Geology topics

Sanjay Giri

Publications and source records attributed to Sanjay Giri.

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↗

USGS2010

Computational modeling of bedform evolution in rivers with implications for predictions of flood stage and bed evolution

Uncertainties in flood stage prediction and bed evolution in rivers are frequently associated with the evolution of bedforms over a hydrograph. For the case of flood prediction, the evolution of the bedforms may alter the effective bed roughness, so predictions of stage and velocity based on assuming bedforms retain the same size and shape over a hydrograph will be incorrect. These same effects will produce errors in the prediction of the sediment transport and bed evolution, but in this latter case the errors are typically larger, as even small errors in the prediction of bedform form drag can make very large errors in predicting the rates of sediment motion and the associated erosion and deposition. In situations where flows change slowly, it may be possible to use empirical results that relate bedform morphology to roughness and effective form drag to avoid these errors; but in many cases where the bedforms evolve rapidly and are in disequilibrium with the instantaneous flow, these empirical methods cannot be accurately applied. Over the past few years, computational models for bedform development, migration, and adjustment to varying flows have been developed and tested with a variety of laboratory and field data. These models, which are based on detailed multidimensional flow modeling incorporating large eddy simulation, appear to be capable of predicting bedform dimensions during steady flows as well as their time dependence during discharge variations. In the work presented here, models of this type are used to investigate the impacts of bedform on stage and bed evolution in rivers during flood hydrographs. The method is shown to reproduce hysteresis in rating curves as well as other more subtle effects in the shape of flood waves. Techniques for combining the bedform evolution models with larger-scale models for river reach flow, sediment transport, and bed evolution are described and used to show the importance of including dynamic bedform effects in river modeling. For example calculations for a flood on the Kootenai River, errors of almost 1m in predicted stage and errors of about a factor of two in the predicted maximum depths of erosion can be attributed to bedform evolution. Thus, treating bedforms explicitly in flood and bed evolution models can decrease uncertainty and increase the accuracy of predictions.

Conference Paper↗

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↗