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Publication Number: FHWA-HRT-07-026
Date: February 2007
Bottomless Culvert Scour Study: Phase II Laboratory Report
Chapter 5: Conclusions
Phase II improved on the Phase I study results by providing additional research data, including the following.
Equations are presented to estimate the maximum expected scour depths at the upstream corners of bottomless culverts under clear-water conditions. New equations are also presented to estimate the riprap sizes needed to protect bottomless culvert footings from scour.
All experiments outlined in the test matrix in table 1 were completed in Phase II, but there were some limitations in the experimental setup. The experimental results were based on laboratory flume experiments with a flat approach cross section with uniform flow conveyance, which is not typical of field conditions. The experiments were also conducted under clear-water approach flow conditions with no sediment being transported into the culvert. The authors attempted to present the results in terms of overbank flow rather than geometric variables; presenting the results is this fashion allows accounting for the reduced conveyance that is typical of overbank flow for natural streams. These results have not been tested for field conditions; however, they are offered as initial guidance for field applications. An anticipated next step is that MDSHA will adopt the results as preliminary design guidelines and test them for field sites using engineering judgment to decide if the applications are reasonable.
The abutment scour concept of using the flow distribution at the culvert entrance to compute the primary scour depth component and adjusting that with an empirical factor based on laboratory data appears to be valid for bottomless culverts. Three different equations for the initial representative velocity and two different equations for the critical incipient motion velocity were tested to compute the flow distribution scour. The Froude numbers in the experiments did not cover the full range that is expected in the field, and the negative slopes presented in table 2 are probably not realistic. In fact, other experiments performed by GKY and Associates, Inc., show that the correlation of ks with the Froude number is positive.(3) For this reason, we recommend changing the Froude number multiplier to zero for equations in table 2 with negative slopes. This change is equivalent to changing the ks equations with a Froude number in them so that ks equals only the intercept. Nevertheless, the laboratory data suggest that calculations of ks as a function of either VRA, VCL, and F1 or VRM, VCN, and Qblocked are the two best functions for calculating scour in an unsubmerged bottomless culvert. The kp results, however, are still too preliminary to suggest the best predictors of scour in submerged bottomless culverts.
The culvert entrance flow conditions were a significant influence on the scour. The flow through various inlet and outlet configurations was investigated as both submerged (pressure flow) and unsubmerged to determine the overall effects of the flow conditions on scour hole formation. The results show that submerged flow conditions induce greater inlet scour depths, while unsubmerged flow conditions induce greater outlet scour depths. The results also show that 45-degree inlet wingwalls are effective at reducing inlet scour, whereas 8-degree inlet wingwalls are not effective.
The outlet scour experimental results showed the effects of using different wingwall configurations at the outlet. Changing the angle of the wingwalls reduces the turbulent shear stress, and thus reduces the scour depth created. The outlet experiments clearly demonstrate that outlet scour can be substantially reduced by using outlet wingwalls with a streamlined shape. The elongated streamlined bevel wingwall was best at reducing scour. Experimental results indicate that turbulence is reduced and “vortex shedding” caused by abrupt changes in pressure is almost eliminated by using this shape. In other words, the streamlined wall eliminates flow separation and decreases turbulence.(10) Hence, with the streamlined bevel, vortices do not propagate downstream and the resulting turbulence is more evenly distributed—not concentrated in a single location. Conversely, the abrupt change in pressure that results from a square exit shape (as found in culverts without wingwalls at the outlet) induces vortex shedding and increased scour depths.
Eight-degree outlet wingwalls were also tested because streamlined wingwalls may not be practical in the field. These results revealed reduced turbulence and scour depth at the outlet. This is an encouraging finding because wingwalls with an 8-degree flare are easy to construct or can be ordered prefabricated, which may make this design more cost-effective than the streamlined design.
Equation 27 is useful for sizing riprap to reduce scour. Chang’s pile dissipators dissipated some of the energy at the outlet and thus reduced the scour depth. The MDSHA Standard Plan for countermeasures did not significantly reduce the scour depth, but it is considered a good practice because the riprap that was employed in this plan moved and fell into the scour holes, after which the riprap stabilized. However, since these results are still preliminary, this report does not make any recommendations about sizing or placing riprap for this design. Cross vanes are not recommended at the inlet because the results show that they contribute to rather than hinder scour due to a spiral current effect.
Additional research could extend and improve upon the Phase I and Phase II study results. This research could include: