| Rain garden is a common ecological facility in urban runoff control of Sponge City and Low Impact Development.With the increasingly serious problems caused by runoff such as non-point source pollution and urban waterlogging,the correlation study on the rain garden had been deepened gradually.But,most of them were concentrated in Beijing,Shanghai and other cities with high development and serious runoff problems while the study on the rain garden in island area was still in the preliminary stage.Therefore how to design the rain garden rationally and optimally according to the hydrogeological conditions,climatic characteristics in specific areas,that to maximize the engineering and environmental benefits has become one of the most hot issues in applied research of the rain garden.According the climate,geography,rainfall and runoff characteristic of Zhoushan archipelago,the optimum design of the rain garden,indoor simulation device experiment,demonstration application engineering and model simulation were combined to study the application of the suitable rain garden in island region.The effect of saline-alkali on the fillers and plants design was discussed in the optimal design of the sea-island rain garden.The indoor simulation devices were constructed to discuss the effect and influence rule of its treatment on runoff pollutants from inflow load,plant species and salinity,and to discuss the effect and change rule of its hydrological regulation on runoff from infiltration rate,runoff outflow time and water storage.Shengsi island was chosen as the application demonstration site,the design and construction of the rain garden,the control effects,and the evaluation of engineering benefits were discussed.The research results can provide scientific basis and technical guidance for the design and construction of sponge facilities such as rain gardens in island and saline-alkali areas,as well as the comprehensive evaluation of the runoff control effect.Following main conclusions are obtained:（1）The combination of salt-alkali tolerance measures and the rain garden design was a design characteristic of the suitable rain garden in island region.The adsorption and its influence of fillers on pollutants and the tolerance of plants under different salt-alkali conditions were studied.Salt-alkali had a negative effect on the adsorption of NH3-N by the fillers,which were dominated by ion exchange adsorption,and promoted the TP adsorption of low concentration.When the combined fillers and its proportion was ten percent perlite,ten percent expanded vermiculite,red soil and at least forty percent sand,it could meet the requirement on permeability coefficient of the rain garden,and maintain better removal efficiency of nitrogen and phosphorus under low salt-alkali conditions（salinity was 0.5%,pH was 8.0）.The salt-alkali tolerance of the four plants was from the highest to the lowest:Phyllostachys heteroclada Olive,Lythrum salicaria L,Iris lacteal Pall.var.chinensis（Fisch.）Koidz,Canna indica L.When the salinity was 0.74%and pH was 8.5,plants began to be affected by saline-alkali stress.Salinity of 2.08%was the salt-tolerance threshold of Canna indica L.（2）The experiment on purification effect of runoff with simulation device results showed that:the planting of plants was beneficial to the removal of pollutants on runoff and the stability of the system;inflow load had great influence on pollutant removal,the comprehensive removal rate of the rain garden was the best under the condition of inflow load three,and its value was 68.55%;the rain garden has a certain salt-alkali tolerance,the removal rate of PO43+and TP,which were mainly removed by filtration and adsorption of the fillers,was less affected,and the average removal rate was above 78%;the salt-alkali has different degrees of negative effects on COD and nitrogen which were mainly removed by microorganism;under the condition of low salt-alkali（salinity was 0.74%,pH was 8.5）,the average removal rate of COD and NH3-N remained above 60%,and the average removal rate of NO3-N was less than 20%;the removal rate of TSS was the best,which was stable above 90%,and basically unaffected by external influences.（3）The experiment on hydrological regulation of runoff with simulation device results showed that:the infiltration rate of the rain garden gradually decreased and then tended to be stable with the prolonging of run time,the decrease of infiltration rate was related to the initial infiltration rate of the system and the characteristics of plant root system;the stable infiltration rate of the two simulators was 3.68×10-66 m·s-1and 2.88×10-5m·s-1,respectively,the planting of Phyllostachys heteroclada Olive and Canna indica L was conducive to the stability of the infiltration rate of the system;the actual runoff outflow time and storage range of the rain garden were 62.37122.37 s and 0.2l13.01 L,respectively,which were significantly different from the theoretical value.This was mainly affected by the initial water content of the filler before inflow,which was related to seasonal temperature.（4）The demonstration project combined a infiltration ditch with the impervious rain garden,which could prevent saline-alkali lateral seepage and infiltration to pollute the surrounding soil and groundwater.The planting soil layer was Shengsi soil with ten percent nutritive soil,the filler layer was fifty percent zeolite and fifty percent expanded vermiculite.The results of rainfall event analysis were as follows:though the Shengsi soil has a high sand content,its particles were fine and soft,the stable infiltration rate was only 2.53×10-66 m·s-1,and the runoff infiltration and outflow were gentle;there was no overflow during the rainfall event,the outflow time of the rain garden was 3050 min,the peak reduction rate was 70.52%91.23%,in the case of small and moderate rainfall events,the total runoff control rate could more than 80%,and the peak flow reduction rate could more than 70%;the average EMC removal rates of the TSS,COD,NH3-N,NO3-N,TN,PO43+and TP were 82.25%,42.17%,66.70%,64.94%,58.63%,65.24%and 76.46%,respectively;when the average conductivity of the inflow was 12.5 ms·cm-1,the alkalinity reduction rate was64.5%.The simulation results of the SWMM were as follows:when the recurrence period was 0.510 a,the runoff amount control rates was 26.33%57.59%,the peak flow reduction rate was 16.11%61.64%,the peak time lay was 15 min;SWMM could well simulate and predict the running effect of the rain garden,the rain garden has better comprehensive hydrological control effect of runoff on medium and small rainfall events. |
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