Level II scour analysis for Bridge 8 (DANVTH00020008) on Town Highway 2, crossing the Morrill Brook, Danville, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
DANVTH00020008 on Town Highway 2 crossing Morrill Brook, Danville, 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). Results of a Level I scour investigation also are included in Appendix E of this
report. A Level I investigation provides a qualitative geomorphic characterization of the
study site. Information on the bridge, gleaned from Vermont Agency of Transportation
(VTAOT) files, was compiled prior to conducting Level I and Level II analyses and is
found in Appendix D.
The site is in the New England Upland section of the New England physiographic province
in North-East Vermont. The 4.74-mi²
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover is forest with a residence on the
upstream right bank.
In the study area, Morrill Brook has an incised, sinuous channel with a slope of
approximately 0.03 ft/ft, an average channel top width of 60 ft and an average channel
depth of 8 ft. The predominant channel bed material is cobble with a median grain size
(D₅₀) of 67.0 mm (0.220 ft). The geomorphic assessment at the time of the Level I and
Level II site visit on September 9, 1995, indicated that the reach was stable.
The Town Highway 2 crossing of Morrill Brook is a 59-ft-long, two-lane bridge consisting
of one 57-foot steel-beam span (Vermont Agency of Transportation, written
communication, March 24, 1995). The bridge is supported by vertical, concrete abutments.
The channel is skewed approximately 5 degrees to the opening while the opening-skew-toroadway is 0 degrees.
The scour protection measure at the site included type-2 stone fill (less than 36 inches
diameter) along the base of the left abutment. There was type-1 stone fill (less than 12
inches diameter) along the base of the right abutment. There was also type-3 stone fill (less
than 48 inches diameter) along both upstream banks at the location of previous bridge
abutments. 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, 1995). 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 modelled flows ranged from 0.1 to 0.4 ft. The worst-case contraction
scour occurred at the 500-year discharge. Abutment scour ranged from 4.0 to 8.7 ft. The
worst-case abutment scour occurred at the 100-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, 1995, p. 47). Usually,
computed scour depths are evaluated in combination with other information including (but
not limited to) historical performance during flood events, the geomorphic stability
assessment, existing scour protection measures, and the results of the hydraulic analyses.
Therefore, scour depths adopted by VTAOT may differ from the computed values
documented herein.