Application of modified Manning formula in the determination of vertical profile velocity in natural rivers

Seldom studied before, the vertical profile velocity is indicative of the flood process and nutrient transportation process. In this paper, a substitution of cross section hydraulic radius with vertical depth was made to the Manning formula, which was then applied in the vertical profile velocity determination. Simultaneously, the determination accuracy and its relationship with hydraulic conditions were discussed, based on the 1050 vertical profiles sampled from 140 cross sections in flood and moderate level seasons. The observations show the following. (1) The modified Manning formula provides a simplified approach for vertical profile velocity determination with acceptable accuracy. (2) The fitting quality of the profile velocity from the middle region of the cross section and the flood season were higher than that from near the bank or the moderate level season. The coefficient of determination ( R 2 ) of the regression for the moderate level season and the flood season were 0.55 and 0.58, while the Nash–Sutcliffe coefficients were 0.64 and 0.82, respectively. (3) Analysis of the determination error and the coefficient of variation showed a positive correlation with the river aspect ratio. This seems to suggest that the modified Manning formula tends to be more applicable in narrow and deep rivers. More measurements from rivers or channels with a high aspect ratio would be meaningful for future research.