Implications of Pool and Riffle Sequences for Water Quality Modeling

Water quality models like QUAL2E and WASP employ a constant hydraulic geometry to describe a river reach. However, hydraulic geometry is known to vary along pool to riffle sequences. This paper examines the magnitude of the hydraulic geometry variation between riffle and pool sequences for different flow levels and develops a mathematical model to simulate the downstream effects of the variation. The hydraulic geometry relationships—relating average velocity and cross-sectional area to discharge—were derived for seven hydrometric stations in a study of the Assiniboine River in Canada. These stations span a section of river 387 kilometers long, and each station has over 20 years of recorded flow data. There is a marked variation in the exponents and coefficients of the at-a-station hydraulic geometry. However, when the variation is mapped against riffle or pool sections, a graphical pattern of curves emerges. This pattern depicts the hydraulic reversal hypothesis, postulated by Keller in 1971. The curves show the variation in average velocity and cross-sectional area between riffles and pools for differing flow conditions. Distinct patterns emerge for each river reach. Subsequently surveyed cross-sections chosen to reflect riffle, pool and transition sections support the contention that the reversal pattern is unique for each reach. Water quality models simulate river reaches as uniform stretches exhibiting constant hydraulic geometry. However, river reaches show a variable hydraulic geometry due to riffle-pool sequences. This paper formulates a periodic function to describe the variability of riffle-pool hydraulic geometry along a reach. The function is incorporated into the mass balance equation and the resultant model applied to the Assiniboine River. Implications of the revised mass balance to water quality are discussed.