In flows around three-dimensional surface obstacles in laminar or turbulent streamsthere are a number of points where the shear stress or where two or more component,s of the mean velocity are zero. In the first part of this paper we summarize and extend the kinematical theory for the flow near these points, particularly by emphasizing the topological classification of these points as nodes or saddles. We show that the zero-shear-stress points on the surface and on the obstacle must be such that the sum of the nodes ΣNand the sum of the saddles Σssatisfy\[ \Sigma_N -\Sigma_S = 0. \]If the obstacle has a hole through it, such as a passageway under a building,\[ \Sigma_N -\Sigma_S =-2. \]If the surface is a junction between two pipes,\[ \Sigma_N -\Sigma_S =-1. \]We also consider, in two-dimensional plane sections of the flow, the points where the components of the mean velocity parallel to the planes are zero, both in the flow and near surfaces cutting the sections. The latter points are half-nodes N′ or half-saddles S′. We find that\[ (\Sigma_N +{\textstyle\frac{1}{2}}\Sigma_{N^{\prime}}-(\Sigma_{S^{\prime}}+{\textstyle\frac{1}{2}}\Sigma_{S^{\prime}}) = 1-n, \]where n is the connectivity of the section of the flow considered.In the second part new flow-visualization studies of laminar and turbulent flows around cuboids and axisymmetric humps (i.e. model hills) are reported. A new method of obtaining a high resolution of the surface shear-stress lines was used. These studies show how enumerating the nodes and saddle points acts as a check on the inferred flow pattern.Two specific conclusions drawn from these studies are that:for all the flows we observed, there are no closed surfaces of mean streamlines around the separated flows behind three-dimensional surface obstacles, which con-tradicts most of the previous suggestions for such flows (e.g. Halitsky 1968);the separation streamline on the centre-line of a three-dimensional bluff obstacle does not, in general, reattach to the surface.

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The Hapchon Dam, located in the headwaters of the Hwang River, Korea, was completed in 1988. Due to low storage levels, the spillway has not been operated over the past decade. Thus, a new ecosystem has become established in the river downstream of the dam. This is not a common phenomenon in Korean rivers. This study investigates the effect of flow regime change on the river morphology and vegetation cover in a reach downstream of the dam following its construction. Geomorphological effects shown by pre-and post-construction surveys of the channel include channel incision over a stream length of approximately 25 km immediately downstream of the dam. Analysis of pre-and post-construction aerial photography reveals significant encroachment of riparian vegetation onto previously active bar surfaces.

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