Eco-hydrological Effects Of Bioretention Facility

Under the global urbanization trend, the impervious surface replaced the open space, it alters the natural hydrologic cycle and brings a series problems of water environment, such as runoff pollutions, water logging and so on. Low impact development(LID) is aimed at minimizing this change and simulating the process of natural hydrologic cycle, it plays an important role on promoting a well hydrologic cycle in regional scale.Bioretention is proved one of the most efficient LID practices. It can remove the runoff pollutions, cut the runoff volume and the peak flow, relieve the thermal pollution, and reduce the impact of regional development on hydrologic cycle. This study had taken an experiment on simulating the running of bioretention units. Using the Stormwater Management Model(SWMM) to simulate the rainfall, compared the simulated results to the experimental results, and then analyzed the capacity on carbon emission reduction of bioretention units. Based on the analysis, discuss the bio-hydrologic effect of bioretention units. The main contents and conclusions of this study are as follows:(1) Study on runoff quality control of bioretention unitsBioretention is a measure which can relieve the water environment problems and remove a lot of types of pollutions. The bioretention units can remove chemical oxygen demand(COD)、total phosphorus(TP)、ammonia nitrogen(NH4+-N) well, but not well on the removal of total nitrogen(TN)、nitrate nitrogen(NO3--N). The removal rate and the stability on removal rate of zinc is better than other heavy metals. Cooper, lead and chromium are all in high removal rates, but cadmium is in a low removal rate. The removal rates of the PAHs are greater than 60% for bioretetion units except naphthalene. Among them, the removal rates of fluorene, phenanthrene, anthracene, fluoranthene, flexion, benzo[ghi] pyrene are relatively stable. The performance of silver sand is better than other media in removing pollutants.(2) Study on runoff quantity control of bioretention unitsBioretention units with different medium depth had obviously control on runoff volume, peak flow and lag time in different return period. Removal effect of peak flow is better than runoff volume, the removal rate of runoff volume are all under 40%. Under the same return period, the larger medium depth, the higher removal rate of runoff volume; the peak flow had not obviously changed when medium depth increased; the lag time increased with the increase of medium depth at a small return period, but had no obvious change at a large return period. When the return period is less than 3 year, the bioretention units can well control the runoff. When the return period is greater than or equal to 3 year, the bioretention units began to overflow. The proportion of overflow increased with the largen of return period. So the assistant storage facilities are needed. Under the same medium depth, the removal rate of runoff volume increased when return period became larger, the peak flow and the lag time were all decreased when the return period became smaller. Silver sand had a good control on the runoff.(3) Contributions of bioretention units to reduce thermal pollution, evapotranspiration and carbon emissionWhen runoff temperature was between 20~35℃, the outflow temperature of bioretention units were between 14~18℃. The bioretention units had obvious control effects to runoff temperature. With the increase of medium depth, the effect became more prominent. The reduction amplitude of runoff temperature increased with the rising of inflow temperature. The humidity of surface soil and the total mass of bioretention units reduced quickly in greenhouse conditions per 24 hours, it can increase the volume of evapotranspiration. On the perspective of the runoff water quality improvement, one square meter bioretention unit can reduce 0.14 kg methane emission per year on average. On the perspective of the runoff water quantity control, one square meter bioretention unit can reduce 0.81 kg carbon dioxide emission per year on average. The benefit to carbon emission reduction of bioretention unit with silver sand is better than other media.(4) Analyze the biohydrologic effects of bioretention unitsBioretention units can well promote the balance of regional water and heat, it has considerable economic benefits. It makes quite a lot contribution to restore the ecohydrologic balance of developed regions.