A storm drain system is designed to drain excess rain and ground water from paved streets, parking lots, and sidewalks. Storm drains vary in design from small residential dry wells to large municipal systems. They are fed by street gutters on most motorways and freeways as well as in towns which experience heavy rainfall or flooding and additionally in coastal towns. Storm drainage systems are typically designed to drain storm water into rivers or streams. The two main types of stormwater drain inlets are side inlets and grated inlets. Side inlets are located adjacent to the curb and rely on the ability of the opening to capture flow. Grated inlets have gratings or grids to prevent large objects and debris from falling into the sewer system. However, their bars are fairly widely spaced so that the flow of water is not impeded.Consequently sediment and many small objects can fall through. Some of the heavier sediment and small objets may settle in a catchbasin, or sump,which lies immediately below the outlet, where water from the top of the catchbasin reservoir overflows into the sewer proper. The catchbasin serves much the same function as the "trap" in househole wastewater plumbing in trapping objects. In the United States, unlike the plumbing trap, or more commonly known "P-Trap", the catchbasin does not necessarily prevent sewer gases such as hydrogen sulfide and methane from escaping.