Storage Structures collection efficiency


Clean water from the roof can be stored in Underground sumps or Rain barrels for further use after the suggested filtration methods.
Calculation of storage/recharge volume based on the average rainfall and collection efficiency: The storage or Recharge structure capacity can be calculated by the amount of runoff from your roof taking into consideration the losses due to evaporation, leakages etc.…
The optimum storage = Roof Area in Sq Runoff Coefficient(0.9)x Average Rainfall in mm, Runoff co-efficient is the percentage of water from the roof that comes down in the rainwater pipes which is taken as 90% for RCC roofs, as about 10% will be lost by evaporations, leakages etc.… Average design Rainfall of Bangalore can be taken as 32mm (970mm/30days).
Eg: If the roof area of a building is 112 Sq M, then the storage or recharge structure capacity will be 112 x 0.9 x 32=3225 Liters.
We shall first calculate the maximum amount of rainfall that can be harvested from the RCC rooftop Following details are available:

  • Area of the catchment 112 Sq.m.

(C) Runoff coefficient = 0.9 (R) Average annual rainfall 970 mm (also call it as height of rainfall 0.97meters) (R)=0.97, Annual rain water potential=(A)Roof area (in Sq.m) X(C)Co-efficient run-off from roof)X(R) Annual rainfall (in mm) Annual water harvesting potential from 112 Sq.m. Roof = A x C x R= 112 x 0.9 x 0.97= 97.7 cu.m. (97776 liters) Rain water volume Calculation Area of =112 sq.m. Height of rainfall=0.97m=970mm or 39 inches Volume of rain fall=Area of plot x height of rainfall Over the plot 112 sq.m. x 0.97 m =105.38 cu.m.(105,380liters Assuming that only 60% of the total rainfall is effectively harvested. Volume of water harvested =105,380 Liters x 0.6=63288 Liters This volume I about thrice the annual drinking water requirement of a 5-member’s family. The average drinking water requirement per person maximum 10 liters a day.


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