2016, Milad Mohammadi Graduated

Characteristics of the A-type hydraulic jump at a positive step

Abstract

Many researchers studied local scour phenomenon to predict the maximum scour depth and pattern. It is difficult to analyze this phenomenon by using analytical and numerical methods because of the three dimensional flow features, the sediment transport and the changing of flow boundaries. This phenomenon is often studied experimentally. Most scour studies have been focused on free surface flow condition, while few have been conducted at pressure flow condition. Pressure flow scour occurs when the flow piles up upstream the bridge deck (partially submerged) or passes over the bridge deck (fully submerged). In both cases, the flow under the bridge deck is under pressure.

In this thesis, the time series and scour pattern in partially submerged flow bridge deck with and without apron are studied. The apron case refers to situation where the channel bed under the deck is rigid. The testing program is performed in the Hydraulic Laboratory in the Civil Engineering Faculty of Isfahan University of Technology (IUT). The experimental flume is 9.5 m long, 0.4 m wide, and 0.7 m height. Model bridge deck and apron are rectangular 0.26 m wide. Uniform sand of median diameter d₅₀ = 0.72 mm and relative density of 2.65 is used. The flow depths in all tests are kept constant and clear-water condition prevails. The tests are fulfilled for three bridge opening, with and without the apron. The time series are obtained at four time periods of 24, 48, 72 and 120 hours.

In all cases, the equilibrium scour depth reaches after 45-55 hours. In cases without apron, local scour starts upstream of the deck. Maximum scour depth cross-section shifts, with time, to the downstream. The maximum scour depth section is located near the downstream edge of the deck. In the apron cases, the maximum scour depth occurs at downstream section of the apron. When the bridge opening reduces to 70% of upstream flow height, the downward flow at the upstream plunges toward the upstream section of the apron and the generated vortexes create a second scour hole upstream of the apron. Maximum scour depths in apron cases are 10-40% more than scour depths without apron. In all cases, increasing the submerged height or reducing the bridge opening height increases the scour depth and reduces the equilibrium scour time. Empirical equations are derived to predict the maximum scour depth.