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When the main sewers of John Wignall Leather's scheme were designed, very few streets in the areas served were paved and the population of Leeds was only of the order of 160,000. By the early 1880's the population of the Borough had leapt to more than 300,000, numerous streets had been paved, and the sewerage system had been extended to many of the outlying townships. So thoroughly was the area being drained of surface water during rain storms, that the principal intercepting sewer was completely unable to cope. The result was that many premises lying in the lower levels of the town were subject to an increasing frequency of back flooding during heavy rainfalls. Something had to be done and yet it was out of the question to provide sewers big enough to conduct all the storm water to Knostrop.

The Borough Engineer of the time, Thomas Hewson, proposed a solution to the Streets and Sewerage Committee of the Council in June 1883. In his "Report on Leeds Intercepting Sewers and Disposal of Rainfall" Hewson suggested that no more than two volumes of rainwater to every one volume of foul sewage should be allowed to enter the Intercepting Sewers. This would prevent them flowing more than two thirds full for the forseeable future and would be achieved by a system of storm overflows. He explained the principle of operation as follows:

"Intercepting sewers receive along their routes at irregular intervals, the main and street sewers running right and left from them. On the main and street sewers, arrangements of leaping weirs as storm overflows or diverters of the rainfall from them to the becks, would be made. These should be so remote, laterally from the intercepting sewer, as to reach an altitude on the main or street sewer above the highest flood level of the beck, and so prevent the beck storm water from flowing into the sewer. They would have two outlets to discharge into, one near, for when the flow was sluggish, and one farther off, for when the rainfall increased the flow and velocity so as to carry it over the first opening."

The Local Government Board, set up in 1871 by Disraeli, was the central authority responsible for sewage and sewage disposal. Apparently, one of the Board's "requirements" was that storm overflows should not come into operation until a flow equal to six times the dry weather flow was being conveyed to the sewage works for treatment. Hewson's two of rainfall to one of sewage was consistent with this, since in his calculations he assumed that at peak flow the foul sewage flowed at twice the average daily rate (i.e. 50 gallons per head per day, compared with an average of 25 gallons per head per day).

Of course, this temporary 'overflow' expedient tackled only the tip of the iceberg. Conditions in some parts of Leeds still gave testimony to what life could be like without proper surface water drainage and street paving. Take for example the situation in Thornhill Road (Upper Wortley), as described by an irate correspondent of the "Yorkshire Post" in late 1890:

"... It is perfect cruely to animals to attempt passing along this road with a load. A number of minor accidents have already taken place, such as sacks of flour falling off waggons, carts upset, horses sticking completely fast and having to be pulled out with extraneous aids"

At least Hewson - the Borough and Sewerage Engineer of the time - and his contemporaries were a little better equipped than their predecessors, as regards hydraulic theory. Discussion of the proposed measures for dealing with excess storm flow, did not involve all the arguments about pipe capacities, which plagued earlier engineers.

By the 1890's, relatively reliable flow tables were being used. Experiments by Darcy, Bazin and others, provided the basis for the popular formulae of Kutter or Crimp and Bruges. Tables published by the latter pair in 1897 are still widley used today for analysing old egg-shaped sewers. Indeed, the copy currently held by the Council's sewerage engineers bears Hewson's name inside the cover!

6. High Level Treatment