Movie:TidalBoreAK

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Tidal Bore in Turnagain Arm



Key Attributes

Domain: coastal
Keywords: Tidal Bore
Keywords: Alaska
Model name: Animation model name
Name: Lynn, L'Archeveque
Where: Turnagain Arm, Cook Inlet, Alaska
When: August, 2007


Short Description

Grade level: Under graduate (13-16)

Statement: Tidal bore in the Turnagain Arm of Cook Inlet, near Anchorage, Alaska.

Abstract: This movie features the tidal bore in the Turnagain Arm of Cook Inlet, near Anchorage, Alaska. This tidal bore can be up to 2 metres and travels at 20 km per hour. The bore in the movie smaller than 2m (pers. comm. L'Archeveque).

Theory

A tidal bore, also called aegir, is a tidal phenomenon in which the leading edge of the incoming tide forms a wave of water that travel up a river or narrow bay against the river current. As such, it is a true tidal wave. Bores occur in relatively few locations, but they do occur worldwide, usually in areas with a large tidal range (typically more than 6 m between high and low water). All these locations are shallow, narrowing rivers or fjords in which water is funneled via a broad bay. The funnel-like shape not only increases the height of the tide, but it can also decrease the duration of the flood tide down to a point where the flood appears as a sudden increase in the water level. The rising tide may force the tidal wave-front to move faster that a shallow water wave can propagate into water of that depth:

T=L/(c+u)

T = wave period
L = wave length
c = wave speed
u = speed of current

If the current flows counter the direction of wave propagation, then L will increase and the wave will get shorter and higher (upto the point of breaking). Bore tides come in after extreme minus low tides created by the full or new moon (Chanson, 2004).

Bores take on various forms, ranging from a single breaking wavefront —like a shock wave — to ‘undular bores’ comprising a smooth wavefront followed by a train of secondary waves (whelps). Large bores can be dangerous for shipping. Rivers that do have a tidal bore include the Amazon and Orinoco Rivers, in South America, the Hoogly River in the Ganges-Brahmaputra delta, several rivers in the UK, and rivers draining into the Bay of Fundy. The largest tidal bore occurs in the Qiantang River in China, it is 9m high and travels at 40 km/hr.

Tidal bores have distinct influence on sediment transport. The arrival of the borefront is associated with intense bed shear stress and bed scour. Suspended sediment is advected upwards in the wake of the tidal bore. This phase is associated with turbulent structure. The suspension of sediment is sustained by wave motion for several minutes to half an hour after the bore has passed (Chanson, 2004).

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