Article

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

Water Resources Research (Impact Factor: 3.71). 10/2007; 43(10). DOI: 10.1029/2006WR005330

ABSTRACT 1] 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.

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    • "Herein, we model the contemporary characteristics of sediment transport in the Waipaoa River as a response to climate change in the 21st Century. We use a one-dimensional sediment transport model (TUGS) to simulate gravel and sand (bed load) transport (Cui, 2007), and a climate-driven hydrological model (HydroTrend) to simulate the water discharge and suspended sediment load in the lower reaches of the river (Kettner and Syvitski, 2008). The supply of fluvial sediment is a key factor affecting: 1) the sustainability of beaches in Poverty Bay; and 2) the operational lifetime of the flood control scheme that protects property and ~ 100 km 2 of high value agricultural land on the Waiapoa River floodplain "
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    ABSTRACT: A one-dimensional sediment transport model (TUGS) and a climate-driven hydrological model (HydroTrend) are used to predict changes to the sediment transport regime of the Waipaoa River that may occur in response to forecast 21st Century variations in climate. Climate change may reduce the mean flow in the Waipaoa River at Matawhero by an average of 13% in the 2030s and 18% in the 2080s. In the 2030s, the maximum simulated change in the mean annual suspended sediment discharge of ± 1 Mt y− 1 may be difficult to discern because of the large variation in the contemporary suspended sediment load (13.4 ± 7.3 Mt y− 1). Depending on the climate change scenario, in the 2080s the suspended sediment discharge may either decline by 1 Mt y− 1 or increase by 1.9 ± 1.1 Mt y− 1. Adverse impacts have the potential to be offset or ameliorated by a modest (35%, ~ 12000 ha) increase in forest cover across the basin headwaters. Size-selective transport and deposition throughout the lower reaches of the Waipaoa River currently limit the amount of bed load exported at the coast to 10.2 ± 24.3 Kt y− 1. In the 2030s this may decline to 6.3 ± 16.1 Kt y− 1, but in the 2080s it may rise to 9.4 ± 20.1 Kt y− 1 as aggradation reduces the amount of accommodation space and modifies the long profile of the simulated river. The bed in the lower 27 km of the river could aggrade by an average of 0.31 m in the 2030s, and 0.85 m in the 2080s. This is likely to be the most costly consequence 21st Century climate change, because rising bed levels have the potential to cause a loss of capacity throughout 75% of the Waipaoa River flood control scheme.
    Global and Planetary Change 03/2009; 67(67):153-166. DOI:10.1016/j.gloplacha.2009.02.002 · 3.71 Impact Factor
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    • "For simulations longer than 10 years, the 10-year series was run repeatedly until the last year of simulation is reached. The main reason for Cui and Wilcox (2007) to use a 10-year series for simulation is that their study needs only to be simulated for a 10-year period and this study inherited the same 10-year hydraulic series. Applying a longer hydrologic series does not change the general behaviour of the simulated results. "
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    ABSTRACT: This paper describes the application of The Unified Gravel-Sand (TUGS) model for the simulation of the Sandy River, Oregon. TUGS model employs Wilcock and Crowe's (2003) bedload transport equation for simulation of gravel and sand transport, and is capable of simulating the dynamics of bed material grain size distributions, including the fractions of sand in the deposits. The model has been examined with three large-scale flume experiments and a flushing flow flume experiment with good agreements in bed profiles, gravel characteristic grain sizes and fractions of sand in the deposits. These examinations, along with descriptions of model development, are presented in a manuscript submitted concomitantly elsewhere (Cui, 2007), and this paper expands model examinations to a natural river. Eight runs are conducted with the Sandy River, Oregon as the prototype river, focusing on the responses of bed material grain size distributions under different hydrologic and sediment supply conditions. Simulation with the recorded discharge and the best understanding of sediment supply in the Sandy River produced good agreement in river longitudinal profile. This simulation also produced decreased median grain size and increased sand fraction in the downstream direction that match well with the general observations in the field. Examinations with varied sand supply indicate that bed material sand fractions are positively correlated with sand supply. Examinations with varied water discharge indicate that increased discharge under the same sediment supply conditions results in decreased bed material sand fraction. Examination of a hypothetical backwater effect from the Columbia River indicates that backwater effect results in increases in bed material sand fractions. Simulation of the sedimentation process upstream of a 14-m tall dam during dam operation produced similar current bed profile and a stratified sediment deposit very similar to those observed in the field.
    River Research and Applications 09/2007; 23(7):732 - 751. DOI:10.1002/rra.1012 · 1.97 Impact Factor
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    ABSTRACT: Recent research on sediment transport in gravel bed rivers shows evidence of the influence of sub-threshold flow history on values of entrainment thresholds (Paphitis and Collins, 2005; Monteith and Pender, 2005; Haynes and Pender, 2007; Haynes and Ockelford, 2008). The research presented here analyses the effect of the characteristics of hydrograph rising limbs (flow magnitude and duration) on entrainment thresholds of gravel beds, with discharges ranging from 0.25-1.6 times the estimated bed threshold flow and durations from 0.5h to 6h. This analysis uses results from flume experimentation. Entrainment thresholds were determined by two well documented methods: a) particle movement counts (visual method, Yalin, 1977); and b) the reference transport method (RTM) (Parker et al., 1982a; Shvidchenko et al., 2001). Results obtained with near-uniform and uniform bed material sizes show a clear influence of flow magnitude and duration on entrainment thresholds, with bed resistance increasing up to c. 25% for longer durations of antecedent flows when using the visual method, similarly to Paphitis and Collins (2005). The results from the unimodal gravel bed suggest an intermediate duration of rising limb (c. 2h) producing the strongest bed, with more mobile beds resulting from both shorter and longer rising limbs. Total bedload transport rates reduce with increased bed resistance, this effect is also noted during the stability test phase. These results are used to develop a new simplified method for estimating critical bed shear stress using only total bedload data. The performance of a new formulation for bedload rates derived in this thesis is tested against a number of traditional bedload transport equations and appropriately discussed. In-depth analyses of bed surface and bedload size composition and surface grain structure show that bed surface undergoes little change of size composition, with a slightly proportionally larger reduction of fine content, suggesting penetration of fines below the surface. The analysis of coarse-grain bed surface structures, mobility and clustering, based on the size class containing D90 and using digital images taken under UV light, suggests that the surface distribution of coarse grains has a primary role on bed stability.
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