Urban stormwater: why urban flooding is not a natural disaster

When a street floods after twenty minutes of rain, the reason is rarely that "too much water fell". Usually it is that the impermeable surface grew while the network stayed the same. Urban flooding is almost always a design problem, not a weather problem.

Urbanisation changes the equation

On a natural surface, most of the rain infiltrates or is held by vegetation. When you cover it with concrete and asphalt, the runoff coefficient goes from around 0.2 to 0.9. In practice, the same rainfall produces several times more surface water, and produces it much faster: the time of concentration shortens, so the peak comes higher and earlier.

How large the network should be

Sizing starts from the return period you accept to risk. A secondary street can be designed for a 2-year storm; a main road or an underpass where the water has nowhere to go requires a much longer period. This choice is an engineering decision with financial consequences, so it must be made consciously and documented, not inherited from the previous project.

Intensity must be local

The point where mistakes are most often made is rainfall intensity. IDF curves vary significantly from one area of the country to another, and using a foreign curve or a national average value distorts the whole calculation. In Albania, data from meteorological stations exist; using them is not academic perfectionism, it is the minimum.

How AlbGIS helps

• Stormwater Network — model the network on a map using the Rational Method; each manhole receives an area and a C coefficient, while the intensity comes from the stations' IDF curves.
• Rational Method for Flows — peak flow for small catchments, with the time of concentration according to Kirpich.
• Culverts — sizing of crossings under roads according to HEC-22 / HDS-5, with inlet and outlet control checks.