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

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Case 33 - Sand Creek at WheelIng, Aurora

This drop IS designed to handle 21,500 cfs. It appears stable as seen In Figure 1lI-

5, although there IS concern regarding abutment stabIlity when major flows overflow

the ends. It may be preferable to have a deeper trickle notch as there are some

signs of aggradation upstream. Rock SIZing could not be verified.

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Case 60 - Bear Canyon Creek Downstream of GilpIn, Boulder
This drop, depicted In Figure 1I1-4, was origInally a sloping drop with much of the
rock installed on a four to one slope. Over time, the chute rock moved and a
plunge pool was created. The design drawings call for a 4-foot layer of 24-inch
d50 riprap to be prOVided for 30 feet downstream. We suspect that much of the
material has settled In place resulting in a stable scour pool and that problems
occurred durIng constructIon. It is very likely that the upstream wall Influences
the discharge pattern over the crest, thus Increasing the Intensity of scour. There
are other indIcatIons of transitIOn and rlprap displacement problems downstream as
noted by bank scour.

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The structure was primarily designed for the 5-year event due to lImIted channel

capacity. However, the structure .was desIgned to be stable for the 100-year flood.

DespIte the vIsible problems, It appears that the facIlity IS reasonably stable but

Will eventually require more rock work for bank and drop stabilIty.

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Case 80 - BIg Dry Creek at C-470, Arapahoe County

ThiS drop utIlizes precast concrete components to form the drop and stillIng basins,

as shown In Figure llI-S. 12-inch rock is specified for placement In the basIn and 9-

Inch IS called for downstream of the sill. It IS apparent that there IS excessive

rock movement, secondary drops downstream of the Sill, and a fair amount of channel

Instability that will requIre corrective work.

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CONCLUSIONS
Vertical riprap drops appear to be a more satisfactory alternative than the present
UDFCD sloping rock drops. However, there are fewer existing vertical drops than
slopIng drops. The likely reasons for this are expense and the engineering required
for the wall. To a degree, the success of the drop IS less senSitive to

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