[1] A new model for the depth-averaged velocity for flow in presence of submerged vegetation is developed. The model is based on a two-layer approach, where flow above and through the vegetation layer is described separately. Vegetation is treated as a homogeneous field of identical cylindrical stems, and the flow field is considered stationary and uniform. It is demonstrated that scaling considerations of the bulk flow field can be used to avoid complications associated with smaller scale flow processes and that still the behavior of depth-averaged flow over vegetation is described accurately. The derived scaling expression of the average flow field is simple in form, it follows fundamental laws of fluid flow, and it shows very good agreement with laboratory flume experiments. The new model can be used for quick evaluation of a river's hydraulic response in cases where vegetated floodplains are inundated.

Abstract. High-resolution IRAS maps are used to search for the presence of stellar-wind bow-shocks around high-mass X-ray binaries (HMXBs). Their high space velocities, recently confirmed with Hipparcos observations, combined with their strong stellar winds should result in the formation of wind bow-shocks. Except for the already known bow-shock around Vela X-1 (Kaper et al. 1997), we do not find convincing evidence for a bow-shock around any of the other HMXBs. Also in the case of (supposedly single) OB-runaway stars, only a minority appears to be associated with a bow-shock (Van Buren et al. 1995). We investigate why wind bow-shocks are not detected for the majority of these OB-runaway systems: is this due to the IRAS sensitivity, the system's space velocity, the stellar-wind properties, or the height above the galactic plane? It turns out that none of these suggested causes can explain the low detection rate (∼40%). We propose that the conditions of the interstellar medium mainly determine whether a wind bow-shock is formed or not. In hot, tenuous media (like inside galactic superbubbles) the sound speed is high (∼100 km s −1 ), such that many runaways move at subsonic velocity through a low-density medium, thus preventing the formation of an observable bow-shock. Superbubbles are expected (and observed) around OB associations, where the OB-runaway stars were once born. Turning the argument around, we use the absence (or presence) of wind bow-shocks around OB runaways to probe the physical conditions of the interstellar medium in the solar neighbourhood.

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A. Wesselink et al. / Trends in flood risk management in deltasFlood-risk management (FRM) is shaped by context: a society's cultural background; physical possibilities and constraints; and the historical development of that society's economy, political system, education, etc. These provide different drivers for change, in interaction with more global developments. We compare historical and current FRM in six delta areas and their contexts: Rhine/Meuse/Scheldt (The Netherlands), Pearl River (China), Mekong (Vietnam), Ganges/ Brahmaputra/Meghna (Bangladesh) Zambezi/Limpopo (Mozambique), and Mississippi (USA). We show that in many countries the emphasis is shifting from 'hard' engineering, such as dikes, towards non-structural 'soft' measures, such as planning restrictions or early warning systems, while the 'hard' responses are softened in some by a 'building with nature' approach. However, this is by no means a universal development. One consistent feature of the application of 'hard' FRM technology to deltas is that it pushes them towards a technological 'lock-in' in which fewer and fewer 'soft' FRM alternatives are feasible due to increased flood risks. By contrast, 'soft' FRM is typically flexible, allowing a range of future options, including future hard elements if needed and appropriate. These experiences should lead to serious reflection on whether 'hard' FRM should be recommended when 'soft' FRM options are still open.