Contraction Reach Lengths (Lc on Figure)

While the apparent contraction ratios of the five field prototype cases were all below 1:1, the contraction ratios (CR on figure above) for the idealized cases ranged from 0.7:1 to 2.3:1. As with the expansion reach lengths, these values correlated strongly with the same Froude number ratio. A more important independent variable, however, is the decimal fraction of the total discharge conveyed in the overbanks (Q_ob/Q) at the approach section. A strong regression equation was developed for the contraction length and is presented later in this appendix.

Because the mean and median values of the contraction ratios were both around 1:1, there is some support from this study for the rule of thumb which suggests the use of a 1:1 contraction ratio. There is no support, however, for the concept of the contraction reach length being equal to or proportional to the bridge opening width.