EXAMPLE ON DRAINAGE DESIGN

A pavement surface $)=(0\mathrm{.}8$ cambered on both sides has a total width of $7\mathrm{.}0m$ and a grass shoulder with adjoining land $)=(0\mathrm{.}25$ of $8\mathrm{.}0m$ on each side of the carriageway. A trapezoidal drainage ditch, $400m$ long, runs parallel to the pavement and shoulder, with adjoining land, and is to drain half the pavement. On the other side of the ditch, water flows across from the road reserve land with a grass cover $)$ and a $2$ percent cross$-$slope towards the drainage ditch; the width of this strip of land is $25\mathrm{.}0m.$

$($i$)$ Estimate the amount of run$-$off in the drain assuming a $20$ year return period and storm duration of $15$ minutes.

$($ii$)$ Design the cross section and the longitudinal slope of the side ditch in a clayey soil $)=0\mathrm{.}022,V_{\text{m}\text{a}\text{x}}=(0\mathrm{.}8\frac{m}{s}.$

Assumptions:

Roadside slope $-1$:$4\mathrm{.}5$

Verge side slope $-1$:$1$

Width of bottom of ditch $-0\mathrm{.}5m$