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A Flow net is a graphical representation of flow of water through a soil mass. It is a curvilinear net formed by the combination of flow lines and equipotential lines. Properties and application of flow net are explained in this article.
Flow lines represent the path of flow along which the water will seep through the soil. Equipotential lines are formed by connecting the points of equal total head.
Properties of Flow Net
Properties of flow net are as follows:
- The angle of intersection between each flow line and an equipotential line must be 90o which means they should be orthogonal to each other.
- Two flow lines or two equipotential lines can never cross each other.
Equalquantity of seepage occurs in each flow channel. A flow channel is a space between two flow lines.
- Head loss is the same between two adjacent potential lines.
- Flow nets are drawn based on the boundary conditions only. They are independent of the permeability of
soiland the head causing flow.
- The space formed between two flow lines and two equipotential lines is called a flow field. It should be in a square form.
- Either flow lines or equipotential lines are smoothly drawn curves.
Applications of Flow Net
Flow net is useful to determine the following parameters in seepage analysis of soil :
- Rate of Seepage loss
- Seepage Pressure
- Uplift Pressure
- Exit Gradient
1. Rate of Seepage loss (Q)
Using flow net, the rate of seepage loss or seepage quantity can be determined using the below expression :
k = coefficient of permeability
H = Head causing flow
Nf = Number of flow lines
Nd = Number of Equipotential lines
2. Seepage Pressure (Ps)
Seepage pressure at any point is determined by using the below mentioned formula :
h = Hydraulic potential after “n” potential drops. It can be expressed as :
3. Uplift Pressure (Pu)
The uplift pressure at any point within the soil mass can be found using the undermentioned formula. It is also called as hydrostatic pressure.
4. Exit Gradient (iexit)
The exit gradient is the hydraulic gradient at the downstream end of flow line where seepage water from the soil mass joins with free water at the downstream. Exit gradient can be expressed as :