A rain garden is a very simple landscape feature that traps rain water runoff from streets, walkways and parking lots. The purpose is to increase the value of rain water by allowing the water from excess rainfall to percolate down into the soil where it can be used by plants, and then flow into the local water table, where it has a valuable use, versus running into the city drainage system.
You can see that the construction of a rain garden is very simple. Some soil modification and drains may be necessary in very clay soil types.
No curbs in a parking lot allow the rain water to flow into the rain garden.
This rain garden is typical of landscpaing around the country. The use of ground cover, perennials and trees can easily handle most rain events.