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Question: THE FORMATION OF A TIDAL BORE

Answer: Brief Introduction

A tidal phenomenon in which the incoming tide forms a wave (or waves) of water against the direction of the current, is known as Tidal Bore or Benak in Malay. It occurs in relatively few locations worldwide, usually in areas with a large tidal range, typically more than 6 metres, between high and low water. It is a phenomenon that has never been truly understood, although the mechanism of its passage and size are fairly predictable.

The main key features of a tidal bore are the intense turbulence and turbulent mixing generated during the bore propagation. A tidal bore creates a powerful roar that combines the sounds caused by the turbulence in the bore front and whelps that can be heard far away because its low frequencies can travel over long distances.

It moves upriver as incoming tidal current surges against the flow of a more narrow and shallow river, bay, or estuary. It can be 10 to 16 feet high and move rapidly 10 to 15 knots upstream faster than the rising tide. As for the Sri Aman Bore, it start appeared at Pulau Seduku, Bakong and travel up to 45 km passing Sri Aman Town till it ended somewhere at Berangan. Some local also believe the movement of the bore ended as far as Engkelili, 47 km from Sri Aman Town.

A tidal bore may take on various forms, ranging from a single breaking wavefront with a roller, somewhat like a hydraulic jump to undular bores, comprising a smooth wavefront followed by a train of secondary wave (whelps).

The river mouth shape and channel Bathymetry

In a case of Batang Lupar’s Tidal Bore, it occur when the incoming tides re funneled a shallow, narrowing Batang Lupar, where the area of wetted cross-section decrease upstream, accordingly the speed of flood current increases. The submerged sand bar in the channel causes the depth of water to shoal, thus the celerity of tidal waves decrease. This will cause the incoming tide that propagating to upstream turns to rising. Such condition is known as positive surge. As the surge progresses inland, the river flow is reversed behind it. A surge in an open channel is a sudden change of flow depth where it is a function of the channel depth.

An abrupt increase in flow depth is called a positive surge while a sudden decrease in depth is termed a negative surge. A positive surge looks like a moving hydraulic jump. The front of a positive surge absorbs random disturbances on both sides of the surge and this makes the positive surge stable and self-perpetuating. With appropriate boundary conditions, a tidal bore may travel long distances upstream of the river mouth.

Type of Tidal and Its Amplitude

Another factor that contributes to the formation of a tidal bore other than the shape and the depth of the channel is the large tidal amplitude. The tides are forced oscillations generated by the attractions of the Moon and Sun, and have the same periods as the motion of the Sun and Moon relative to the Earth. Every fourteenth day at full moon or new moon, the attraction forces of the Sun and Moon reinforce one another, and these conditions give the spring tide conditions. The tidal range may be locally amplified further by a number of factors including when the natural resonance of the bay and estuary is close to the tidal period. This coincidence implies that the general sloshing of the waters around the inlet or bay becomes synchronized with the lunar tides and amplifies their effect, yielding often the best tidal bores a couple of days after the date of the maximum tidal range.

Weather and Stormunoff Surge

Weather also plays significant contribution to the formation of a tidal bore. During landas season, stormrunoff generated from certain event of rainfall accumulated at the river channel that will further drain to the sea. During the process, the surge from the upstream meets the incoming tide thus forming a tidal bore where the large volume of upstream flow further amplifies the magnitude of a tidal bore. Therefore, during monsoon season, it can be observed that the height of a tidal bore in Batang Lupar can reach more than 5 feet which potential posed navigational threat for inland shipping.

Conclusion

A tidal bore is fascinating geophysical phenomenon where its existence based upon fragile processes. The bore development is closely linked with the tidal range, the shape of the river mouth, channel bathymetry and the freshwater river flow conditions. Once formed, the bore existence relies upon the exact momentum balance between the initial and new flow conditions. Some simple theoretical considerations show that this balance may be easily disturbed by some changes in the boundary conditions and freshwater runoff that may affect adversely the bore existence. Dredging and river training are some examples that potentially yielded the disappearance of the tidal bore.

(Source: Pustaka Negeri Sarawak, 28 Mar 2022)

Source Link: https://did.sarawak.gov.my/page-0-315-517-Tidal-Bore.html