Estimating storm water quantity

The quantity of storm water from a locality is estimated by the Rational method, which is the standard method for small and medium urban catchments (up to a few hundred hectares).

$$Q = \frac{C\,i\,A}{360}$$

where $Q$ = peak runoff (m³/s), $C$ = coefficient of runoff, $i$ = rainfall intensity (mm/hr) for a duration equal to the time of concentration, and $A$ = catchment area (ha).

Steps

1. Find the catchment area $A$ from the map, and divide it into surfaces (roofs, roads, lawns, open land).
2. Find the weighted coefficient of runoff: $C=\dfrac{\sum C_jA_j}{\sum A_j}$. Typical values: roofs 0.75-0.95, asphalt road 0.70-0.95, gravel 0.25-0.40, lawns 0.05-0.20, parks 0.10-0.25. For future development use values for the fully developed area.
3. Find the time of concentration: $t_c=t_e+t_f$, where $t_e$ is the inlet (entry) time (usually 5-20 min) and $t_f=L/v$ is the flow time in the drain.
4. Find the intensity $i$ for duration $t_c$ and a chosen return period (2-10 years for urban drains) from the intensity-duration-frequency (IDF) curve or formula of the local Department of Hydrology and Meteorology. A common empirical form is $i=\dfrac{a}{t+b}$.
5. Compute $Q=CiA/360$, and design each drain section for its own $t_c$ and area, adding the area of each branch downstream.

Example

A residential-commercial locality of 40 ha: 30% roads and pavements ($C=0.85$), 20% roofs ($C=0.90$), 20% yards ($C=0.30$), 30% gardens ($C=0.15$). Entry time = 10 min, flow time in drain = 10 min. Using $i=760/(t+10)$ mm/hr (assumed).

$$\begin{aligned} C &= 0.3(0.85)+0.2(0.90)+0.2(0.30)+0.3(0.15) = 0.540 \\ t_c &= 10+10 = 20\ \text{min} \\ i &= \frac{760}{20+10} = 25.33\ \text{mm/hr} \\ Q &= \frac{0.540 \times 25.33 \times 40}{360} = 1.520\ \text{m}^3/\text{s} \end{aligned}$$

Answer: Q = 1.520 m³/s; the storm drain is designed for this discharge.