Scour at submerged bridge is shown in Figure 6.43. Submergence of the superstructure leads to an increase in scour at the bridge foundations compared to that at an unsubmerged bridge. Some guidance is available for estimation of scour depths at submerged bridges from recent research by Abed (1991), Jones et al (1993) and Richardson and Devis (1995). The scour that develops can be considered to comprise two components:
· local scour, which can be estimated by neglecting the effects of submergence; and
· scour due to bridge superstructure submergence, termed pressure scour in the United States because the flow through the bridge opening does not have a free surface. The pressure scour component can also be considered to be a form of vertical contraction scour.
The second component can arise from partial submergence, where the water level rises part way up the bridge superstructure and all the flow is forced beneath the bridge; or from complete submergence, where flow is both over and under the bridge. The combined local and pressure scour at bridge piers has been measured at up to 10 times the local (free surface) scour. Richardson and Devis (1995) represent an equation based on 81 laboratory measurements of the pressure scour component at partially and completely submerged bridges, where condition in the approach channel were clear-water. The following expressions, in which the constant and exponent have been, rounded -of, were shown to fit the data reasonably well:
(1)
where w = depth of overflow, and Hb = height from undistributed bed level to the “low steel” elevation of the bridge superstructure. The measurements covered the following ranges of variables: D/Hb = 1.06 ® 2.0; and w/D = 0 ® 0.19; and V/Vc » 0.4 ® 1.0. Equation 1.14 shows that pressure scour is deepest when the flow is about to spill over the bridge deck (water level at bridge deck level and w = 0). For that condition, (1.14) reduces to
(2)
in which (ds + Hb) represents the flow depth beneath the bridge. The equation is very similar to an expression given by Gill (1981) for (horizontal) clear-water contraction scour.
