IT IS ESSENTIAL THAT ALL SULLAGE DRAINS HAVE SELF-CLEANSING VELOCITIES, SO THAT THERE ARE NO ACCUMULATIONS IN THE SEWERS AND SEWAGE DOESN'T BECOME SEPTIC.

Drainage schemes are prepared for the next thirty years effects on the design of sewers as regards self cleansing velocities. It becomes essential to drain out the surface water of the ground (rain water) for successful building operations or for the sanitary reasons. Open jointed drains are generally laid below the ground level, which are of the size that will permit draining out all the water and minimum size should not be less than 50 mm. The drains are laid in the direction of the greatest slope, which ensures the greatest velocity and capacity. High velocities of water assist in scouring out the drains and keeping them clean. It is very important that drains should be laid as straight as practicable. There are two principal systems known as the combined and the separate systems. In the former system one set of drains or sewers is provided for the removal of both the soil sewage and the rain water. In the separate system, two sets of drains are provided, one for the soil sewage and the other for rain water (or one underground sewer for sewage and one surface drain for rain water) . Both the systems have their advantages and the disadvantages and in most cases a partially separate system is considered most suitable. While preparation of the drainage schemes estimated it is necessary to prepare the layout plan of the whole area and the ground levels and the lowest floor levels, direction of flow of the existing drains or sewers into which drainage is to be taken must be straight and having good slope. For calculating the sizes of the drains or sewers, rainfall must be considered in regions of the average rainfall, as such the run-off decreases with increase in the drainage area. It is necessary to follow design requirements while calculating rainfall run-off drainage of the surface drains as such in towns and developed areas more of the rainwater finds its way into the drains than in the rural areas. Where there is no water supply and the inhabitants depends upon the wells and the hand pumps water will be used much less and a lot of it will be spilled outside and will not be taken by the drains.