Spacing effects of discrete bed roughness on sequent depth, jump length, and energy loss in hydraulic jumps

This study examines the effect of roughness spacing on the characteristics of hydraulic jumps in open-channel flows, based on experimental results in a rectangular flume with a smooth bottom and three different sets of discrete roughness (spacings S = 12 cm, 24 cm, and 46 cm; fixed height K = 2 cm). Tests were conducted with supercritical incoming flows at discharges of 15, 20, and 25 L/s. Flow visualization revealed regime transitions from skimming-like flow at close spacing ( S/K = 6) to wake-interference at intermediate spacing, and wake-type hydraulic jumps at the largest spacing ( S/K = 23), with increasing discharge promoting unsteady wake–jump interactions, surface rolling, and enhanced turbulence. Results show that increasing roughness spacing significantly reduces relative jump length ( L j /y ₁ ) from 20 to 35 ( S = 12 cm) to 10–17 ( S = 46 cm), while relative energy dissipation (ΔE/E 1 ) decreases with larger spacing due to reduced flow–roughness interaction density; optimal dissipation occurs at smaller spacings. Velocity profiles, pressure distributions, and roller lengths were also affected, with closer spacings yielding more compact jumps and higher turbulence near the bed. Empirical predictive relationships were developed for relative jump length and a modified conjugate depth ratio incorporating a roughness correction factor, showing good agreement with measurements. These findings highlight the critical role of roughness spacing in optimizing energy dissipation and jump control in stilling basins and hydraulic structures.