Level II scour analysis for Bridge 45 (CHELTH00440045) on Town Highway 44, crossing first Branch White River, Chelsea, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
CHELTH00440045 on town highway 44 crossing the First Branch White River, Chelsea,
Vermont (figures 1–8). A Level II study is a basic engineering analysis of the site, including
a quantitative analysis of stream stability and scour (U.S. Department of Transportation,
1993). A Level I study is included in Appendix E of this report. A Level I study provides
a qualitative geomorphic characterization of the study site. Information on the bridge
available from VTAOT files was compiled prior to conducting Level I and Level II
analyses and can be found in Appendix D.

The site is in the Green Mountain physiographic province of central Vermont in the town of
Chelsea. The 32.5-mi²
drainage area is in a predominantly rural and forested basin. In the
vicinity of the study site, the banks have low to moderate woody vegetation coverage except
for the upstream right bank, which is grass covered. The immediate vicinity of the site is
suburban and the overbank areas are occupied by houses, driveways, and lawn areas. The
upstream right bank area is a dirt parking lot for a small auto repair garage.

In the study area, the First Branch White River has an incised, sinuous channel with a slope
of approximately 0.003 ft/ft, an average channel top width of 41 ft and an average channel
depth of 4 ft. The predominant channel bed material is gravel (D₅₀ is 43.1 mm or 0.141 ft).
The geomorphic assessment at the time of the Level I and Level II site visit on November
17, 1994, indicated that the reach was stable.

The town highway 44 crossing of the First Branch White Riveris a 31-ft-long, two-lane
bridge consisting of one 27-foot clear-span concrete-encased steel beam deck
superstructure (Vermont Agency of Transportation, written commun., August 25, 1994).
The bridge is supported by vertical, concrete abutments with wingwalls. The channel is
skewed approximately 10 degrees to the opening while the opening-skew-to-roadway is 5
degrees.

Both abutment footings were reported as exposed and the left abutment was reported to be
undermined by 0.5 ft at the time of the Level I assessment. The only scour protection
measure at the site was type-1 stone fill (less than 12 inches diameter) along the left
abutment which was reported as failed. Additional details describing conditions at the site
are included in the Level II Summary and Appendices D and E.

Scour depths and rock rip-rap sizes were computed using the general guidelines described
in Hydraulic Engineering Circular 18 (Richardson and others, 1993). Total scour at a
highway crossing is comprised of three components: 1) long-term streambed degradation;
2) contraction scour (due to accelerated flow caused by a reduction in flow area at a bridge)
and; 3) local scour (caused by accelerated flow around piers and abutments). Total scour is
the sum of the three components. Equations are available to compute depths for contraction
and local scour and a summary of the results of these computations follows.

Contraction scour for all modelled flows ranged from 0.4 to 5.1 ft. with the worst-case
occurring at the 500-year discharge. Abutment scour ranged from 9.9 to 20.3 ft. The worst-case abutment scour also occurred at the 500-year discharge. Additional information on
scour depths and depths to armoring are included in the section titled “Scour Results”.
Scoured-streambed elevations, based on the calculated scour depths, are presented in tables
1 and 2. A cross-section of the scour computed at the bridge is presented in figure 8. Scour
depths were calculated assuming an infinite depth of erosive material and a homogeneous
particle-size distribution.

It is generally accepted that the Froehlich equation (abutment scour) gives “excessively
conservative estimates of scour depths” (Richardson and others, 1993, p. 48). Many factors,
including historical performance during flood events, the geomorphic assessment, scour
protection measures, and the results of the hydraulic analyses, must be considered to
properly assess the validity of abutment scour results. Therefore, scour depths adopted by
VTAOT may differ from the computed values documented herein, based on the
consideration of additional contributing factors and experienced engineering judgement.