Key Concept Review (Answers to in-text “Concept Checks”)
Chapter 11
1. Tide waves are called forced waves because they are never free of the forces that
cause them. In contrast, after they are formed, wind waves, seiches, and tsunami are
free waves -- they are no longer being acted upon by the force that created them and
they do not require a maintaining force to keep them in motion.
2. The position and proximity of the moon makes the most important contribution to
tidal patterns. The sun’s influence on the tides is only 46% that of the moon’s.
3. The pull of gravity between two bodies is proportional to the masses of the bodies but
inversely proportional to the square of the distance between them.
4. The combined outward-flinging force of inertia and inward-pulling force of gravity
are called tractive forces. Gravity and inertia don’t always act in exactly the same
balanced way on each particle of Earth and moon; the tractive forces are the net
strength and direction that result when the two forces are combined (Figure 11.5).
The key to understanding tides is to imagine Earth turning beneath bulges of water
formed by tractive forces (as in Figure 11.7).
5. Because of its proximity to Earth, the moon generates the strongest tractive forces.
6. Spring tides occur when Earth, moon, and sun are aligned (Figure 11.11a). Neap
tides occur when Earth, moon, and sun form a right angle (Figure 11.11b).
7. The dynamic theory correctly treats tide waves as shallow-water waves. As Earth
turns, landmasses divert, slow, and otherwise complicate the movements of tidal
crests. This interference produces different patterns in the arrival of tidal crests at
different places.
8. Because of their immense wavelength, tides can never be in “deep” water (that is,
water deeper than half the wavelength), even though their crests may traverse abyssal
depths.
9. Some coastlines experience semidiurnal (twice daily) tides: two high tides and two
low tides of nearly equal level each lunar day. Others have diurnal (daily) tides: one
high and one low. Coastlines with mixed (or semidiurnal mixed) tides have
successive high tides or low tides of significantly different heights, caused by
blending of diurnal and semidiurnal tides. Figure 11.13 shows an example of each
tidal pattern.
10. Tidal crests rotate around amphidromic points – “no tide” points in the open ocean
(Figure 11.15). Because of the shape and placement of land masses around ocean
basins, the tidal crests and troughs cancel each other at these points.
11. The largest tidal ranges occur at the edges of the largest ocean basins, especially in
bays or inlets that concentrate tidal energy because of their shape. If the basin is
narrow and restricted, the tide wave crest cannot rotate around an amphidromic point
and simply moves into and out of the bay. In other cases, arriving tide crests
stimulate natural oscillation periods of around 12 or 24 hours, resulting in extreme
tides (Figures 11.16 and 11.17).
12. A tidal bore is a steep wave moving upstream generated by the action of the tide crest
in the enclosed area of a river mouth.
1|Page
13. Meteorological tides are weather-related alterations to predicted tidal cycles, such as
those associated with the storm surge of tropical cyclones.
14. Arrival of a storm surge on top of a high tide can be especially devastating to coastal
regions. Some of the astonishing destructiveness of Hurricane Katrina in 2005 can be
attributed to the arrival of the surge (and wind-driven masses of water) coincidentally
with a high tide.
15. Within the intertidal zone, organisms are exposed to varying amounts of emergence
and submergence. The animals and plants sort themselves into horizontal bands
based on the amount of exposure they can tolerate. Each distinct zone is an
aggregation of animals and plants best adapted to the conditions within a particular
narrow habitat.
16. There are major tidal power stations in France on the estuary of the river Rance
(Figure 11.21) and on the Annapolis River in Nova Scotia.
17. Tidal power plants can be damaged by storms and corroded by seawater. Computer
simulations have suggested that installing a dam would change the resonance modes
of a bay or estuary—and therefore the height of the tide wave. Studies also suggest
that sensitive planktonic and benthic marine life would be disrupted.
2|Page