Figure $5\mathrm{.}37$ shows the cross$-$section of a concrete dam founded on an isotopic layer of silty sand, which is underlain by an impermeable layer.

Fig. $5\mathrm{.}37$

$($a$)$ Sketch a flow net and estimate the seepage quantity under the dam on its own.

$($b$)$ Sketch two further flow nets for the seepage, after a sheet piling cut$-$off has been installed: $($i$)$ at A$-$A$,$ and $($ii$)$ at B$-$B$.$

$($c$)$ Comment on the effects of installing the sheet piling cut$-$off at $C-C.$