Geoscience Reference
In-Depth Information
Fig. 3.27
Types of vortices
responsible for bedrock
sculpturing by tsunami. Based on
Kor et al. (
1991
), and Shaw
(
1994
)
comma mark
muschelbrüche
sichelwanne
V-shaped groove
pothole
trough
into a model of multiple-vortex tornado formation that may
be more appropriate in explaining the formation of whirl-
pools in tsunami-sculptured terrain.
Multiple-vortex tornadoes are produced by vortex
breakdown within a tornado as air is pulled downwards
from above into the low pressure of the tornado (Fujita
1971
; Grazulis
1993
). Smaller secondary vortices rotating
in the same direction develop around the circumference of
the larger parent vortex. Tsunami-generated whirlpools are
formed by the vertical vortices embedded in flow over-
washing a headland (Bryant and Young
1996
). These vor-
tices expand and increase in rotational velocity with time.
When the vortex is wide enough, water is pulled down into
the center of the vortex and lifted upwards at the circum-
ference (Fig.
3.28
). When velocity is high enough, the
vortex bores into bedrock and locks into position. Velocities
exceed that necessary for cavitation as shown by the pres-
ence of drill marks in the whirlpool at Atcheson Rock
(Fig.
3.26
). Towards the center of the vortex system, the
directions of water movement in the mini-vortex and parent
vortex are opposite and begin to cancel each other out. Here
the resultant flow velocity is the rotational velocity of the
parent vortex minus that of the mini-vortex. These lower
rotational velocities aid the collapse of water into the center
of the vortex, but at velocities that are too low to erode
bedrock for part of the time. This process leaves a plug of
bedrock in the middle of the whirlpool. Once multiple
vortices form, the system of flow becomes self-perpetuating
Secondary
vortex
Parent
vortex
Secondary
vortex
T - V
T +V
T - V
T +V
T, rotational speed of parent vortex
V, rotational speed of secondary vortex
Fig. 3.28
Model for multiple-vortex formation in bedrock whirl-
pools. Based on Fujita (
1971
) and Grazulis
(1993)
as long as there is flow of water to maintain the parent
vortex. The fact that the plug height is always lower than
the pothole walls suggests that multiple vortices develop in
the waning stages of tsunami overwash after the crest of the
tsunami has swept past and established the parent vortex.
Whirlpools form when flow velocities increase first
through convergence of water over bedrock and then
through funneling at preferred points along the coast.
Critical rotational velocities required to erode resistant
bedrock also take time to build up; however, the flow under
