Find the upstream and downstream water level difference $H$ for the dam to be safe against instability problem$($s$)$ that can occur due to seepage. The saturated unit weight of the riverbed soil is ${}_{\text{s}\text{a}\text{t}}=20k\frac{N}{m^3}.$ Note that the circles in the flow net are to illustrate that all flow net elements are square. Assume that the pressure head from A to B decreases linearly and the distance between A and B is $30$ m$.$ The dam is a concrete structure with a unit weight of ${}_{\text{c}\text{o}\text{n}\text{c}}=23k\frac{N}{m^3}$