Artifi cially straight river channels tend to be unstable, and ultimately develop into river meanders through bank erosion and point-bar deposition. In this paper account is taken of the effects of riparian and fl oodplain vegetation on bank strength, fl oodplain fl ow resistance, shear stress partitioning, and bedload transport. This is incorporated into an existing 2D hydrodynamic-morphological model. By applying the new model to an initially straight and single-threaded channel, the way that its planform and cross-sectional geometry evolve for different hydraulic and fl oodplain vegetation conditions is demonstrated. The results show the formation and upstream migration of gravel bars, confl uence scouring and the development of meandering and braiding channel patterns. In cases where the channel becomes unstable, the instability grows out of bar formation. The resulting braiding patterns are similar to analytical results. The formation of a transition confi guration requires a strong infl uence from vegetation.

Two-layer exchange flow through a contraction with both friction and barotropic forcing is modelled in terms of three parameters reflecting the friction and the strength and period of the barotropic forcing. In the appropriate limits, the results for steady flow with and without friction, and inviscid barotropically forced flow are recovered. The predicted time-dependent interface position compares well with laboratory experiments, improving on the inviscid formulation. The concurrent effects of friction and barotropic forcing on average exchange flow rate are determined. When friction is weak barotropic forcing increases the exchange rate. However, when friction is high, tidal forcing can result in a reduced exchange rate, a phenomena that we call tidal inhibition. When friction is weak maximal exchange occurs throughout the tidal cycle, but as friction is increased submaximal flow develops for longer and longer periods. As friction is increased even further the flow becomes hydraulically uncontrolled. The parameter range for major sea straits includes tidally enhanced and tidally inhibited flows, as well as maximal, submaximal and uncontrolled flows.

The management of ice-covered rivers presents special issues with respect to planning, design and operation of a water resources project. To address these issues properly entails a good understanding of the hydraulics of icecovered rivers, where the composite Manning's coefficient is a fundamental parameter. The aim of this paper is to obtain field estimates of the composite coefficient associated with the ice cover and riverbed. Using four different methods, this paper analyses a large volume of winter measurements of water velocities made from shallow icecovered rivers in Canada. The results show that the composite Manning's coefficient ranges from 0 . 013 to 0 . 040 as winter averages, but can vary up to seven-fold through the winter. The slope of the energy grade line is difficult to measure in the field and thus is rarely available; it appears to be about 30% of the water slope. The method for determining the coefficient based on the logarithmic velocity distribution fitted to measurements has shown advantages, as revealed by comparing the results of the four methods. The estimates of the composite Manning's coefficient reported in this paper are useful for modelling ice-covered river flow and determining winter discharges. This is particularly true when site-specific data are unavailable. Notation a slope of a semi-log velocity profile b intercept of a semi-log velocity profile (m/s) g gravity (m/s 2 ) J total number of subsections across the channel width j index for subsection k s average roughness height of the ice underside (m) n i ice Manning's coefficient corresponding to an individual velocity profile n c ice Manning's coefficient for the entire cross-section n b bed Manning's coefficient n o composite Manning's coefficient associated with the ice and riverbed P i wetted perimeter of an individual subsection (m) R hydraulic radius of the ice-affected layer (m) Re Reynolds number R h hydraulic radius of a river section (m) S slope of the energy grade line (m/m) s water slope (m/m) V cross-sectionally averaged velocity (m/s) v velocity at a given depth (m/s) v i depth-averaged velocity of the ice-affected layer (m/s) v m maximum velocity of a velocity profile (m/s) v* friction velocity (m/s) x cross-channel distance (m) Y local flow depth (m) y depth below the ice underside (m) y i thickness of the ice-affected layer (m) å ratio of the energy-grade-line slope to water slope k Karman constant í kinematic viscosity of water (m 2 /s)

A lack of reliable hydraulic parameters has been a main factor hindering the progress in predicting ice‐covered river flows; the predictions need input hydraulic parameters such as the energy, momentum and drag coefficients (α, β and CD). In this paper, a large volume of winter measurements of flow velocity collected from 26 ice‐covered rivers is analysed to determine the coefficients. Using cross‐sectionally distributed streamwise velocities, α and β are evaluated directly. They are also derived from empirical relationships. For both the riverbed and ice cover, CD is evaluated on the basis of turbulent boundary‐layer theory and the quadratic law for friction. The results show that ice‐covered river flows feature a number of velocity distributions: a single core of high velocities in the thalweg, a single core of high velocities off the thalweg and multiple cores of relatively high velocities at the cross section. The velocity distributions are significantly non‐uniform. Direct evaluations give overall averages of α = 1.23 and β = 1.08. They represent 22% and 8.3% corrections to the literature values (overestimates). An examination of the velocity distributions reveals that the ratio of the maximum velocity to the cross‐sectionally averaged velocity equals 1.356. It is recommended that values of CD = 0.004 ± 0.0005 and 0.002 ± 0.0005 be used for the riverbed and ice, respectively. This paper discusses turbulence shear stress and the associated length scale in the boundary layer as well as winter discharges. The results have applications to aquatic ecology, water resources development and flood prevention. Copyright © 2012 John Wiley & Sons, Ltd.

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