FLOOD09485

• Designation Number: 206
• County: Larimer
• Community: Fort Collins
• Stream Name: Fossil Creek Drainage Basin
• Basin: South Platte
• Title: Master Drainageway Planning - Fossil Creek Drainage Basin, Fort Collins, Larimer County, Colorado
• Date: 8/1/1982
• Designation Date: 1/1/1983
• Floodplain - Doc Type: Floodplain Report/Masterplan

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VI. ROADWAY AND RAILROAD EMBANKMENTS

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Most roadway and railroad embankments are not designed to serve as deten-

tion structures. If they are used for detention structures, stability analy-

sis and dam break analysis should be performed. Roadway and railroad embank-

ments which serve as water detention structures during storms are subject to

three types of failure; (1) overtopping, (2) piping and liquefaction, and (3)

slope instabilities. A short discussion of each of the failure modes follows.

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6.1 OVertopping

Embankment overtopping occurs when the storage capacity behind the

embankment is fully utilized and inflow to the detention area exceeds the

outflow capacity of the culvert. Under such conditions, high velocities may

result on the downstream face of the embankment. Flow with high velocities

has an extremely high erosive potential, and an embankment subjected to such a

velocity would soon be breached. In practice, an embankment should be pro-

tected against overtopping failures by protecting the downstream face with

riprap, rock sausages (ref. 13), or similar protection.

Embankments that are overtopped during floods act as broad-crested weirs.

As a result, to protect the downstream slope from the high-velocity, supercri-

tical flow, very large riprap would be required for a spillway type design

(ref. 16). In all likelihood, the lengths of the embankments, the velocities

against which they must be protected, and the infrequency of overtopping will

make downslope protection economically infeasible. As such, the only reliable

alternative to protect against overtopping failures is to design the culvert

system to convey the de~ign discharge without the occurrence of overtopping.

Table 24 lists the major roads that are overtopped during the laO-year flood

for developed conditions in the study area. The depth of overtopping and

possible protection measures are also presented.

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6.2 Piping and Liquefaction

A second mode by which embankments may fail is piping and liquefaction.

Piping (a "quick" condition) in cohesionless soils occurs when the effective

stress of the soil approaches zero (i.e., the shear strength approaches zero).

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This commonly occurs in two situations.

(1) an upward flow of water of such a

magnitude that the total upward force of the water equals the total soil

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Table 24. Detention Data for Embankments, 100-Year FJ ood - Develq>ed ConditIons.

Locatfon

De tent fan
Volume
(ae-ft)

MaxImum
Water
Depth
1ft)

Maxlrrum
Depth of
Flow Over
Road
1ft)

Embankment
He I ght
(ft)

Comments

F05S I L CREEK

County Road 1 I

Un Ion PacIfIc RR &

Trilby Road ICty.
Road 34)

Lemay Avenue
(cty. Road 13)

Colo. & Southern RR

Fossl I Creek Dr.

Sh lelds Street
Taft HI II Road
STANTON CREEK

County Road 32

Lemay Avenue
(cty. Road 13)

LANG GULCH

Colo. & Southern
RR 12

Tr 1 Iby Road (cty.
Road 34) and
Co 10. & Southern
RR 13

Colo. & Southern
RR 14

Sh lelds Street

SMI TH CREEK

Sh lelds Street

Taft HI I I Road
8URNS TRI8UTARY

Sh lelds Street

Taft HI II Road

210

570

H

19

12
19

18

54

5

9

12

56

40

53

32
47

11

25
18

15

12

15
10

14

13

13

14

2

3

2

8

2

9

52
15

13

33

6

14
22

12

12

18

17
11

11

11

11
16

28
17

En J arge channel
cross I ng.

Perform s tab I jJ ty ana-
lysIs
De"Velop adequate deten-
tion

En I erge channel
crossing

Unstable wIng wa j I at
U/S face of crossing

Increase culvert and
weIr capacity

En J arge channel
crclSslng

En J arge channel
crossing

Check st.bl I Ity of
embankment"s

Check stabIlity of
embankments

2

5

9

2
2

7

2

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