Methods to Reduce Urban Runoff

A parking lot is basically a waterproof sheet. Rain that would have soaked into soil instead sheets across the asphalt, picking up oil, salt, and litter, and arrives at the creek fast, hot, and dirty, all at once instead of trickling in over days. A grass strip between the lot and the creek works because soil has pore space and roots. Water slows down, sinks in, and the soil and root hairs filter out sediment and grab onto nutrients like nitrogen and phosphorus before they reach the stream. That strip is a riparian buffer, and the slower the water moves through it, the more sediment drops out and the more time roots and soil microbes have to pull nutrients out of solution.

Permeable pavement, rain gardens, and green roofs all attack the same problem from different spots. Permeable pavement has gaps or pores so water sinks straight through instead of running off the top. A rain garden is a shallow planted depression that catches roof or lot runoff and lets it soak in slowly, with plant roots and soil filtering it on the way. A green roof is a layer of soil and plants on a rooftop that intercepts rain before it ever becomes runoff. All three cut the runoff coefficient, the fraction of rainfall that becomes surface flow, and spread the water's arrival at the sewer or stream over more time.

This matters most where storm drains feed into combined sewers, pipes carrying both sewage and stormwater to one treatment plant. Those pipes were sized for the rain patterns of their era. A storm beyond that capacity triggers a combined sewer overflow, dumping raw sewage and stormwater straight into the river to keep it out of streets and basements. Every rain garden or buffer that keeps water out of the pipe in the first place is one less gallon pushing the system toward overflow.