A Gradual Rehabilitation Programme for Combined Sewerage Systems

The Kannai-Yamashita district, which is Yokohama's administrative and economic heart, was the first part of the city to be installed with a modern sewerage system. This district is served by combined sewers, many of which have extensive pipe deterioration, shallow covering, and insufficient capacity. Whilst rehabilitation of these sewers has therefore been an urgent priority, flood alleviation has been given even greater precedence. It was consequently decided (a) to construct supplementary storm trunk sewers and a stormwater pumping station to mitigate flooding, and (b) to implement a systematic rehabilitation of the sewers on a gradual basis. Studies of branch sewer rehabilitation also used a surcharge model in the interest of achieving an economical and efficient design.