Downstream fining of bed sediment in alluvial rivers is usually gradual, but often an abrupt

decrease in characteristic grain size occurs from about 10 to 1 mm, i.e., a gravel-sand transition (GST) or

gravel front. Here we present an analytical model of GST migration that explicitly accounts for gravel and

sand transport and deposition in the gravel reach, sea level change, subsidence, and delta progradation.

The model shows that even a limited gravel supply to a sand bed reach induces progradation of a gravel

wedge and predicts the circumstances required for the gravel front to advance, retreat, and halt. Predicted

modern GST migration rates agree well with measured data at Allt Dubhaig and the Fraser River, and

the model qualitatively captures the behavior of other documented gravel fronts. The analysis shows

that sea level change, subsidence, and delta progradation have a significant impact on the GST position

in lowland rivers.

Downstream fining of bed sediment in alluvial rivers is usually gradual, but often an abrupt

decrease in characteristic grain size occurs from about 10 to 1 mm, i.e., a gravel-sand transition (GST) or

gravel front. Here we present an analytical model of GST migration that explicitly accounts for gravel and

sand transport and deposition in the gravel reach, sea level change, subsidence, and delta progradation.

The model shows that even a limited gravel supply to a sand bed reach induces progradation of a gravel

wedge and predicts the circumstances required for the gravel front to advance, retreat, and halt. Predicted

modern GST migration rates agree well with measured data at Allt Dubhaig and the Fraser River, and

the model qualitatively captures the behavior of other documented gravel fronts. The analysis shows

that sea level change, subsidence, and delta progradation have a significant impact on the GST position

in lowland rivers

Downstream fining of bed sediment in alluvial rivers is usually gradual, but often an abrupt

decrease in characteristic grain size occurs from about 10 to 1 mm, i.e., a gravel-sand transition (GST) or

gravel front. Here we present an analytical model of GST migration that explicitly accounts for gravel and

sand transport and deposition in the gravel reach, sea level change, subsidence, and delta progradation.

The model shows that even a limited gravel supply to a sand bed reach induces progradation of a gravel

wedge and predicts the circumstances required for the gravel front to advance, retreat, and halt. Predicted

modern GST migration rates agree well with measured data at Allt Dubhaig and the Fraser River, and

the model qualitatively captures the behavior of other documented gravel fronts. The analysis shows

that sea level change, subsidence, and delta progradation have a significant impact on the GST position

in lowland rivers

Downstream fining of bed sediment in alluvial rivers is usually gradual, but often an abrupt decrease in characteristic grain size occurs from about 10 to 1 mm, i.e., a gravel-sand transition (GST) or gravel front. Here we present an analytical model of GST migration that explicitly accounts for gravel and sand transport and deposition in the gravel reach, sea level change, subsidence, and delta progradation. The model shows that even a limited gravel supply to a sand bed reach induces progradation of a gravel wedge and predicts the circumstances required for the gravel front to advance, retreat, and halt. Predicted modern GST migration rates agree well with measured data at Allt Dubhaig and the Fraser River, and the model qualitatively captures the behavior of other documented gravel fronts. The analysis shows that sea level change, subsidence, and delta progradation have a significant impact on the GST position in lowland rivers.

To compute the GST migration celerity with equation (1) or (B1), various parameters, among which the gravel flux, need to be specified. Closure relations for these parameters can be determined from (a) measured field data, possibly with the rarely available gravel flux estimated from measured values of slope and characteristic flow rate or (b) imposing a mean gravel flux and applying the approach described below.

Here we apply closure relations for slope and surface texture that are based on the assumption that both the gravel and the sand reach are in a state of quasi-equilibrium. We introduce the term “extended model” to indicate the combination of the analytical model of GST migration in equation (B1) and the quasi-equilibrium closure relations. The quasi-equilibrium state refers to the fact that, despite their transient state due to GST migration, the gravel reach and the sand reach are at grade with the water discharge and their gravel and sand supply rates. This assumption holds provided that boundary conditions change slowly (i.e., the time scale of the changing boundary conditions is larger than the time scale of channel response to the changing boundary conditions; Howard, 1982). Under these assumptions we can apply the formulations for slope and surface texture in an equilibrium alluvial reach derived by Blom et al. (2016) to the upstream ends of the gravel and sand reaches. The resulting values are assumed to represent reach-averaged values.

Quoted from: Advance, Retreat, and Halt of Abrupt Gravel-Sand Transitions in Alluvial Rivers