Yes of course, it rains on every garden, but does that mean every garden functions as a rain garden? Read on and learn what you can do to make a functioning rain garden in your yard. In urban areas, rain falls on land that has 30-50% impervious surfaces. The polluted run-off is collected by storm sewers and piped directly into streams. After heavy rain, urban streams can’t hold all that water, and this can lead to erosion and flooding. Worse yet, water in storm sewers may get polluted by contamination from old leaky sanitary sewer pipes. The purpose of rain gardens is to slowly soak the rainwater into the ground