Alluvial cover on bedrock channels: applicability of existing models

Abstract

Abstract. Several studies have demonstrated the importance of alluvial cover; furthermore, several mathematical models have also been introduced to predict the alluvial cover on bedrock channels. Here, we provide an extensive review of research exploring the relationship between alluvial cover, sediment supply and bed topography of bedrock channels, describing various mathematical models used to analyse the deposition of alluvium. To test one-dimensional theoretical models, we performed a series of laboratory-scale experiments with varying bed roughness under simple conditions without bar formation. Our experiments show that alluvial cover is not merely governed by increasing sediment supply and that bed roughness is an important controlling factor of alluvial cover. A comparison between the experimental results and the five theoretical models shows that (1) two simple models that calculate alluvial cover as a linear or exponential function of the ratio of the sediment supplied to the capacity of the channel produce good results for rough bedrock beds but not for smoother bedrock beds; (2) two roughness models which include changes in roughness with alluviation and a model including the probability of sediment accumulation can accurately predict alluvial cover in both rough and smooth beds; and (3), however, except for a model using the observed hydraulic roughness, it is necessary to adjust model parameters even in a straight channel without bars.

Controls of alluvial cover formation, morphology and bedload transport in a sinuous channel with a non‐alluvial boundary

“…In the case of Fernández et al (2020), the bedrock material had higher roughness than the alluvial cover, so that the formation of an alluvial cover layer decreased the grain roughness but increased the form roughness within the channel. The relatively smooth material of the bed material in our context means that the grain roughness also likely increases as the alluvial cover forms, similar to observations made by Mishra and Inoue (2020). Such a process results in areas of bare bed becoming preferential transport paths (as observed in our experiments; Video S2).…”

“…The theoretical model by Nelson et al (2014) suggests that the difference in grain roughness between the alluvial cover and the bare bed influences the alluvial cover characteristics, although this effect was secondary to the effects of the channel curvature (Nelson et al, 2014). Recent flume experiment studies have demonstrated that the formation of an alluvial cover alters the flow resistance within the channel by introducing spatial variation in both the local grain and bedform roughness (Fernández et al, 2020; Mishra and Inoue, 2020). In the case of Fernández et al (2020), the bedrock material had higher roughness than the alluvial cover, so that the formation of an alluvial cover layer decreased the grain roughness but increased the form roughness within the channel.…”

“…Johnson and Whipple, 2010; Inoue et al, 2014; Hodge and Hoey, 2016a, 2016b), sediment supply grain size (e.g. Inoue et al, 2014; Hodge et al, 2016) and bedrock roughness (Inoue et al, 2014; Johnson, 2014; Hodge and Hoey, 2016b; Mishra and Inoue, 2020) have also been shown to influence the extent of the alluvial cover. Despite advances in the understanding of the controls on the extent of alluvial cover, several knowledge gaps persist.…”

Numerical Simulations of Meanders Migrating Laterally as They Incise Into Bedrock

“…(2020) studied the heuristic for randomness in particle trajectories driven by bedrock topographic roughness. However, since accurate prediction of hydraulic roughness should take into account not only the bedrock topographic roughness but also the arrangement of bed unevenness (Mishra & Inoue, 2020), we do not link the hydraulic roughness height and the roughness effect coefficient in this study.…”

Upscaling sediment-flux-dependent fluvial bedrock incision to long timescales

“…The tools effect arises because fluvial bedrock erosion is driven by the impacts of moving bedload particles, implying that an increasing number of moving particles leads to an increasing number of impacts and therefore higher erosion rates (e.g., Beer & Turowski, 2015;Cook et al, 2013;Foley, 1980;Inoue et al, 2014). The cover effect arises because sediment residing on the bed can shield the bedrock from impacts, thereby decreasing erosion rates (e.g., Chatanantavet & Parker, 2008;Mishra & Inoue, 2020;Turowski, Hovius, Hsieh, et al, 2008). Yet, in landscape evolution models designed for long timescales, fluvial bedrock erosion is commonly described by the stream power incision model (SPIM), in which incision rate is a power function of water discharge and channel bed slope (e.g., Barnhart et al, 2020;Seidl & Dietrich, 1992).…”