1st. Storm water does not require extensive underground removal, or where it can be concentrated in a few shallow underground channels.
2nd. Drainage areas are short and steep facilitating rapid flow of water over street surfaces to the natural water courses.
3rd. The sanitary sewage must be pumped.
4th. Sewers are being built in advance of the city’s development to encourage its growth.
5th. The existing sewer is laid at grades unsuitable for sanitary sewage, it can be used as a storm sewer.
A combined system must be relatively larger than a separate storm sewer as the latter may overflow on exceptional occasions, but the former never.
A combined system of sewerage should be used where:
1st. It is evident that storm and sanitary sewerage must be provided soon.
2nd. Both sanitary and storm sewage must be pumped.
3rd. The district is densely built up.