FLOOD03313

• County: Denver
• Basin: South Platte
• Title: Drop Structures in the Denver Metropolitan Area
• Date: 12/1/1986
• Prepared For: UDFDC
• Prepared By: McLaughlin Water Engineers, Ltd.
• Floodplain - Doc Type: Floodplain Report/Masterplan

The check structure is categorized here because the performance characteristics are
considered similar to the overflow jet and plunge pool concept, Illustrated by Figure
lll-l.

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This rehabll1tatlve maIntenance project was designed for a flow rate of 500 cfs. It

has experienced flows on the order of 1100 cfs. Rock was originally desIgned with

a minimal safety factor In mInd; thus, one can see the displacement of rock, the

resulting scour hole, and the secondary drop downstream. The structure appears to

be stable but we suspect that future maintenance Will require either heavier rock to

be installed utilizing UDFCD guidelines for vertical loose rlprap drops, the Corps'

Derrick Stone Approach, or a grouted rock basin.

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We note that trickle flow scour exists downstream of the drop and aggradation

occurs upstream of the drop. The observation here is that the grass-lined channels

will naturally agrade or degrade to form a stable trickle channel configuration, and

that trickle channels and corresponding invert elevations will rise to stay above the

effective crest elevation. (Minor not~hes do not prOVide sufficient capacIty to

prevent aggradation). The trickle channel should be stabilized between checks.

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Case 31 - Spring Creek (trIbutary to little Dry Creek) downstream of County line

Road, Arapahoe County

These vertical rlprap drops were originally desIgned to comply wIth UDFCD interim

standards (Figure lll-2). Unfortunately, the rock prOVided was Rhyolite which has a

low speCIfIc gravity, it had to be grouted to be stabIlized. Thus, It was debatable

whether thiS drop was in a vertical rlprap or vertical hard basIn category. The

grouting was sub-standard In our OpiniOn, thus It was categorized here. Also, the

rock is deteriorating. It IS IrOniC that very large granIte rock was used for Ithe

retaining wall whIch had a speCific gravity of 2.7.

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The basins appear stable, however, the trickle and main channels upstream show

signs of aggradation. Also, the loose riprap is not well graded or placed properly

and problerTlll are likely. The design flow was 1160 cfs.

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