Geology topics

Satomi Yamaguchi

Publications and source records attributed to Satomi Yamaguchi.

3 recordsLinked to original sources

USGS2019

Morphological computation of dune evolution with equilibrium and non-equilibrium sediment-transport models

This paper presents an exploratory study that comprises the implementation and comparison of different approaches and parameterization of sediment transport mechanisms in a process-based morphological model for simulating river dunes. The purpose of this study was to assess the underlying physical processes associated with sediment transport and dune evolution simulated by the model with two different bedload transport models: a non-equilibrium approach considering saltation distance and an equilibrium approach with the incorporation of the effect of the local bed slope. The advancement of this study is that it comprised detailed analysis of the bed shear stress and sediment transport over a dune during time varying flows, comparing both transport models and revealing distinctive transient features of bedload transport and dune evolution. We also improved a non-equilibrium transport model by incorporating a formulation for sediment saltation distance as a length-scale. Using both sediment transport approaches, the morphological model was applied to replicate a large-scale field experiment. Notably, the model yielded the quasi-equilibrium dune feature reasonably well using either sediment-transport formulation. However, detailed analysis of simulated spatial and temporal features of sediment transport and the dune evolution process were found to be noticeably different. Even though both sediment transport mechanisms appear to work adequately at a basic level, the major difference between the two approaches is the underlying transport process over the dunes and time-scale of the dune evolution process.

Water Resources Research↗

USGS2017

The effect of wet-dry weathering on the rate of bedrock river channel erosion by saltating gravel

Previous work has shown that the bedrock erosion rate E because of collisions of saltating bedload can be expressed by E = βq b (1- P c ), where q b is the sediment transport rate, P c is the extent of alluvial cover, and β is the abrasion coefficient. However, the dependence of the abrasion coefficient on the physical characteristics of the bedrock material is poorly known, and in particular, the effects of wet-dry weathering on the saltation-abrasion bedrock incision has not been specifically characterized. Observation suggests that the typical wet-dry cycling of exposed bedrock in river beds gives rise to cracks and voids that are likely to alter the incision rate of the material when subjected to impacts of moving sediment. In this study, flume experiments are performed to develop an understanding of how wet-dry cycling affects the rock tensile strength and the bedrock erosion rate. To represent the physical effects of weathering, boring cores taken from natural bedrock channel are exposed to artificial wet-dry cycles. The experimental results suggest the following: (1) the abrasion coefficient for fresh bedrock is estimated by β = 1.0 × 10 − 4 σ T − 2 ( d / k sb ) 0.5 , where σ T is the tensile strength, d is the diameter of colliding gravel, and k sb is the hydraulic roughness height of bedrock; (2) the tensile strength of the bedrock decreases exponentially as a result of repeated wet-dry cycles, σ T /σ T 0 = exp (- C T NW a 0 /σ T 0 ), where σ T 0 is the initial tensile strength, W a 0 is the initial normalized rate of water absorption., N is the number of wet-dry cycles, and C T is a constant; (3) the erosion rate of fresh bedrock depends on the inverse of the square of tensile strength, but the erosion rate of weathered bedrock depends on the − 1.5 power of tensile strength.

Geomorphology↗

USGS2009

Numerical simulation of dune-flat bed transition and stage‐discharge relationship with hysteresis effect

This work presents recent advances on morphodynamic modeling of bed forms under unsteady discharge. This paper includes further development of a morphodynamic model proposed earlier by Giri and Shimizu (2006a). This model reproduces the temporal development of river dunes and accurately replicates the physical properties associated with bed form evolution. Model results appear to provide accurate predictions of bed form geometry and form drag over bed forms for arbitrary steady flows. However, accurate predictions of temporal changes of form drag are key to the prediction of stage‐discharge relation during flood events. Herein, the model capability is extended to replicate the dune–flat bed transition, and in turn, the variation of form drag produced by the temporal growth or decay of bed forms under unsteady flow conditions. Some numerical experiments are performed to analyze hysteresis of the stage‐discharge relationship caused by the transition between dune and flat bed regimes during rising and falling stages of varying flows. The numerical model successfully simulates dune–flat bed transition and the associated hysteresis of the stage‐discharge relationship; this is in good agreement with physical observations but has been treated in the past only using empirical methods. A hypothetical relationship for a sediment parameter (the mean step length) is proposed to a first level of approximation that enables reproduction of the dune–flat bed transition. The proposed numerical model demonstrates its ability to address an important practical problem associated with bed form evolution and flow resistance in varying flows.

Water Resources Research↗