Hello Pritam You can model the sloshing effect by refering IS 1893 Part 2 in that the water will be divided into two masses impulsive mass and moving convective mass that will give you the way of indian Standard method of design actually it is ,reffered from ACI_350_3-01_Seismic_Design_of_Liquid-Containing_Concrete_Structures you can either refer it for having some equations to find the masses and respective heights :-)