Donald W. Knight

Turbulent open-channel flows with variable depth across the channel

The flow of water in straight open channels with prismatic complex cross-sections is considered. Lateral distributions of depth-mean velocity and boundary shear stress are derived theoretically for channels of any shape, provided that the boundary geometry can be discretized into linear elements. The analytical model includes the effects of bed-generated turbulence, lateral shear turbulence and secondary flows. Experimental data from the Science and Engineering Research Council (SERC) Flood Channel Facility are used to illustrate the relative importance of these three effects on internal shear stresses. New experimental evidence concerning the spatial distribution of Reynolds stresses τ yx and τ zx is presented for the particular case of compound or two-stage channels. In such channels the vertical distributions of τ zx are shown to be highly nonlinear in the regions of strongest lateral shear and the depth-averaged values of τ yx are shown to be significantly different from the depth mean apparent shear stresses. The importance of secondary flows in the lateral shear layer region is therefore established. The influence of both Reynolds stresses and secondary flows on eddy viscosity values is quantified. A numerical study is undertaken of the lateral distributions of local friction factor and dimensionless eddy viscosity. The results of this study are then used in the analytical model to reproduce lateral distributions of depth-mean velocity and boundary shear stress in a two stage channel. The work will be of interest to engineers engaged in flood channel hydraulics and overbank flow in particular.

Turbulence measurements in a shear layer region of a compound channel

The paper describes some turbulence measurements carried out in the SERC Flood Channel Facility at Hydraulics Research Ltd., Wallingford, U.K. The facility represents a large scale model of a river system with floodplains, and is designed to produce fully developed boundary layer flows with transverse shear. This article presents some of the open channel flow data, including measurements of the primary velocity, Ū/u ∗, the distribution of turbulent intensities, u1/u∗ , v1/u∗ , and w1/u∗ , the kinetic energy, k/u2∗ , and the Reynolds stresses τzx and τyx in the region of strong lateral shear induced by transverse variation in depth. Attention is focussed on the non linear nature of the Reynolds stresses in the shear layer, flow structures and the lateral variations in eddy viscosity and local friction factor.

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.

Stage-discharge prediction for rivers in flood applying a depth-averaged model

The prediction of the stage-discharge relationship for rivers in flood is described by a finite element model of depth-averaged turbulent flow, calibrated using (hree hydraulic coefficients governing local bed friction, lateral eddy viscosity and depth-averaged secondary flow. The resulting lateral distributions of depth-averaged velocity are subsequently integrated to yield the stage-discharge relationship. The calibration of the model involves the establishment of simplifying hypotheses for certain coefficients in order to give the correct depth-mean velocity and boundary shear, both across the channel and with stage. Comparisons against some experimental data from the UK Flood Channel Facility, for channels with trapezoidal and compound cross-sections, help develop the calibration philosophy for both inbank and overbank flows. Numerical experiments with the coherence method for a hypothetical river are used to extend the model calibration to rivers with homogeneous and heterogeneous roughness. Applications of the model to simulating the flow in a number of natural valley and mountain rivers serve to test hypotheses and results obtained at a real scale.

Some field measurements concerned with the behaviour of resistance coefficients in a tidal channel

Measurements of the individual terms in the one-dimensional unsteady flow momentum equation in a 1.2 km tidal reach of the Conwy estuary, N. Wales, have yielded values for the resistance coefficients commonly adopted in open channel flow hydraulics. The results indicate clearly that Mannings n or Darcy-Weisbachs f vary significantly with stage, and to a lesser extent with flow direction. The relative importance of the various terms in the one-dimensional momentum equation are illustrated, and the techniques employed in measuring them indicated. Values of Nikuradses roughness parameter, k s , have also been computed and are shown to vary significantly with water level. Mean bed shear stresses have been evaluated, and the variation in bed shear velocity throughout different tidal cycles determined. The results indicate the inadvisability of selecting a single roughness parameter to represent the hydraulic characteristics of any individual element in a numerical tidal model.

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.

River basin modelling for flood risk mitigation

This book is broad in content, but integrated, and covers topics such as: a European perspective on flooding, climate change, rainfall and river flow forecasting systems, decision support systems, river flood hydraulics, sediment & dam-break modelling, risk & uncertainty, social issues and developments in flood forecasting and warning systems.

Energy losses due to secondary flow and turbulence in meandering channels with overbank flows

An investigation of energy losses due to boundary friction, secondary flow, turbulence, expansions and contractions in meandering compound channels with overbank flow is described.The compound meandering channel was divided into three sub-areas, namely the main channel below the bankfull level, the meander belt width above the bankfull level and a region outside the meander belt above the bankfull level, and turbulence data obtained by a Laser Doppler anemometer system.The energy loss due to the shear stress on the horizontal plane at the bankfull level was estimated using the measured Reynolds stresses and sectional averaged velocity, and the energy loss due to secondary flow below the bankfull level was then estimated.Both energy losses were found to make a significant contribution to the total energy loss in the lower layer for shallow overbank flow.The energy losses due to the contraction and expansion in the meander belt were evaluated and found to be a significant component of the total energy loss ...

Entropy-based design approach of threshold alluvial channels

A new approach has been developed for designing the shape and dimensions of the cross section of a straight threshold channel based on the concepts of entropy and probability. A formula for the lateral distribution of transverse slopes, and hence the cross-sectional bank profile, was derived by the entropy-maximisation principle and the calculus of variations. The bank profile is a new type of simple parabolic curve. The results of numerical experiments based on the approach of boundary shear stress distribution support the conclusion that the entropy-based channel bank profile is at threshold. Bank profile equations are coupled with an appropriate frictional relationship to obtain a new design method. Channel dimensions and bank profiles predicted by this method are compared with those given by other design methods. The predicted channel dimensions are in reasonable agreement with laboratory experimental data.

1-D modelling of conveyance, boundary shear and sediment transport in overbank flow

Several sets of experimental data on conveyance capacity, the division of flow between a main river channel and its floodplains, boundary shear, and sediment transport in overbank flow are presented. The strengths and weaknesses of various 1-D modelling approaches are highlighted. The coherence method (COHM) of Ackers [J. Hydraul. Res. 31 (1993) 509] and the weighted divided channel method (WDCM) of Lambert and Myers [Proc. Inst. Civil Engineers Water Maritime Energy 130 (1998) 84] are shown to be two useful 1-D methods for dealing with overbank flows. They are relatively simple to apply, and give not only the stage–discharge relationship, but also the division of flow between the main channel and the floodplains. Their general validity has been extended by testing the two methods against data sets other than those used in their original formulation.A modified version of the WDCM is presented that gives better predictions for homogeneously roughened channels. It is shown that the WDCM does not give good predictions for heterogeneously roughened compound channels. A comparison is made between the COHM and WDCM concerning their relative accuracy and ease of application for conveyance and boundary shear calculations. Finally, the COHM is applied to some mobile boundary compound channel data and shows that once the total and zonal channel discharges are calculated correctly, the sediment transport concentration may be predicted reasonably well for high discharges but not so well for low discharges and shallow floodplain depths, unless the boundary shear stress on the main channel bed is specified or calculated.