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The Correct Answer is "D" (18.6 ft)

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Question

A trapezoidal open channel has a base width of six ft. and both sides are sloped at the ratio of one vertical to three horizontal. The depth of storm water flow in the channel is two ft. Determine the wetted perimeter.

Answer

Solution: The wetted perimeter is the sum of the base width (six ft.) plus the lengths of the two sloped sides in contact (wetted) with the storm water.

Use the Pythagorean Theorem to calculate the lengths of the two sloped sides.

With a depth of flow of two ft. and side slope ratios of one vertical to three horizontal, each sloped side calculates to 6.3 ft. Therefore, the wetted perimeter is the base plus the two sloped sides 6 ft. + 2(6.3 ft.) = 18.6 ft.