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

INTRODUCTION

In 1985, the COE (US. Army Corps of Engineers) constructed a dam 300 feet downstream of
Union Avenue in Englewood on the South Platte River. The dam serves as a diversion structure
for a water supply intake for the city of Englewood and also controls the grade of the South Platte
River for flood control. The reinforced concrete dam is 18.5 feet high and has a 3 to I slope on
the face of the spillway. The total head drop is 15 feet to the downstream channel. A section
through the existing dam is shown on figure I. The energy is dissipated in a hydraulic jump at the
base of the spillway. This type of energy dissipator has been used successfully at hundreds of
structures throughout the world. However, the reverse roller in the jump can trap a boat or person
and is hazardous when a boat goes over the dam. Recreational boating has become rnore popular
on the South Platte in recent years (fig. 2); therefore, boater safety has become a primary concern.

The CWCB (Colorado Water Conservation Board) is investigating structural alternatives to improve
boater safety at the Union Avenue dam. One alternative is to construct a series of boatchutes,
each with a drop of 2 to 4 feet, to handle the entire 15-foot drop. The CWCB selected WWE
(Wright Water Engineers, Inc.) to plan the boatchutes at the site. The CWCB also requested
assistance on the project from Reclamation (the Bureau of Reclamation). Reclamation agreed to
construct a physical model of the boatchute in the Hydraulic Laboratory. Funding for the model
study was provided primarily by the Great Plains Region's Assistance to States Program; the CWCS
provided partial funding for the work. The testing was divided into three phases:

. Phase A. - Model tests to achieve desired flow patterns for boating at discharges between
100 and 1,500 ft'!s.

. Phase B. - Sediment tests to observe deposition and scour patterns as they affect the
Englewood water intake structure upstream of Union Avenue dam and deposition and
scour patterns in the boatchute 1 pool.

. Phase C. - Floodflow tests up to the 100-year flood of 16,400 ft'!s.

CONCLUSIONS AND RECOMMENDATIONS

I. A double-ramp boatchute configuration was developed as a result of the rnodel study. This
boatchute configuration eliminates dangerous reverse roller hydraulic jumps (figs. 3-5) and provides
boatable waves downstream throughout the entire flow range. Boating safety will be greatly
improved by the addition of the proposed features.

I'
I

2. Guidelines were developed for designing the boatchutes. These guidelines can also be applied
to other sites with head drops between 2 and 3.5 feet.

3. With the present Englewood water intake design, boaters would be drawn into the intake area
during sluicing.

4. A solid wall [the same height as the intake wall (elevation 5296.5)] is recommended in front
of the primary Englewood intakes to protect boaters and to enhance the present sluicing capacity
(fig. 3).