also estimate the wave power. While wave power near
the beach is estimated to be 0.034 Â 10
7 erg/s on
average for the Mississippi delta, it is 10 Â 10
7 erg/s for
the Nile, and 20.6 Â 10
7 erg/s for the Magdalene River
(Wright 1978). In rivers like the Mississippi, which
has a shallow profile beyond the delta front, most of
the wave power dissipates before it reaches the delta
front. Deltas which develop into deep water right by
the continental slope are exposed to the greatest
wave power, because waves are not damped before
Fig. 2.34 Vertical section through modern Mississippi delta sediments. From Coleman and Prior (1980)
Water flow out through a channel
Low rate of flow
Fresh water
At low flow of water a wedge of salt water may reach far up
in the fluvial channel
Standing internal waves
strong turbulence
Quick speed
reduction
Ocean
Salt water
Mouth bar
Strong turbulence
River water
Salt water
Flood
Erosion
At flood the wedge of salt water is forced back over
the mouth bank of the channel
a
b
Fig. 2.35 Flow of fresh and salt water at the mouth of a river channel (after Wright and Coleman 1974)
72
K. Bjørlykke