By Jennifer Drake
More than half the world’s population lives in metropolitan centers. The built environment of a city is very different from that of rural and natural areas. When it rains over a rural landscape, much of the rainwater sinks into the ground or is evaporated or transpirated by trees, crops and other plants.
Transpiration is the biological process in which plants pull moisture out of the soil by their roots and release water vapor to the atmosphere through small openings in their leaves. Every day, plants release large quantities of water vapor. Growing plants can transpire up to 10 times as much water as they hold in their stems and leaves. These slow, natural processes allow precipitation to replenish groundwater and sustain vegetation, leaving only a small amount of water as overland flow or runoff.
In urban areas, though, when it rains, it pours! City streets, rooftops and parking lots make for hard surfaces that convert huge amounts of rainwater directly into runoff. In a typical city block, stormwater is collected by drains and catchbasins which then convey it underground through sewers. Finally, it’s discharged to a natural water system such as a creek, river or lake. Often stormwater is released without any pollution management. That’s a problem, since as water runs over urban surfaces, it picks up bacteria, heavy metals, nutrients and particulates.
In cities, the source of water that replenishes our aquifers is completely altered. Under natural conditions, aquifers are sustained through the infiltration of clean rainwater and streamwater into the ground. But for urban environments, these sources are cut off by the impervious landscapes. Instead, water from leaky sewers, water mains, septic tanks and landscape irrigation becomes the source for groundwater recharge. Since many of these sources are wastewater, they’re poor-quality sources and can lead to groundwater contamination.
Recent innovations in how to deal with urban stormwater are working to prevent runoff near its source and to provide pollution prevention and quantity control.
Imitating nature to solve an urban problem
Low Impact Development (LID) is a planning and design approach that aims to mimic naturalized water balances. It combines infiltration, evaporation and transpiration while limiting runoff. The goal of LID is to restore processes that are lost in a built-up urban environment. LID includes several types of low-level new and innovative stormwater technologies that together let water infiltrate the ground and evapotranspire into the air.