Abstract:The drainage system in the old urban areas is dominated by the combined drainage system in the current situation and the short term. The separation system will be mainly in some urban areas after the rainwater and sewage are separated in the long term. In order to control the overflow pollution of the combined drainage system, the storage tank is an important engineering measure. Taking the CB small watershed in the old urban areas of Wuhan as an example, the rainfall data in the urban areas of Wuhan is analyzed, and the relationship between the rainfall flow and rainfall intensity in the urban areas of Wuhan is obtained. And the relationship between the interception ratio and overflow out-discharge of the combined drainage system in the CB small watershed. Referring to the effective volume of the storage tank required for the control of runoff pollution in the long term, the recommended effective volume of the overflow storage tank of combined drainage system in the CB small watershed is obtained.