Hydraulic Resistance of Emergent Macroroughness at Large Froude Numbers: Design of Nature-Like Fishpasses
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The mean flow in a nature-like fishpass can be highly modified by the Froude number. It is important to understand this evolution to correctly design the structure. The studied configuration is an emergent staggered arrangement of obstacles. The hydraulic resistance of a fishpass is experimentally investigated that depends on several geometric parameters: block shape, ramp slope, block density, and bed roughness. An analytical model based on the balance momentum allows one to quantify the influence of each hydraulic parameter. The bed roughness has a weak influence, whereas the block shape and the Froude number are significant. The variation of the drag coefficient was analyzed to improve the stage-discharge relationship. To this end, a correlation with the block diameter and water level is proposed. The maximal velocity reached in the fishpass can also be estimated. These results have to be compared with the fish swimming ability to assess the fishpass passability.Keywords:
Froude number
Hydraulic roughness
Hydraulic resistance
Hydraulics
Momentum (technical analysis)
Flow resistance
The hydraulic resistance of vegetation can play a major role in the hydrodynamics of vegetated streams or rivers with extensive natural floodplains. Contrary to commonly used wall roughness methods, vegetation penetrates the flow field causing drag and, subsequently, additional energy losses. Klopstra et al. (1997) derived an analytical solution of the flow velocity profile over and through a field of homogeneously distributed rigid cylinders. This solution has been adopted as a standard method to determine vegetation roughness in the Netherlands as described in the handbook for hydraulic resistance of vegetation in floodplains (Van Velzen et al., 2003). Although the analytical solution gives a good description of the flow over submerged cylinders some assumptions oversimplify the problem. In this paper these assumptions are evaluated and suggestions for improvement are made.
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The paper reports the results of laboratory experiments to investigate the effect of vegetation patch mosaics on hydraulic resistance. Experiments were run for seven levels of vegetation coverage with square patches of flexible plastic grass in aligned and staggered configurations and a wide range of hydraulic conditions. Hydraulic resistance was substantially higher for staggered than aligned configurations, particularly for intermediate ranges of vegetation coverage. The results indicate that hydraulic resistance differs between regimes of isolated roughness flow, wake interference flow, and skimming flow. Two types of models are proposed to predict hydraulic resistance (i.e. Manning's coefficient n) for aligned and staggered configurations, one as a function of the nondimensional spatially-averaged hydraulic radius and another as a function of relative submergence and surface area blockage factor. To account for the effects of vegetation patch alignment, an additional factor α is introduced. This work provides comprehensive datasets and models that can be used to improve the prediction of hydraulic resistance in open-channel flows with vegetation patches.
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In environmental flood management, an essential task is to improve channel conveyance using environmentally preferable methods, which aim to preserve natural morphological and hydraulic characteristics of a river. This requires a reliable channel design method that accounts for complex hydraulics, i.e. two-stage channel or considerable bank vegetation. Hydraulic field measurements were carried out in two rivers to find out how different factors affected flow resistance. In one of the study reaches, the effects of bioengineering on channel hydraulics were investigated under boreal climatic conditions. The Darcy-Weisbach friction factor, the Manning coefficient and the roughness height were related to the characteristics of channel geometry and flow. Comparison between the field data and the investigated channel design methods gave accurate results only in reaches having simple hydraulic properties. In reaches with complex hydraulics the results were poor.
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Preliminary results from the Flood Channel Facility sponsored by the British Science and Engineering Research Council and Hydraulics Research Ltd. are presented. The Facility takes the form of a large compound channel and the first series of experiments have been analysed to assess flow resistance characteristics of simple and compound channels having smooth boundaries. The effects of momentum transfer from main channel to flood plains on the capacity of the compound and main channel sections have been demonstrated. Flow resistance relationships for the compound section, main channel and flood plains have been presented in terms of Manning's and Darcy-Weisbach resistance coefficients, and compared with simple channel shapes. This has emphasised the complexity of compound channel flow resistance, while also pointing out the likely errors involved in applying simple channel methods of analysis and design to rivers and channels of compound geometry.
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A laboratory study on the hydraulics of flow in an open channel with circular cylindrical roughness is presented. The laboratory study consists of an extensive set of flume experiments for flows with emergent and submerged cylindrical stems of various sizes and concentrations. The results show that the flow resistance varies with flow depth, stem concentration, stem length, and stem diameter. The stem resistance experienced by the flow through the vegetation is best expressed in terms of the maximum depth-averaged velocity between the stems. Physically based formulas for flow resistance, the apparent channel velocity, and flow velocities in the roughness and surface layers are developed. The formulas are validated with the flume data from the present study as well as those from past studies. A method for calculating channel hydraulic conditions using these formulas is presented.
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Based on the observed data,the characteristics of hydraulic geometry forms of delta river channel in reservoirs,ripples and dunes evolving,flow resistance change,and water temperature influencing flow resistance are analyzed.Based on the past studying results,the formulas between the resistances of dunes,total resistances,resistances of icing period and intensity of stream flow are established.At last,by examining the observed data of river bed forms and resistances of a reservoir,these formulas are satisfactory.
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Gravel bed flow resistance is affected by the shape and size of the roughness elements and their arrangement on the channel bed surface (spacing between elements, direction with respect to flow streamlines, and protrusion of the elements from the channel bed). Previous studies demonstrated that the flow resistance of open channel flows can be obtained by integrating the power velocity profile. This paper aims to study flow resistance in gravel-bed channels with different concentrations of boulders having staggered arrangements. At first, the equation relating Γ coefficient of the power velocity profile, and the Froude number was calibrated using measurements performed in a flume covered by hemispheric roughness elements for partially submerged and completely submerged hydraulic conditions. The roughness elements were evenly spaced (staggered) and arranged using three different concentrations of 9, 25, and 49%. Moreover, the relationship between Γ, slope, and the Froude number, calibrated using literature measurements performed with the same experimental setup but with a square arrangement, was tested for the measurements obtained with the staggered arrangement. The results showed that i) the Darcy-Weisbach friction factor can be accurately estimated by the proposed flow resistance equation, ii) the differences in flow resistance behavior between the two different investigated arrangements (staggered, square) occur only for the partially submerged hydraulic condition, and iii) for the staggered arrangement, skimming flow is reached for lower element concentrations as compared to the square one.
Froude number
Flume
Hydraulic roughness
Flow resistance
Hydraulics
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Inflow
Power function
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The mean flow in a nature-like fishpass can be highly modified by the Froude number. It is important to understand this evolution to correctly design the structure. The studied configuration is an emergent staggered arrangement of obstacles. The hydraulic resistance of a fishpass is experimentally investigated that depends on several geometric parameters: block shape, ramp slope, block density, and bed roughness. An analytical model based on the balance momentum allows one to quantify the influence of each hydraulic parameter. The bed roughness has a weak influence, whereas the block shape and the Froude number are significant. The variation of the drag coefficient was analyzed to improve the stage-discharge relationship. To this end, a correlation with the block diameter and water level is proposed. The maximal velocity reached in the fishpass can also be estimated. These results have to be compared with the fish swimming ability to assess the fishpass passability.
Froude number
Hydraulic roughness
Hydraulic resistance
Hydraulics
Momentum (technical analysis)
Flow resistance
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Citations (30)