Specifications Part 1 - Water Supply, Sewerage and Tube Well Works

14.3.5.4.3 (c) By drop in previous successive manholes instead of providing the total droprequire at the junction manholes, the same may be achieved by giving smallerdeeps in successive manhole preceding the junction manhole. Thus, forexample, if a total drop of 2.4m is required to be given, 0.6m drop maybe givenin each of the previous three manholes and the last 0.6m-drop maybe given atthe junction manhole.14.3.6. Scraper (Service) Type Manhole: -All sewers above 450mm diameter should have manhole at intervals for 110 to120 m of scraper type. This manhole should have clear opening of 1200 X 900mm at top to facilitate lowering of buckets.14.3.7. Flushing manholes14.3.7.1 Where it is not possible to obtain self-cleaning velocities due to flatness of thegradient specially at the top end of branch sewer which receive very little flow,it is essential that same form of flushing device be incorporated in the system.14.3.7.2 This can be done by making grooves at intervals of 45 to 50m in the maindrains in which wooden planks are inserted & water allow to head up which willrush on with great velocity when the planks are removed.14.3.7.3 Alternatively, an overhead water tanks is built, from which connection aremade through pipe flushing hydrants to rush water to the sewer. The relevantIndian standard IS: 4111(part two).Flushing can be very convenientlyaccomplished by use of fire hydrant or tanker.14.3.7.4 Where flushing manhole is provided, they are located generally at the head of asewer. Sufficient velocity shall be imparted in the sewer to wash away thedeposited solid. The flush is usually effective up to a certain distance afterwhich the imparted velocity gets dissipated.14.3.7.5 The automatic systems which are operated by mechanical units gets oftencorroded by the sewer gases and do not generally function satisfactorily andhence are not recommended. Care should be exercised to be ensuring thatthere is no possibility of back flow of sewer into the water supply mains.14.4 Constructional Details: -Manhole is usually constructed directly over the centre line of the sewer theyare usually constructed with brickwork. However in areas where sewers are tobe laid in high water condition manhole shall be constructed in R.C.C. They arecircular, rectangular or square in shape. Manholes should be of such size aswill allow necessary cleaning and inspection of manholes.14.4.1 Rectangular Manholes - The minimum internal sizes of rectangular manholesbetween brick face should be as follows:14.4.1.1 For depth of manholes less than 0.9m, 900mm x 800mm14.4.1.2 For depths of manholes from 0.9m and up to 2.5m, 1200mm x 900mmUrban Administration & Development Department Page 162