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

during the sluicing tests. During these tests, the flow through the Englewood intake was set at
30 ft' /s.

The city of Englewood currently operates at radial gate settings ranging from 2 to 8 feet to sluice
sediment for riverflows ranging from 200 to 3,000 ft' /s. At 3,000 ft' /s, the upstream water intakes
must be closed due to sediment buildup, even with the sluice gate fully opened. These sluicing flows
would draw boaters to the barrier and pin the boats against it; therefore, the sluicing flows would
be unacceptable for boating.

In order to improve sluicing and reduce the risk to boaters, the low wall currently in front of the
intakes was increased in height to match the present elevation of the primary intake wall
(5296.5 feet). All of the water entering the intake area was forced to enter at the upstream end
of the intake. The high wall extends downstream to the crest of the existing dam and has a I-foot
opening, as shown on figure 3, to allow small amounts of debris to exit the sluiceway. In order to
assess the effect of this change, velocities were measured in the intake area with and without the
high wall in place. Table 2 summarizes the data.

As shown by the data, the sluicing action is greatly enhanced by adding the high wall. The same
velocity can be obtained at a 25-percent gate opening with the high wall as with a 100-percent gate
opening with a low wall. When the sediment test was conducted with 3,OOO-ft' /s riverflow and a 30-
percent gate opening with the high wall, the primary intake area was almost entirely sluiced out;
therefore, sluicing is much more effective with the high wall.

The radial gate can be operated automatically (as it is now) to maintain a constant water surface
of about 5290 feet. At riverflows of 50 to 150 ft' /s, all or most of the flow enters the Englewood
intake channel. As the riverflow increases, the radial gate can be adjusted to facilitate sluicing. The
model tests indicated that a sluiceway gate opening of about 50 percent would provide maximum
sluicing capacity (table 2).

Approach flow to Englewood intake. - With the high wall in place, an open bar barrier or other type
of open barrier will be required to prevent boaters from entering the sluice at the upstream end.
A series of tests were conducted to determine the required placement of the barrier. Figures 41
through 43 show velocities measured for various operating conditions, including river discharges of
240, 811, and 1,450 ft' /s. Three different barrier locations were considered. The tests show that
the angle of the velocity vector with the barrier would pin the boats against the barrier unless the
barrier extends upstream parallel to the angle of the Englewood primary intakes (recommended
barrier location on the figures). For this position, the magnitude of the velocity component
perpendicular to the barrier does not exceed 2.5 ft/s and the component parallel to the barrier is
larger. This would tend to move the boats downstream parallel to the barrier rather than pinning
them against the barrier.

Sediment tests. - A typical bedload particle size distribution curve was obtained from WWE for this
section of the South Platte River near Oxford Avenue, located approximately] mile downstream
of Union Avenue. The particle size distribution curve was scaled based on techniques outlined in
the Sediment Scaling section of this report. Estimates of bedload discharge rate were made for a
large spring flow of 3,000 ft' /s, which has a return period of 10 years (Wright Water Engineers.
1987).

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