Some field measurements concerned with the behaviour of resistance coefficients in a tidal channel

Abstract

Measurements of the individual terms in the one-dimensional unsteady flow momentum equation in a 1.2 km tidal reach of the Conwy estuary, N. Wales, have yielded values for the resistance coefficients commonly adopted in open channel flow hydraulics. The results indicate clearly that Manning's n or Darcy-Weisbach's f vary significantly with stage, and to a lesser extent with flow direction. The relative importance of the various terms in the one-dimensional momentum equation are illustrated, and the techniques employed in measuring them indicated. Values of Nikuradse's roughness parameter, k s , have also been computed and are shown to vary significantly with water level. Mean bed shear stresses have been evaluated, and the variation in bed shear velocity throughout different tidal cycles determined. The results indicate the inadvisability of selecting a single roughness parameter to represent the hydraulic characteristics of any individual element in a numerical tidal model.