lag for the low waters is 195 min. (3.24 hr). Therefore, flow into the inlet and pond can
only occur when the water level in the sound is above msl.
The long-term average water level in Goldsmith Pond is expected to be constant.
The time duration of the high water is much shorter than the duration of the low water
about the mean water level. Because the same amount of water must enter on flood as
leaves at ebb to maintain the average water level in the pond, but in a shorter time, the
average of the inlet channel cross-sectional current velocity on flood must be much
greater than on ebb. Such an inlet is called "flood dominant," referring to the greater
magnitude, but shorter duration of the flood tide.
Tidal asymmetry of coastal inlets has been well studied (e.g., Boon 1975; Boon and
Byrne 1981; Aubrey and Speer 1985; Speer and Aubrey 1985; Speer et al. 1991, as
summarized by Walton (2002). For example, shoaling in channels truncates the lowest
portion of the tide, resulting in a longer falling tide and a weaker ebb current as compared
to the flood current. Such a truncation is a hypsometric effect, the control of water-
surface elevation by the bathymetry or depth. In the case of Goldsmith Inlet, the
elevation of the entire inlet entrance is located near msl datum in the Long Island Sound.
At the lower water levels of ebb tide, the sills at the flood shoal and shoreline become
effective in retarding flow. In addition, water entering the fringing marsh of Goldsmith
Pond on flood tide has greater velocity than when it exits on ebb. The effective friction
of the marsh, creating storage capacity, releases water slowly on ebb as compared to its
entrance at flood tide.
Current
The hand-held current meter (Current Meter 3) was located approximately at
middepth position (Figure 4-49). Along-channel current velocity at this location is
plotted in Figure 3-48. The flood current velocity reached 1.30 m/sec, when the current
meter had to be removed because of concerns over the rising water level and strong
current on the pole holding the meter. Corresponding to discussion of water level, there
must be a strong asymmetry in current velocity at Goldsmith Inlet, with flood current
being significantly stronger than ebb current. Sediment transport is proportional to a
power of water velocity, typically the third power. Therefore, a flood-dominant inlet will
tend to have net sediment transport directed into the bay, or into Goldsmith Pond in the
present situation.
91
Chapter 3 Field Data Collection and Analysis
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