Tidal processes • Waves • Sediments • Flocculation

Waves in Estuaries & Bays

Waves which affect estuaries and bays may be sub-divided into two main categories: open sea waves and internally generated wind waves. Wave generation is governed by four factors: wind speed, wind duration, fetch and water depth. In the open sea all of these factors are important and the largest waves will develop when high wind speeds blow over a large distance of sea (fetch) for extended time periods. However, in the open sea, water depths and fetch are rarely limiting since very large sea distances and depths are involved. This means that wave height is governed by wind speed and duration. Since both of these factors are variable in time, so are wave conditions. Small waves are more frequent than large waves, with the result that the return intervals increase progressively with wave height.

Open sea waves can enter the mouths of estuaries and bays and play a major role in modifying the morphology of their shorelines. Shorter period waves can penetrate further upstream in an estuary than longer period waves since they are less affected by the shallow water conditions they meet in the estuary. Conversely, longer waves refract more markedly than short waves and, therefore, tend to enter confined estuary/bay mouths or those protected by spits or bars more easily than short waves. As a result, the effect of waves within estuaries/bays differs greatly depending on physiographic setting and degree of exposure. For example, the estuaries of the Camel in north Cornwall or the Dovey in Wales are affected by long Atlantic swell waves which can penetrate their outer parts, resulting in the formation of extensive sandy intertidal flats with restricted saltmarsh development and a characteristic wide, flat channel. By contrast, the estuaries of East Anglia are affected only to a much smaller extent by open sea waves from the North Sea , which lack the oceanic swell component and tend to be shorter and steeper.

The second type of wave is generated within the confines of the estuary system itself. Since the fetch distances here are normally very short, usually no more than 10 to 20km, wind speed and duration are not critical and only fetch length and water depth are limiting. Thus the height of waves generated within estuaries and confined bays is controlled almost entirely by fetch, so that a given shoreline with a fixed fetch across the estuary/bay to the far bank, and which is remote from the influence of sea waves, can only experience waves of a fixed maximum height. However, it should be noted that, in most estuaries, both fetch length and water depth vary with tidal stage, so that the largest waves generally occur at about high water. This is important when considering the development of both mudflats and saltmarshes in estuaries, since the existence of an upper limit to wave height means that these shorelines can develop in response to this fixed upper limit, whereas shorelines facing open sea waves are forced to respond to the continually increasing wave heights associated with longer return intervals.

It should not, however, be assumed that because waves generated within estuaries are relatively small they have little geomorphological impact. Small waves are active for a high percentage of the time and can produce a significant cumulative effect.