We decided to study silt scouring because power station cooling water may be drawn into the intake through an open channel or through a tunnel; a channel is cheaper to build but may suffer from siltation and so may need dredging, which can be very costly. Clearly, a better understanding of the siltation process could lead to significant savings in the cost of future stations. 43.A substantial part of our investigation is described in the Paper and I will only mention briefly the reconstitution tests described in 4-16. In addition some work was done on the measurement of shear strength with a vane shear meter and on the effect of hydrostatic pressure on the compaction of muds, and a paper on the sedimentation of muds was published in the Proceedings of this Institution in 1966.a Dr Peirce will describe aspects of this work and some recent calculations on the shear force needed to scour muds from stable estuary inlets.44. Three problems deserve further attention: the erosion of mud by water waves, the effect of bed slope on erosion rates, and the natural hydrodynamic roughness of the beds of rivers, estuaries and coastal waters. I hope that contributors will mention some progress on these problems. Dr PeirceThe described studies have shown that the measured fluid shears necessary for sustained erosion were in the range 16-50dyn/cm2 and, exceptionally, up to about 160 dyn/cma. In order to demonstrate that these values are realistic, reference is made to the work of Bruun and Gerritsen.6 These authors examined data for a number of stable tidal inlets and showed a relationship between measured values of maximum tidal flow, Qm (spring tide conditions) and the corresponding channel cross-sectional area, A . The relationship was of the form:where p is the fluid density and g the acceleration due to gravity. C is a roughness coefficient given by the equation: C = 30+5 log A ( A has the dimensions of m2; C, mlla S-l). (2) T. is defined as the stability shear stress which may be considered as the maximum shear applied under spring tide conditions and that necessary to remove previously deposited material, i.e. to maintain channel stability. From the measurements, the deduced average value of T~ was approximately 38 dyn/cma. This level of 7. is similar to those measured in the current work.46. The relevance of spring tide conditions to channel stability was further demonstrated by studies of the erosion/deposition characteristics of Portishead mud on the estuary foreshore. Measurements were made of the variation of mud level with time and it was established that there was a cycle of erosion and deposition with maximum erosion occurring under spring tide conditions. 47. Measurements were made of the sedimentation properties of a range of estuarine muds and the results have been described fully in an earlier paper.a In particular, it was shown that the muds existed as fiocs, containing groups of solid uarticles with occluded water. From an analysis of the variation of sedimentation
An experimental laboratory study of the deposition of droplets on dead house-flies (Musca domestica L.) was made, using a spinning-top sprayer to produce a spray of uniformly sized oil droplets and a cascade impactor to measure the concentration of the spray of droplets, which were dyed. The deposits obtained on a dead house-fly and a cascade-impactor slide when these were exposed in turn to a wind of 1 m. per sec. in a wind tunnel were compared colorimetrically, and determinations thus made of the collection efficiency of the flies, defined as the volume of liquid deposited on an object expressed as a percentage of the volume that would have passed through the same cross-section as the object had that not been there.The measured collecting efficiency of a fly varied from about 70 per cent. (droplet dia. 27μ) to about 200 per cent. (droplet dia. 75μ), and was approximately twice that of a sphere with a cross-sectional area twice the projected frontal area of the fly. From theoretical calculations of the filtering effect of different elements of the vegetation, it is concluded that the optimum droplet diameter for deposition on flies in woodland is 20–40μ.
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