The Main Drainage and Sewage-Disposal of Edinburgh.” by WILLIAMFAIRLEY, Assoc
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226 FAIRLEY ox THE MAIN DRAINAGE OF EDINBURGH. [Selected (Paper No. 2841.) ‘‘ The Main Drainage and Sewage-Disposal of Edinburgh.” By WILLIAMFAIRLEY, Assoc. M. Inst. C.E. THECity of Edinburgh occupies an area of about 6,000 acres, and has a populationestimated at 261,000. Its site varies in level between 26 feet and 330 feet above Ordnance datum, and may be said to consist roughly of four valleys, into which all the drainage gravitates.These valleys form part of the catchment-areas of three streams : (1) The Water of Leith, a river with an average flow of about 2,000 cubic feet per minute ; (2) The Craigentinny Burn, now a common sewer; and (3) The POWBurn, with an average flow of about 200 cubicfeet per minute. The rock substratum belongs tothe calciferoussandstones of the lower carboniferous period, and over a large area crops up to within a fewfeet of the surface. On the northernside of the city it is overlaid by thickbeds of sand, while patches of boulder clay occur over the whole area, most frequently in the western district. The average annual rainfall is 28.40 inches. A large number of the houses are completely fitted with modern sanitary appliances, and an abundant supply of waterfrom the Pentland and Moorfoot Hills is distributed and laid on to all but a fewof the oldest class of buildings. The consumption of water is at present at the rate of about 40 gallons per head per day. Previously to the year 1870 all the mainsewers were constructed of rubble masonry of various cross sections; in many cases, where the cutting was through rock, a mere facing of masonry being required. That part of the city known as the “New Town,” laid out in the years 1767 to 1800 and 1802 to 1822, wasprovided with a system of main sewers, large enough in almost every case for a man to walk through them forinspection, the size being frequently 6 feet by 3 feet, with flat bottom, battered sides and arched roofs. In the parts of the city more recently sewered, fire- claysalt-glazed pipes are employed,jointed with neat Portland cement and rope yarn ; or brick egg-shaped sewers. No general system of ventilation has as yet been introduced, but on all new Downloaded by [ UNIVERSITY OF OXFORD] on [13/09/16]. Copyright © ICE Publishing, all rights reserved. Papers.] FAIRLEY ON THE MAW DRAINAGE OF EDINBURGH. 227 works ventilators havebeen placed at distances of 100 yards apart. During the year 1888 a beginning was made to carry out a system of ventilation for the main sewers in the central district; generally this has been accomplished by introducing ventilating-shafts with gratings in the centre of the roadways,and, in the vicinity of public parks, by forming outlets in the shrubberies, at some dis- tance from the foot-walks. A difficulty will always present itself in districts which contain breweries andmanufactories, discharging hot liquids directly into thesewers, owing tothe emission of steam from the gratings on the roadways or streets, and consequent risk to horse traffic. For the pastfew years the averageexpenditure in drainage work andnew sewers has been about S11,OOO perannum. In designingnew sewers, the rule generally adoptedhas been to allowadischarging capacity equal to 42 gallonsper diem perhead of population inthe drainagearea, one-half of that quantity being assumed to pass off in eight hours, together with a rainfall of 2 inches in twenty-four hours. This rule has been adhered to in designing the greater portion of the recently con- structed sewers, and gives a capacity sufficient for all ordinary requirements. Except in special cases no allowance is madefor subsoil water. The city is divided into three main drainage-districts (Plate 5). (1) The Water of Leith, or northern district; (2) The Craigen- tinny,or central district ; and (3) ThePowburn, or southern district, each area discharging by a separate outfall to the sea. In allthese districts the sewagehas been utilizedby private enterprise for irrigating grass lands, the total area thus dealt with being about 360 acres. On the Water of Leith outfall there is an irrigation farm,at the head of the intercepting sewer, on the lands of Roseburn, with an area of 33 acres. This farm was formerly of much larger dimen- sions,being at onetime about 250 acres in extent, and almost entirely outside the city boundary; it has, however, beengradually curtailed,owing to the use of theland for building purposes. The Lochrin outfall-sewer passes in an open stream, through the portion at present under irrigation, the sewage being drawn off by a main carrier and distributed in leaders by gravitation, so as to flow evenly over the ground. Onlya small portion of the entire volume of the sewage is now required. The subsoil is mainly a stiff clay, and no proper system of subsoil- or under-drainage has been adopted ; the effluent water is caught by intercepting ditches and delivered direct into the Water of Leith. The whole area is Q2 Downloaded by [ UNIVERSITY OF OXFORD] on [13/09/16]. Copyright © ICE Publishing, all rights reserved. 228 FAIRLEY ON THE MAIN DRAINAGE OF EDIKBUROH. [Selected under natural grass, and yields between four and five crops each season, the average rental beingbetween S20 and S30 per acre. On thecentral or Craigentinny drainage-area thereare two irrigation farms, both situatedon the north-eastern outskirtsof the city ; viz., the Lochend and the Craigentinny meadows. Lochend farm has an areaof 43 acres, of which 35 acres are commanded by main carriers led from the point at which the main outfall-sewer from thecity debouches intoan open stream. The sewage is pumped up on to the remaining 8 acres by an undershot water- wheel, 14 feet in diameter by 3 feet G inches broad, driven by the sewage flowing to the lower area, and it is then distributed by means of underground pipes and surface hydrants. The Craigen- tinny irrigation farm extends from the point where the stream leaves the Lochend farm, to the foreshore of the Firth of Forth, along which it spreads for a distance of about 14mile. This farm, [which has an area of about 236 acres, may be taken as typical of the others. The greater portion of these meadows was laidout upwards of two hundred years ago, and a part under Italian grass and other crops about fifty years ago. The surface of the ground is undulating, with a general slope seawards, and a large portion of it was formerly waste land, on which only sea-grass and whim could be grown. The subsoil varies between a stiff boulder clay and light sands and gravels. There is a complete system of under- drainage, consisting of 2-inch and 3-inch agricultural tile-drains, laid 10 yards apart and 3 feet deep. The sewage is led from the regulating-sluices and settling-tank along the higher grounds by main carriers 6 feet by 2 feet, delivering into secondary carriers 24 feetby 2 feet. From these carriers smallleaders andturf drains, 12 inches by 6 inches, are cut so as to distribute thesewage as evenly as possible over the surface of the meadow. The effluent water from the surface and the under-drains is caught by ditches, to be again used in irrigating lower levels, or delivered by the outletsinto the Firth of Forth.The secondary carriersdivide the ground into plots, each of about $-acre in area. Four crops areusually taken off theland annually, and occasionally, in dry and favourable years, five or six. The ground was originally sown with a mixture of natural and aquatic grasses, the latter in smallquantities, and it has not been broken up since, nor has it been cropped inany other way. The 50 acres under Italian grass are situated at a higher level than the main area of the farm, and are cropped in rotation with the arable land, the sewage being raised by a pumpdriven by asteam-engine at the farmbuildings. Two or three men, withthe occasional Downloaded by [ UNIVERSITY OF OXFORD] on [13/09/16]. Copyright © ICE Publishing, all rights reserved. Papers.] FAIRLEY ON THE MAIN DRAIXAGE OF EDINBURGH. 229 use of a horse and cart, do all the labour required, regulating the sluices, repairing roads, andkeeping the carriers clear. From the month of May until the end of October, the whole of the sewage, amounting to about 800 cubicfeet per minute is utilized on the land, each group of plots being under water about twenty-four hours continuously once every three weeks. During the winter, irrigation work is only carried on during fresh open weather. The average rental received is about S36 per acre per annum. The dairymen, and otherswho take thecrops, cut the grass themselves as they require it, the proprietor attendingonly to the regular supply of water to the land, and the repairof the roads. Another sewage farm, theCraigmillar meadows, is situated outside the city boundary, on the Powburn or southern outfall. This farm has an area of about 48 acres, of which 32 acres were laidout about fifteen years ago, andthe remainder aboutthe year 1885. The subsoil is a stiff clay, and there is no system of under-drainage. The sewage is led on to the farm by an 18-inch pipe, and is distributed by two main leaders 3 feet wide by 2 feet 6 inches deep, the effluent mater flowing into an intercepting drain to again join the outfall sewer. The plots are laid out so as to secure as far as possible a uniform flow over the land, which is constantlyunder grass, and is divided into &-acre plots.