A flownet is given below for flow under a concrete dam. The dam is $100m$ long and

has two $10m$ sheet piles driven partially into the granular soil layer. The datum is at

the surface of impervious bedrock, shown in the figure. The water level is $20m$

above soil layer in the upper stream and at the soil layer surface in downstream. The

coefficient of permeability of soil is $0\mathrm{.}2\frac{m}{?}$ day. The porosity of soil is $0\mathrm{.}5.$ Calculate

the actual seepage velocity, vs$,$ at Point $P$ under the middle of the dam. Unit: m$/$day$.$

Use the scale shown at the bottom of the figure to estimate the length.