Storm water estimation and its limitations

Determination of storm water quantity

For a densely built urban area the rational method is used:

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

1. Measure the catchment area $A$ (ha) of each drain, from a contour map and drainage layout.
2. Find the weighted runoff coefficient $C$ from the land use (roofs, paved roads, courtyards, open land). For a dense urban area $C$ is high, about 0.7-0.9.
3. Find the time of concentration $t_c=t_e+t_f$ (entry time 5-10 min in dense areas plus flow time in the drain).
4. Read the intensity $i$ for $t_c$ and the design return period (5-10 years for main drains) from the IDF curve of the nearest rain gauge station.
5. Compute $Q$ for each section and design the drains using Manning's formula.

For large or complex areas, hydrograph methods or computer models (unit hydrograph, SWMM) are used.

Limitations in a highly populated urban area

1. Assumption of uniform rainfall over the whole catchment and uniform intensity during $t_c$ is not true for large areas.
2. Runoff coefficient is uncertain. It changes with soil moisture, storm intensity and quick urban growth. A fixed $C$ may under- or overestimate.
3. Time of concentration is hard to find in a complex network with many inlets, varied slopes and encroached drains.
4. Lack of rainfall data: short-duration records are often few or missing for Nepali towns, so IDF curves are doubtful.
5. The method gives only the peak, not volume or hydrograph; it cannot account for storage in ponds or detention.
6. Backwater and blockage: the method ignores blocked inlets and drains, solid waste in drains, river backwater and surcharging, so actual flow differs.
7. Valid only for small catchments (generally below 400-500 ha).
8. Changing land use: future urbanisation, filling of ponds and drains, climate change increasing rainfall, are difficult to predict.
9. Interaction with the sewage flow and illegal connections are not included.

To reduce errors a factor of safety is used, local data are collected, and drains are checked after construction.