| Per capita water resources in Kunming Dianchi Lake is less than 300m3, meanwhile that remain at extremely low levels in the country. The coexistence of resource, water quality and engineering type water shortage had become a serious restriction to sustainable economic and social development of modern Kunming. While, for the past few years, the tail water recycling has been widely used in wastewater treatment plant, therefore, exploring the characteristics of water quality residential district and rainwater resource utilization system (especially the shallow green on underground space roof, sunken greenbelt and other indirect use system) can provide technical and practical support for the construction of rainwater harvesting facilities of Kunming.Kunming downtown is a basin around Dianchi Lake, the construction area of city is divided as lakeside plains, alluvial plains and plateau, hills and mountain edges three parts. There are huge difference between the surface soil structure and the underground water level of different regions, but the main factor controlling infiltration of rainwater are the characters of topsoil and the underground water level. The seepage system construction of sunken green in Kunming need to take full consideration to the permeability and the effective depth of backfill soil, even the depth of underground water and the impact of the rainy season. The water level difference between underground water level and epipedon should be greater than 100 ～ 150cm when construction, and greater than 150cm is more better. What’s more, the sunken lawn also required to have good soil permeability coefficient (steady infiltration rate between 1 X 10"3 and 1 X 10"5 cm/s) and the certain effective thickness of aquifer (0.5～1m) to make sure that soil has benign stagnant water and water absorption capability. The water level difference between underground water level and epipedon plus the lifting height of underground water equal to the total thickness of sunken greenbelt. The lifting height of underground water in Kunming area is about 0.2m, the soil thickness of sunken green should≥1.7m. In order to ensure effective regulation and storage, infiltration and use of rainwater, What’s more, the sunken green would not be flooded under normal circumstances, so we can not consider the impact of flooding on green plants while designing, and rainwater can be completely infiltration in 2-3 h when the thickness of hydrops is 10cm.The pollutants of the rainwater mainly originate from the erosion to the underlying surface during the process of rainfall and the transport of pollutants in surface soil. The water quality of non-landing rainwater in Kunming is better than Grade I water quality standards of "Surface Water Quality Standards"(GB3838-2002). As for the urban residential district, the underlying surface can be divided as:roofing, green, and roads. The Variation of runoff rainwater is that the content of pollutants in it are increased in the order of roads>roofing>green. While, the pollutant content all present the trend of dropping rapidly at first and then decreasing slowly or tending to steady with the duration of rainfall run-off increasing. What’s more, the quality of rainwater runoff mainly due to the impact of rainfall conditions, land surface conditions, rainfall two consecutive intervals, regional meteorological conditions and atmospheric deposition. When the amount, intensity, duration of rainfall increase, the quality of rainwater runoff would improve correspondingly. In contrast, increasing the time interval of two consecutive rainfall will make the quality of rainwater runoff worse. However, the pollutant varies quickly when the average temperature in the region is high. The more stable the atmospheric air environment is, the worse the quality of rainwater runoff would be.The content of COD and SS that in the runoff rainwater of roofs, roads and green are at high levels in Kunming urban residential area, wherein pollutants in the first rain of the rainy season and the runoff rainwater of roads is most serious, the initial rainwater or the EMC value is worse than the water quality of Dianchi Lake. Collection, processing and resource utilization to the runoff rainwater of residential district in Kunming are the necessary means to achieve sponge city of Kunming and protect water quality of Dianchi Lake.For the collection and utilization project of rainwater in Kunming, its investment depends on the design scale, and the scale depends on the local daily rainfall. Although rain specification calculation method is proposed by Rainwater Technical Specifications, but there is no specific parameters. The reasonable indicator be confirmed through research daily rainfall selected as 25.5mm, that rainwater collection rate (control rate) can reach 85%, to achieve a good environment and economic benefits under the investment be reduced largely, and the achievements have been identified as the main parameters of rainwater utilization engineering design scale; At the same time research has identified as the collect scale of rainwater, the abandoned flow of initial rainwater and pretreatment of initial rainwater of different underlying surfaces under different conditions, providing theoretical and practical application guide for the rainwater utilization project.Artificial improvement shallow green on the urban underground space roof is a new type of artificial penetration and storage utilization system, and it is one of the effective methods to solve efficient use of urban rainwater and inadequate water storage capacity of the regional construction projects with its regulating and flood storage, purification, storage utilization and other function. Shallow green on the top of underground space roof is conducive to sponge city construction of Kunming because of in most case it will not storage water saturated and produce runoff.The implementation of rainwater utilization project in Kunming can take good advantage of utilizing rainwater, reduce runoff rainwater effectively and reduce the amount of pollutants in runoff rainwater. It provide technical and practical support for the large-scale treatment and utilization of rainwater in Kunming. |
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