Xiaonan Tang

The concept of roughness in fluvial hydraulics and its formulation in 1D, 2D and 3D numerical simulation models

This paper gives an overview of the meaning of the term “roughness” in the field of fluvial hydraulics, and how it is often formulated as a “resistance to flow” term in 1D, 2D and 3D numerical models. It looks at how roughness is traditionally characterized in both experimental and numerical fields, and subsequently challenges the definitions that currently exist. In the end, the authors wonder: Is roughness well understood and defined at all? Such a question raises a number of concerns in both research and practice; for example, how does one modeller use the roughness value from an experimental piece of work, or how does a practitioner identify the roughness value of a particular river channel? The authors indicate that roughness may not be uniquely defined, that there may be distinct “experimental” and “numerical” roughness values, and that in each field nuances exist associated with the context in which these values are used.

Variable parameter Muskingum-Cunge method for flood routing in a compound channel

(1999). Variable parameter Muskingum-Cunge method for flood routing in a compound channel. Journal of Hydraulic Research: Vol. 37, No. 5, pp. 591-614.

Analytical models for velocity distributions in open channel flows

Various models for the analytical solution to the depth-integrated Navier-Stokes equations, applied to open channel flows, are discussed. These models include the effects of bed friction, lateral turbulence and secondary flows. The model developed by Shiono and Knight (1988, 1991) is compared with those by Ervine et al. (2000) and Castanedo et al. (2005) through numerical experiments on both simple and compound channels. These tests show that the impact of the secondary flow term on the results is significant. It is also shown that the dimensionless eddy viscosity coefficient has less influence on the results than the secondary flow terms for overbank flows, but is important for inbank flows. A comparison of the analytical results with experimental data shows that the model by Shiono and Knight produces well velocity distributions within open channels of various prismatic shapes.

Evaluation of Flow Resistance in Smooth Rectangular Open Channels with Modified Prandtl Friction Law

Flow resistance in open channels is usually estimated by applying the approach that is developed originally for pipe flows. Such estimates may be useful for engineering applications but always differ from measurements to some extent. This paper first summarizes empirical approaches that have been proposed in the literature to reconcile the resistance difference. These include various modifications of the pipe friction for applications to rectangular ducts and open channel flows. An improved friction equation is then derived for evaluating flow resistance of smooth rectangular open channels. Comparisons are made with experimental data reported by previous researchers and those collected in the present study. It is shown that the new proposed equation is applicable for both narrow and wide channels and is more accurate than those available in the literature.

Flow discharge computation over compound sharp-crested side weirs

Abstract Side weirs are generally rectangular sharp-crested in form, but for accurate flow measurement and management in a wide range of flow discharges, compound side weirs perform better. Such a commonly used one is rectangular compound side weir, which has a small rectangular weir in lower section for measuring low flow discharges, and a wide rectangular section alongside for measuring higher flows. Determination of discharge coefficient for such a compound side weir is difficult and needs to be calibrated using a large set of laboratory tests. In this paper, based on the practical design method of May et al., a new approach was proposed for predicting flow discharge over compound side weirs. This new approach has been evaluated against experimental data in subcritical flow conditions, and has shown very good agreement with the experimental data, with the mean overall and absolute relative errors being 1.6 and 7.8%, respectively. Because the proposed method is independent of discharge coefficient for overflow rate computation, it will provide a reliable and convenient tool for flow measurement of compound side weirs and their water surface profiles.

Optimal prediction of lateral velocity distribution in compound channels

ABSTRACT Many rivers have deep main channels in the centre with one or two adjoining floodplains. Prediction of the lateral velocity distributions over the entire river cross-section is necessary for solving many river-related and hydro-engineering problems. Using Genetic Algorithm (GA), this paper proposes a simple model with two separate equations for predicting the lateral velocity distribution in the main channel and floodplains of straight compound channels. The proposed model is based on two key parameters of compound channels, i.e. depth ratio and the coherence parameter. The constants and exponents of the model are obtained by using a GA based on both experimental data of several compound flumes and measurements of the river Severn in the UK. Using several statistical measures, it is shown that the predictions of lateral velocity distribution and stage-discharge by the model agree well with the observed laboratory data and natural river measurements used for calibration and validation. Moreover, the model is shown to outperform the conventional vertical divided channel method with 11.2% less error on average in predicting the velocity distribution.