Tidal Bores Essay

4514 words - 19 pages

Tidal Bores
Report for the course “Physics of Coastal Systems 2010”

Werner de Groot (3558835)
1

Table of contents
Abstract ............................................................................................................................................. 3 Introduction ...................................................................................................................................... 3 Results: a model for tidal bores ........................................................................................................ 4 Physical explanation ...view middle of the document...

(2004), which examines an undular bore observed in the Daly Estuary.

Introduction
A tidal bore is a phenomenon in which an incoming tidal wave travels up a river against the direction of the current and forms a steep and high front. It occurs when the flood tide gets confined to a narrow and shallow channel. This may not sound very exciting but it is, in fact, a fairly violent process in which two to three meters tide comes in all at once. To illustrate: a tidal bore on the Indus River apparently wiped out the entire fleet of Alexander the Great in 325 BC [1]. This might seem surprising at first – we usually regard tidal processes as being relatively smooth and quiet, so how can it produce these huge, fast waves? – but a simple model shows that, with a large enough tidal range and especially shallow waters, truly enormous waves can occur. In fact, a lot of videos and images of tsunamis circulating on the web are actually tidal bores! Nowadays, tidal bores can be accurately predicted and have become, more than anything else, a great opportunity for surfers and rafters.

Figure 1: A tidal bore on the Turnagain Arm in Canada, which can reach a height up to 2.5 meters, travelling with speeds up to 25 kilometers per hour. With the appropriate boundary conditions, the bore may travel as much as 80 kilometers. Source: http://www.photolib.noaa.gov/htmls/line1769.htm.

Tidal bores are extremely rare. Of all the rivers in the world, only a tiny subset have a large enough tidal range and the necessary relatively shallow waters to support them, and even then, often only during spring tide. The most famous tidal bores are probably those of the Seine River, France where it is called the Mascaret (which has been greatly reduced in magnitude recently due to man-made interventions), the Qiantang River, China where it is called the Black Dragon and those of the Severn River (see Abbott, 1956 for a discussion of the Severn River Bore) and the Humber, both in the United Kingdom. The occurrence of tidal bores has a significant impact on river mouths and estuarine systems. Erosion and scour take place beneath the bore front, while suspended matters are carried upstream 3

by the large velocities caused by the bore. The existence of tidal bores, however, is based upon a truly fragile hydrodynamic balance which may easily be disturbed by changing the boundary conditions and the freshwater inflow. Man-made interventions led to the disappearance of several bores, with often a strong (negative) influence on the ecosystem, like in the case with the Mascaret of the Seine River [2]. Although these rivers gained considerably in safety in most cases (these rivers were generally regarded as very dangerous, as one can imagine) the elimination of several fish and bird species was also a direct consequence. Fish eat matter in suspension after the passage of the bore while birds fish directly behind the bore. Despite their impact on estuarine processes and their often...

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