| In the background of urbanization, water system and natural water process of many cities and areas around the world are affected by different factors nowadays. It triggered a series of problems, such as the impervious surface increased, the runoff cofficient increased, and the haversting time decreased, so the problems of the stormwater flood and water pollution became more and more prominent. How to control the negative effects resulting from stormwater management failures became a hot point which has attracted attention of many countries. In fact, an effective approach of stormwater management is not only beneficial to environment, but also good for economy and aesthetics. Low Impact Development(LID) is a sustainable landscape way to simulate natural hydrological status and maintain natural water course, currently LID technology research and application development has become the focus in China.Based on the concept of LID in Sponge City, combined with the green building community ecological planning and construction needs, a three-beds rainwater gravel wetland was designed, a pilot test has been carried out in the demonstration base. The first flush effect of runoff was studied, the correlation between the various pollutants and the performance of the device to control rainwater runoff water quality and quantity was discussed as well, a relatively complete evaluation method of the gravel wetland was given out.The experiment analysis of runoff qualities showed that, in runoff, the EMC(Event Mean Concentration) of suspended solid(SS) was 26.38~432.33 mg/L, the EMC of chemical oxygen demand(COD) was 15.21~ 187.35 mg/L, the EMC of total phosphorus(TP) was 0.06~1.57 mg/L,the EMC of total nitrogen(TN) was 1.23~5.33 mg/L, the EMC of ammonium nitrogen was 0.27~2.31 mg/L,the EMC of nitrate nitrogen was 0.28~3.52mg/L,, and rainwater runoff water quality was generally worse than the V class standard of surface water quality. Under different rainfall conditions, there was first flush effect of different pollutants, different pollutants exhibited different flush characteristics, in which, the first flush curves of SS, COD and TP deflected obviously above the balance line. In runoff, COD and TP maintained a good correlation with SS, the correlation coefficients were 0.797 and 0.572. In order to reduce the pollution load and congestion of the device, so the content of each pollutant can be reduced through sedimentation and filtration by using pretreatment measures.The analysis of overflow and penetration water quality showed that, the overflow water quality did not meet the landscape water standard, the pollutant reduction of overflow mainly depended on the co-adsorption and interception of plants and soil, pollutants had a progressively decreasing trend,and the redution had a great relationship with external sources, which requires cleaning regularly. The penetration water quality was pretty good, the EMC of SS was 6.22~9.21mg/L and the removal rate of SS was 45.4%-95.4%, the EMC of TP was less than 0.5 mg/L and the removal rate of TP was 9.1%-82.9%, the EMC of TN was 0.63~3.16mg/L the removal rate of TN was 30.8%-66.8%, the total removal rate of ammonium nitrogen was 28.3%-90.6%, the total removal rate of nitrate nitrogen was 14.1%-80.2%, the EMC of COD was 5.11~75.95mg/L and the total removal rate of COD was 9.2%-90.2%, the penetration water quality was up to the landscape water quality standard.The study on the control effect of runoff amount was as following, gravel wetland had a significantly effect on delaying the runoff peaks, and the delayed time was 5-28 minutes, the peak flow reduction rate was 40.5%-75.3%, and the return period affect the rate greatly, it manifested that, greater the retutn period was, greater the runoff peak was, and smaller the peak reduction rate was. The runoff volume reduction of gravel wetland was 1.77-3.94 cubic meter, the reduction rate was 26.78%~100%, and it was affected by rainfall patterns, rainfall intensity, rainfall duration, and sunny days since last rain, smaller the rainfall intensity was and longer the sunny days were, bigger the runoff reduction volume was. When the service area ratio was 1:2, the landscaping ratio could meet the standard, the delayed time of runoff peaks was 4.51~17.93 minutes and the reduction of runoff peaks was 166.36~220.60 m L/s, the reduction rate was 11.1%~38.5%. The pollutant average reductions were as follows: for TP, the reduction was-0.59g-1.91g; for TN, the reduction was 0.45g-10.07g; for ammonium nitrogen, the reduction was 0.15 g-2.57 g; for nitrate nitrogen, the reduction was 0.10g-6.65g; for COD, the reduction was 2.624g-576.56g; for SS, the reduction was-14.80g-780.30 g. The reduction of TP mostly was negative, because the TP can be hardly removed only through the adsorption of plant roots.When the return period was 0.25 a, the service area ratio was 1:5, the runoff can be completely treated by the gravel wetland, besides, the runoff can be completely treated as well on the condition that the return period was 0.5a, the service area ratio was 1:2 and the return period was 3a, the service area ratio was 1:1. The reduction of runoff volume was independent of service area ratio and was decided by the return period. In condition of the same service area ratio, longer return period led to small peak flow reduction rate.The analysises above showed that the gravel wetland had an obvious effect on reducing and delaying runoff peak, so did on purifying water quality. The design and evaluation method was proposed for the gravel wetland, the maintence advices were given as well. This study will provide a theoretical basis for the implementation of this technology in the green building community, it has an important practical significance to the technology implementation in the demonstration project. |
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