Level II scour analysis for Bridge 42 (RANDVT00120042) on State Highway 12, crossing Third Branch White River, Randolph, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
RANDVT00120042 on State Highway 12 crossing the Third Branch White River,
Randolph, 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 division of central Vermont in the town of
Randolph. The 61.9-mi²
drainage area is in a predominantly rural drainage basin. In the
vicinity of the study site, the left and right banks have moderate tree cover with residential
and commercial buildings on the overbank areas.

In the study area, the Third Branch White River has a sinuous channel with a slope of
approximately 0.013 ft/ft, an average channel top width of 133 ft and an average channel
depth of 5 ft. The predominant channel bed material is gravel with a median grain size (
D₅₀) of 49.6 mm (0.163 ft). Bank material is sand and gravel (D₅₀ is 3.08 mm or 0.010 ft).
The geomorphic assessment at the time of the Level I site visits on July 8, 1994 and
December 13, 1994, indicated that the reach has experienced vertical degradation. A drop
structure has been constructed downstream of the bridge to prevent further degradation. In
addition to the degradation of the stream bed, there is local pier scour at the bridge site as
well. At the nose of the pier in the main channel, the bed is approximately three feet below
the mean thalweg and two feet below the bottom of the pier footing.

The State Highway 12 crossing of the Third Branch White Riveris a 220-ft-long, two-lane
bridge consisting of four concrete spans. The maximum span length is 57 ft. (Vermont
Agency of Transportation, written commun., July 29, 1994). The bridge is supported by
vertical, concrete abutments and three concrete piers. The toe of the left abutment is at the
channel edge. The toe of the right abutment is set back on the right over-bank. The roadway
centerline on the structure has a slight horizontal curve; however, the main channel is
skewed approximately 5 degrees to the bridge. 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). Scour depths were
calculated assuming an infinite depth of erosive material and a homogeneous particle-size
distribution. The scour analysis results are presented in tables 1 and 2 and a graph of the
scour depths is presented in figure 8.