Small-Scale Inlets as Tidal Filters

  • John A. Moody
Conference paper
Part of the Lecture Notes on Coastal and Estuarine Studies book series (COASTAL, volume 29)


The tidal distortion by small-scale tidal inlets has been investigated at six locations on or near Cape Cod, Mass. For these inlets the ratio of the inlet cross-sectional area, a, and the surface area of the adjoining estuary, A, ranged from 0.36 × 10-5 to 111 × 10-5, while the length of the inlets, ranged from 42 m to ~380 m.

The amplitude of the semidiurnal M2 tide on the ocean side of the inlets varied from 18 cm to 98 cm with corresponding M4/M2 ratios of 0.28 and 0.01. The response function L2 (the squared ratio of the basin tide amplitude, 03B7;b, to the ocean tide amplitude, 03B7;o), for the semidiurnal tides (N2, M2, and S2) was 0.35 for six inlets, reflecting the constricted nature of these inlets.

Three approximations are made to simplify the equations of motion for constricted inlets (a/A<2.0 × 10-5) and as a result the response function is proportional to a \( Q = \frac{g} {{\eta _0 }}\left( {\frac{a} {{\omega A}}} \right)^2 \), dimensionless number where g is the acceleration of gravity and ω is the frequency of the tidal constituent. The logarithms of L2 and Q for six constricted inlets and for 12 tidal constituents (01, Kb N2, M2, S2, MK3, MN4, M4, MS4, MK5, M6, and M8) were fit by linear regression to the equation ln(L2) = ln(cQm). The correlation coefficient was 0.82 with m= 0.90 and c= 0.09.

Deviations from this relationship occur for large ratios of a/A (~100 × 10-5) because the approximation u2 α ηo is not valid, and when a large fraction of the basin’s surface area consists of tidal flats (~50%) which generate harmonic constituents within the basin such that L2> 100.

The results show that a constricted inlet acts as a tidal filter which is a function of the ratio. a/A, the tidal frequency, ω, and the tidal amplitude, η0, outside the inlet.


Tidal Flat Flood Tide Tidal Amplitude Tidal Constituent Tidal Inlet 
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Copyright information

© Springer Science+Business Media New York 1988

Authors and Affiliations

  • John A. Moody
    • 1
  1. 1.Water Resources DivisionU.S. Geological SurveyLakewoodUSA

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