Here selected properties of the new data set will be noted in relation to the
analytical model. Trends are investigated without reference to scaling of field
conditions. One concept explored analytically was that the elongation rate of a spit
under constant longshore transport decreases with approach to a channel. Two
possible reasons were implemented, one for which the depth of active movement
increases, and the other was the sweeping action of the tidal current flowing through
the inlet, which would remove material from the distal end of the spit.
Spit evolution in the physical model for two cases, plotted in Fig. 10,
qualitatively displays the behavior calculated in the numerical mode (Fig. 4). Wave
conditions were the same for both cases, with one being conducted with no tidal
current and the other with a 17-cm/sec flood current, but no change in water
elevation. Elongation of the spit follows a fairly smooth trend. Because the flood
current enhances the wave-induced longshore current and increases the longshore
transport rate, spit elongation is more rapid as compared to the case with only wave
action. In the presence of the flood current, the tip of the spit reaches the channel
(2.1-m distance) after about 80 min. Thereafter, the flood current swept material off
the spit and it did not grow. In contrast, the spit created by wave action only had not
yet arrived at the channel after 120 min.
2.5
H = 2.4 cm, T = 1.1 sec
17 cm/sec flood current
Constant Water Level
2.0
No tidal current
1.5
1.0
0.5
0.0
0
20
40
60
80
100
120
Elapsed Time, min
Fig. 10. Spit elongation with and without flood current, constant waves.
Spit width through time for the same case above with the flood current is plotted
in Fig. 11. Each line in the figure shows width measured on cross-shore lines at
0.3048-m (1-ft) increments of distance alongshore downdrift from the spit genesis
location on the barrier island. After 20 min, the spit had elongated slightly less than
1 m (Fig. 10); therefore, the first three cross-shore lines (at 0.30, 0.61, and 0.91 m)
show non-zero width. As the spit reaches other downdrift lines, width can be
measured on other lines in succession. Width of the spit at the more mature lines
closest to the genesis point shows a trend for reaching equilibrium under the constant
wave action. The lines at the distal end of the spit show a weaker but consistent
trend. Although not shown here, a direct correlation was found in the physical
Kraus
14
Previous Page
