How do you calculate total head in ground water flow?

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Discussion Overview

The discussion revolves around the calculation of total head in groundwater flow, including the definitions and relationships between total head, pressure head, and pore water pressure. Participants explore theoretical concepts and practical implications related to these terms.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant expresses confusion about calculating total head and asks for clarification on the process, indicating they understand elevation but struggle with pressure head.
  • Another participant questions the definitions of total head, pressure head, and pore water pressure, suggesting a need for simpler explanations.
  • A different participant attempts to define head as stress or pressure multiplied by depth, proposing that total head represents total pressure.
  • One participant challenges the previous definition, suggesting that pressure is not necessarily related to depth and provides an example involving a "Pressure Casserol" to illustrate their point.
  • A participant provides a mathematical definition of head, including a formula that relates pressure, density, and elevation, and describes specific head values at different points in a hypothetical scenario.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the definitions and relationships between total head, pressure head, and pore water pressure. Multiple competing views and interpretations remain present throughout the discussion.

Contextual Notes

Some participants express uncertainty regarding terminology and definitions, particularly in relation to different cultural or educational backgrounds. There are also unresolved questions about the relationship between pressure and depth in various contexts.

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Its attached below. I don't understand how you work out the total head. Can someone explain how you do it? I'm really struggling.

I know how to do the elevation, which is just the height above/below the datum level. And I've checked the solutions as well, it seems like you're supposed to work out the total head before the pressure head.
 

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What are the "total head", "pressure head" and "pore water pressure"?
Can you explain that to you grand-mother?
 
maajdl said:
What are the "total head", "pressure head" and "pore water pressure"?
Can you explain that to you grand-mother?

'head' is simply stress (or pressure) mulitplied by depth

for total head is the total pressure as it were
 
Do you think your grand mother could understand?
Could you explain those things in a tangible way?
For example, by comparing what you measure if free water versus water in a soil?

In addition, I don't think that "pressure x depth" is related to the "head".
As I am from Belgium, I am not used with this terminology.
However, I looked at http://en.wikipedia.org/wiki/Pressure_head and it looks like that "pressure head" is the height of fluid that would produce by gravity the same pressure. In some way, it is the pressure expressed in meters of fluid.

In a "Pressure Casserol", the pressure is unrelated to the depth of water insided the "Pressure Casserol". Yet, it can be expressed in head of fluid, and the numerical value might be meters while the "Pressure Casserol" would only be 20 cm high.

See also this picture, where head is shown by the measuring tubes.

320px-Venturifixed2.PNG
 
Last edited:
The head is defined as H=\frac{p}{ρg}+(z-z_0), where z-z0 is the distance measured upward from the Ordinance Datum z0. So, at points P and C, the head H is 1 meter (there is no flow resistance between points P and C). At point A, the figure shows that the head is 6 meters above the Ordinance Datum. At point B, because the flow is steady, the head is the average of points A and C, or 3.5 meters. The upward seepage velocity is given by:

v=- K\frac{(H_A-H_C)}{(z_A-z_C)}

where K is the hydraulic conductivity (10-4 m/s).
 

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