Numerical simulation of downstream fining by selective transport in gravel bed rivers: Model development and illustration

Abstract

A numerical model has been developed for the routing of gravel‐sized sediment along a river channel which is free to adjust both its long profile and surface texture. Hydraulic calculations use a step‐backwater approach, and sediment transport is predicted with the method of Parker (1990a), which uses a low degree of size selectivity. Exchange of sediment between the surface and subsurface is described using the modified Exner equation of Parker and Sutherland (1990). The model is applied to an idealized channel based on the highly concave Allt Dubhaig, Scotland, in which fining by particle wear is minor. The rapid downstream fining observed in this river is closely matched by model predictions after a time equivalent to <102 years under the present flow regime of the river. The evolution of the fining pattern during the model run and associated changes in sediment transport and bed aggradation are described. It is concluded that strong profile concavity can force rapid downstream fining even though bed load transport is only slightly size selective. This run of the model serves as a basis for testing of the sensitivity of downstream fining to alternative choices of parameter values and boundary conditions, which are summarized here and will be described in a subsequent paper.

A one‐dimensional model for simulating armouring and erosion on hillslopes: 1. model development and event‐scale dynamics

“…This is consistent with the low sensitivity of the calibration to armour depth except when the assumed armour depth was small. This suggests that it may be possible to eliminate the calibration of armour depth and instead use published results from river and flume studies such as (1) one grain size (Proffitt, 1982), or (2) 2d 84 (Hoey and Ferguson, 1994). The authors, however, caution against arriving at this conclusion because the long-term erosion predictions (up to 100 years) appear to be more sensitive to the armour depth (Sharmeen and Willgoose, 2006).…”

“…For Northparkes any value of β could be adopted, and this reflected the relative insensitivity of its lightly armoured materials to hiding characteristics. A value of β = 0 implies perfect equal mobility of sediments while β = 1 is perfect (Hoey and Ferguson, 1994). With a small value of the hiding parameter β (size selectivity near equal mobility) the fit was significantly better than for pure equal mobility mode.…”

“…The armour grading evolves with time. The literature suggests that the depth of the armour layer lies between one grain size (Proffitt, 1980) and 2d 84 (Hoey and Ferguson, 1994). Field observations by the first author suggest that erosion depths with stable armour may be as much as a few hundred millimetres as a result of rilling.…”

“…The exponent β determines the degree of size selectivity on the entrainment process. Its value lies in the range from zero (perfect equal mobility, equivalent to mode 2) to one (purely size selective transport, equivalent to mode 1) (Hoey and Ferguson, 1994). The rate of sediment entrainment ER k for hiding is estimated using the approach of Andrews and Parker (1987):…”

Tracer‐pebble movement along a concave river profile: Virtual velocity in relation to grain size and shear stress

“…Its gravel bed fines rapidly downstream, and since abrasion and lateral inputs are negligible this fining is attributed to selective transport and deposition [Ferguson et al, 1996]. A numerical model of these processes simulates the pattern of fining quite well without any calibration [Hoey and Ferguson, 1994]. However, predictive ability does not prove that a model simulates processes faithfully, so we made field measurements to verify that bedload transport in this river is size-selective and that its formulation in the model is realistic.…”