# Experimental Study on the Near-Red Flow Characteristics of Alluvial Channel with Seepage
## 2 Experimental Setup and Methodology  
The laboratory experiments were carried out in a 17.20 m long, 1.00 m wide, and 0.72 m deep channel (Figure 1) to analyze the flow characteristics at moderate Reynolds number in a seepage bed. The length of the channel was sufficient to attain fully developed flow conditions in the test section. The flow depth over a rectangular notch was used to measure the flow discharge. It was controlled by a tailgate fixed at the outlet end of the channel. The test section was 5 m long and started 5 m downstream of the channel entrance to avoid any influence of the inflow and outflow of the channel. A digital point gauge attached to a moving trolley measured the flow depth in the test section. The water surface slope in the channel was calculated by using a pitot-static tube, connected to a digital manometer, and also attached on a moving trolley in the channel. Further experimental details and a thorough description of the procedure are available in [PERSON] et al. [3]. The longitudinal slope of the channel bed was maintained as 0.001 for all the experimental runs. The aspect ratio (channel width/flow depth) in the present experiments was greater than 6; thus, the influence of secondary currents on the main flow was negligible in the current work [22], and, eventually, the flow features in the channel bed were not affected by the three-dimensional effects. The downward seepage was allowed by forming a seepage chamber(i.e., the pressure chamber in Figure 1) that was 15.20 m long, 1.0 m wide, and 0.22 m deep. The seepage chamber was employed just below the channel bed, which extracted the water from the channel in the form of downward seepage. The seepage discharge was measured by a pair of electromagnetic flow meters, which were connected through pipes to the seepage chamber [3]. The present experiments used a non-uniform sand as the bed material. The sieve analysis to determine the grain size distribution was performed according to the Indian Standard Code IS: 2386 (Part I) [23] (Figure 2). The geometric standard deviation \(\sigma_{g}=(d_{84.1}-d_{15.9})/d_{50}\) of the used sand material was 1.65, where \(d_{84.1}\), \(d_{15.9}\), and \(d_{50}\) represent the size of the particles, for which the percentages of finer particles are 84.1, 15.9, and 50, respectively. Therefore, the sand used in this study was non-uniform (though slightly) because \(\sigma_{g}>1.4\)[24]. Details on the bed material and flow conditions are provided in Table 1. The sand bed was 17.2 m long, 1.0 m wide, and 0.22 m deep. The experiments were carried out always with the same approaching discharge \(Q=0.0242\) m\({}^{3}\)/s but under three different conditions: no seepage, seepage discharge equal to 10% of \(Q\), and seepage discharge equal to 15% of \(Q\). In the no-seepage experiment, the valve, installed at the inlet tank, was regulated slowly in order to supply the water flow into the channel until the required no-seepage discharge and flow depth were attained. The sediment bed channel was prepared again on subsequent completion of the no-seepage run, and, thereafter, seepage experiments were carried out by considering the seepage through the channel bed, which was 10% and 15% of the no-seepage discharge, as stated above. The flow depth \(h\) was kept constant to allow the comparison of turbulence quantities between no-seepage and seepage runs. Laboratory environments provide the opportunity to detect and measure comprehensive procedures from reach to grain scale, with the abilities of the measurement equipment being the only limiting factor. In this study, the Reynolds number, though moderate (around 20,000), was such as to preserve the conditions of fully rough flow, as typically happens in rivers. Then, the Froude number (around 0.20) becomes the main criterion for the up-scaling from the model to the prototype [25].