Location of the maximum scouring depth at the outlet of partially-blocked and non-blocked box culvert

Among various hydraulic structures, culvert is the most likely one which usually blocks by the debris that are carrying by flow during large flood events. The size of the structure and its location where a waterway crosses a road or railway increases the possibility of clogging. The blockage of culvert accelerates bed scouring at the outlet hence affects bed scouring profile. This leads to increase the risk of culvert collapsed and produce different kinds of damages to the society. The present study concerns the effect of upstream blockage on the scouring profile at the outlet of a box culvert. Therefore, the experimental program was designed to investigate the relationship between the scouring geometry and blockage ratio. The experimental tests were carried out under non-blocked and partially blocked conditions. The sediment material used in this study was uniform non-cohesive sand material. Results showed that the scouring bed profile is different in partially blocked condition when compared to the non-blocked condition. Additionally it was found that the maximum scouring depth in a partially blocked culvert occurred at a distance very close to the outlet of the box culvert. scale on maximum depth of scouring based on experimental test of circular culverts. Liriano et al. (2002) studied scour at culverts influenced by turbulent flow and pointed out that the peak values of turbulence intensities over the fixed bed coincide with the location of the maximum scour depth for the fully developed scour hole. Emami and Schleiss (2010) conducted some experimental tests to evaluate the natural mobile bed erosion without any protection. They compared their work with some previous studies and proposed equation for maximum scour depth based on their experimental tests. According to previous studies it can be concluded that main factors influencing the maximum scour depth downstream of culverts are recognized as sediment properties such as median grain size and geometrical standard deviation, flow conditions and depth of tail water. Some research also indicated that geometry of the culvert and its slope also affects the formation of scour hole. Most formulas proposed in this regard are function of densimetric Froude number, median grain size of bed material, depth of water downstream the culvert and size of culvert opening. In all of previous studies, it is attempted to consider scoring pattern at the outlet of culvert in a non-blocked condition, however, in flood events the culverts are not usually works in non-blocked condition as they are partially blocked by the debris. Therefore, the focus of this study is to investigate the flow characteristics and scouring pattern in a partially blocked condition and the results are reported in this paper 2 EXPERIMENTAL SET UP An experimental program was designed to investigate the relationship between the maximum scour depth, blockage ratio of the culvert and flow characteristics. The experimental tests were carried on, in two different conditions; non-blocked and partially blocked condition and the effect of culvert blockage are studied. The sediment material used in this study is uniform non-cohesive sand. The median grain size for sands equals 0.85 mm and 2.0 mm. Experimental tests conducted in the scour testing facility depicted in Figure 1. Water was supplied to the static tank from the laboratory supply. The water introduced to the flume through a valve which controls the water flow rate, and then it runs through the box-culvert that is settled in the sand basin. At the end of the flume a sluice gate is installed to the downstream water depth and velocity. The flume in UTS Hydraulic Laboratory is a 19 m long concrete flume. The width and depth of the flume are 605 mm and 600 mm, respectively. A test section with the length of 4 m is provided at a distance of 9 m from the entrance. The thickness of the bed material in the test section was 150mm. The culvert model is shown in Figure 2. The opening of culvert is 200×200 mm and the length of culvert’s barrel is 900 mm. There are transitions with 30° flare at the inlet and outlet of the culvert. To make the blockage a plate was installed at the opening of the culvert. Two sizes of plates (200×80 mm and 200×120 mm) were used so far to get the variant blockage ratio in the experimental tests. Figure 1 Experimental test facility in Hydraulics Laboratory Figure 2 Culvert model; (a) Plan view (b) Profile view (c) inlet view (all units are in mm) Table 1 depicts the flow conditions, sediment properties and test classifications. In this table H is the depth of water at the outlet of the culvert, u h is the upstream water level, t h represents the tail water depth and d F is the densimetric Froude number. The blockage of culvert is represent by B which defines as   u B h h where B h is the height of the plate used as the blockage in the inlet of culvert. Table 1Test classification for the experimental tests Test d50 g  hB H ht hu Q B Fd dsm (mm) (mm) (mm) (mm) (mm) (lit/s) (mm) S2B01 2 1.23 0 125 125 125 10.1 0 2.2 240 S2B02 2 1.23 0 155 155 155 12.3 0 2.2 255 S2B03 2 1.23 0 100 105 105 12.8 0 3.6 250 S2B04 2 1.23 0 120 120 120 8.6 0 2 210 S2B05 2 1.23 0 100 105 100 6.6 0 1.8 250 S1B01* 0.85 1.37 0 200 197 198 14.61 0 2 280 S1B02 0.85 1.37 0 157 157 160 15 0 2.7 270 S1B03 0.85 1.37 0 170 170 174 18.7 0 3.1 270 S1B04 0.85 1.37 0 138 145 150 13.2 0 2.7 240 S1B05** 0.85 1.37 0 95 108 95 14.2 0 4.2 360 S1B06 0.85 1.37 0 90 95 90 4.9 0 1.5 255 S1B07 0.85 1.37 0 55 50 75 7.04 0 3.6 135 VS1B04 0.85 1.37 0 148 148 147 7.25 0 1.4 ----VS1B05 0.85 1.37 0 142 142 142 7.8 0 2.4 ----S2B401 2 1.23 80 140 145 170 10.7 0.5 2.1 240 S2B402 2 1.23 80 155 160 185 12.3 0.4 2.2 250 S2B403 2 1.23 80 105 105 175 12.8 0.5 3.4 240 S2B404 2 1.23 80 110 115 145 8.8 0.6 2.2 240 S2B405 2 1.23 80 105 110 125 6.6 0.6 1.7 250 S1B401* 0.85 1.37 80 195 195 210 14.67 0.4 2.1 265 S1B402 0.85 1.37 80 165 168 196 14.61 0.4 2.5 240 S1B403* 0.85 1.37 80 60 60 195 15.12 0.4 7 300 S1B405 0.85 1.37 80 210 213 230 19.8 0.3 2.6 300 S1B406* 0.85 1.37 80 150 145 200 20.65 0.4 3.8 260 S1B407* 0.85 1.37 80 80 70 230 25 0.3 8.7 30 S1B408 0.85 1.37 80 70 70 120 4.86 0.7 1.9 210 S1B409 0.85 1.37 80 80 95 160 10.11 0.5 3.5 180 VS1B403 0.85 1.37 80 147 147 160 7.2 0.5 1.4 ----VS1B405 0.85 1.37 80 143 143 153 7.8 0.5 2.4 ----S2B601 2 1.23 120 130 130 190 10.3 0.6 2.2 270 S2B602 2 1.23 120 170 170 210 12.5 0.6 2 250 S2B603 2 1.23 120 50 100 195 12.8 0.6 7.1 240 S2B604 2 1.23 120 110 115 160 8.8 0.8 2.2 250 S2B605 2 1.23 120 90 105 135 6.6 0.9 2 255 S1B601 0.85 1.37 120 60 70 150 7 0.8 5 280 * Culvert without transition 3 RESULTS AND DISCUSSION