Effect Of Double Substrates On Runoff Quantity And Quality

Rapid urbanization and industrialization lead to various problems in urban ecosystem,giving rise to a significant threat to its sustainable development.In order to address these problems,various methods have been proposed,in which the green roof technology has aroused great concern.According to the rainfall water quality and runoff in Tianjin area,testing device of double substrate layer green roof was designed.In order to provide reference data for future design and construction of urban green roofs,this study focused on the effects of green roof substrate type and thickness on regulation of the roof runoff water quality and quantity.Water quality data in terms of green roof,exposed roof and simulated rainfall was collected and analyzed.Obtained primary conclusions are listed as follows:(1)Green roof with double substrate layer could delay the runoff producing time for 10 to 15 minutes and reduce roof runoff by 40% to 50%,which can retard the flood peak flow well and reduce load of the urban piped drainage network;(2)Recorded pH during the test are: PH of simulated rainfall was 4.75,and PH of water on green roof with double substrate layer was 6.45 ~ 6.87,while PH of water on exposed roof was 5.27,indicating that green roofs can increase original rainfall pH,and also reduce the harm of acid rain;(3)Compared to bare turf soil and building roof,simulated runoff turbidity of green roof testing device showed a dramatic downward trend under the combined effect of vegetation and substrate layer;(4)The substrate material of green roof is considered as a release source of runoff conductivity,and the green roof with double substrate layer could reduce runoff conductivity;green roof with double substrate layer showed good performance in terms of removing most heavy metals within runoff.The removal rate was high and was hardly affected by the intensity of rainfall;(5)Lower rainfall runoff velocity,stable hydraulic conditions and a thicker adsorption substrate layer of green roof can be beneficial to reduce the effluent COD;it was indicated that there is a certain correlation between runoff effluent COD and turbidity,although it is not fully consistent.This is probably because either part of the material generating turbidity did not carry considerable organic matter,or organic substances in substrate was degraded with the runoff outflow;(6)Green roof had feasible performance in terms of removing ammonium,nitrate and phosphate;whereas its total phosphorus removal effect was minimal,indicating that the substrate layer of green roof released organic phosphorus;effluent total nitrogen content of the green roof saw a dramatic increase in comparison with that of the building roof,indicating that the substrate layer of green roof released enormous organic nitrogen.