FLOOD06658

• County: Arapahoe
• Community: Littleton
• Basin: South Platte
• Title: Union Ave Dam Boatshute
• Date: 9/1/1989
• Prepared For: Littleton
• Prepared By: BOR
• Floodplain - Doc Type: Community File

left. Flows ranging between 100 ft'/s and 1,500 ft'/s were tested in the model. Boats moved toward
the sluice wall while passing through the Union Avenue dam boatchute.

Reclamation conducted two demonstrations of the second design configuration. The first
demonstration on January 18, 1989, showed the model operating at a range of flows between 100
and 16,400 ft'/s. Attendance at the first demonstration included personnel from the cities of
Englewood, Bowmar, and Sheridan; CWCB; UDFCD (Urban Drainage and Flood Control District);
COE; and WWE.

During the first demonstration, both chutes were narrowed by placing large boulders (5-foot
prototype) along each side of the chute to help boating conditions at smaller flows. Photographs
of this flow condition are shown on figures 15 and 16. Narrowing the boatchutes improved lower
flow conditions, but an undesirable wave still existed downstream of the Union Avenue dam
boatchute. The boats had a tendency to turn toward the sluice wall while going through
boatchute 1. A high center wave also existed downstream of boatchute 2.

The wave height was adjusted by placing a ramp in the center of the boatchute. The purpose of
the ramp was to prevent boaters from taking on bow water by reducing the center wave. Initially,
a single ramp 15 feet long and 7.5 feet wide was tested in the model (fig. ]7). The single ramp
reduced the height of the wave and caused a more desirable wave pattern for boating. However,
the wave was still too high for safe boating conditions at higher t10ws at Union Avenue dam
boatchute. A long wedge-shaped block was placed on the downstream face of the Union Avenue
dam to the left of the boatchute. The wedge blocks will alleviate the dangerous roller that developed
over the dam when flows exceeded 500 ft'ls (fig. 18). The wedge block placed on the face of the
dam greatly improved flow conditions over the dam and was recommended for final design. The
block was placed far enough down the dam face to avoid altering the discharge coefficient of the
crest. If the discharge coefficient was altered, the river would not remain within its banks at the
100-year flood due to the reduction in efficiency.

During the second demonstration (January 31,1989), the model was run for flows ranging from 100
to 8,000 ft'/s. A video tape of the 100-year flood of ]6,400 ft'ls was shown to the participants.

Several design changes were attempted on boatchute 1. The height of the rocks on the left side
of Union Avenue boatchute was raised to prevent backflow into the boatchute from the left. This
change improved the boating flow conditions. The wave moved up on the ramp, and boats turned
left toward boatchute 2 instead of heading toward the sluice wall. However, slight changes in the
placement of the rocks would affect the flow, and the proximity of the riprap to the boatchute on
the left side was a safety concern.

Reduction in the height of the roller on boatchute 1 was accomplished by using a combination of
two ramps in the model. One ramp was set at elevation 5287.71, and the lower ramp was set at
5286.24 feet. Initially, the ramps only extended across the center of the boatchute (fig. 19), and
rocks still extended along either side of the boatchute. This boatchute configuration reduced the
height of the wave at the bottom of the boatchute, but boats still turned toward the sluice wall
unpredictably.

The next design change attempted in the model included two ramps completely across the boatchute
with a center trough up to the first ramp (figs. 20 and 21). Flow was uniform through the

7