The Unified Gravel‐Sand (TUGS) Model: Simulating Sediment Transport and Gravel/Sand Grain Size Distributions in Gravel‐Bedded Rivers

This paper presents The Unified Gravel‐Sand ( TUGS ) model that simulates the transport, erosion, and deposition of both gravel and sand. TUGS model employs the surface‐based bed load equation of Wilcock and Crowe (2003) and links grain size distributions in the bed load, surface layer, and subsurface with the gravel transfer function of Hoey and Ferguson (1994) and Toro‐Escobar et al. (1996), a hypothetical sand transfer function, and hypothetical functions for sand entrainment/infiltration from/into the subsurface. The model is capable of exploring the dynamics of grain size distributions, including the fractions of sand in sediment deposits and on the channel bed surface, and is potentially useful in exploring gravel‐sand transitions and reservoir sedimentation processes. Simulation of three sets of large‐scale flume experiments indicates that the model, with minor adjustment to the Wilcock‐Crowe equation, excellently reproduced bed profile and grain size distributions of the sediment deposits, including the fractions of sand within the deposits. Simulation of a flushing flow experiment indicated that the sand entrainment function is potentially capable of simulating the short‐term processes such as flushing flow events.