Discriminating between pore‐filling load and bed‐structure load: a new porosity‐based method, exemplified for the river Rhine

Abstract

Sediments contained in the river bed do not necessarily contribute to morphological change. The finest part of the sediment mixture often fills the pores between the larger grains and can be removed without causing a drop in bed level. The discrimination between pore‐filling load and bed‐structure load, therefore, is of practical importance for morphological predictions. In this study, a new method is proposed to estimate the cut‐off grain size that forms the boundary between pore‐filling load and bed‐structure load. The method evaluates the pore structure of the river bed geometrically. Only detailed grain‐size distributions of the river bed are required as input to the method. A preliminary validation shows that the calculated porosity and cut‐off size values agree well with experimental data. Application of the new cut‐off size method to the river Rhine demonstrates that the estimated cut‐off size decreases in a downstream direction from about 2 to 0·05 mm, covariant with the downstream fining of bed sediments. Grain size fractions that are pore‐filling load in the upstream part of the river thus gradually become bed‐structure load in the downstream part. The estimated (mass) percentage of pore‐filling load in the river bed ranges from 0% in areas with a unimodal river bed, to about 22% in reaches with a bimodal sand‐gravel bed. The estimated bed porosity varies between 0·15 and 0·35, which is considerably less than the often‐used standard value of 0·40. The predicted cut‐off size between pore‐filling load and bed‐structure load (Dc,p) is fundamentally different from the cut‐off size between wash‐load and bed‐material load (Dc,w), irrespective of the method used to determine Dc,p or Dc,w. Dc,w values are in the order of 10−1 mm and mainly dependent on the flow characteristics, whereas Dc,p values are generally much larger (about 100 mm in gravel‐bed rivers) and dependent on the bed composition. Knowledge of Dc,w is important for the prediction of the total sediment transport in a river (including suspended fines that do not interact with the bed), whereas knowledge of Dc,p helps to improve morphological predictions, especially if spatial variations in Dc,p are taken into account. An alternative to using a spatially variable value of Dc,p in morphological models is to use a spatially variable bed porosity, which can also be predicted with the new method. In addition to the morphological benefits, the new method also has sedimentological applications. The possibility to determine quickly whether a sediment mixture is clast‐supported or matrix‐supported may help to better understand downstream fining trends, sediment entrainment thresholds and variations in hydraulic conductivity.

Using sediment impact sensors to improve the morphological sediment budget approach for estimating bedload transport rates

“…7), which gives porosity values using Carling and Reader's (1982) equation of 0.15 at Hubberholme and 0.23 at Starbotton. These values are similar to those recorded in other gravelbed rivers by Lane et al (1995) who found ε to be 0.2 and Frings et al (2008) who suggested that values between 0.15 and 0.35 are feasible. Yet most studies in gravel-bed rivers use higher values of between 0.25 and 0.3 (e.g., Martin and Church, 1995;Ham and Church, 2000;Martin, 2003).…”

“…Yet observations from the field surveys indicate that within the Wharfe, rapid downstream fining occurs -a common feature of gravel-bed river systems. Carling and Reader (1982) and more recently Frings et al (2008) explored the effect of grain size and sediment sorting on sediment porosity. Carling and Reader (1982) found that porosity is negatively correlated to the median grain size, whilst Frings et al (2008) explored the sediment size threshold separating pore-filling load and bed-structure load.…”

“…Carling and Reader (1982) and more recently Frings et al (2008) explored the effect of grain size and sediment sorting on sediment porosity. Carling and Reader (1982) found that porosity is negatively correlated to the median grain size, whilst Frings et al (2008) explored the sediment size threshold separating pore-filling load and bed-structure load. Thus, a single value for porosity is unlikely to represent the full range of sedimentary characteristics down the channel.…”

Quantifiable effectiveness of experimental scaling of river- and delta morphodynamics and stratigraphy

“…Although the silt is not nearly as cohesive as clay, Lick and Gailani (2004) show that the critical shear stress increases for particles smaller than 50 µm, so that the added silica flour is expected to increase the threshold for channel erosion on a floodplain. At the same time, the silt particles percolate through the pores into the bed and silt smaller than a certain cutoff size does not contribute to bed level change and roughness (Frings et al, 2008). We calculated that silt-sized material finer than 20 µm, that is, 40% of the silt-sized silica flour, is accommodated entirely in the pore space of the sand mixture, which means that more than half of the silt is deposited on top of the sand to form slightly cohesive layers with smooth surfaces.…”

Three-dimensional numerical modelling of levee depositions in a Scandinavian freshwater delta

“…The dominating grain size on the river bed of most channels was medium sand, with a low percentage of fine sand. The fine-sand fraction probably occurred as pore-filling load in the unimodal sand mixture, and their mean grain size almost coincided with the washload cutoff size between 0.06 and 0.1 mm (Frings et al, 2008). Therefore, these fine particles were entrained more easily and rode higher in the flow, causing them to be transported at a higher rate than the medium sand fraction and appear with higher concentrations in the upper layer of the flow near the floodplain elevation (James, 1985;Garcia and Parker, 1991).…”