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BAY/WETLAND RESPONSE TO TIDE

Tidal bays and wetlands are critical to the nations ecological balance as they provide protective environments for the growth of juvenile marine life. Additionally, increased pressure for development of the coastal zone assures that population will rapidly grow along coastal bays and wetlands. Changes to these natural systems should be studied intensively when man-made alterations are planned. Additional potential for flooding due to storms brought about by change could put both the ecosystem and the population that live in these areas at risk. This note addresses one issue of possible change, that of mean water level elevation change in a bay/wetland which occurs due to changing tidal hydraulics in a larger water body (where a connecting channel joins the bay/wetland to the larger water body). In this note, the larger water body is assumed to have a tide consisting of both a first harmonic constituent at the dominant tidal period (=12.5 hours) and a second harmonic constituent at twice the dominant tidal period. In the case presented below, bay surface area (=1000000 meters²), channel cross section (=250 meters²), length of channel (=5000 meters), friction characteristics of channel (Darcy-Weisbach friction factor f=.05), and amplitudes of first and second harmonic components of forcing tide, are all held constant. The only changing parameter is the phasing of the second harmonic component of the tide.

Governing equations for the mass/momentum exchanges of water are not given here but are provided in the references cited below. The mean water level change (from the "no tide" situation) is brought about by the non-linearities in the system equations. Approximate solutions to the non-linear equations are given in Figures 1 and 2 for two different phase values of the second harmonic forcing tide constituent and constant values of the bay surface area, inlet cross sectional area, channel length, channel depth, friction coefficient, and amplitude of the ocean tide. Parameters (i.e. A_b , A_c , L, h, f, a_o, etc.) are the same in both cases below. It should not be assumed that examples different from the cases provided will show similar characteristics. In shallow channels, changing cross sectional area and bay surface area can provide very different answers than those provided here.

Figure 1 shows a forcing tide that is rising faster than it falls and is associated (for the simplified scenerio provided) with a "setdown" of the water level in the bay (i.e. bay water level is 0.13 meters below that of the water body for which the tide is specified). The top graph in Figure 1 shows the forcing tide as the solid line and the bay tide response as the dash-dot line. The flow in the connecting channel is shown in the lower graph of Figure 1.

Figure 2 shows a forcing tide that is falling faster than it rises and is associated (for the simplified scenerio provided) with a "setup" of the water level in the bay (i.e., bay water level is 0.13 meters above that of the water body for which the tide is specified). Again, the top graph shows the forcing tide as the solid line and the bay tide response as the dash-dot line. The flow in the connecting channel is shown in the lower graph.

Should you have any questions concerning your specific project needs please call Todd Walton, Ph.D., P.E. (850-644-2847).

REFERENCES:

DiLorenzo, J.L.(1988). `The Overtide and Filtering Response of Small Inlet-Bay Systems,' Hydrodynamics and Sediment Dynamics of Tidal Inlets, ed. D.G. Aubrey and L. Weishar, 24-53 Springer-Verlag Publishing, New York, N.Y.

Dronkers, J.J.(1964). Tidal Computations, North Holland Publishing Company, Amsterdam.

Escoffier, F.F. and Walton, T.L.Jr.(1979). `Inlet Stability Solutions for Tributary Inflow,' Jour. Waterways and Harbors Division, WW4, 105, ASCE, 341-355.

Keulegan, G.H.(1967). Tidal Flow in Entrances: Water Level Fluctuations of Basins in Communication with the Seas, Committee on Tidal Hydraulics Technical Bulletin No.14, U.S. Army Engineers Waterways Experiment Station, Vicksburg,MS.

Walton,T.L.Jr. and Escoffier, F.F.(1981). `Linearized Solution to the Inlet Equation with Inertia,' Jour. Waterways and Harbors Division, WW3, ASCE, 191-195.

Walton, T.L.Jr.(2002). Setup and Setdown in Tidal Bays and Wetlands, Journal of Estuarine, Coastal, and Shelf Science, Vol. 55, 789-794, Elsevier Science Ltd.




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