{"id":165,"date":"2019-06-10T08:59:30","date_gmt":"2019-06-10T08:59:30","guid":{"rendered":"http:\/\/geop16.epgpbooks.inflibnet.ac.in\/?post_type=chapter&p=165"},"modified":"2019-06-10T09:03:24","modified_gmt":"2019-06-10T09:03:24","slug":"river-regimes-and-channel-flows","status":"publish","type":"chapter","link":"https:\/\/ebooks.inflibnet.ac.in\/geop16\/chapter\/river-regimes-and-channel-flows\/","title":{"rendered":"River Regimes and Channel Flows"},"content":{"raw":"
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\r\n\r\n\u00a0 \u00a0<\/strong>\r\n\r\n \r\n\r\nIntroduction<\/strong>\r\n\r\n \r\n

The Characteristic temporal pattern of rivers flow quantity and variability is called as River Regime. The flow of river varies on time scales of hours, days, seasons, years and longer.<\/p>\r\n\r\n<\/div>\r\n

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River regimes show geographical patterns, mainly determined by river size, climate, geology, topography and land use and land cover. The river regime is defined by five critical components viz. Magnitude, Frequency, Duration, Timing and Rate of Change (Poff and Ward, 1989; Richter et al., 1996; Walker et al., 1995). Magnitude of discharge is the amount of water moving past a fixed location per unit time. Magnitude can be referred either absolute or relative. Maximum and minimum flow magnitudes of flow vary with climate and watershed size. The frequency refers to how often a flow above a given magnitude recurs over some specified time interval. The Duration is the period of time associated with specific flow conditions (like flood). The timing of flows of defined magnitude refers to the regularity with which they occur. The rate of change or flashiness refers to how quickly flow changes from one magnitude to other.<\/p>\r\n \r\n\r\nClassification of River Regimes<\/strong>\r\n\r\n \r\n

There are three basic types of regimes (Pard\u00e9, 1955) viz. (i) Simple regime - one maximum and one minimum per year; (ii) Mixed regime\/double regime - two maximums and two minimums per year; and (iii) Complex mode - several extremes. The simple regimes can be further classified in nival, pluvial or glacial, depending on the origin of the water.<\/p>\r\n \r\n\r\nSimple river regimes<\/strong>\r\n\r\n \r\n\r\nGlacial regime<\/em><\/strong>\r\n\r\n \r\n

The glacial regime is characterised by very high discharge during summer season due to ice melt and very low discharge from the end of autumn to early spring. The amplitude of monthly variation of discharge is greater than 25. In glacial regime very high daily variability in discharge is found during the year. Mostly such regimes are found at high altitudes, above 2,500 metres (8,200 ft).<\/p>\r\n \r\n\r\nNival<\/em><\/strong>\r\n\r\n \r\n

Snowmelt-dominated (or nival) regimes occur in the interior plateau and mountain regions, and in higher-elevation zones of the Coast Mountains. In these zones, winter precipitation dominantly falls as snow and remains in storage until spring melt. As a result, these regimes exhibit low flows through winter, and high flows in May, June, and July. The nival regime is similar to the glacial, but attenuated and the maximum takes place earlier, in June. The plain nivalregimes have short and violent flood in April\u2013May following massive spring thawing ofwinter snows.<\/p>\r\n \r\n\r\nPluvial<\/em><\/strong>\r\n\r\n \r\n

The pluvial regime is characterized by high water in winter and spring along with low discharge in summer. There is a great inter-annual variability in discharge, flow is generally rather weak. It is typical of rivers at low to moderate altitude (500 to 1,000 metres or 1,600 to 3,300 feet).<\/p>\r\n \r\n\r\nTropical pluvial<\/em><\/strong>\r\n\r\n<\/div>\r\n

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The tropical pluvial regime is characterized by very low discharge in the cold season and abundant rainfall in the warm season and minimum can reach very low values. There is great variability of discharge during the year.<\/p>\r\n \r\n\r\nMixed regimes or double regime<\/strong>\r\n\r\n \r\n\r\nNivo-glacial<\/em><\/strong>\r\n\r\n \r\n

There is only one true maximum in the nivo-glacial regime, which occurs in the late spring or the early summer (from May to July in the case of the Northern hemisphere). Relatively high diurnal variations during the hot season is noticed with significant yearly variation, but less than in the snow regime.<\/p>\r\n \r\n\r\nNivo-pluvial<\/em><\/strong>\r\n\r\n \r\n

In this type of river regime two maximums occur, the first occurring in the spring and the other in autumn. A main low-water is observed in October and a secondary low-water is found in January. Significant inter-annual variations are found in the discharge.<\/p>\r\n \r\n\r\nPluvio-nival<\/em><\/strong>\r\n\r\n \r\n

The river regime has a period of rainfall in late autumn due to abundant rainfall, followed by a light increase due to snow melt in early spring. The single minimum occurs in autumn and the amplitude is low.<\/p>\r\n \r\n\r\nComplex regimes<\/strong>\r\n\r\n \r\n

The complex regime is characteristic of large rivers, the flow of which is diverly influenced by numerous tributaries from different altitudes, climates etc. The influences diminish extreme discharges and increase the regularity of the mean monthly discharge from upstream to downstream.<\/p>\r\n \r\n\r\nHydrologic Processes and the Flow Regime<\/strong>\r\n\r\n \r\n

All river flow derives ultimately from precipitation, but in any given time and place a river's flow is derived from some combination of surface water, soil water, and ground-water. Climate, geology, topography, soils, and vegetation help to determine both the supply of water and the pathways by which precipitation reaches the channel. The different water movement pathways in different settings have different flow regimes and that is why flow is variable in virtually all rivers. Collectively, over-land and shallow subsurface flow pathways create hydrograph peaks, which are the river's response to storm events. By contrast, deeper groundwater pathways are responsible for baseflow, the form of delivery during periods of little rainfall. Variability in intensity, timing, and duration of precipitation (as rain or as snow) and in the effects of terrain, soil texture, and plant evapo-transpiration on the hydrologic cycle combine to create local and regional flow patterns(Figure 1).<\/p>\r\n \r\n

The natural flow regime organizes and defines river ecosystems. In rivers, the physical structure of the environment and, thus, of the habitat, is defined largely by physical pro-cesses, especially the movement of water and sediment within the channel and between the\u00a0channel and flood-plain. To understand the biodiversity, production, and sustainability of river ecosystems, it is necessary to appreciate the central organizing role played by a dynamically varying physical environment. The physical habitat of a river includes sediment size and heterogeneity, channel and floodplain morphology, and other geomorphic features. These features form as the available sediment, woody debris, and other transportable materials are moved and deposited by flow.<\/span><\/p>\r\n\r\n<\/div>\r\n \r\n\r\n\"\"\r\n

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Figure 1: Stream valley cross section depicting various basic principles about natural pathway of water<\/strong><\/p>\r\n \r\n\r\nChannel Flow<\/strong>\r\n\r\n \r\n

Open-channel flowsare those that are not entirely included within rigid boundaries; a part of the flow is in contract with nothing at all in other words the flow in an open channel or in a closed conduit having a free surface is referred to as free-surface flow or open-channel flow. Water travels downhill from points of higher energy to points of lower energy (unless forced to do otherwise) until it reaches a point of equilibrium, such as an ocean. This tendency is facilitated by the presence of natural conveyance channels such as brooks, streams, and rivers. The water\u2019s journey may also be aided by man-made structures such as drainage swales, pipes, culverts, and canals. Hydraulic concepts can be applied equally to both man-made structures and natural features. The open channel flow has following parameters to define its geometry.\"\"<\/p>\r\nArea, Wetted Perimeter, and Hydraulic Radius<\/em><\/strong>\r\n\r\n \r\n

The term area refers to the cross-sectional area of flow within a channel. When a channel has a consistent cross-sectional shape, slope, and roughness, it is called a prismatic channel. If the flow in a conveyance section is open to the atmosphere, such as in a culvert flowing partially full or in a river, it is said to be open-channel flow or free-surface flow. If a channel is flowing completely full, as with a water distribution pipe, it is said to be operating under full-flow conditions. Pressure flow is a special type of full flow in which forces on the fluid cause it to push against the top of the channel as well as the bottom and sides. These forces may result from, for example, the weight of a column of water in a backed-up sewer manhole or elevated storage tank. A section\u2019s wetted perimeter is defined as the portion of the channel in contact with the flowing fluid. The geometry of an open channel flow is defined as follows:<\/p>\r\n\"\"\r\n\r\nOpen Channel Geometry<\/em><\/strong>\r\n\r\n \r\n\r\nDepth of flow (y) -Vertical distance from the bottom to surface\r\n\r\n \r\n\r\nTop width(B) \u2013 The width of the channel at the free surface\r\n\r\n \r\n\r\nFlow area(A) \u2013 Cross-sectional area of the flow\r\n\r\n \r\n\r\n\"\"\r\n

\r\n\r\nSteady and Unsteady Flows<\/em><\/strong>\r\n\r\n \r\n

If the flow velocity at a given point does not change with respect to time, then the flow is called steady flow. However, if the velocity at a given location changes with respect to time, then the flow is called unsteady flow. It is possible in some situations to transform unsteady flow into steady flow by having coordinates with respect to a moving reference. Typical example of such a situation is the movement of a flood wave in a natural channel, where the shape of the wave is modified as it propagates in the channel.<\/p>\r\n \r\n\r\nUniform and Non-uniform flows<\/em><\/strong>\r\n\r\n \r\n

If the flow velocity at a given instant of time does not vary within a given length of channel, then the flow is called uniform flow. However, if the flow velocity at a time varies with respect to distance, then the flow is called nonuniform flow, or varied flow. Depending upon the rate of variation with respect to distance, flows may be classified as gradually varied flow or rapidly varied flow.<\/p>\r\n \r\n\r\nLaminar and Turbulent Flows<\/em><\/strong>\r\n\r\n \r\n

The flow is called laminar flow if the liquid particles appear to move in definite smooth paths and the flow appears to be as a movement of thin layers on top of each other. In turbulent flow, the liquid particles move in irregular paths which are not fixed with respect to either time or space.<\/p>\r\n\"\"\r\n\r\n \r\n

The relative magnitude of viscous and inertial forces determines whether the flow is laminar or turbulent: The flow is laminar if the viscous forces dominate, and the flow is turbulent if the inertial forces dominate. The ratio of viscous and inertial forces is defined as the Reynolds number,\"\"<\/p>\r\n \r\n

in which Re = Reynolds number; V = mean flow velocity; L = a characteristic length; and \u03bd = kinematic viscosity of the liquid. Hydraulic radius may be used as the characteristic length in freesurface flows. Hydraulic depth is defined as the flow area divided by the top water-surface width and the hydraulic radius is defined as the flow area divided by the wetted perimeter.<\/p>\r\n \r\n\r\nSubcritical, Supercritical, and Critical Flows<\/em><\/strong>\r\n\r\n \r\n

A flow is called critical if the flow velocity is equal to the velocity of a gravity wave having small amplitude. A gravity wave may be produced by a change in the flow depth. The flow is called subcritical flow, if the flow velocity is less than the critical velocity, and the flow is called supercritical flow if the flow velocity is greater than the critical velocity. The Froude number, Fr, is equal to the ratio of inertial and gravitational forces and, for a rectangular channel, it is defined as:<\/p>\r\n \r\n\r\nFr = V \u221agy\r\n\r\n \r\n\r\nwhere, y = flow depth.\r\n\r\n \r\n

Depending upon the value of Fr, flow is classified as subcritical if Fr< 1; critical if Fr = 1; and supercritical if Fr> 1.<\/p>\r\n \r\n\r\nPressure Distribution<\/strong>\r\n\r\n \r\n

The pressure distribution in a channel section depends upon the flow conditions. Let us consider several possible cases, starting with the simplest one and then proceeding progressively to more complex situations.<\/p>\r\n\r\n

\r\n\r\n\u00a0 \u00a0 <\/em><\/strong>\r\n\r\nStatic Conditions<\/em><\/strong>\r\n\r\n \r\n

Let us consider a column of liquid having cross-sectional area \u0394A. The horizontal and vertical components of the resultant force acting on the liquid column are zero, since the liquid is stationary. If p = pressure intensity at the bottom of the liquid column, then the force due to pressure at the bottom of the column acting vertically upwards = p\u0394A. The weight of the liquid column acting vertically downwards = \u03c1gy\u0394A. Since the vertical component of the resultant force is zero, we can write p\u0394A = \u03c1gy\u0394A or p = \u03c1gy. In other words, the pressure intensity is directly proportional to the depth below the free surface. Since \u03c1 is constant for typical engineering applications, the relationship between the pressure intensity and depth plots as a straight line, and the liquid rises to the level of the free surface in a piezometer. The linear relationship, based on the assumption that \u03c1 is constant, is usually valid except at very large depths, where large pressures result in increased density.<\/p>\r\n \r\n\r\nHorizontal Parallel Flow<\/em><\/strong>\r\n\r\n \r\n

Let us now consider the forces acting on a vertical column of liquid flowing in a horizontal, frictionless channel. Let us assume that there is no acceleration in the direction of flow and the flow velocity is parallel to the channel bottom and is uniform over the channel section. Thus the streamlines are parallel to the channel bottom. Since there is no acceleration in the direction of flow, the component of the resultant force in this direction is zero. Referring to the free-body diagram and noting that the vertical component of the resultant force acting on the column of liquid is zero, we may write \u03c1gy\u0394A = p\u0394A or p = \u03c1gy = \u03b3y in which \u03b3 = \u03c1g = specific weight of the liquid. Note that this pressure distribution is the same as if the liquid were stationary; it is, therefore, referred to as the hydrostatic pressure distribution.<\/p>\r\n \r\n\r\nParallel Flow in Sloping Channels<\/em><\/strong>\r\n\r\n \r\n

Let us now consider the flow conditions in a sloping channel such that there is no acceleration in the flow direction, the flow velocity is uniform at a channel cross section and is parallel to the channel bottom; i.e., the streamlines are parallel to the channel bottom. The cross-sectional area of the column is \u0394A. If \u03b8 = slope of the channel bottom, then the component of the weight of column acting along the column is \u03c1gd\u0394Acos \u03b8 and the force acting at the bottom of the column is p\u0394A. There is no acceleration in a direction along the column length, since the flow velocity is parallel to the channel bottom. Hence, we can write p\u0394A = \u03c1gd\u0394Acos \u03b8, or p = \u03c1gdcos \u03b8 = \u03b3dcos \u03b8. By substituting d = y cos \u03b8 into this equation (y = flow depth measured vertically), we obtain p = \u03b3y cos2 \u03b8. Note that in this case the pressure distribution is not hydrostatic in spite of the fact that we have parallel flow and there is no acceleration in the direction of flow. However, if the slope of the channel bottom is small, then cos \u03b8 1 and d y.Hence, p \u03c1gd\u03c1gy. In several derivations we assume that the slope of the channel bottom is small. With this assumption, the pressure distribution may be assumed to be hydrostatic if the streamlines are almost parallel and straight, and the flow depths measured vertically or normal to the channel bottom are approximately the same.<\/p>\r\n \r\n\r\nCurvilinear Flow<\/em><\/strong>\r\n\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n

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In the previous three cases, the streamlines were straight and parallel to the channel bottom. However, in several real-life situations, the streamlines have pronounced curvature. To determine the pressure distribution in such flows, let us consider the forces acting in the vertical direction on a column of liquid with cross-sectional area \u0394A.<\/p>\r\n \r\n

If r = radius of curvature of the streamline and V is the flow velocity at the point under consideration, then Centrifugal acceleration = V 2 r and Centrifugal force = \u03c1ys\u0394AV 2 r. Dividing the centrifugal force by the area of the column and converting the pressure to pressure head, we obtain the following expression for the pressure head, ya, acting at the bottom of the liquid column due to centrifugal acceleration ya = 1 g ys V 2 r. The pressure due to centrifugal force is in the same direction as the weight of column if the curvature is concave, and it is in a direction opposite to the weight if the curvature is convex. Therefore, the total pressure head acting at the bottom of the column is an algebraic sum of the pressure due to centrifugal action and the weight of the liquid column, i.e., Total pressure head = ys(1 \u00b1\u00a0 1 g V 2 r ). A positive sign is used if the streamline is concave, and a negative sign is used if the streamline is convex. Thus, the liquid in a piezometer inserted into the flow rises. In other words, pressure increases due to centrifugal action in concave flows and decreases in convex flows. Boussinesq derived a formula for solving problems with small water surface curvatures.<\/p>\r\n \r\n\r\nThe Energy Principle<\/strong>\r\n\r\n \r\n

The first law of thermodynamics states that for any given system, the change in energy (\u2206E) is equal to the difference between the heat transferred to the system (Q) and the work done by the system on its surroundings (W) during a given time interval.<\/p>\r\n \r\n

The energy referred to in this principle represents the total energy of the system, which is the sum of the potential energy, kinetic energy, and internal (molecular) forms of energy such as electrical and chemical energy. Although internal energy may be significant for thermodynamic analyses, it is commonly neglected in hydraulic analyses because of its relatively small magnitude.<\/p>\r\n \r\n

In hydraulic applications, energy values are often converted into units of energy per unit weight, resulting in units of length. Using these length equivalents gives engineers a better \u201cfeel\u201d for the resulting behavior of the system. When using these length equivalents, the engineer is expressing the energy of the system in terms of \u201chead.\u201d The energy at any point within a hydraulic system is often expressed in three parts:<\/p>\r\n\r\n<\/div>\r\n

\r\n\r\n\u00a0 \u00a0Pressure head\r\n\r\n \r\n\r\nElevation head\r\n\r\n \r\n\r\nVelocity head\r\n\r\np\/\u03b3\r\n\r\n \r\n\r\nz\r\n\r\n \r\n\r\nV2\/2g\r\n\r\n<\/div>\r\n
\r\n\r\n\u00a0 \u00a0 where\u00a0 p = pressure (N\/m2, lbs\/ft 2 )\r\n\r\n \r\n\r\n\u03b3\u00a0 = specific weight (N\/m3 ,lbs\/ft3 ) z = elevation (m, ft)\r\n\r\n<\/div>\r\n

\u00a0 \u00a0 \u00a0V = velocity (m\/s, ft\/s)<\/p>\r\n \r\n\r\nThe Energy Equation<\/strong>\r\n\r\n \r\n

In addition to pressure head, elevation head, and velocity head, energy may be added to a system by a pump, and removed from the system by friction or other disturbances. These changes in energy are referred to as head gains and head losses, respectively. Because energy is conserved, the energy across any two points in the system must balance. This concept is demonstrated by the energy equation:\"\"<\/p>\r\n

where\u00a0 p = pressure (N\/m2 , lb\/ft 2 )<\/p>\r\n \r\n

\u03b3 = specific weight of the fluid (N\/m3, lb\/ft3 ) z = elevation above a datum (m, ft)<\/p>\r\n \r\n

V = fluid velocity (m\/s, ft\/s)<\/p>\r\n \r\n

g = gravitational acceleration (m\/s 2, ft\/s2 ) HG = head gain, such as from a pump (m, ft) HL = combined head loss (m, ft).<\/p>\r\n \r\n\r\nHydraulic Grade<\/strong>\r\n\r\n \r\n

The hydraulic grade is the sum of the pressure head (p\/\u03b3) and elevation head (z). For open channel flow (in which the pressure head is zero), the hydraulic grade elevation is the same as the water surface elevation. For a pressure pipe, the hydraulic grade represents the height to which a water column would rise in a piezometer (a tube open to the atmosphere rising from the pipe). When the hydraulic grade is plotted as a profile along the length of the conveyance section, it is referred to as the hydraulic grade line, or HGL.<\/p>\r\n \r\n\r\nEnergy Grade<\/strong>\r\n\r\n \r\n

The energy grade is the sum of the hydraulic grade and the velocity head (V2 \/2g). This grade is the height to which a column of water would rise in a Pitot tube (an apparatus similar to a piezometer, but also accounting for fluid velocity). When plotted in profile, this parameter is often referred to as the energy grade line, or EGL. For a lake or reservoir in which the velocity is essentially zero, the EGL is equal to the HGL.<\/p>\r\n \r\n\r\nEnergy Losses and Gains<\/strong>\r\n\r\n \r\n

Energy (or head) losses (HL) in a system are due to a combination of several factors. The primary cause of energy loss is usually the internal friction between fluid particles traveling at different velocities. Secondary causes of energy loss are localized areas of increased turbulence and disruption of the streamlines, such as disruptions from valves and other fittings in a pressure pipe, or disruptions from a changing section shape in a river. The rate at which energy is lost along a given length of channel is called the friction slope, and is usually presented as a unitless value or in units of length per length (ft\/ft, m\/m, etc.). Energy is generally added to a system with a device such as a pump.<\/p>\r\n \r\n\r\nSummary<\/strong>\r\n\r\n \r\n

In this chapter, commonly used terms for river regimes and channel flow were defined, classification of flows using several different criteria was outlined, and the properties of a channel section were presented. The distribution of velocity and pressure in a channel section was discussed. The distribution of pressure was discussed and a brief description of the energy principle and energy flow was presented to facilitate its application for further references.<\/p>\r\n \r\n\r\n\r\n\r\n
you can view video on River Regimes and Channel Flows<\/strong><\/td>\r\n\"\"<\/a><\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n \r\n
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References<\/h3>\r\n
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