We have proposed a water disaster prevention mesh network - Rainwater Grid (RGS) . It utilizes the water storage function of tanks to temporarily store rain water during heavy rainfalls. This will reduce the amount of water flowing into the rivers and the ground and suppress the flood damage. In this paper, we explain our design and development of the Smart Rainwater Tank System (SRT). We have confirmed that the system continuously monitors the water level of the tank, stores data and visualizes it as a webpage. It will be pre-discharge when heavy rainfall is predicted. This prediction will be made by pattern recognition of weather data by Self Organized Map (SOM). Live Demo shows to sensing and control of the multiple model tanks as Rainwater Grid System. This concept is recently proposed as Distributed Multi-purpose Civil Dam and is advanced.