It involves various factors such as intensity of rainfall, characteristics of catchment area, duration of storm, etc. The quantity of storm water, which is known as wet weather flow (WWF) entering the sewer is to be carefully determined. Stagnant waterpools serve as breeding places for mosquitoes. Low lying areas get flooded and transport system is paralysed. Initial washings of streets by storm water contain organic matter and hence such water requires to be collected and to be taken to the treatment plant Wherever possible, the storm water is to be collected and conveyed in sewers at proper places for the following reasons:ĭamp conditions are created which are unhygienic as they provide flourishing ground for micro organismsĮxistence of waterpools affects the foundations of structures Estimates of such flows have to be made separately. Industries and commercial buildings often use water other than the municipal supply and may discharge their liquid wastes into the sanitary sewers. Once the flow is estimated as per the above Table, the design infiltration value shall be limited to a maximum of 10% of the design value of sewage flow. Since sewers are designed for peak discharges, allowances for groundwater infiltration for the worst condition in the area should be made as in Table. The minimum flow may vary from 1/3 to 1/2 of average flow.Įstimate of flow in sanitary sewers may include certain flows due to infiltration of groundwater through joints. The peak factors also depend upon the density of population, topography of the site, hours of water supply and therefore individual cases may be further analysed if required. The peak factor or the ratio of maximum to average flows depends upon contributory population as given in following Table. But for the purpose of hydraulic design, estimated peak flows are adopted. The flow in sewers varies from hour to hour and also seasonally. Non-conventional sewers shall be designed as the case may be. However, the conventional sewers shall be designed for a minimum sewage flow of 100 litres per capita per day or higher as the case may be.
In arid regions, mean sewage flows may be as little as 40% of water consumption and in well developed areas, flows may be as high as 90%. However, the observed dry weather flow quantities usually are slightly less than the per capita water consumption, since some water is lost in evaporation, seepage into ground, leakage, etc. The entire spent water of a community should normally contribute to the total flow in a sanitary sewer. The tributary area for any section under consideration has to be marked on a key plan and the area can be measured from the map.
The need to finance projects within the available resources necessitates the design to be restricted to political boundaries. However, in designing sewers for larger areas, there is usually an economic advantage in providing adequate capacity initially for a certain period of time and adding additional sewers, when the pattern of growth becomes established. For larger drainage areas, though it is desirable that the sewer capacities be designed for the total tributary area, sometimes, political boundaries and legal restrictions prevent the sewers to be constructed beyond the limits of the local authority. The natural topography, layout of buildings, political boundaries, economic factors, etc., determine the tributary area. There are several methods for estimation or forecasting of population which can predict or forecast population for a specific design period, usually three to four decades. Because the flow is largely a function of population served, population density and water consumption, lateral and sub main sewers are usually designed for peak flows of the population at saturation density as set forth in the Master Plan. In fixing a period of design, consideration must be given for the useful life of structures and equipment employed, taking into account obsolescence as well as wear and tear. The length of time up to which the capacity of a sewer will be adequate is referred to as the design period.
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