A study of air mixing and discharge in rainwater sewers by hydraulic model testing

Abstract

It has been pointed out in recent years that intensive rainfall has frequently lifted and displaced manhole covers, threatening public safety. This study was performed to establish a method of designing rainwater sewers accounting for air eruptions. First, model testing of three models with identical shapes but different scales was carried out. Based on the results, the hydraulic behavior of water containing air in a pressured pipe was analyzed to study its law of similitude. The followings were found.
(1) According to the air mixing test, a mixing ratio α (amount of mixed air/flow rate of running water) can be expressed by a linear regression of h/H (water level in the manhole/depth of the manhole) .
(2) It was clarified that the mixing ratio α is affected by the scale reduction, deriving an equation for estimating the mixing ratio of inrush-style manhole.
(3) The mixing ratio α of a stepped manhole is about a half that of the inrush-style manhole when the water level in the manhole is low; however, when the h/H is larger than 0.4, α is about 5% in any cases.
(4) The air discharge testing showed that the discharge ratio β (maximum amount of air discharged/flow rate of running water) in upstream and downstream manholes are of the same level depending upon its flow rate. Therefore, countermeasures for an upstream manhole on the same scale as the downstream manhole would be necessary.
(5) The discharge ratio β is also clearly affected by the scale reduction, deriving an equation for estimating the discharge ratio β for a downstream manhole.