ERDC/CHL CETN-IV-29
September 2000
Some inlets exhibit tidal currents in which the mean velocity through the inlet is not zero, yet the
net discharge over a tidal cycle is zero. This situation occurs if the tidal curve for water level is
not in phase with that for the current through the inlet. Because discharge is a function of water
level and current, the phase difference between the two variables creates a time-varying
discharge that has a net value of zero over a tidal cycle.
As an example, water level plotted to mean tide level (mtl), current speed, and discharge tidal
curves for a flood-dominated inlet are shown in Figure 3. The mean current speed is 0.1 m/s
(ebb-directed) owing to the longer duration of ebb current. Peak tidal current precedes peak
water level by 2.5 hr (0.105 day). When the current switches from flood to ebb, the water level
is near its peak flood value. Maximum ebb current coincides with lower water level. Flood
current commences during lower water level and peak current occurs during higher water level.
Peak discharge is greater during the strong flooding current because the water level is higher
than during peak ebb current (peak ebb and flood velocities are nearly equal). Although the ebb
discharge is smaller than the flood, it has longer duration. Thus, the phase difference between
water level and current combined with the asymmetrical tide produces a discharge curve that
balances flood and ebb, giving a zero net discharge.
1.0
4000
3
flood
Flood
current
max
3000
water
2
level
0.5
2000
1
1000
0.0
0
0
-1000
-1
-0.5
-2000
min
water
-2
-3000
level
ebb current
Ebb
-1.0
-3
-4000
308.0
308.0
308.5
309.0
309.5
310.0
Year Day 1998
Water level
Current Speed
Discharge
Figure 3. Example relationship between water level, current speed,
and discharge at a flood-dominated inlet
Ebb and flood dominance can play a role in preferred sand transport direction at tidal inlets. The
bias in peak velocity creates a difference in the amount of material being transported on flood
and ebb tides. For example, a flood-dominated inlet may transport sand into bay channels and
can also create multiple flood shoals (FitzGerald 1988).
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