# Experiments on Longitudinal and Transverse Bedload Transport in Sine-Generated Meandering Channels
## 3 Results
### The 110\({}^{\circ}\) Flume
(**b**) Sample volume (unit: m\({}^{3}\)) of grains D\({}_{1}\), D\({}_{2}\), D\({}_{3}\), D\({}_{4}\) at each section along the left bank. (**c**) Sampling volume (unit: m\({}^{3}\)) of grains D\({}_{1}\), D\({}_{2}\), D\({}_{3}\), D\({}_{4}\) at each section along the right bank. D\({}_{1}\), D\({}_{2}\), D\({}_{3}\), D\({}_{4}\) grains are represented by green, yellow, red, and cyan colors, respectively. Sand grains of D\({}_{3}\) and D\({}_{4}\) are initially paved along the left (convex) and right (concave) bank of the B\({}_{2}\) bend of the 110\({}^{\circ}\) flume (sections 37-61, as shown in Figure 1). On the contrary, to the bend of B\({}_{1}\), the B\({}_{2}\) bend presents a quasi-equilibrium regime. Figure 2c indicates a large quantity of D\({}_{4}\) grains (total volumes of 28.4 \(\times\) 10\({}^{-4}\) m\({}^{3}\)) were eroded from the right (concave) bank. Within the B\({}_{2}\) bend, most of those eroded D\({}_{4}\) grains (75%) were longitudinally transported to the next bend B\({}_{3}\), and only a small proportion (25%) crossed the channel centerline and contributed to the point bar deformation in the same bend, i.e., the B\({}_{2}\) bend (as shown in Figure 2b). The ratio of longitudinal to transverse transport of D\({}_{4}\) in the B\({}_{2}\) bend is about 3.0, even larger than the longitudinal-transverse ratio of D\({}_{1}\) in the B\({}_{1}\) bend (about 2.6). In contrast to the traditional bend theory [16; 17], which assumes that the point bar is mainly composed of the other-bank sediment moved by laterally asymmetrical longitudinal flow pattern, the observed behavior suggests that, especially in highly curved channels, the point bar is composed by the combination of a dominant longitudinal transport and a cross-sectional motion. The evacuating amount of D\({}_{3}\) in the B\({}_{2}\) bend is relatively small (14.0 \(\times\) 10\({}^{-4}\) m\({}^{3}\)) compared to the scouring near the concave bank (about a half in volume). About 73% of the D\({}_{3}\) grains eroded from the B\({}_{2}\) bend were longitudinally transported to the next bend B\({}_{3}\), i.e., the transverse transport of D\({}_{3}\) grains accounts for 27% (as shown in Figure 2c). The high ratio obviously resulted from the direction of near-bed flow, which constrains the bedload from moving toward the concave bank. White-color grains are initially paved along the entire area of the B\({}_{3}\) bend of the 110\({}^{\circ}\) flume (sections 61-85, as shown in Figure 1). In contrast to the bends of B\({}_{1}\) and B\({}_{2}\), the B\({}_{3}\) bend presents a mild aggradation regime. As D\({}_{3}\) and D\({}_{4}\) grains moved into the B\({}_{3}\) bend, the longitudinal-transverse sorting continues. From Figure 2b,c, in the B\({}_{3}\) bend we measured the volumes of 4.1 \(\times\) 10\({}^{-4}\) m\({}^{3}\) and 2.8 \(\times\) 10\({}^{-4}\) m\({}^{3}\) of D\({}_{3}\) grains in the left (longitudinally transported) and right (transversely transported) half-width, respectively. The ratio of longitudinal to transverse transport for D\({}_{3}\) in the B\({}_{3}\) bend is about 1.5, while for D\({}_{4}\) grains, we measured the volumes of 12.5 \(\times\) 10\({}^{-4}\) m\({}^{3}\) and 10.4 \(\times\) 10\({}^{-4}\) m\({}^{3}\) in the right (longitudinally transported) and left (transversely transported) half-width channel, respectively. The ratio of longitudinal to transverse transport for D\({}_{4}\) in the B\({}_{3}\) bend is about 1.2.