Q$4\_$ In a constant central angle arch dam, the length of the chord on the downstream face of the arch, $80$ m below the crest, is $65$ m$.$ Determine the thickness of the arch at this depth. If a variable radius and central angle arch dam were constructed in this valley, the axial radius at a depth of $80$ m below the crest would be $30$ m$.$ Calculate the thickness of the arch ring at this level. The allowable stress of the concrete is $300$ t$/$m$^2,$ and the modulus of elasticity is $2$x$10\text{'}6$ t$/$m$\text{'}2$