Comparison of Local and Global Optimization Methods for Calibration of a 3D Morphodynamic Model of a Curved Channel

Vahid Shoarinezhad,Silke Wieprecht,Stefan Haun

doi:10.3390/w12051333

Abstract

In curved channels, the flow characteristics, sediment transport mechanisms, and bed evolution are more complex than in straight channels, owing to the interaction between the centrifugal force and the pressure gradient, which results in the formation of secondary currents. Therefore, using an appropriate numerical model that considers this fully three-dimensional effect, and subsequently, the model calibration are substantial tasks for achieving reliable simulation results. The calibration of numerical models as a subjective approach can become challenging and highly time-consuming, especially for inexperienced modelers, due to dealing with a large number of input parameters with respect to hydraulics and sediment transport. Using optimization methods can notably facilitate and expedite the calibration procedure by reducing the user intervention, which results in a more objective selection of parameters. This study focuses on the application of four different optimization algorithms for calibration of a 3D morphodynamic numerical model of a curved channel. The performance of a local gradient-based method is compared with three global optimization algorithms in terms of accuracy and computational time (model runs). The outputs of the optimization methods demonstrate similar sets of calibrated parameters and almost the same degree of accuracy according to the achieved minimum of the objective function. Accordingly, the most efficient method concerning the number of model runs (i.e., local optimization method) is selected for further investigation by setting up additional numerical models using different sediment transport formulae and various discharge rates. The comparisons of bed topography changes in several longitudinal and cross-sections between the measured data and the results of the calibrated numerical models are presented. The outcomes show an acceptable degree of accuracy for the automatically calibrated models.

Highlights

Numerical models have become useful behavioral representatives of complex environmental systems
In order to evaluate the performance of the four selected calibration methods, in the first step, the numerical model of Run#5 is calibrated as a test case, using Wu’s bedload transport formula, which considers the fractional transport of nonuniform sediments
Namely roughness height, active layer thickness, and the sediment volume compared to water at the bed, are selected for calibration, using their initial values and a reasonable range based on the literature

Summary

Introduction

Numerical models have become useful behavioral representatives of complex environmental systems. In water-related domains, the growth of knowledge about the underlying processes and recent developments regarding numerical solvers and computational techniques have increased the performance and speed of simulations. A significant challenge remains for modelers to minimize the misfit between simulation outputs and corresponding physical observations, which is necessary to obtain a reliable predictive model [1,2]. Hydro-morphodynamic models are characterized by a large number of input parameters dealing with a considerable amount of uncertainty. This uncertainty arises from the sophisticated behavior of environmental fluid systems, the simplified structure of models, implemented empirical equations, unknown boundary conditions, Water 2020, 12, 1333; doi:10.3390/w12051333 www.mdpi.com/journal/water. Water 2020, 12, 1333 and imprecise input data. Some of the physical input parameters, which are subject to calibration, can only be quantified at specific locations over a limited period, and for some it is not physically feasible to measure them

Objectives

Results

Conclusion