The Effect of Tidal Inlet Currents on the Characteristics and Energy Propagation Of Ocean Waves
Authors: Herbich John B. , Hales Lyndell B. 
Published: October 2008

When surface gravity waves are propagated through coastal inlets, changes occur in their characteristics because of the interaction with the non-uniform currents through the tidal inlet. Results of an experimental study indicate the wave period is invariant under all flow conditions and a shape factor can be applied to the mean flow to predict wave length alterations. The relative depth and wave steepness are pertinent quantities, and the functional relationship is displayed in graphical form.

Introduction

When surface gravity waves are propagated toward coastal inlets, certain changes occur in their characteristics. The non-uniform currents created by the flow of tidal water through the relatively narrow opening either increase or decrease the wave lengths with corresponding alterations to the wave heights. This has a direct bearing on the energy propagation through the inlet and into the bay or estuary, as well as implications regarding the flushing of littoral drift and sediment from the inlet

An experimental study was conducted in a wave basin of 150 ft length, 50 ft width and 3 ft depth in which was simulated a tidal inlet capable of creating a non-uniform current representative of those found in the natural state. Current measurements and wave characteristics were determined at selected points from the ocean, through the inlet and into the bay, for pre-determined flow conditions based on the Freudians criteria for scaling. Initial wave characteristics were selected to produce a range of relative depths and wave steep nesses corresponding to those found in the prototype. Results of the preliminary analysis of the data are presented in dimensionless form to apply to all scales. It appears that shape of the velocity profile through the inlet is a pertinent parameter and a shape factor should be applied to a representative value of the flow. Also, the wave length to water depth ratio and wave steepness are critical quantities in the functional relationship between the initial and the altered wave state and the non-uniform current producing these changes.

Statement of the Problem

Many bays and estuaries are connected to the open ocean by relatively narrow inlets which may be required to pass large volumes of flow during the filling and emptying of these bodies by tidal action. These same tidal inlets are often required to conduct shipping and navigation vital to the nation's commerce, as well as an ever-increasing traffic in pleasure and recreation craft.

Coastal inlets in a natural state are subjected to opposing forces which alternately try to close or enlarge the passage. During the flood stages, the littoral transport of sand in the surf zone tends to be swept into the tidal inlet by the wave action and strong currents created by the rising tide. On the ebb flow, the estuary experiences a flushing condition and sediment, suspended and bed load, may be swept through the tidal inlet and passed down coast or lost to deep water. A river may also drain into the bay augmenting the ebb flow. When a balance of these forces exists the inlets tend to be stable and remain open. Otherwise the inlets may try to migrate or close completely.