Specific Energy

# Specific Energy | Irrigation Engineering - Civil Engineering (CE) PDF Download

Total energy head at a section in an open channel flow is given by:
E = z + h + (v2/2g)
Where, z is the elevation of the bottom of the channel from an arbitrary datum. If we take the datum as the bottom of the channel itself, the equation becomes.
E = h + (v2/2g)
This represents the specific energy of the channel flow.
Thus the specific energy at any section of a channel is the sum of the depth of flow and the kinetic head at that section.
In the case of a steady uniform flow, depth of flow and velocity is same at all sections. Therefore specific energy head is same at all sections.

Considering a non-uniform flow taking place in an open channel of rectangular cross-section of width b, at a steady rate Q.
Discharge per unit width,
q = Q/b = Constant.
Velocity at the section,
v = Q/bh
= q/h
or q = vh
Thus the product of velocity and depth of flow is constant at all sections.
E = h + (v2/2g)

Where, K = q2/2g.

Thus, for a given value of q specific energy head is a function of depth as follows.
E = E1 + E2
Here, E1 = h
and
Where

If we plot static energy head, kinetic energy head and specific energy head against depth flow, the graph obtained is as follows:

From the above, it follows that specific energy head first becomes lesser and lesser when depth increases and reaches a minimum and then it increases as the depth is increased.

The depth corresponding to minimum specific energy is called the critical depth (hc). When the depth of flow is greater than the critical depth, the flow is called streaming flow or tranquil flow or sub-critical flow.

Critical flow: When the flow is such that specific energy is minimum, the flow is called a critical flow. Therefore at critical flow, depth of flow is equal to the critical depth (hc).
The velocity of flow at critical depth is called the critical velocity.

The document Specific Energy | Irrigation Engineering - Civil Engineering (CE) is a part of the Civil Engineering (CE) Course Irrigation Engineering.
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## FAQs on Specific Energy - Irrigation Engineering - Civil Engineering (CE)

 1. What is specific energy in civil engineering?
Ans. Specific energy in civil engineering refers to the energy required to excavate or remove a unit volume of material. It is typically measured in kilowatt-hours per cubic yard or megajoules per cubic meter. This parameter is important for determining the efficiency and cost of excavation operations.
 2. How is specific energy calculated in civil engineering?
Ans. Specific energy in civil engineering can be calculated by dividing the total energy consumed during excavation by the total volume of material removed. The formula is: Specific Energy = Total Energy / Total Volume The total energy can be obtained by measuring the fuel consumption or electrical energy used by the excavation equipment, while the total volume can be determined by measuring the volume of excavated material.
 3. What factors affect specific energy in civil engineering projects?
Ans. Several factors can influence the specific energy in civil engineering projects. These include the type and hardness of the material being excavated, the efficiency of the excavation equipment, the depth of excavation, the soil condition (e.g., dry or wet), and the skill level of the operators. Additionally, factors such as the haul distance and the presence of obstacles or irregularities in the excavation area can also impact the specific energy.
 4. How can specific energy be reduced in civil engineering projects?
Ans. Specific energy can be reduced in civil engineering projects by implementing various strategies. These include selecting the appropriate excavation equipment that is optimized for the specific material and conditions, maintaining the equipment in good working condition to ensure efficiency, using advanced technology such as GPS-guided excavation systems to minimize over-excavation, and optimizing the excavation sequence to minimize unnecessary movements. Additionally, proper operator training and supervision can also contribute to reducing specific energy.
 5. What are the advantages of considering specific energy in civil engineering projects?
Ans. Considering specific energy in civil engineering projects offers several advantages. Firstly, it allows project managers to evaluate the efficiency and cost-effectiveness of different excavation methods, helping them make informed decisions. Secondly, it helps in identifying areas where improvements can be made to reduce energy consumption and increase productivity. Thirdly, by optimizing specific energy, projects can reduce their environmental impact by minimizing fuel consumption and associated emissions. Overall, considering specific energy promotes sustainable and efficient excavation practices in civil engineering projects.

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