Velocity and flow resistance in step-pool streams

Flow resistance in steep streams with step-pool morphology consists of form drag as well as skin friction. It therefore alters rapidly as discharge increases and is not expected to show constant Manningʹs n or simple log law behaviour, but there is no widely accepted alternative representation. We report measurements of at-a-station hydraulic geometry, n, and (1/f)0.5 for a range of discharges at each of six field sites and in flume experiments loosely Froude-scaled to the prototypes. Velocity and resistance vary strongly with discharge in all cases. The flume data extend to formative flows, and when rescaled extend the range of the field data, but plot in line with them. This suggests a single resistance law can describe the full range from trickle to formative flow. Equations based on the log law with ks∝step D84 perform unexpectedly well, especially when allowance is made for flow blocking and/or the ks/D84 multiplier is optimised separately for each site. The latter suggests step D84 is not a good summary representation of effective roughness, but none of a variety of alternative grain-size and microtopographic measures performed much better.