Probabilistic and Economical Design of Roof Top Rainwater Harvesting Tank by Simulation Technique

Introduction: Rainwater harvesting is the collection of rainwater directly from the surface(s) it falls on. Rainwater harvesting through collection tank is an effective method. Numerous methods are available for determining the size of the storage capacity required to satisfy a given demand. These methods vary in complexity and sophistication.

Methods: The tank design method includes general thumb rule (5% of annual runoff), sequential peak analysis (simulating twice the length of the record), optimization (best one that suits objective criteria), simulation, probabilistic and economical design. Simulation water balance model which works on daily basis, normal probability distribution and economics are used in designing the capacity of tanks and it is presented in a graphical form. The tanks are designed for two different purposes like domestic use and toilet flushing only.

Place and Data: Trichy city daily rainfall records from 1951-2011 is used. If a person living in Trichy city wants to construct a rainwater harvesting tank for toilet flushing purpose (6 Nos * 25l = 150l demand per day), the graphs can used.

Results: At a chosen exceedance probability (EP) of failure (how much time the tank fails to supply water), the engineer can decide the storage size under a preset deficit rate and also the cost of each tanks (per 1000 l) from the curves generated in this study. These relationships can be used by engineers in the design process.