A storm grate is a lined metal opening usually along a curb on residential streets in cities, that is a pipe inlet for rainwater runoff from storms. Typically, a storm grate channels rainwater into an independent piping system that carries it to a natural waterway in untreated form, such as a river or ocean. Many cities, however, tie their drainage grills directly into one set of piping that is part of the sewer system, instead of building two independent sets of underground pipe. This increases the volume load on waste water treatment facilities during storms and can cause untreated sewage to be released into the environment at such times.
Storm grate designs vary by nation and climate location. Cities or urban areas close to a large natural waterway may have streets that contain no storm grates at all, such as some beach regions in the US where runoff is channeled directly to the Atlantic Ocean. Ocean City, Maryland, in the US, is a modern city that uses the same practice of eliminating storm drains entirely in urban sections close to the Atlantic shore. By contrast, some cities like Kobe, Japan, are rated as being among the cleanest in the world, in part because they have completely separate roadway drainage systems for rain runoff so that no untreated sewage is released into the environment.
Cities like New York City, New York, in the US that have an extensive underground subway, often have a dual-purpose air ventilation and storm grate system on surface streets. This can result in flooding of the subway system during intense storms, as happened in New York City in August 2007. The solution to such problems in this location was to elevate the storm grate above the surface of the road, so that it minimized the amount of rain that it captured during storms, but it also continued to act as a ventilation shaft for the subway.
Storm drains in large metropolitan areas are sometimes seen as very detrimental to the local ecology of the region, because the rain they capture carries along with it any pollution or debris it picks up from city streets and buildings. Part of the solution to this is to make the storm grate openings of a fine enough pattern of slits or grid work that only the smallest possible debris can wash past them. Immediately underneath a storm grate is a depression known as a catch basin, which is designed to trap debris that gets past the storm grate. While these rain water pits, known as gulley-pots in the UK, are effective short-term solutions, they must be periodically pumped clean by the city with vacuum-powered equipment. The water itself is often still channeled into the waste water sewage system, which can overload it and cause a premature discharge of untreated sewage during intense storms.