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Potential Drift Accumulation at Bridges
Various methods were used to locate relevant publications. The literature search began with computer searches of the following databases:
A citation search was performed for papers that cite a key paper on drift transport and jam formation (Likens and Bilby, 1982). Citations in articles obtained were examined for relevant material.
State Departments of Transportation were asked to provide reports on drift problems at bridges. A standard report form provided to them requested values for specific variables and solicited comments on several aspects of the site (figure 5). This form resembled that used by Chang and Shen, but contained additional data categories (Chang and Shen, 1979). Most reports were completed by district maintenance engineers or their staff. Photographs attached to several reports aided in data interpretation.
Figure 5. Sample State drift-accumulation report form.
Several State Departments of Transportation, with support from the FHWA, have sponsored cooperative studies by the USGS of scour potential at bridges (Bryan and others, 1995; Huizinga and Waite, 1994). Compiled data were available from Indiana, Maryland, Massachusetts, Tennessee, and South Carolina (Noel Hurley, USGS, written commun., 1992; Ron Thompson, USGS, written commun., 1992; Bernard Helinsky, USGS, written commun., 1992; G.W. Parker, USGS, written commun., 1993). Data include size of drift accumulations; pier location, skew, and type; span length; channel width; and bank height. Other variables, including width of drift accumulations, percentage of channel blocked, effective span width, and ratio of drift width to span length, were calculated from the reported data.
Contingency-table analysis was used to examine the association of bridge and channel characteristics with the frequency and size of drift accumulations (Mendenhall and others, 1981). Despite random variation in this data (particularly due to the random number and size of floods intervening between the inspection and the most recent removal of drift), the large number of bridges inspected justified statistical inferences.
Field studies were the major source of information on drift size, drift
characteristics related to origin and transport, and the shape and
structure of drift accumulations. Drift accumulations were studied at
144 drift-accumulation sites in 11 States and the District of Columbia
(table 1). The typical study of a drift-laden bridge included
measurement of bridge characteristics and channel dimensions, mapping
of the drift accumulation in plan view, estimation of the river stage
at which the accumulation occurred, and photographs of the bridge and
drift. Studies performed during high flow included observations of flow
characteristics and abundance and position of drift approaching the
bridge. Some studies included measurements of water depth, flow
velocity, and log dimensions. Sites of drift accumulation other than
bridges included channels, bars, islands, and flood plains. Several
sites were revisited to observe changes in accumulations, the process
of drift removal, and recurrent accumulation following drift removal.
The bridges across the main stem of the Harpeth River in Middle Tennessee were visited repeatedly during the 3-year study period. Drift, bridge, and channel characteristics were analyzed to determine why particular bridges trapped most of the accumulated drift. Drift stored in the channel network of the West Harpeth River, a tributary of the Harpeth River, was inventoried to determine the volume, size distribution, and dominant sources of drift present (Diehl and Bryan, 1993).
Scanning sonar was used at the FM 2004 bridge over the Brazos River near Lake Jackson, Texas, with the cooperation of the Texas Department of Highways and Public Transportation, to measure the size and shape of a large drift accumulation. Observations of the underwater drift accumulations and the scour holes around them were made during base flow and near the peak of a major flood. The sonar was lowered from the bridge deck at two or more locations near each drift-laden pier, and scanned the area within about 24 meters (m) [80 feet (ft)]. During field work, the sonar and associated processing software were used to produce local bathymetric maps showing bridge piers, scour of the river bed, and drift. Bathymetric maps of the entire site were prepared in the office by combining the data from all scans.
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