Water Resources: Hydraulics and Hydrology
on the rock up to the top of the jetty. Most of the tetrapod protection is required on the
channel side of the jetties. When high waves attack the back side of a jetty, as frequently.
occurs at the seaward end of jetties, then similar tetrapod protection should be provided
there.
The use of tetrapod protection was accepted. Tests were made at Bonneville Hydraulic
Laboratory on a sectional model of the jetty. Different rock and tetrapod sizes were tested
for different wave heights. This provided design criteria for rock and tetrapod sizes for
different wave heights.
Another problem involved maintenance of the navigation channel which was 40 feet deep.
Vessels coming in from the ocean passed through the navigation channel for several miles
to the seaward end of the jetties, then passed between the jetties for several miles and then
continued up the navigation channel to Portland. The jetties are 800 to 1,000 feet apart
at the seaward ends. During large storms,
waves would cause the vessel to pitch
so much, that if its draft was more than about 25 feet, it would strike the bottom of the
foot channel. The navigation interests and the Corps in the Pacific Northwest came up
with the idea of enlarging the depth of the channel to 48 feet, increasing the depth by 8
feet. The next question was how to do it.
One plan was to dredge it. The second plan was to construct spur dikes near the inside
end of the jetties. The spur dikes would be attached to the jetties, and aimed toward the
navigation channel. Those spur dikes would constrict the opening between the two jetties,
which would reduce the amount of water flowing up the river, during rising tides and
storms. Also, less sediment would go up the river, so the amount of dredging required
would be less. It was concluded that the dredging probably would not be completely
eliminated, but it might be reduced enough for the
channel to justify the
construction of the spur dikes.
Another question that was raised was whether the constriction at the end of the jetties
caused by the spur dikes would produce erratic wave patterns and sandbar erosion with
the navigation channel. It was concluded that the only way to answer this question was
to make a model study of the whole thing. The last time I had anything to do with this
project, the decision had been made to adopt the first plan of dredging the
channel
to determine how much more dredging would be required than for the
channel.
If the increase is not excessive, then the channel would continue to be maintained by
dredging. If the dredging is greatly increased, then a cost estimate would be made of the
construction of the spur dikes.
Hydraulic model tests would need to be made to determine whether one or two spur dikes
are required. It'd be more uniform. With only one, the amount of dredging I think would
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