Example Calculation of Wave Transformation at
Tidal Inlet
Field data are lacking for tidal inlets that would allow for detailed testing
of NMLong-CW. Thus, a hypothetical case was simulated by specifying
conditions representative for the flow through an inlet during ebb and flood
tide. The example discussed here involves inlet currents and waves
representative of hydrodynamic conditions as observed at Shinnecock Inlet,
Long Island, NY. An inlet channel was simulated with a water depth of 4 m
at the throat and linearly sloping offshore to a depth of 12 m (assumed
boundary for this inlet). Waves are assumed to travel along the channel (zero
incident wave angle), and the tidal current (flood or ebb) decreases linearly
from the throat to the offshore end of the channel, where it was set to zero (at
the 12-m water depth). The deepwater rms wave height was set to 1.0 m, and
the mean wave period 8 s (typical for Shinnecock Inlet). Standard parameter
values were employed in the NMLong-CW simulations, and the roller model
was not included (negligible effect on the wave transformation in this case).
Figure 17 displays the results of the simulations for two different current
speeds at the inlet throat for the flood and ebb current, together with a
calculation with no current. In the no-current case, a small increase in wave
height is observed because of shoaling associated with the decrease in water
depth. For the flood current, the waves experience a reduction in height as
they approach the inlet because of the following current. The opposite trend
occurs if the waves encounter an ebb flow, and a pronounced wave height
increase might take place. For example, for the situation of an ebb current of
3.0 m/s at the inlet throat, wave breaking occurs and causes a reduction in
wave height, as seen in Figure 17. Also, for the stronger ebb current, wave
blocking takes place before the waves reach the inlet throat. Figure 18 is a
photograph taken from the west jetty at Shinnecock Inlet and shows wave
breaking and blocking on an ebb current. Note that the waves cannot
penetrate against the current, with turbulent water to the left (south) and calm
water to the north, inside the inlet.
By this example, it can be seen that the wave climate in a long, straight
inlet channel can be investigated with NMLong-CW under the assumption of
longshore uniformity. For example, for a given ebb current and offshore
wave height and period, the increase in wave height and wave steepness,
defined as H/L, owing to the presence of the tidal current can be calculated.
Steep waves make navigation difficult if the wavelength approaches that of
the vessel transiting the inlet.
Concluding Remarks
The enhanced wave model based on the wave action equation was
evaluated by comparison with two data sets from laboratory experiments
involving the shoaling and breaking on an opposing current. One of the
experiments (Smith et al. 1998) was conducted in shallow water, implying
that wave transformation over the bottom was marked, whereas the other
experiment (Chawla and Kirby 1999) was carried out essentially for deep-
water conditions (negligible influence of the sea bottom). The former
experiment characterizes situations for which NMLong-CW would typically
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Chapter 4 Verification of Wave Model
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