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When the flow increases due to wet weather, the excess flow can overwhelm the plant. Wastewater treatment plants work best with a nice steady inflow. The main idea behind wastewater collection systems is to get wastewater to the treatment plant. Hopefully, if you are reading this blog you know that RDII stands for Rainfall Derived Infiltration and Inflow, while RTK is not an acronym at all. In the above example if the density of population is 300persons/hectare and the rate of water supply is 250 litres/capita/day, calculate the quantity of sanitary sewage for (a) separate system and (b) for partially separate system.That’s a lot of acronyms for one title. Impervious factor or overall runoff coefficient ‘c’ If the maximum intensity of rainfall is 40 mm/hour, calculate the quantity of storm water which will reach sewer lines. The catchment area of a city is 200 hectares.Īssuming that the surface on which the rain falls is classified as follows:Ĭalculate the impervious factor. (c) For storms occurring once in 10 yrs.: It gives intensity of rainfall which will occur once in 5 yrs. This formula is adopted for area having heavy and frequent rainfall. (b) For localities where rainfall is frequent: (ii) a = 40 and b = 20 when duration of storm is 20 to 100 minutes. (i) a = 30 and b = 10 when duration of storm is 5 to 20 minutes. T = duration of storm in minutes a and b are constantsĪccording to Ministry of health, U.S.A., the values of constant a and b are as follows: In case, where rainfall records are not available, the intensity of rainfall is obtained by applying suitable empirical formula.įollowing are some of the empirical formulas which are used for this purpose:
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The rainfall intensity may be taken from rainfall records of that area for which storm sewers are to be designed. The intensity of rainfall can be worked out from the rainfall records of the area under consideration. S = slope of the area in metre per thousand metre.Įmpirical Formulae for Rainfall Intensities : unit formulae have been converted into M.K.S. These have been developed suiting a particular region after long practical experience and collection of field data. All the empirical formulae are applicable only under certain specific conditions such as slope of land, imperviousness, rate of rainfall etc. Empirical Formulae Method :įor determining runoff from very large areas, generally empirical formulae are used. Table 4.2 gives overall runoff coefficient for different types of localities. Therefore runoff coefficients are chosen by inspection of the localities, which directly depends on the density of population. The calculation of area of each type of surface is very tedious and impracticable. Are the different types of area and C 1, C 2, C 3 are their runoff coefficients respectively.įrom the above expression, it is clear that for calculating runoff coefficient, area of each type of surface is to be measured and then substituted in the formula. Table 4.1 gives the common values of runoff coefficient, which are commonly used in determining the quantity of storm water reaching the sewer lines.Īs every locality consists of different types of surface areas, therefore, for calculating the over-all runoff coefficient the following formula is used. The runoff coefficient is a fraction which is multiplied with the quantity of total rainfall, to determine the quantity of rain water which will reach the sewers.Īfter continuous rainfall for some time, the ponds and ditches are filled up and the atmosphere becomes nearly saturated, therefore the runoff coefficient mainly depends on the characteristics of ground surface as porosity, wetness, ground cover etc. A portion of it percolates in the ground, a portion evaporates, a portion is stored in ponds and ditches, and only the remaining portion of rain water reaches the drains and sewers. The whole quantity of rain water that falls over ground does not reach the sewer lines or drains. In Rational Method, the value of runoff coefficient ‘C’ is required.