numerical simulation of armour layer development under ... - AIIA2011

G. Kaless, L. Maoparticles and a following re-arrangement of the coarser fractions. Using a laser scannerdevice, Mao et al. (2011) recently depicted the highly structured and imbricated natureof the static armour layers, which became topographically less complex at higherforming flow strengths, with significant implications for sediment mobility and bedroughness. Static armour development as observed in flume experiments has beensimulated numerically with success using different approaches (e.g. Ashida & Michiue,1971; Proffitt, 1980; Bettess & Frangipane, 2003). The aim of the present work is topresent a simple approach for reproducing numerically static armour development, todiscuss the simulation of the sediment transport rate reduction through time, and toassess the best performing bedload formula.2 MATERIALS AND METHODS2.1 Numerical modellingThe numerical model is intended to predict the evolution of the bed profile and thesurface grain size distribution given a certain flow strength and a sediment transportcapacity. The model is based on mass and energy conservation principles and fractionssediment transport process is described by means of the sediment conservation equationand a sediment transport model. Being developed for poorly‐sorted sediment grain‐sizemixtures, the model is based on the Parker & Sutherland (1990) extension to theExner’s conservation equation for the case of heterogeneous mixtures. Parker &Sutherland (1990) model considers a surface layer that directly interacts with sedimenttransport and a subsurface layer that interacts with the upper layer by means ofdegradation/aggradation:∂z∂t = − 1 ∂q bT1 − λ p ∂x(1)∂F si∂t = 1 ∂(z − L a )*fL Iia ∂t− F si∂L a∂t − ∂q bi∂x 4where q bT is the total volumetric sediment transport, λ P is the sediment porosity, F si ,and f Ii are the volume fraction of material in the i th grain size range corresponding to thesurface layer, and the exchanged material between layers, respectively; L a is thethickness of the active layer and q bi is the volumetric sediment transport of the i th grainsize range. In case of bed degradation, the surface grain size distribution equals thesubsurface mixture, whereas in the case of bed aggradation a linear mixing modelconsidering bedload grain size and the armour layer is applied (Hoey & Ferguson,1994). As to the active layer, its thickness is comparable to the surface D 90 (size such90% of material is finer).The sediment transport process can be described by many models based on thesurface material. For this study two models based on field data (Parker, 1990 andPowell et al., 2003) and three models derived from flume experiments (Wilcock &Crowe, 2003; Wu et al., 2000 and Hunziker & Jaeggi, 2002) have been selected. All themodels are suitable for computing fractional sediment transport (i.e. the transport ofeach grain size classes is calculated separately).The model has been applied to simulate static armour development in a closed(2)