Flow Net for SY civil Engineering students

flow net

1. FLOW NET

2. Flow net 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.

3. Properties of Flow Net The angle of intersection between each flow line and an equipotential line must be 90 o which means they should be orthogonal to each othe r. Two flow lines or two equipotential lines can never cross each other. Equal quantity 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 soil and 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

4. Applications of Flow Net Rate of Seepage loss Seepage Pressure Uplift Pressure Exit Gradient

5. 1. Rate of Seepage loss (Q) Using flow net, the rate of seepage loss or seepage quantity can be determined using the below expression : Where, k = coefficient of permeability H = Head causing flow Nf = Number of flow lines Nd = Number of Equipotential lines Δ B= Width of flow field ΔL= Length of flow field

6. 2. Seepage Pressure (Ps) Seepage pressure at any point is determined by using the below mentioned formula : γ w =Unit weight of water h = Hydraulic potential after “n” potential drops. It can be expressed as : ΔH= Potential drop or drop in head between two equipotential lines

7. 3. Uplift Pressure (P u ) The uplift pressure at any point within the soil mass can be found using the undermentioned formula. It is also called as hydrostatic pressure. Where, γ w =Unit weight of water h w = Pezometric head or pressure head= total head -elevation head

8. 4. Exit Gradient (i exit ) 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 : ΔH= Potential drop or drop in head between two equipotential lines ΔL= Length of flow field