Stormwater is rainwater made dangerous. Of all the water on earth, less than 2% of it is fresh and drinkable, and 75% of that is frozen in glaciers. Rainwater is the primary way that many cities get all of their freshwater for all of its needs, either by river or reservoir. Two centuries ago, we did not have the wherewithal to do anything with rainwater but accept it when it came, soaking the ground. It beat the drier alternative of drought. This worked well in cities with dirt streets and brick sidewalks, as it helped to wash off the horse and human wastes collecting in the streets By the latter half of the 19th century, however, paving the street was becoming more useful than leaving it as bare earth. So drainage systems were installed between 1865 and 1900 to catch the runoff (stormwater) from the streets to convey to the local river. This was the first time the technology for piping, culverts and drainage had become advanced enough to actually do anything about flooding and muddying of the streets. Jettisoning rainwater seemed like a great solution at the time.
Many of these storm drains were built in the 1880s to handle all runoff, both rain from the streets and sanitary from the buildings. It was a tremendous improvement from the middens and open sewers in use then, but a liability today. Now that our cities are (much) more paved and extensive than they were in 1880, with parking lots that did not even exist until the 1930s, even middling rains can overflow these storm sewers and send untreated sewage into local streams. There are three solutions that have been tried in American cities. None of them are cheap. The first is to build a large catch basin to hold the stormwater volume under the combined sewer network, deeper underground. This has been tried in Washington and Atlanta. The second is to separate the sewers, which involves digging up every street in the historic core of the city, widening the trench and doubling the amount of sewer pipe laid under the city, then rearranging all the building and street connections so storm flow goes to storm sewers, and sanitary goes to sanitary sewers. This is the way most of Alexandria built since 1940 was built. The third option is to use green infrastructure and inground filtration practices to retain stormwater within the soil so that it merely trickles into the sewers. As far as I know, only Philadelphia is trying this.
A problem with the jettison model of stormwater is scour. Engineering keeps the road or parking lot dry, but it also pushes in minutes the rainwater that would have percolated or flowed to the ground and streams over hours. In the 1960s and 1970s, civil engineers began to work on this problem with thousands of small retention basins, but this was not enough to reverse the damage of almost a century of flash floods to streams that had been receiving rainwater gradually over millions of years. As the technology of “stormwater management” progressed, stormwater ponds became more sophisticated, larger, and able to handle more of the “first flush volume” and “flood volume”.
At the same time, stormwater management became distributed, not centralized. Alexandria, VA was an innovator in this in the 1990s, with “ultra-urban” storm water management facilities that fit into the tree lawn between street and sidewalk. The notion of using each parcel as a watershed with its own rainwater management became the focus of “low impact development”. Not every parcel is equal in this regard, however. Some soils can soak up water at a safe rate, but most soils are too impervious to percolate quickly enough. Some limestone soils, called Karsts, percolate so quickly that cavitation and collapse can result from too much runoff. Natural soil percolation, runoff, and erosion are kind of a Goldilocks situation.
The typical American suburban stream looks almost like a stream in the desert southwest. A century of flash floods from paving have scoured out the walls to a new equilibrium, with no floodplain remaining. The floodplain has been replaced by a deep trench, with trees and soil falling into the edge of the stream. The width of this trench is determined by the flood volume. Once the trench gets wide enough, the stormwater scour force at the erosion face is less than the cohesive force of the soil.
Look at nearly any stream in the suburbs and you will see this pattern. The typical urban stream is in a pipe by now, as real estate became valuable enough to channelize, bury and fill in the ground over the stream. This disaster for the stream works well enough for real estate, unless the flood volume exceeds the capacity of the pipes. Then basements start flooding.
CityLab, based on data from the Center for Neighborhood Technology
Acknowledging this, stormwater engineers have designed bioinfiltration swales to store rainwater without admitting it to the ground underneath. If we cannot recharge aquifers in developed sites through the soil, at least we can keep the runoff from destroying stream banks and habitats downstream. Stormwater is an evolving field, and it is expensive; involving the movement of cubic yards of soil and rock and placement of tons of concrete and square yards of geotextiles.
All because we needed to keep the ground dry for the use of bicycles and cars.