Module 14
Two-dimensional seepage and flow nets
Water seeping around walls and beneath dams: flow lines and equipotentials, the flow net as a graphical solution, the seepage flow rate, and the exit gradient that governs piping and heave.
What this module covers
- Describe two-dimensional seepage with flow lines and equipotentials
- Read a flow net as a mesh of curvilinear squares and count Nf and Nd
- Derive and use the flow-net flow rate q = k·h·(Nf/Nd)
- Find pore pressure at a point from the equipotential drops
- Compute the exit gradient and the factor of safety against piping/heave
Lessons
From one-dimensional Darcy flow to a two-dimensional flow net.
Start lesson →- 14.2
Using a flow net
The seepage flow rate and pore pressure from the net.
Start lesson → Where seepage emerges, and how it can lift or wash away the ground.
Start lesson →
Module checkpoint
Check what you have taken in
3 questions
Question 1
A flow net has Nf = 3, Nd = 9, head loss h = 12 m and k = 5 × 10⁻⁷ m/s. What is the seepage flow rate per metre width, in m³/s/m?
Question 2
With h = 8 m, Nd = 16 and an exit-square length l = 1.0 m, what is the exit gradient?
Question 3
To increase the factor of safety against piping beneath a cutoff wall, you should: