![]() 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. The calculation of area of each type of surface is very tedious and impracticable. Therefore runoff coefficients are chosen by inspection of the localities, which directly depends on the density of population. Table 4.2 gives overall runoff coefficient for different types of localities. Empirical Formulae Method :įor determining runoff from very large areas, generally empirical formulae are used. All the empirical formulae are applicable only under certain specific conditions such as slope of land, imperviousness, rate of rainfall etc. These have been developed suiting a particular region after long practical experience and collection of field data. S = slope of the area in metre per thousand metre.Įmpirical Formulae for Rainfall Intensities : unit formulae have been converted into M.K.S. The intensity of rainfall can be worked out from the rainfall records of the area under consideration. The rainfall intensity may be taken from rainfall records of that area for which storm sewers are to be designed. 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: ![]() (i) a = 30 and b = 10 when duration of storm is 5 to 20 minutes. (b) For localities where rainfall is frequent: (ii) a = 40 and b = 20 when duration of storm is 20 to 100 minutes. This formula is adopted for area having heavy and frequent rainfall. (c) For storms occurring once in 10 yrs.: It gives intensity of rainfall which will occur once in 5 yrs. 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. 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. 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. 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. ![]() The main idea behind wastewater collection systems is to get wastewater to the treatment plant. Wastewater treatment plants work best with a nice steady inflow. When the flow increases due to wet weather, the excess flow can overwhelm the plant. ![]()
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