Water-level measurements obtained offshore of Mattituck Inlet from
19 September to 8 October 2002 drove the model. It was found by numerical
experimentation that raising the water-surface elevation in this data set by 0.25 m
produced a successful model simulation that accurately represented the tidal
signature recorded at Goldsmith Pond, while not drying the channel. The authors
have never observed the inlet channel to dry, even during low tide. DYNLET
model water-level calculations are referenced to NAVD88. The results were
adjusted by subtracting 0.25 m to account for the datum shift of the input water-
surface elevation. A shift upward in the driving water level is functionally
equivalent to shifting the entire bathymetry grid down by the same amount.
DYNLET was calibrated by specifying larger values of the bottom friction
coefficient in the Goldsmith Inlet channel, where small rocks are present and can
protrude above the water surface, some of which may be remnants from jetty
construction. The default value of Manning's n of 0.025 m/sec1/3 was maintained
at most nodes, but in the channel where rocks and roiling water are seen, the
value was increased to 0.03 to 0.04. The time-step in the model was set to
30 sec.
In initial model runs, calculated water level at the pond gauge lagged the
measurements by 36 min. A lag between calculations and measurements is
expected, because Goldsmith Inlet lies 5 km east of Mattituck Inlet, and the tidal
wave travels from east to west. The tidal record offshore of Mattituck Inlet was
therefore adjusted forward 36 min to account for the time of tidal wave travel
between the location of the tidal record and the location of the input driving the
model. This adjustment implies that the tidal wave moves westward at about
0.23 m/sec along the shallow water of this portion of the north shore of Long
Island.
The input boundary condition (Node 1) and the first DYNLET water-level
calculation (Node 2) for the period of data collection (20 September 8 October
2002) are plotted in Figure 5-22a. Figure 5-22b shows these water levels for the
5-8 October 2002, spring tide. A comparison of water-level measurements at
Goldsmith Inlet for 20 September 8 October 2002 and the corresponding
DYNLET calculations (at Node 30) are shown in Figure 5-22c, and Figure 5-22d
gives this comparison for a period of spring tide.
Current velocity measurements taken within Goldsmith Inlet for a short
interval on 8 October are compared to corresponding DYNLET current velocity
calculations (at Nodes 13 and 14) in Figures 5-23a and 5-32b. The calculations
well reproduce the limited length of the measurements. The current velocity is
seen to be strong, exceeding 1 m/sec, and the calculated current is flood
dominant, meaning that the flood current has a higher peak velocity than ebb, but
shorter duration. This dominance has implications for sediment transport in the
inlet, discussed in the following paragraphs.
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Chapter 5 Circulation Analysis
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