# Total & Specific Energy

## Presentation on theme: "Total & Specific Energy"— Presentation transcript:

Lecture Notes 3: (Chapter 12) Energy principles in Open-Channel
Energy generated at an overfall (Niagara Falls). prepared by Ercan Kahya

Total & Specific Energy
Specific Energy: the energy per unit weight of water measured from the channel bottom as a datum ► Note that specific energy & total energy are not generally equally. At section 1: Specific Energy Total Energy

Total & Specific Energy
► Specific energy varies abruptly as does the channel geometry ► Velocity coefficient (α) is used to account nonuniformity of the velocity distribution when using average velocity. ► It varies from 1.05 (for uniform cross-sections) to 1.2 (nonuniform sections). ► For natural channels, a common method to estimate α: Weighted mean velocity: A channel section divided into three sections

Specific Energy Diagram (SED)
Assuming α equal to 1, it is convenient to express E in terms of Q for steady flow conditions f(E, Q, y) = 0 Specific Energy Diagram (SED) SED is a graphical representation for the variation of E with y. Let`s write E equation in terms of static & kinetic energy: where and

Specific Energy Diagram
Es varies linearly with y Ek varies nonlinearly with y Horizontal sum of the line OD & the curve kk` produces SED For given E: alternate depths (y1 & y2) They are two depths with the same specific energy and conveying the same discharge Emin vs critical depth The specific energy diagram

Specific Energy Diagram
An increase in the required Emin yields bigger discharges. Fn : Froude number equals to V square / gD The specific energy diagram for various discharges

Critical Flow Conditions
General mathematical formulation for critical flow conditions: Assume dA/dy = B

Critical Flow Conditions
At the critical flow conditions, specific energy is minimum: Then, which can also be expressed as --> Then, In wide or rectangular section, D = y at critical depth

Critical Velocity The general expressions for
Used to determine the state of flow Critical state condition: Critical velocity for the general cross section: Velocity head at critical conditions: In wide or rectangular section, D = y

Critical Depth For a certain section & given discharge:
Critical depth is defined as the depth of flow requiring minimum specific energy This equation should be solved … For the trapezoidal cross section: Solve this by trial & error … For the rectangular cross section: Critical depth trapezoidal and circular sections

Critical Energy Critical Energy is the energy when the flow is under critical conditions. Recall for any cross section: Then, For wide or rectangular section, D = y

Critical Slope Critical slope is the bed slope of the channel producing critical conditions. ► depends discharge; channel geometry; resistance or roughness For Chezy equation: Then, For Manning equation: In English unit: For direct computation:

Critical Slope Critical slope is very important in open-channel hydraulics. WHY? The summary given above encompasses much of the important concepts of the energy & resistance principles as applied to open channels.

Discharge-Depth Relation for Constant Specific Energy
Now assume Eo constant, then evaluate Q-y relation: For the condition of the Qmax: It reduces to Then substitute this into Q equation at the top: implies that the Qmax is encountered at the critical flow condition for given E.

Discharge-Depth Relation for Constant Specific Energy
Q-y relation for constant specific energy For wide or rectangular section, D = y can be written as Differentiating this w.r.t. y and equating to zero:

Transitions in Channel Beds
Consider an open-channel with a small drop ∆z in its bed Assume that friction losses and minor losses due to drop are negligible The method provides a good first approximation of the effects of the transition First step: compare the given conditions to critical conditions to determine the initial state of flow. A small drop in the channel bed (subcritical flow): (a) change in water levels, and (b) steps for solution.

Transitions in Channel Beds
Consider an abrupt rise ∆z in the open-channel bed Assume that upstream conditions are subcritical & initial E1 Note that ∆z should be subtracted from E1 & While TEL unchanged, E reduced

Transitions in Channel Beds
Consider an abrupt rise ∆z in the open-channel bed Assume that upstream conditions are supercritical & initial E1 Note that ∆z should be subtracted from E1 & While TEL unchanged, E reduced RESULT : Water depth must rise after the step

Chokes Chokes can only occur when the channel is constricted, but will not occur where the flow area expanded such as drops or expansions. In designing a channel transition that would tend to restrict the flow, engineer wants to avoid forcing a choke to occur if at all possible.

Chokes Figure 12.16: Rise in a channel bed: (a) a small step-up, (b) a bigger step-up

Chokes Figure 12.16: Rise in a channel bed: (c) a still bigger step-up, and (d) changes in the specific energy.

Enlargements and constructions in channel widths
(b) (c) (d) (a) A contracted channel. (b) Water levels in a contracted channel. (c) SED for a contracted channel. (d) Water level in a contracted channel-supercritical flow.

EXAMPLE A 6.0 m rectangular channel carries a discharge of 30 m3/s at a depth of 2.5m. Determine the constricted channel width that produces critical depth.

EXAMPLE: s o l u t i o n b2 = Q/ q2 = 30 / 7.56 = 3.07 m

Weirs & Spillways y2=0 To control the elevation of the water
Functions as a downstream choke control Classified as sharp crested or broad crested depending on critical depth occurrence on the crest y2=0 Head on the weir crest Orifice equation:

Weirs & Spillways Assume V1=0 Immediate region of weir crest
Discharge through the element: Integrate across the head (0 - H): Total discharge across the weir:

Coefficient of Discharge
Losses due to the advent of the drawdown of the flow immediately upstream of the weir as well as any other friction or contraction losses; To account for these losses, a coefficient of discharge Cd is introduced. (Henderson, 1966) where, H is the head on the weir crest, Z is the height of the weir. Use this equation up to H/Z = 2

Discharge Measurements
Weirs Flume Orifices Weirs and flumes not only require a simple head reading to measure discharge but they can also pass large flow without causing the upstream level to rise significantly and causing flooding. Discharge Control - Orifices are rather cumbersome for discharge measurements, but they are very useful for discharge control Practical Hydraulics by Melvyn Kay Copyright © 1998 by E & FN Spon . All rights reserved.

Discharge Control Practical Hydraulics by Melvyn Kay

WEIRS

WEIRS

FLUMES Practical Hydraulics by Melvyn Kay