Evaluation of the effect of green roofs to reduce the risk of flooded networks

A short study has been performed in order to assess the potential effect of green roofs concerning reduced demands for pipe dimension. The study has been performed using hydraulic models of the stormwater network. The stormwater model is fictitious even though it is a model of a real network the amount of impermeable surfaces connected to the network are augmented to make them critical to changes i.e. that increased loads causes flood. Two different types of roofs have been tested at two different climate conditions from Sweden. The study was performed with mike urban (Dhigroup.com) a market leading modeling software in Sweden. The modeling software has tools for supporting calculations of different SUDS (sustainable urban drainage systemes) such as green roofs, bio retention cells and infiltration trenches. The tools are quite similar to the tools included in another software SWMM (EPA) The build-up of the roofs in the model are modelled after a real roof in Kiruna. One roof consists of a thicker sedum-herbs-grassroof with a thickness of approximately 110-140 mm and the other 40 mm thick. None of the roofs had any technical constructions under in order to delay the runoff. Only water absorbing mats for the vegetation. Two different climates were simulated using precipitation, and temperature calculated evaporation using open data from the Swedish Meterological and Hydrologic Institute, SMHI. The locations chosen was from Kiruna in the north of Sweden and from Malmö. Only summer periods were studied. The main mechanisms for stormwater reduction and detention are 1. Reduction of the total runoff due to wetting of the materiel. When the water content in the soil is under the field capacity there will be an additional loss for saturating the material to make it drip through. 2. Slowing the runoff for more intense rains by detention. Green roof substrates are quite permeable but for heavier rains it can be limiting and thus leading to temporary storage in the pores. Sometimes there can be underlaying constructions for detention. First yearly simulations were performed for the two different types of roofs at two locations. In the study yearly runoff was evaluated which differs between different roofs and different climates. Furthermore, the variation of saturation of the roofs were analysed. Secondly development was simulated by increasing the connected impermeable surfaces connected to the model by 10% and 40%. Then design rains with a return time of 10 year was simulated with floodings as a result (since the models were altered to be critical). Then the number of green roofs were increased until the flooding vanished. Different initial saturations were tested. For 30-70% of all rains >6mm in summertime in Kiruna no runoff was calculated. For Malmö the result was between 50-95%. The cause of the reduction is due to storage in unsaturated soils which has been dried up between rains. The increased area connected impermeable surfaces had to be matched with a little more than an equal number of green roofs. The main and in a way an obvious conclusion was that a warmer climate results in a roof that dries up more often thus allowing it to absorb all water from many smaller rain events. Another conclusion was that a thicker roof has more capacity than a thinner roof to absorb water. It is also quite logical that an increase of permeable surfaces has to be matched against the same number of green roofs to remain at status quo.