Variation of Resistance Coefficients
INTRODUCTION: Natural streams, rivers, and man-made surface drainage channels often overflow their banks during the occurrence of high rainfall thus, causing loss of life and extensive damage to nearby properties in many parts of the world. Accurate assessment of the discharge capacity of meandering channels are therefore essential in suggesting structures for flood control and in designing artificial waterways etc. A river is the author of its own geometry. It is an established fact that meandering represents a degree of adjustment of water and sediment laden river with its size, shape, and slope such that a flatter channel can exist in a steeper valley. Distribution of roughness coefficients in a channel section is an important aspect that needs to be addressed properly. A roughness value underestimates the discharge and a low value can overestimate. Sinuosity and slope have significant influences for the evaluation of channel discharge. Water that flows in a natural channel is a real fluid for which the action of viscosity and other forces cannot be ignored completely. Owing to the viscosity, the flow in a channel consumes more energy.
Major factors affecting Manning’s roughness coefficient are the: (i) surface roughness,(ii) Vegetation, (iii) channel irregularity, (iv) channel alignment, (v) silting and scouring, (vi) shape and the size of a channel, and (vii) stage-discharge relationship.
PREVIOUS WORK DONE: Visual estimation of n values can be made at each site using Barne’s (1976) guideline. Jarrett (1984) developed a model to determine Manning’s n for natural main channels having stable bed and in bank flow without meandering coefficient. James and Wark (1992) proposed a simple relationship for determination of Manning’s n, which is called LSCS method, considering different value of meandering effect in simple meandering channel. A summary of floodplain hydraulics is given by Knight and Shiono (1996). Jana & Panda et.al (2006) have performed dimensional analysis and predicted the stage-discharge relationship in simple meandering channels of low sinuosity. Pang (1998), Patra (1999), Patra and Kar (2000), Khatua and Patra (2006), Khatua (2007) have shown tha Manning’s n not only denotes the roughness characteristics of a channel but also the energy loss in the flow.
EXPERIMENTAL SETUP: The experiments are carried out using in the Fluid Mechanics and Hydraulics Laboratory of the Civil Engineering Department at the National Institute of Technology, Rourkela. Four types of flumes are cast with channels of varying sinuosity, geometry and slope. Out of this data 12 m long tilting flum having trapezoidal cross section with side slope 1:1, constructed of transparent perspex sheets with smooth walls, has been used for the present work. Details of the geometrical parameters of the channel are given in Table- 1. A hand-operated tailgate weir is constructed at the downstream end of the channel to regulate and maintain the desired depth of flow in the flume. Manning’s n, Chezy`s C and Darcy- Weisbach f are determined for the flume by measuring the depth and discharge. The bed slope is set by adjusting the whole structure, tilting it upwards or downwards with the help of a leaver, which is termed as slope changing leaver. Readings are taken for the different slopes.
MANNING’S RESISTANCE FACTORS FOR VARIOUS CHANNEL SURFACES: Suggested values for Manning's n are tabulated in Chow (1959), and Henderson (1966).Roughness characteristics of natural channels are given by Barnes (1967). Though there are large numbers of formulae/procedures available to calculate Manning’s n for a river reach, the following four methods are found to be more useful. Jarrett’s (1984) equation for high gradient channels
RESULTS AND DISCUSSION: Variation of Manning’s n with Depth of Flow for Simple Meandering Channel the experimental results for Manning’s n with depth of flow for in-bank flows of meander channels. Manning’s n is found to decrease with increase of aspect ratio (ratio of width of the channel to the depth of flow) indicating that simple meander channel consumes more energy as the depth of flow increases. For this reason, with increase of aspect ratio Manning’s n decreases. For different slopes also Manning’s n varies with aspect ratio. For narrow channels the decrease in the value of n with depth can be mainly due to the decrease of the resistance to flow and wider channels of type III the values of n increases with sinuosity and channel slope. the values of n increases with sinuosity and channel slope. Therefore it can be noted that steeper channels consume more energy than the milder/flatter channels. Again, for highly sinuous channels the values of n become large indicating that the energy loss is more for such channels.