FLOOD05064

• Designation Number: 205
• County: Larimer
• Community: Fort Collins
• Basin: South Platte
• Title: Master Drainageway Planning - Cooper Slough, Boxelder Creek, Fort Collins, Larimer County, Colorado
• Date: 8/1/1981
• Designation Date: 1/1/1983
• Floodplain - Doc Type: Floodplain Report/Masterplan

.

.

.

.

available right of way a concrete-lined channel is required from Highway 14 to

approximately 1,000 feet downstream on Cooper Slough. Downstream of this sec-

tion of reach 1, a grasslined trapezoidal channel would parallel the Lake

Canal. Along the concrete-lined section, security fencing should be provided.

The north bank of the Cache la poudre Reservoir Inlet should be built up to

prevent flow from overtopping the existing banks and flowing south. The top-

width of the proposed open channel ranges from 60 feet for the 10-year design

flow between vine Drive and the Colorado and Southern Railroad to 150 feet for

the 100-year design flow along Reach 1 of Boxelder Creek. Table 22 presents a

summary of the channel design data.

To maintain a velocity of 5 fps along Reach 2 of Cooper Slough and Reach

1 of Boxelder Creek drop structures are required. The channel for the 10-year

design flow requires one 4-foot drop structure between Vine Drive and Highway

14. The channel for the 100-year design flow required three 4-foot drop

structures along this reach. For Reach 1 of Boxelder Creek, one four-foot

drop structure is required along with five erosion control checks downstream

of Prospect Street.

Under Alternative 2 it is also necessary to enlarge openings at channel

crossings for Cooper Slough. Enlarged openings are necessary at Cooper Slough

and Highway 14, Vine Drive and Colorado and Southern Railroad. A new crossing

is required at Cooper Slough and Sherry Road. Also enlarged openings are

required at Boxelder Creek and Prospect Street. Enlarged culverts at private

road crossings along Boxelder Creek below Prospect should be designed to carry

the ten-year flow of 1200 cfs and act as a weir during a less frequent flood.

A summary of the design data for the conceptualized bridges and culverts are

presented in Table 23 for Cooper Slough, Alternative 2. Table 24 presents the

design data for the conceptualized bridges and culverts for Boxelder Creek for

both Alternatives 2 and 3.

.

.

Table 22. Alternative 2, Design Data for Conceptualized
Grasslined Channel

.

.
i.

Location

I

Recurrence
Interval
(years)

Design
Flow
(cfs)

Slope
(ft/ft)

Bottom
Width
(ft)

Depth
(ft)

ROW
Width
(ft)

.

Cooper Slough

.

vine Drive to
Colorado and Southern
Railroad
Reach 2

I

I
I

Colorado and Southern
Railroad to
State Highway 14
Reach 2

I

I

I

.

.

*State Highway 14 to
approximately 1,400 feet
downstream
Reach 1

I

.

*1,400 feet downstream
of Highway 14 to 400'
D/S Sherry Road
Reach 1

I

.

.

I

400' D/S of Sherry Road
to Boxelder Creek
Confluence
Reach 1

I

.

Boxelder Creek'"

I.

Other necessary structural improvements under Alternative 2 are required

to separate the floodwater of Cooper Slough from the Lake Canal and provide a

low flow diversion to the concrete pipe where the existing Cooper Slouqh enters

Lake Canal. A crossing under Lake Canal near the confluence of Boxelder Creek

is most practical because the proposed channel is below the grade of Lake

Canal.

To prevent erosion along the design channels, a maximum velocity of 5 fps

was allowed. To prevent scour at erosion control checks and at the downstream

I

I

1-25 to
Prospect Street

10
50
100

10
50
100

10
50
100

10
50
100

10
50
100

Flow at
1-25

330
900
1100

500
1240
1700

500
1300
1800

500
1300
1800

500
1300
lBOO

2000

.0044
.0038
.0036

.0041
.0035
.0033

.0033
.0033
.0033

.0015
.0015
.0015

.002
.002
.002

.0033

20
33
45

30
64
BO

10
20
28

15
30
42

40
85
120

100

3.0
3.5
3.5

3.5
3.5
3.5

4.0
4.0
4.0

4.0
4.0
4.0

3.5
3.5
3.5

3.5

70
88
100

85
120
135

20
30
36

25
40
52

80
125
160

160

*Concrete lined channel

I

I

*Same data for Alternative 3

I
I

.

64

I

I

65