FLOOD06919

• County: Arapahoe
• Stream Name: South Platte
• Basin: South Platte
• Title: Union Avenue Boat Drop
• Date: 1/1/1983
• Prepared For: CWCB
• Prepared By: CWCB
• Floodplain - Doc Type: Easement

DRAFT

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. 17). 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 flows 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' /s (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
loo-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' Is. A video tape of the 100-year flood of 16,400 ft' /s 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 feel. 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
boatchute, but boats did not pass through at low flows without hitting the first ramp. Boats tended
to turn sideways as they went over the second ramp. This configuration significantly reduced the
wave heights.

The next boatchute design configuration constructed in the physical model for boatchute 1 included
two ramps with a center trough extending through the first ramp but not through the second ramp
(figs. 22 and 23). The elevation of the upper ramp was reduced to 5287.25 feet and the second
ramp was increased to 5286.5 feel. Wave patterns were acceptable for boating at 100 ft' /s and
800 ft' /s with a V-wave pattern (figs. 24 and 25); however, the wave in the center was larger than
the previous configuration. The boats remained straight as they passed through the chute.

A third demonstration was held on March 22, 1989, with personnel attending from Reclamation,
WWE, COE, CWCB, UDFCD, GS (Geological Survey), and the city of Englewood. The model was
demonstrated for flows between 100 and 8,000 ft' Is. UDFCD proposed a change in the alignment
of the second dam and boatchute 2 and reorientation of boatchute 2. Boatchute 2 was directed
away from the left bank to prevent development of a scour hole and possible bank erosion. The

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