Small-Scale Inlets as Tidal Filters

  1. David G. Aubrey and
  2. Lee Weishar
  1. John A. Moody

Published Online: 23 MAR 2013

DOI: 10.1029/LN029p0137

Hydrodynamics and Sediment Dynamics of Tidal Inlets

Hydrodynamics and Sediment Dynamics of Tidal Inlets

How to Cite

Moody, J. A. (1988) Small-Scale Inlets as Tidal Filters, in Hydrodynamics and Sediment Dynamics of Tidal Inlets (eds D. G. Aubrey and L. Weishar), Springer-Verlag, New York. doi: 10.1029/LN029p0137

Publication History

  1. Published Online: 23 MAR 2013
  2. Published Print: 1 JAN 1988

ISBN Information

Print ISBN: 9783540968887

Online ISBN: 9781118669242



  • Bournes Pond Inlet;
  • Herring River, Menemsha and Nauset;
  • Inlet-basin characteristics;
  • Ocean tide characteristics;
  • Small-scale tidal inlets;
  • Tidal measurements


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, ηb, to the ocean tide amplitude, η0), 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 dimensionless number $${\rm Q} = {{\rm g} \over \eta }_o \left( {{{\rm a} \over {\omega {\rm A}}}} \right)ˆ2 $$ where g is the acceleration of gravity and co is the frequency of the tidal constituent. The logarithms of L2 and Q for six constricted inlets and for 12 tidal constituents (01, K1, 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 coefficientwas 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 ∝ η0 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 ofthe ratio. a/A, the tidal frequency, ω, and the tidal amplitude, ηo, outside the inlet.