A note on critical flow section in collector channels

Generalized solution for the location of critical flow section in collector channels is presented. Based on the concept of the singularity, the dynamic equation of spatially varied flow (SVF) is solved using the flow resistance equations of von Karman (for rough regime) and Jain (for transitional and smooth regimes). The advantage of using Jain's equations is that they provide the explicit forms of the Colebrook-White and Nikuradse equations. Computational steps for the determination of critical flow section in a collector channel, being dependent on channel geometry, roughness, longitudinal bed slope and inflow discharge, are given for different channel shapes

Incipient motion of gravel and coal beds

An experimental study on incipient motion of gravel and coal beds under unidirectional steady-uniform flow is presented. Experiments were carried out in a flume with various sizes of gravel and coal samples. The critical bed shear stresses for the experimental runs determined using side-wall correction show considerable disagreement with the standard curves. The characteristic parameters affecting the incipient motion of particles in rough-turbulent regime, identified based on physical reasoning and dimensional analysis, are the Shields parameter, particle Froude number, non-dimensional particle diameter and non-dimensional flow depth. Equations of critical bed shear stress for the initial movement of gravel and coal beds were obtained using experimental data. The method of application of critical bed shear stress equations is also mentioned

Theory of free surface flow over rough seeping beds

A new theory is developed for the steady free surface flow over a horizontal rough bed with uniform upward seepage normal to the bed. The theory is based on the Reynolds averaged Navier-Stokes (RANS) equations applied to the flow domain that is divided into a fully turbulent outer layer and an inner layer (viscous sublayer plus buffer layer), which is a transition zone from viscous to turbulent regime. In the outer layer, the Reynolds stress far exceeds viscous shear stress, varying gradually with vertical distance. Near the free surface, the velocity gradient in vertical direction becomes lesser giving rise to wake flow. On the other hand, in the composite inner layer close to the bed, the viscous shear stress exists together with the turbulent stress. Thus, for the outer layer, a logarithmic law having modified coefficients from the traditional logarithmic law is obtained for the streamwise velocity, whereas for the inner layer, a fifth-degree polynomial including effective height of protrusions holds. The exact velocity expressions for inner and outer layer, which contain principal terms in addition to infinitesimally small terms, are in agreement with the experimental data obtained from laboratory measurements through an acoustic Doppler velocimeter. The experiments were run on two conditions of no seepage and a low upward seepage. Expressions for the Reynolds stress are also derived and computed for validation by the experimental data

A Reynolds Averaged Theory of Turbulent Shear Flows over Stable Sinusoidal Beds and Formation of Sand Waves

Source: ICHE Conference Archive - https://mdi-de.baw.de/icheArchiv

Hydraulics of free overfall in Δ-shaped channels

In this paper, two methods are presented to analyse the free overfall in Δ -shaped (equilateral triangle-shaped) channels. First, the flow upstream of a free overfall from smooth horizontal or mildly sloping Δ -shaped channels is analysed theoretically to determine the end-depth-ratio (EDR), applying the momentum equation based on the Boussinesq approximation. Second, an alternate method for analysing free overfall in Δ -shaped channels is also presented where the flow over a free overfall in a Δ -shaped channel is simulated by that over a sharp-crested weir to calculate the EDR. The method of estimation of discharge from the known end depth is also presented for both the methods. These approaches eliminate the need of an experimentally determined pressure coefficient. Experiments are conducted to verify the results obtained from the present methods. Comparisons of the computed and experimental results are satisfactory

Influence of decelerating flow on incipient motion of a gravel-bed stream

An experimental study on incipient motion of gravel-bed streams under steady-decelerating flow is presented. Experiments were carried out in a flume with two median grain sizes, d50 = 16.7 mm for a fixed-bed case and d50 = 8 mm for a mobile bed case. In addition, an effort is made to determine a simplified method for the estimation of bed shear stress in decelerating flow over fixed and mobile beds for use in field situations. From the observation of eleven fixed-bed and nine mobile-bed velocity profiles, it is revealed that the parabolic law method (PLM) and the Reynolds stress method are comparable for estimation of shear velocity in general. Also, the results show that the shear stress distribution adopts a convex form over fixed and mobile beds. Due to this form the critical Shields parameter value for decelerating flow is less than the reported values in literature. This paper supports Buffington & Montgomery (1997) statement that less emphasis should be given on choosing a universal shields parameter, and more emphasis should be given on choosing defendable values based on flow structure

The National Rural Employment Guarantee Scheme in Birbhum

This study of the functioning of the National Rural Employment Guarantee Scheme between February 2006 and July 2009 in Birbhum district, West Bengal reveals that in order to serve as an effective “employer of last resort”, the programme should provide proportionately more job-days during the agricultural lean season and wages should be paid in a timely manner

Evaluating India's national rural employment guarantee scheme

The world's biggest Employment Guarantee Programme, India's National Rural Employment Guarantee Scheme (NREGS) has been in operation in rural India since February 2006. In principle, the scheme is a self-targeted programme designed to provide 100 days of employment to rural households and to serve as a safety net. More broadly its aim is to reduce rural poverty through the creation of sustainable rural infrastructure which is expected to foster rural economic growth. This study looks at the performance of the NREGS from three perspectives - it examines the targeting aspect of the programme, the efficiency of the implementing PRI bodies and the impact of the program on various outcomes at household level. The study is based on primary data collected from 500 randomly selected households, 2249 individuals and 70 schemes located in 13 Gram Panchayats in Birbhum District of West Bengal, India. On the basis of this primary data, the study reveals that at least in Birbhum District the programme is far more likely to be accessed by poorer households (defined in terms of land holding, monthly per-capita income and other household related characteristics). At the same time there is a clear and substantial impact of left political inclination in terms of enabling access to a greater number of days of work under the scheme. In terms of the efficiency impact, the analysis reveals a clear violation of the formal clauses and the spirit of the NREG Act and thereby undermining the potential of the programme in terms of providing a safety net. In terms of the impact, the study finds no statistically significant impact on economic outcomes at household level but does find a statistically significant and substantial relation between reduction of stress related to joblessness and access to the NREGS. The estimates suggest that while the NREGS may not be creating any new employment, and may indeed be substituting for existing employment opportunities, the scheme is still considered valuable as it offers better working conditions