FLOOD10328

• County: Boulder
• Community: Lyons
• Stream Name: North Saint Vrain and Saint Vrain Creeks
• Title: Floodplain Information Report, North Saint Vrain Creek, Saint Vrain Creek, Floodplain Update, Town of Lyons, Boulder County, Colorado
• Date: 6/1/1998
• Prepared For: Lyons
• Prepared By: BRW, Inc
• Floodplain - Doc Type: Floodplain Report/Masterplan

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Floodplain Information Report
K:\35600\004\FLOODPLAIN INFORMATION REPORT.doc

8

June

998

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Access Bridge
(1828+50)

Assumed
washed out

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u.s. Highway 36
(1811+23)

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u.s. Highway 36
(1786+30)

5387.5

5399.2

5404.7

59

690

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Highway
(1750+05)

5364.5

5372.3

5375.9

58

344

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Footbridge
(1743+80)

7

5329.0

5340.6

5342.0

60

552

Assumed
washed out

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2nd Avenue
(1729+ 10)

5308.5

5320.8

5323.0

113

1.2

1636

25%

Assumed
washed out

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Location
U.s. Highway 36
(1662+55)

Invert
(MSL)
5253.5

Low
Chord
(MSL)
5264.9

Top of
Road
(MSL)
5267.4

Bottom
Width
(fO
65

Pier
Width
(fO
4.0

Opening
Area
(s9.:..!t.)
822

Side
Slope
(H:V)
2

Blocka
25%

e

Notes

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TABLE 1
SUMMARY OF CULVERT/BRIDGE INFORMATION

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As in the previous two studies, several culverts/bridges were assumed to be washed out during
the 100-year flood. This assumption was based on a visual inspection of the structural integrity
of the crossing by Boulder County, the Colorado Water Conservation Board, and the consultant
for the 1987 FIR. The crossings assumed to wash out are noted in Table 1, summary of
culvert/bridge data.

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Bridge and culvert debris blockage remains consistent in this re-analysis with the 1987 FIR.
Bridges with center piers and multiple culvert installations have a 25% debris blockage of the
opening area. No blockage was assumed for bridges without piers or for single culvert
installationso To model the blockage, the low chord of the culvert or bridge was lowered to
reduce the opening area. The culvert information obtained by the U.so army Corps of Engineers
for the 1987 FIR was used where appropriate.

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channel constructions at bridges and culverts, thus creating a damming effect. This damming
effect may cause overbank flow, which in turn may erode bridge abutments and approaches.
Where debris had collected at channel constrictions, increasing flood flows may cause a mass of
debris to break loose, causing a wall of water and debris to surge downstream until another
obstruction is encounteredo In some instances, debris may collect to the point where structural
capacity of a bridge is exceeded, causing the bridge to fail.

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North St. Vrain & St.
Floodplain Update

Vrain Creeks

Town of Lyons