Flow threads in surface run-off: Implications for the assessment of flow properties and friction coefficients in soil erosion and hydraulics investigations

Soil surface microtopography produces non-uniform surface run-off, in which narrow threads of relatively deep and fast flow move within broader, shallower, slower-moving regions. This kind of flow is probably widespread, given that microtopography is itself common. Methods used to record the properties of surface run-off include grid- or transect-based depth observations, with a single mean flow speed derived by calculation from V = Q/WD, and the use of dye timing to estimate velocity, with an effective mean depth calculated from D = Q/WV. Because these methods allow only single, flow-field mean values to be derived for V or D, neither is well suited to non-uniform flows. The use of depth data to derive a flow-field mean V furthermore implicitly applies area weighting to the depth data: likewise, the use of dye speeds for V inherently overestimates mean V because dye dominantly follows the faster flow threads. The associated errors in derived parameters such as friction coefficients are not readily quantified and appear not to have been addressed previously. New field experiments made on untilled soil surfaces in arid western NSW, Australia, explore these circumstances and the implications for deriving meaningful measures of flow properties, including friction coefficients. On surfaces deliberately chosen for their very subtle microtopography, average thread velocities are shown to be commonly 2.5 times greater than the flow-field mean, and locally 6-7 times greater. On the other hand, non-thread flow speeds lie below the flow-field mean, on average reaching only 84 per cent of this value, and often considerably less. Flow-field means conceal the existence of regions of the flow field whose properties are statistically distinct. Results confirm that a reliance on flow-field average depths yields estimates of friction coefficients that are biased toward the shallower, high-roughness parts of the flow, while if dye speeds are relied upon the results are biased toward the deeper, smoother threads of flow. A new approach to the evaluation of friction coefficients in non-uniform flows is advanced, involving the determination of separate coefficients for threads and non-thread zones of the flow field. In contrast, flow-field friction coefficients as they are customarily derived in run-off plot experiments subsume these distinct coefficients in proportions that are generally unknown. The value of such coefficients is therefore questionable. Copyright (C) 2004 John Wiley Sons, Ltd.