Design steps of side drain

A side drain carries the runoff from the carriageway, shoulder and nearby catchment along the road. The design steps are:

1. Collect data: catchment area ($A$), slope, soil/ground cover, rainfall intensity for the chosen return period (5 to 10 years for side drains), and the longitudinal gradient of the road.
2. Find the time of concentration $t_c$ and read the rainfall intensity $I$ for that duration.
3. Compute the discharge with the rational formula:

$$Q=\frac{C\,I\,A}{360}\ \ (\text{m}^3/\text{s})$$

with $I$ in mm/h and $A$ in hectares, and $C$ the runoff coefficient. 4. Select the shape and material: trapezoidal (side slope 1:1 to 1.5:1 in earth), rectangular (masonry or concrete, in hills) or V-shaped (small flows), and fix the bed slope, equal to road gradient but not less than 0.5%. 5. Size the drain by Manning's equation:

$$V=\frac{1}{n}R^{2/3}S^{1/2},\qquad Q=A\,V$$

Assume dimensions, find $A$, $P$, $R=A/P$, and revise until $Q_{capacity}\ge Q_{design}$. 6. Check the velocity is more than the non-silting velocity (about 0.6 to 0.75 m/s) and less than the non-scouring limit of the soil/lining. 7. Add a free board (about 0.15 to 0.3 m) and provide lining or check dams where velocity is too high. 8. Fix the discharge points (mitre drains, cross drains) so that the length of drain does not exceed the capacity, and design the outfall protection.