Why do urban or street drainage systems overflow?
Most urban underground drainage systems, including street systems, are designed for frequent storm events and not large, less frequent storms. Therefore, from time to time the capacity of underground systems will be exceeded during a significant/large rainfall event. It is not feasible to design drainage systems to cope with infrequent large storms. It is important that all inlets such as stormwater drains are kept clear and are free from obstructions at all times, as blocked inlets will cause stormwater to divert elsewhere as overland flow, often resulting in localised flooding.
