I learned that term.."Thousand year rainfall" back when the ex was a city official for a small town of 5,000. I asked him too what they meant. He said it is actually referring to a 100 year flood probability, and municipalities hired the city engineers to design and implement the drainage culverts to accommodate a certain amount of water. So the town we lived in, installed larger storm drains for the larger amounts of rain.
according to wiki;
"Regulations and local building codes[edit]
Building codes and local government
ordinances vary greatly on the handling of storm drain runoff. New developments might be required to construct their own storm drain processing capacity for returning the runoff to the
water table and
bioswales may be required in sensitive ecological areas to protect the
watershed.
In the United States, cities, suburban communities and towns with over 10,000 population are required to obtain discharge permits for their storm sewer systems, under the
Clean Water Act.
[20] The
Environmental Protection Agency (EPA) issued stormwater regulations for large cities in 1990 and for other communities in 1999.
[21] The permits require local governments to operate stormwater management programs, covering both construction of new buildings and facilities, and maintenance of their existing municipal drainage networks. Many municipalities have revised their local ordinances covering management of runoff. State government facilities, such as roads and
highways, are also subject to the stormwater management regulations.
[22] Many local municipalities have placed ordinances for both commercial and residential stormwater management practices to be designed, implemented, and approved before an occupancy permit is released."
https://en.wikipedia.org/wiki/Storm_drain
