By Jörg Breuning
Any impervious area of urban development prevents precipitation from infiltrating into the ground. Instead of soaking up the water, it redirects it to another area. By redirecting the stromwater into other areas, the run-off will accumulate elements that are on this impervious area and pollute the run-off water.
Looking for Stormwater Retention (direct contact of humans with water)
Over the last 5 decades multiple studies show that these pollutants have a measurable negative impact to the watersheds and ultimately the lakes, bays or oceans. Since humans are in direct or indirect contact with these bodies of water (through swimming, potable water, irrigation of land grown food, aquatic or seafood) these pollutants have great impact on our health and causes billions of dollars to be spent on health care. The majority of pollutants in our water sheds are caused by chemicals of farmland, substances from vehicles (antifreeze, oil rubber) or simple trash that is carelessly discarded. For example, in many areas people are advised to reduce the consumption of local fish to less than one time a week because this food source is too polluted. It is questionable whether vegetables from farms (including rooftop farms in polluted cities) or other meat from mass husbandry are a good alternative.
With smart stormwater best management practices (BMPs), pollution can be controlled effectively. It can't be diminished to a value that equals the natural environment previously - before the location was populated. Decreasing polluted stormwater run-off is a responsibility of everybody (not only the government) and it will pay-off by creating a livable environment for humans. If we don't make this investment and make it mandatory, our living quality will decrease and the value of our environment will drop over time. People will eventually move to healthier places, but these places are limited. We only have one earth.
Considering that an extensive green roofs with a 4 inch depth can retain 50% or more of the annual rainfall, they are the most efficient “direct on source” solution. If these extensive green roofs are engineered, installed and maintained properly, there is no pollution in the reduced, delayed or remaining run-off and for sure substantially less comparing to an impervious area on the ground. Comparing to other on-source stromwater BMP’s (e.g. pervious pavements) the costs of extensive green roofs are minimal – although still an investment for the building owner. Green roofs can be perfectly combined with any other stormwater BMP what increases the efficiency.
Official numbers show that the imperiousness of residential properties is around 30% and commercial land at around 65%. The implementation of extensive green roofs on commercial properties is typically cheaper because the roof areas are bigger and easier accessible. Green roofs on large scale commercial buildings (warehouse belts around cities) will reduce the run-off dramatically and in addition will reduce the heat island effect by lowering the air temperature that is transported to the surrounded city.
If these commercial owners would choose integrated vegetation and solar (PV) systems, the costs of the green roof installation and maintenance could be off-set in less than 10 years and there would be no additional costs that increase the product of this company. Since this environmental step is an investment, these eco-conscious companies would also reduce their tax on profits in the short term. However, reducing short term profits doesn't seem popular in the current economic environment and as long as there is no eco-rating of these companies. In this case LEED is counterproductive.
Water is the number one source for life on earth and any on-source reduction of run-off from the production process of goods or impervious areas will increase the living conditions tremendously, avoid expensive battles for clean water, and allow the human population to grow healthy and protect the native environment.