TRANSACTIONS OF SECTION G. 639 



THURSDAY, SEPTEMBER 9. 

 The following Papers were read : — 



1. Tho Manchester Drainage Scheme. By de Colrcey Meade. 



The drainage of the City of Manchester has occupied the attention of the 

 City Council for many years, and the subject has been considered and reported 

 upon by experts at various times during the past forty years. The natural 

 drainage of the city is by the rivers Irwell, Mersey, Medlock, Irk, and their 

 tributaries. The Irwell below Manchester is now absorbed into the Ship Canal. 

 The area of the city has been largely extended since 1885. At that time it was 

 only 4,298 acres ; its present area is 21,688 acres — an increase of over 400 per 

 cent. The writer's predecessor, Mr. John Allison, M.Inst.C.E., prepared plans 

 for the drainage of Manchester in 1888. The area of the city at that lime was 

 5,940 acres. The works designed by Mr. Allison were in progress when the 

 writer entered upon his duties as City Surveyor 21 years ago, and those works 

 were completed under his direction. Manchester has far outgrown the capacity 

 of the drainage scheme devised by Mr. Allison, whose plan was calculated 

 to be sufficient to serve a population of 868,522 persons, but before the sewage 

 from 500,000 persons had been connected with the sewers they proved wholly 

 insufficient, and outlets had therefore to be found for the surplus Hood waters 

 through the original channels which discharge into the rivers. That state of 

 things has continued for many years. 



Six years ago the writer was requested to i^repare plans for the drainage of 

 the present city, and also to provide for the requirements of any additional 

 districts which it is probable may hereafter be added to the city. The latter 

 part of the instruction introduced a somewhat difficult problem. After very 

 careful consideration of the whole subject the writer submitted a proposal to 

 the City Council, which was subsequently approved by Parliament, and the 

 works designed by him are now partly completed. The absence of reliable data 

 regarding the ' run off ' of rainfall from ui'ban areas by artificial channels is 

 worthy of note. The want of this information led to an under- estimate of the 

 sewer capacity necessary for the drainage of the Metropolis. The London basis 

 was adopted by the writer's predecessor as sufficient for Manchester, and 

 consequently resulted in a similar deficiency in the size of the conduits he 

 provided for the drainage of this city. Most useful lessons were learned by 

 .these early projects, and information gathered by the writer during the past 

 twenty years, by means of self-recording gauges and otherwise, enabled him to 

 determine with reasonable accuracy the provision necessary for the reception 

 and removal of the city sewage plus rainfall during periods of storm. 



The intensity of rainfall, impermeability and character of surface, and the 

 relative retardation of flow are factors which have an important bearing on 

 the maximum quantity of flood-water which will pass into a system of town 

 drainage. The duration of storm and the condition of the surface immediately 

 preceding a fall largely affect the rate of flow. A rainfall of great intensity 

 and a few minutes' duration may not produce an excessive flood, whilst serious 

 consequences would probably result from a rainfall of similar intensity con- 

 tinuing for, say, a quarter of an hour, other conditions being equal. A chart 

 showing the registered duration and intensity of storms in South Lancashire 

 during the past fifteen years will be shown on the screen. The solid curve 

 thereon indicates approximately the provision which experience has proved to 

 be necessary in the new sewers and storm culverts for sewage plus storm- 

 water ; the dotted curve shows the maximum intensity of rainfall recorded 

 during the period under observation. On tlie 10th ultimo a rainfall intensity 

 of 3'75 inches per hour was recorded in the centre of this city. The storm 

 affected, as is usual in such cases, a very limited area, which is not yet served 

 by the new system of sewers. The surface of this catchment area was wholly 

 impermeable. 



The accompanying table shows the ratio of impervious surface to population : 



