Level II scour analysis for Bridge 38 (JERITH0020038) on Town Highway 20, crossing the Lee River, Jericho, Vermont
This report provides the results of a detailed Level II analysis of scour potential at structure JERITH00200038 on Town Highway 20 crossing the Lee River, Jericho, 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, obtained 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 Green Mountain section of the New England physiographic province and the Champlain section of the St. Lawrence physiographic province in northwestern Vermont. The 12.9-mi2 drainage area is in a predominantly rural and forested basin. In the vicinity of the study site, the surface cover on the upstream and downstream right overbank is pasture while the immediate banks have dense woody vegetation. The surface cover on the upstream and downstream left overbank is forested.
In the study area, the Lee River has an incised, sinuous channel with a slope of approximately 0.02 ft/ft, an average channel top width of 89 ft and an average bank height of 14 ft. The channel bed material ranges from sand to boulder with a median grain size (D50) of 45.9 mm (0.151 ft). The geomorphic assessment at the time of the Level I and Level II site visit on July 2, 1996, indicated that the reach was stable.
The Town Highway 20 crossing of the Lee River is a 49-ft-long, one-lane bridge consisting of a steel through truss span (Vermont Agency of Transportation, written communication, December 12, 1995). The opening length of the structure parallel to the bridge face is 44 ft. The bridge is supported by vertical, concrete abutments with wingwalls. The channel is skewed approximately 10 degrees to the opening while the computed opening-skew-toroadway is 5 degrees.
A scour hole 1 ft deeper than the mean thalweg depth was observed in the center of the channel during the Level I assessment. Scour countermeasures at the site include type-1 stone fill (less than 12 inches diameter) at the downstream left road embankment. Type-2 stone fill (less than 36 inches diameter) protects the upstream left wingwall, the upstream and downstream right wingwalls and the upstream end of the right abutment. Type-3 stone fill (less than 48 inches diameter) protects the left abutment. Additional details describing conditions at the site are included in the Level II Summary and Appendices D and E.
Scour depths and recommended rock rip-rap sizes were computed using the general guidelines described in Hydraulic Engineering Circular 18 (Richardson and others, 1995) for the 100- and 500-year discharges. 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 was zero. Abutment scour ranged from 4.9 to 10.7 ft. The worst-case abutment scour 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, 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.
Additional publication details
|Publication Subtype||USGS Numbered Series|
|Title||Level II scour analysis for Bridge 38 (JERITH0020038) on Town Highway 20, crossing the Lee River, Jericho, Vermont|
|Series title||Open-File Report|
|Publisher||U.S. Geological Survey|
|Publisher location||Pembroke, NH|
|Description||iv, 48 p.|
|Other Geospatial||Lee River|