Fig. 6: Gravel transport travel times and river discharge calculations.

a, b Time taken to transport 2 m thick gravel bed over 10 km distance vs. depth averaged concentration of suspended sediment for flow bulk velocity of (a) 7 m/s and (b) 8 m/s. The coloured lines in (a–d) represent different experimentally constrained coefficients used to modify the Meyer-Peter–Müller bedload transport equation, where φ = 1 is the standard Meyer-Peter–Müller equation (15) and φ = 6, for example, is the modified Meyer-Peter–Müller equation (16) adapted for highly turbulent and erosive flow. c, d Contour plots of suspended sediment concentration and discharge combinations required to transport sufficient gravel to deposit a 2 m thick gravel bed 10 km downstream of the GST in 12 hours (c) and 24 hours (d); dashed lines represent the concentration of suspended sediment (D₅₀ = 0.25 mm, 0.35 mm, 0.5 mm) to discharge magnitude, estimated using standard sediment transport equations. The intersection of coloured and dashed lines are minimum estimated discharges and sediment concentrations required to transport the gravel 10 km downstream. Higher φ values (e.g. φ = 6) represent greater flow turbulance and therefore higher capacity to transport gravel at lower discharges and suspended sediment concentrations. e Estimated return interval for annual peak discharge measurements for the Karnali River, Nepal, from 1962 to 2014 at Chisapani gauging station. The Karnali River is used as a type example of a large trans-Himalayan River. Peak discharge measurements (black circles) were obtained from⁴³). The projected return intervals were estimated using a Gumbel distribution (black dashed line). Red dashed lines represent the upper and lower 95% confidence limits. Shaded blue area represents discharges and associated return intervals to transport a 2 m thick gravel bed 10 km downstream of the GST under 24 h for φ values ranging from 3 to 6.