Theoretical development on the effects of changing flow hydraulics on incipient bed load motion

Abstract

[1] Several decades of flume and field measurements have indicated that in rough turbulent flows the critical Shields stress increases with increasing slope and associated decreasing relative depth. This result contradicts the usual consideration of a decreased critical Shields value on very steep slopes because of increased gravitational effects. However, recent studies have demonstrated that these experimental results could be reproduced with a force balance model if the classical logarithmic velocity profile was replaced with a velocity profile that was more compatible with available velocity measurements over gravel beds. These measurements indicate the existence of a roughness layer that is a zone of almost constant velocity close to the bed, whose properties (mean velocity and turbulence) depended on the flow's relative depth. Unfortunately, velocity profile measurements for low relative depth associated with steep slopes are scarce, and it is still difficult to include such flow properties in a force balance model. Flow resistance data (on the basis of depth average velocity measurements) are very common and cover a wide range of slopes and relative depths. In this paper these data are used to fit a velocity profile including a roughness layer. When used in a force balance model for incipient motion, it adequately reproduced a data set composed of 270 critical Shields values measured in a flume with near-uniform sediments. The relevance of this research to field problems is discussed using a data set composed of 92 critical Shields stresses obtained from field measurements. Finally, a model is proposed for field applications taking into account the slope effect.

Meteorological factors driving glacial till variation and the associated periglacial debris flows in Tianmo Valley, south-eastern Tibetan Plateau

“…This type of channelised run-off could be a combination of three factors: rainfall, melting ice or the overflow that forms when a glacier collapses downward into a water pool. The mechanism of this process lies in the hydrodynamic forces exerted on the surface elements of debris layers and surpassing sediment resistance (Gregoretti and Dalla Fontana, 2008;Recking et al, 2009;Prancevic et al, 2014). The concentration of run-off in the channel bottom causes the erosion of the debris surface layer, forming a solid-liquid current at first, then extends to the layers below with whole or partial mobilisation and debris flows were generated (Gregoretti and Dalla Fontana, 2008).…”

sedFlow – a tool for simulating fractional bedload transport and longitudinal profile evolution in mountain streams

“…The empirical exponent m ranges from 0 to −1, where m = −1 corresponds to the so-called "equal mobility" case in which all grains start moving at the same bed shear stress τ , and m = 0 corresponds to no influence by hiding at all. For x = 50, the values for m, which have been derived from various field observations, typically vary within a range from −0.60 to −1.00 (Recking, 2009), and unfortunately there are only few data points for D i > D 50 (Bathurst, 2013;Bunte et al, 2013). For consistency, the following θ ci,r is used in bedload transport calculations.…”

Predictive Capability of Bedload Equations Using Flume Data

“…Bedload prediction is of primary importance for river engineering, fluvial geomorphology, eco-hydrology, environmental surveys and management, and hazard prediction (Recking, 2009). Bedload transport provides the major process linkage between the hydraulic and material conditions that govern river-channel morphology and knowledge of bedload movement is required not only to elucidate the causes and consequences of changes in fluvial form but also to make informed management decisions that affect a river's function.…”