FLOOD04288

• County: Boulder
• Community: Lyons
• Stream Name: St. Vrain and North St. Vrain
• Basin: South Platte
• Title: North St. Vrain and St. Vrain Floodplain Update
• Date: 6/1/1998
• Prepared For: Lyons
• Prepared By: BRW, Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

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North St. Vrain & St. Vrain Creeks
Floodplain Update

Town of Lyons

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 encountered. In some instances, debris may collect to the point where structural
capacity of a bridge is exceeded, causing the bridge to fail.

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
installations. 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.s. army Corps of Engineers
for the 1987 FIR was used where appropriate.

As in the previous two studies, several culverts/bridges were assumed to be washed out during
the lOG-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.

TABLE 1
SUMMARY OF CUL VERTIBRIDGE INFORMA nON

Low Top of Bottom Pier Opening: Side
Invert Chord Road Width Width Area Slope
Location (MSU (MSL) (MSL) (ft) (ft) (sq. ft.) (H:V) Blockage Notes
U.S. Highway 36 5253.5 5264.9 5267.4 65 4.0 822 2 25%
(1662+55) Assumed
washed out
2nd Avenue 5308.5 5320.8 5323.0 113 1.2 1636 25%
(1729+10)
Footbridge Assumed
(1743+80) washed out
Highway 7 5329.0 5340.6 5342.0 60 552
(1750+05)
U.s. Highway 36 5364.5 5372.3 5375.9 58 344
(1786+30)
U.s. Highway 36 5387.5 5399.2 5404.7 59 690
(1811 +23)
Access Bridge Assumed
(1828+50) washed out

Floodplain Information Report 8
K:1356001004\FLOODPl..AIN INFORMATION REPORT.doc

June 1998