing coastal structures, engineers are often most concerned with extreme event loads. Here,
extreme events may be storm surges and the accompanying storm waves or tsunamis, for example. It is
a present challenge for coastal engineers to determine
these extreme event loads, as there is currently no
accepted design code for extreme wave loads. Furthermore, the sea level rise associated with climate change