Lars Bengtssons green adventure

Lars Bengtssons green adventure 41 Lars Bengtssons green adventure Justyna Czemiel Berndtsson Sydvatten AB, Ideon Science Park, 223 60 Lund, Sweden justyna.berndtsson@sydvatten.se During his fruitful carrier Lars Bengtsson worked mainly with the white and the blue: snow and lakes. However, during the first decade of the 21 st century he had a green period: a green roof adventure. Although he may have been suspecting that other engineering solutions may be more efficient and cost effective in preventing urban flooding than the green roofs can be, he recognized and valued the many diverse benefits green roofs may provide in urban environment. These benefits together would justify the establishment of green roofs. Today we know that besides providing aesthetic values green roofs may contribute to: Reducing and attenuating stormwater runoff which lowers risks of urban floods and improving the urban water balance to approach the natural one (e.g. Bengtsson et al., 2005, Mentes et al., 2006, VanWoert et al., 2005); Achieving thermal benefits: cost of heating and air conditioning and urban heat island effect can be reduced (Fang, 2008, Takebayashi and Moriyama, 2007, Wong et al., 2007, Wong et al., 2003); Obtaining noise reduction (investigated numerically by Van Renterghem and Booteldooren, 2009); Reducing air pollution (Currie and Bass, 2008, Yang et al., 2008); Provision of wildlife habitat and biodiversity enhancement (Brenneisen, 2003, Dunett et al., 2008a, Gedge and Kadas, 2005). Nowadays numerous researchers are working on investigating different aspects of green roofs. However, as shown above most of the publications are rather recent. At the very beginning of the 2000s not much was known and published in the field of green roofs in the international literature in English. It was then that Lars Bengtsson hydrologist invested a part of his time and enthusiasm to answer the water riddles of green roofs. The main questions he posed are

42 Justyna Czemiel Berndtsson What is the runoff reduction rate and what is the seasonal variation? Which factors does the roof storage capacity and runoff dynamics depend on? What is the role of green roofs in reducing flooding in urban drainage? He also supported research on runoff water quality from green roofs. Many answers have been provided and lots have been learned. You find the answers in the following articles, among others: 1. Bengtsson, L., Grahn, L., Olsson, J. Hydrological function of a thin extensive green roof in southern Sweden. Nordic Hydrology, 2005, 36, 3, 259 268. 2. Bengtsson, L. Peak flows from thin sedum-moss roof. Nordic Hydrology, 2005, 36, 3, 269 280. 3. Bengtsson, L. Hydrological response of sedum-moss roof. Conference abstract, AGU Fall meeting, 2004. 4. Bengtsson, L. Runoff from green roofs. Vatten : tidskrift för vattenvård, 2002, 58, 245 250. 5. Czemiel Berndtsson, J., Bengtsson, L., Jinno, K. Runoff water quality from intensive and extensive vegetated roofs. Ecological Engineering, 2009, 35, 3, 369 380. 6. Czemiel Berndtsson, J., Bengtsson, L., Jinno, K. First flush effect from vegetated roofs during simulated rain events. Hydrology Research, 2008, 39, 3, 171 179. 7. Czemiel Berndtsson, J., Emilsson, T., Bengtsson, L. The influence of extensive vegetated roofs on runoff water quality. The Science of the Total Environment, 2006, 355, 1 3, 48 63. 8. Villarreal-Gonzalez, E., Bengtsson, L. Response of a Sedum green-roof to individual rain events. Ecological Engineering, 2005, 25, 1, 1 7. A short summery below provides an overview of the major findings. Bengtsson et al. (2005) shows that the green roofs reduce the amount of storm water runoff. For the investigated extensive green roof at Augustenborg the runoff reduction was found to be 46 %. Bengtsson et al. (2005) report the runoff reduction measured during study period for extensive vegetated roofs on monthly basis with the lowest obtained for February (19 %) and the highest for June (88 %). The runoff reduction rates show seasonal variation. During September February being 34 % and March August 67 %. Villarreal and Bengtsson (2005) showed that weather condi

Lars Bengtssons green adventure 43 Lars Bengtsson supervising rain simulation experiments on green roofs at Augustenborg during summer 2001. While the students simulate rain on the roof Lars stays on the ground to keep an eye on the runoff. tions (dry or wet) affected the retention capacity of studied green roof; for dry conditions 6 12 mm rain were required to initiate runoff; for wet con ditions the response was almost straight. The relationship between a green roof water retention and rain event size and intensity changes has also been investigated. Bengtsson et al. (2005) found that the water storage capacity of the studied green roof was related to the rain intensity variations. The vertical percolation process was domi nating the rainfall-runoff and ibid. suggested that Darcy s law is not appli cable for the horizontal flow in the macro-pore system. Villarreal and Bengtsson (2005) confirmed that water retention within the green roof de pended to a great extent on rainfall intensity the lower the intensity the larger the retention. For rain intensity 0.4 mm/min and slopes 2 and 14 the retention was 62 and 39 % of the simulated precipitation, respectively. For rain intensity 1.3 mm/min and slopes 2 and 14 the retention was 21 and 10 % of the simulated precipitation, respectively. Bengtsson (2005) reports that runoff produced by a rain event on wet green roofs is delayed and the runoff peak reduced compared to the rain intensity peak.

44 Justyna Czemiel Berndtsson Bengtsson (2005) found that the green roof runoff observations were in agreement with the soil characteristics determined in the laboratory. The difference between field capacity (45 %) and wilting point (15 %) corresponded well to 9 mm storage measured for 30 mm soil substrate. Bengtsson et al. (2005) observed that runoff from green roof does not occur until the soil is at field capacity. For the studied extensive vegetated roof it was 9 10 mm of rain if rain occurred after a dry period. Regarding the runoff process it was observed for more intense rain events that the storage on the roof could increase above the max storage capacity, for studied example the storage computed as the difference between the rain intensity and runoff intensity was 12 mm. The different studies on slope influence on green roofs runoff retention capacity bring different result: while some studies find no correlation between roof slope and runoff (Bengtsson, 2005, Liesecke, 1998, Mentes et al., 2006) the others observe that runoff retention may depend on slopes (Getter et al., 2007, VanWoert et al., 2005, Villarreal and Bengtsson, 2005). The effect of the slope on runoff retention combines with the effect of other factors as the physical properties of the roof soil, length and intensity of precipitation event studied (or the length of the observation period) and flow conditions (saturated or unsaturated, overland flow or not), the design of green roof layers and the presence or not of different type drainage materials. Villarreal and Bengtsson (2005) through experiments found out that the slope of a green roof did not influence the shape of the direct runoff hydrograph, i.e. peak flows and stormwater volumes. This means that the slope does not influence the response of the system to different rain events. However the slope (and the rain intensity) influences water retention within the green roof: the lower the slope (and intensity) the higher the retention. For different rain intensities the roof water retention depended on slopes: slope 2 % showed up two double as much retention capacity as compared with slope 14 %. Bengtsson (2005) derived IDF curves for rainfall and runoff from studied extensive green roof in southern Sweden and showed that the runoff with 1.5 year return period corresponds to rain of 0.4 year return period; the runoff IDF curve of 0.5 year return period followed the rain IDF curve of a 0.1 returned period. Key questions regarding runoff quality from green roof that were posed are: does and how much the green roof contributes to increased/decreased urban runoff pollution load? Our study found that the studied green roofs are a source of phosphorus and that most of it appears in the form of phosphates (Czemiel Berndtsson

Lars Bengtssons green adventure 45 et al., 2006, Czemiel Berndtsson et al., 2009). We also found that nitrate nitrogen is retained by the vegetation and soil. If the concentrations of nitrogen in ammonia form are compared in precipitation and roof runoff the later are substantially less. However, the concentrations of total nitrogen in rain and vegetated roof runoff water are very similar. The available studies indicate that there is no substantial release of heavy metals from extensive green roofs. Generally the thin soil of extensive green roofs show no significant influence on runoff quality regarding heavy metals content and the concentrations found in runoff water are the same as concentrations in precipitation. However, if the reduced runoff volume is considered, the green roofs would generally reduce the loads of heavy metals to urban runoff. The concentrations of heavy metals in green roof runoff are generally lower than concentrations of heavy metals in urban runoff from hard surfaces (Czemiel Berndtsson et al., 2009). We have also observed that higher concentrations of a number of studied components (total phosphorus, ammonium nitrogen, nitrate nitrogen, zinc and copper) were found in first runoff samples than in the samples taken at higher runoff depths. This is interpreted as an occurrence of a first flush effect. Potassium and dissolved organic carbon do not show any first flush effect (Czemiel Berndtsson et al., 2008). In conclusion Lars Bengtssons research showed that green roofs contribute to lowering the storm water runoff in urban areas. At the same time another researcher from Lund University, Lars Eklundh 1, finds in his research that more green areas bring more rain even locally. It is enough with grass. Lars Bengtssons green adventure is hopefully never finished: be it on the roof or on the ground. Grey, dear friend, is all theory; green is life s precious tree (J.W. von Goethe). Let us always take time to enjoy it. References Bengtsson, L., Grahn, L., Olsson, J., 2005. Hydrological function of a thin extensive green roof in southern Sweden. Nordic Hydrol, 36 (3) 259 268. Bengtsson, L., 2005. Peak flows from thin sedum-moss roof. Nordic Hydrol, 36(3) 269 280. Brenneisen, S., 2003. The benefits of biodiversity from green roofs- key design consequences. Conference proceedings Greening Rooftops for Sustainable Communities: Chicago 2003. 1 http://sverigesradio.se/sida/artikel.aspx?programid=406&artikel=950593

46 Justyna Czemiel Berndtsson Currie, B.A., Bass B., 2008. Estimates of air pollution mitigation with green plants and green roofs using the UFORE model. Urban Ecosystems, 11, 409 422. Czemiel Berndtsson, J., Emilsson, T., Bengtsson, L., 2006. The influence of extensive vegetated roofs on runoff quality. Science Total Environ, 355 (1 3) 48 63. Czemiel Berndtsson, J., Bengtsson, L., Jinno K., 2008. First flush effect from vegetated roofs during simulated rain events. Hydrol Research, 39 (3) 171 179. Czemiel Berndtsson, J., Bengtsson, L., Jinno K., 2009. Runoff water quality from intensive and extensive vegetated roofs. Ecol Eng, 30, 271 277. Fang, C-F., 2008. Evaluating the thermal reduction effect of plant layers on rooftops. Energy Build 40, 1048 1052. Gedge, D., Kadas, G., 2005. Green roofs and biodiversity. Biologist 52 (3) 161 169. Getter, K.L., Rowe D.B., Andresen, J.A., 2007. Quantifying the effect of slope on extensive green roof stormwater retention. Ecol Eng, 31, 225 231. Liesecke, H.-J., 1998. Das Retentionsvermögen von Dachbegrünungen; Wasserspeicherfähigkeit, Wasserrückhaltung, Abflussverzögerung und Abflussbeiwerte unter besonderer Berücksichtigung von Extensivbegrünungen. Stadt und Grün 47 (1) 46 53. Mentens, J., Raes, D., Hermy, M., 2006. Green roofs as a tool for solving the rainwater runoff problem in the urbanised 21 st century. Landscape urban planning, 77, 217 226. Takebayashi, H., Moriyama, M., 2007. Surface heat budget on green roof and high reflection roof for mitigation of urban heat island. Build Environ, 42, 2971 2979. VanRenterghem, T., Booteldooren D., 2009. Reducing the acoustical façade load from traffic with green roofs. Build Environ, 44, 1081 1087. VanWoert, N.D., Rowe, D.B., Andresen, J.A., Rugh, C.L., Fernandez, R.T., Xiao, L., 2005. Green roofs stormwater retention: Effects of roof surface, slope, and media depth. J Environ Qual, 34, 1036 1044. Villarreal, E.L., Bengtsson, L., 2005. Response of a sedum green-roof to individual rain events. Ecol Eng, 25, 1 7. Wong, N.H., Chen, Y., Ong, C.L., Sia, A., 2003. Investigation of thermal benefits of rooftop garden in the tropical environment. Build Environ, 38, 261 270. Wong, N.H., Tan, P.Y., Chen, Y., 2007. Study of thermal performance of extensive rooftop greenery systems in the tropical climate. Build Environ, 42, 25 54 Yang, J., Yu, Q., Gong, P., 2008. Quantifying air pollution removal by green roofs in Chicago. Atmosph Environ, 42, 7266 7273.