V = volume in litres/second to discharge H = intensity of rain in mm/hour S = surface of drainage area in m2 The simplicity of using PVC FIRST products allows the creation of drainage networks in any type of rain. If a system of evacuation is chosen for points (with traps, grilles, siphons) or for trenches (grilled channels) it is always indispensable to know exactly the volume of fallen rain water in order to plan adequate canalization. EXAMPLE: how to apply the formula for an area of 300 m2 with an average rainfall off 100 mm/h m. 10 3600 V = H x S which must be made round to 4 EXAMPLE 1: witch the SG30 grill (which has a capacity of 2,6 l/s) you get m. 10 m. 30 EXAMPLE 2: you can also start knowing the number of grills to be set up and obtain their size by the water to be drained and the capacity of the chosen grill. If, for example, you just want to set up 3 grills instead of 4you can determine their capacity by the following formula: 2,6 n = 8,33 = 3,20 picture 3 shows that it’s necessary to set SG40 grill (the smallest grill with a capacity of more than 2,77 1/s) N A = V 3 = 8,33 = 2,77 l/s H = 100 mm/h S= 300 m2 Knowing the intensity of the rain (H) and the surface to be drained (S), the volume of water to be channeled is determined (V) with the formula: 3600 V = 100x300 = 8,33 l/s m. 30 m. 10 8.33 is the volume of water to be discharged per second Having obtained the volume of water to be drained, the position of draining point is planned in the intervention area choosing between two possibilities: 1 - the model of grill well is chosen and the quantity of use is calculated 2 - the number of grill wells to be set is decided calculating which model is suited to the volume of water obtained. D R A I N I N G C A L C U L A T I O N S A N D T E C H N I C S ® 34
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