Engineering Wonders of the World
Volume III

THE DRAINAGE SYSTEM OF LONDON. 213 ABBEY MILLS PUMPING STATION, WHERE THE SEWAGE FROM THE NORTHERN LOW-LEVEL SEWERS IS PUMPED INTO THE NORTHERN OUTFALL SEWER. (Photo, Pictorial Agency.) therefore made large enough to carry off some 286,000,000 gallons of rain water per day, in addition to the sewage. This quantity of water represents an average fall of one-sixth of an inch over the area drained. It was assumed, for the purpose of this calculation, that the rainfall would be equally distributed over the twenty-four hours of the day. We all know well enough, however, that a day of heavy rain means a fall greatly exceeding one-sixth of an inch, and that during a thunderstorm as much water will descend in a few minutes as is precipitated in a whole day of soft rain. The old main-line sewers, which, as before stated, run from north to south on the north side of the Thames, and which originally dis- charged their contents into the river, are still utilized for carrying their sewage, but deliver into the intercepting sewers. When the flow in these main sewers and also the intercepting lines becomes too great, owing to excessive rainfall, to be discharged at the .£ „ .4. Storm-Relief outfall, the excess passes into _ ’ 1 Sewers. the river by means of the old outlets. For th© purpose of obtaining addi- tional relief in times of heavy rain, new storm- relief sewers have been constructed. Though this system of coping with heavy rainfalls was in a way a reversion to the old method, it must be noted that the discharge of the storm-relief and other sewers would not begin until the intercepting and main sewers had been well flushed by tlio first inrush of surface water. A compromise was inevitable. The 1891 report of the late Sir Benjamin Baker and of Sir Alexander Binnie stated that a rain- fall of half an inch an hour, flowing off the area drained on the north side of the