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Potential Drift Accumulation at Bridges

Design Log Length and the Span Length of Existing Bridges

Potential is low for blockage of spans with effective width greater than the design log length. Most span blockages have involved spans with an effective width less than the design log length. Exceptions have involved other factors such as sediment accumulation or drifting objects larger than logs such as cabled log rafts.

The design log length, which is also the minimum effective span length for low trapping potential, is intentionally set at the highest level justified by the set of confirmed pier-to-pier accumulations. Additional research may identify situations in which span lengths with low potential for drift trapping are below this threshold. Risk should be balanced against cost in deciding whether this threshold should be modified or used in designing bridges (Pangallo and others, 1992). The cost of designing for drift lies outside the scope of the present study. However, the available data indicate that this threshold span-length is probably not too low. In other words, spans longer than this threshold value belong to a set of spans in which bridging from pier to pier by drift is rare or absent.

A large percentage of existing bridges have spans short enough to be blocked by drift. The 10,352 selected bridges used in the scour-potential studies in Indiana, Maryland, South Carolina, and Tennessee show how the percentage of bridges with spans shorter than the maximum length for span blockage can vary with channel width and from State to State (table 3). About half the selected bridges have spans shorter than the design log length. Variability among States reflects differences in criteria used to select potentially scour-critical bridges for inclusion in the scour-potential studies.

Table 3. Percentage of selected bridges having spans shorter than the design log length.

State Number of selected bridges Percentage of selected bridges with spans shorter than the design log length
Range of channel widths All channel widths
0 to 12 m 12 to 60 m 60 to 300 m
Indiana 2,394 20 32 38 25
Maryland 879 43 45 59 46
South Carolina 3,498 45 78 68 56
Tennessee 3,581 65 75 80 72
All four States 10,352 42 64 68 53


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