Design steps of surface drainage

Surface drainage design of a longitudinal drain (side drain) proceeds as follows (IRC SP:42 / NRS 2070).

1. Collect data: road alignment and longitudinal gradient, cross-section, catchment area and its slope, soil and land use, rainfall records, outfall locations.
2. Fix the design return period (for example 5 to 10 years for side drains) and find the rainfall intensity $I$ for the time of concentration $t_c$.
3. Time of concentration: $t_c=t_{inlet}+t_{flow}$, where $t_{flow}=L/V$ (the length of drain divided by the flow velocity).
4. Calculate discharge by the rational method:

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

where $C$ is the runoff coefficient, $I$ in mm/hour and $A$ in hectares. 5. Select the shape (trapezoidal, rectangular, V-shaped or U-shaped) and the material or lining, and the longitudinal gradient (normally the road gradient, but not less than about 0.5%). 6. Design the section by Manning's equation:

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

Assume a section, find $A$, wetted perimeter $P$ and $R=A/P$, and iterate until the capacity equals or exceeds $Q$. Add a free board of about 0.15 m. 7. Check the velocity: it must be greater than the non-silting velocity (about 0.6 to 0.75 m/s) and less than the permissible non-scouring velocity for the material (for example about 0.3 to 0.6 m/s for sandy soil, 1.0 to 1.5 m/s for hard clay, about 3 m/s for stone masonry and up to about 6 m/s for concrete lining). 8. Provide lining or checks if the velocity is too high, and increase the gradient or reduce the section (to raise the velocity) if it is too low. 9. Fix the spacing of cross drains and outfalls (turnouts or mitre drains) so that the drain length does not exceed the capacity. 10. Prepare drawings and quantities.