Flow Nets & Seepage

What am I seeing?

• A flow net is two families of curves that always cross at right angles. Equipotentials join points of equal head, flow lines follow the path the water actually takes.
• Every square in the net passes the same discharge and drops the same head. That is why q = k·H·Nf/Nd needs nothing more than counting squares.
• Driving the cutoff deeper lengthens every flow path at once, so it cuts the discharge and eases the exit gradient together.
• The exit gradient is what actually fails the structure. When it reaches the critical gradient the upward seepage carries the grain weight, the sand boils and the soil loses all strength.
• Uplift under a dam is the head at the base times the unit weight of water. The diagram is higher at the heel, so the resultant sits upstream of mid base and tries to tip the dam as well as lift it.