Purple-roof

Are Green Roofs True Stormwater Management Tools? Part 2 An audio article by Oscar Warmerdam. ________ Why retention is unreliable for stormwater management at the building scale. Retention can be unreliable because it doesn’t work in repeat storms, large storms, or during cool and cloudy weather. Why? Water that is retained, by definition, leaves the green roof only via evapotranspiration. If there is no time for evaporation, such as during big storms or repeat storms, or if weather conditions do not permit much evaporation, then the retention-oriented green roof sits there like a wet sponge. 2nd-day storms may not be substantial enough on their own to cause trouble but may cause huge problems when combined with the precipitation from the first-day storm. Extra-large storms also often exceed the capacity of the green roof as they outflow with little or no delay. These types of extreme storms happen only a few times a year (5-7), but it is precisely these storms that cause flooding, pollution and river str