Open channel hydraulic
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Open Channel Hydraulic. Hydrology and Water Resources RG. Review of fluid mechanics. Fluid mechanics. Weight Mass Density Specific weight Specific gravity Hydrostatics Continuity equation Types of flow Energy and Energy Head Bernoulli’s Equation Flow through open channel.

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Open Channel Hydraulic

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Open channel hydraulic

Open Channel Hydraulic

Hydrology and Water Resources RG

Review of fluid mechanics

Review of fluid mechanics

Fluid mechanics

Fluid mechanics

  • Weight

  • Mass Density

  • Specific weight

  • Specific gravity

  • Hydrostatics

  • Continuity equation

  • Types of flow

  • Energy and Energy Head

  • Bernoulli’s Equation

  • Flow through open channel

Properties of a fluid

Properties of a Fluid

  • Weight

    W = mg(kN, lb)

    • m = mass of fluid (kg, slugs) g = acceleration due to gravity 9.81 m2/sec, 32.2 ft2/sec

  • Mass Density

    • mass of the fluid per unit volume at a standard temperature and pressure

      r = m/V (kg/m3, slugs/ft3)

    • V = volume of fluid (m3, ft3)

    • In the case of water, neglect the variation in mass density and consider it at a temperature of 4oC and at atmospheric pressure; then r= 1,000 kg/m3

Properties of a fluid1

Properties of a Fluid

  • Specific Weight

    • gravitational force per unit volume

    • Units: kN/m3, lb/ft3

    • In SI units, the specific weight of water at a standard reference temperature of 4oC and atmospheric pressure is 9.81 kN/m3

      g = W/V

  • Specific Gravity

    • ratio of the specific weight of a given liquid to the specific weight of pure water at a standard reference temperature

    • Units????

      Sg (fluid) = gfluid/ gwater

  • Specific Gravity of water = ?



  • A reservoir of glycerin has a mass of 1,200 kg and a volume of 0.925 m3. Calculate

    • Weight of the glycerin

    • Mass density of glycerin

    • Specific weight of glycerin

    • Specific gravity of glycerin

      g = 9.81 ft/sec2, gw = 9800 N/m3.

Open channel flow

Open channel flow



  • Open channel flow – any flow path with a free surface (open to atmosphere)

  • Can be classified as

    • Prismatic channel

      • With constant x-section and a constant bed slope

    • Non-prismatic

      • Varies in both the x-sectional shape and bed slope between any two selected points along the channel length

  • Atmospheric pressure acts continuously, constantly and at every location on water surface therefore is neglected

X section natural channel floodplain

X-section: natural channel & floodplain

Prismatic non prismatic channels

Prismatic & Non-prismatic Channels

X section for open channel flow

X-section for open channel flow

Open channel hydraulics

Open Channel Hydraulics

  • Variables of open channel flow analysis

  • Open channel flow classification based on various criteria

    • Time

    • Depth

    • Space

    • Regime (subcritical or supercritical)

Depth of flow

Depth of Flow

  • Elevation difference between water surface and deepest part of the channel

Channel top width wetted perimeter

Channel top width & wetted perimeter

Channel slope

Channel Slope

  • Difference in the channel invert elevation between two locations divided by the distance between them

  • In prismatic channel the slope is often constant over a significant channel distance

Hydraulic depth hydraulic radius

Hydraulic depth & hydraulic radius

  • Hydraulic depth: average depth across the channel

Discharge velocity

Discharge & Velocity

  • Discharge or flow rate: amount of water moving in a channel or stream system

  • Velocity: speed at which water moves in an open channel

    V = Q/A

    V= average channel velocity, Q= discharge, A = x-sec area

  • Water movement adds kinetic energy to the system

  • Channel velocity is not constant at any location

  • Varies both horizontally and vertically for any given channel cross-section

  • Velocity near the channel banks is less than the velocity in the center of the channel

Velocity profile in channel x sections

Velocity Profile in channel x-sections

Flow classification

Flow Classification

  • Uniform vs. non-uniform

  • Steady vs. unsteady flow

  • One-dimensional vs. multidimensional flows

  • Gradually varied vs. rapidly varied

  • Subcritical vs. supercritical

Types of flow

Types of Flow

  • Uniform Flow

    • in which the flow velocity and depth do not change from point to point along any of the streamlines otherwise it is called non-uniform or varied flow

  • Laminar Flow

    • in which each liquid particle has a definite path and the paths of individual particles do not cross each other

  • Turbulent Flow

    • if each particle does not have a definite path and the paths of individual particles also cross each other, the flow is called turbulent

Types of flow1

Types of Flow

  • Steady Flow

    • in which the depth and velocity at a point are constant with respect to time

  • Unsteady Flow

    • if Q is not constant

  • One-dimensional Flow

    • flow, whose streamlines may be represented by straight lines as opposed to curved lines

Subcritical supercritical flow

Subcritical & Supercritical Flow

  • Classification is based on ratio of inertial to gravitational forces at a stream location – Froude number

  • If Fr > 1 – flow is ‘supercritical’ and inertial forces dominate, associated with steeper slopes (high velocity and shallow depth)

  • If Fr < 1 – flow is ‘subcritical’ – gravitational forces dominate usually calm and tranquil –small slope usually in natural channels - (low velocity and high depth)

  • For Fr = 1 both depth and flow are call ‘critical’





  • What is energy?

    • Ability to do work?

  • Moving fluids possess energy by virtue of its

    • Velocity

    • Position

    • Pressure

Energy and head

Energy and Head

  • 3 kinds of energies that can be stored in a waterbody

    • Potential: due to elevation/position ‘Z’ (elevation above a fixed datum)

      PE = WZ= mgZ

    • Kinetic: due to velocity/motion

      KE = mv2 = (W/g) v2

    • Pressure: amount of work done in moving the fluid element a distance equals to the segment’s length ‘d’

      Force F = PA

      Work done (Pressure energy) = Fxd = PAd = P(Ad) = P(Volume) = PW/ g

Total energy

Total Energy

  • Total Energy = Potential + Kinetic + Pressure

    TE =WZ + (W/g)v2 + PW/ g

  • Energy may be expressed as ‘Head’

    • divide by ‘W’ throughout

    • Represents total energy per unit weight of the fluid

Energy head

Energy Head

  • Total Head

    H = Z + v2/g+ P/ g

  • Z = Elevation Head (units of length)

  • v2/g = Velocity Head (units of length)

  • P/ g = Pressure Head (units of length)

Velocity head at a cross section

Velocity head at a cross-section



  • Given:

    • Water in a 6 in diameter pipe with a velocity of 8 ft/s

    • Fluid pressure is 4 lb/in2

    • Elevation of the center of the pipe above datum is 10 ft

  • Required?

    • What is total energy head?

Bernoulli s equation

Bernoulli’s Equation

Bernoulli s equation conservation of energy

Bernoulli’s Equation – conservation of energy

  • During a steady flow of a frictionless incompressible fluid, the total energy (total head) remains constant along the flow path

    Z + v2/g+ P/ g = constant

    Z1 + v12/g+ P1/ g = Z2 + v22/g+ P2/ g

Continuity equation

Continuity equation

  • Based on the conservation of mass

  • Assumption: flowing fluids have constant mass density (incompressible liquid)

  • States that the quantity of liquid passing per time unit is the same at all sections

    Q1 = Q2 = Q3= ….

  • OR A1V1 = A2V2= A3V3= ….

  • Q = flow discharge [m3/s];

  • V = average velocity of the liquid [m/s];

  • A = area of the cross-section [m2];

  • and 1, 2, 3 = the number of sections 1-3

This is all about rg744 fall semester 2013

This is all about RG744 Fall semester 2013


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