单项选择题

Conditions for crossing a rough bar are usually best at （）.

A.low water slack
B.high water slack
C.high water ebb
D.high water flood

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单项选择题

Clearances of bridges and overhead cables below Bonneville Dam refer to heights in feet above mean （）.

A.lower low water
B.high water
C.low water
D.sea level

单项选择题

Clearance gauges at bridges indicate （）.

A.the height of the tide
B.depth of water under the bridge
C.charted vertical clearance at mean low water
D.distance from the water to low steel of the bridge

单项选择题

Attention is called to the fact that in many cases pipelines are not （） and charted depths may be decreased by up to 2 meters．

A.concealed
B.buried
C.existed
D.built