four generators (x = 18 m), had a standard deviation of only 2.8 percent. In the
inner surf zone, the longshore uniformity in wave height is very good due to the
dominant effect of depth, which limits wave height. The longshore averaged
breaker height index, across the width of the surf zone (Wave Gauge 1 through
6), is calculated to be 0.74, and is tabulated in Table A-3, Appendix A, with
several other parameters. Figure 66b shows that the longshore variation in mean
water surface elevation is approximately +0.0015 m. This value is comparable to
the elevation tolerance of the bridge support rails. Therefore, it can be concluded
that there is no measurable longshore gradient in the mean water level in this
region. Figure 66c shows that the degree of uniformity in the mean longshore
current is quite good. The reduction in magnitude of the longshore current at x =
4.1 m at transect Y31 is caused by the small flow-reversal region farther
upstream, near the shoreline. It is interesting to note that just offshore of the
peak longshore current, the measurements suggest a slight flattening of the
longshore current distribution. This observation is qualitatively consistent with
the present understanding of the interaction of the undertow with the longshore
current; see Putrevu and Svendsen (1992).
Figure 67 quantifies longshore uniformity of the hydrodynamic processes in
the irregular wave experiment. As was found for the regular wave case, all three
hydrodynamic parameters tend to approach a minimum asymptote, if the length
of the testing region is reduced to approximately 12 m, starting at Y19 and
extending upstream to Y31. The values of the standard deviation in the irregular
wave experiment are significantly less than in the regular wave experiment,
especially for the wave and current data. Perhaps the regular wave forcing
generates a basin response that does not occur when using irregular wave forcing.
Figure 68, a through c, illustrates the high degree of longshore hydrodynamic
uniformity for the irregular wave case. Figure 68a shows that the significant
wave height is very uniform in the alongshore direction. The significant wave
height at incipient breaking occurred immediately shoreward of Wave Gauge 7,
based on visual observations and the fact that the gradient in the significant wave
height curve increases significantly at that location. The longshore averaged
breaker height index, across the width of the surf zone (Wave Gauges 1 through
7) is calculated to be 0.75, as tabulated in Table A-3, Appendix A. The
longshore averaged value of the measured maximum wave height also is shown
to illustrate that Hmax > 0.35 m at x = 18 m. As was the case for the regular wave
test, Figure 68b shows that the alongshore variation in mean water surface
elevation is approximately +0.0015 m. The wave setup at x = 4.1 m is only about
60 percent of the value measured in the regular wave case, even though the
incident wave energy was constant by setting Hrms in the irregular wave case
equal to Hreg in the regular wave case. Figure 68c shows that the mean longshore
current is very uniform in the longshore direction. The peak current is 0.34 m/s,
relative to a peak current of 0.42 m/s in the regular wave case. The cross-shore
distribution is broader than in the regular wave case, with the offshore tail
decreasing much more uniformly. Dye was used to investigate the longshore
current in very shallow water. No local increase in longshore current was
detected shoreward of ADV 1 for either the regular or irregular wave case. It is
interesting to note that the total longshore flow rate actively pumped through the
cases, respectively. These values are very similar, since the incident wave
energy was held constant for the two cases, as mentioned previously.
116
Chapter 9
Establishing Uniform Longshore Currents
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