| In order to combat the adverse impact of urbanization on regional water environment,low impact development and sponge city construction have been advocated in many places in China.As small green infrastructures,rain gardens have has been widely adopted as LID measures in the world.Rain gardens are bioretention systems that retain storm water runoff based on ecological engineering.They can infiltrate storm water runoff effectively and improve water quality,modifying the hydrological regime of urban areas to be close to the natural conditions.In this dissertation,we present a field monitoring study and model simulation analysis based on three different sets of rain gardens.First,the inflow and outflow processes of a infiltration rain garden was monitored over 5 years,the effect of rain garden on storm runoff reduction was analyzed;the sustainability of the experimental rain garden was discussed on the basis of soil sampling for particle size analysis and on-site measurements of infiltration rate;subsequently,we predicted the regional hydrological effect of infiltration rain gardens on regional hydrology based on different rain garden installation scenarios in the future.Secondly,we analyzed the effect on runoff reduction and pollutants removal by two rain gardens of different filling mediums.Finally,we analyzed the potential effect of preferential flow on different pollutants removal in two parallel rain gardens that receive road runoff.The major findings of thia research are as below:(1)The experimental rain garden filled with the locally available loess soil has a generally good infiltration capacity,it retained storm runoff from a roof effectively.The rain garden has an area ratio of 20:1 and depth of 15 cm.Over 28 storm events monitored in 5 years,there were only 4 storm events that produced overflow from the rain garden,including 3 in 2013 and 1 in 2014.No overflow occurred in 2011 or 2012.The runoff reduction rate was 100%in 2011 and 2012,96.8%in 2013 and 98.9%in 2014.(2)After 5 year operation of the infiltration rain garden,sand content at the surface layer increased from 7.36%to 20.80%,silt content decreased from 83.19%to 73.49%,and clay content decreased from 9.46%to 5.71%.The infiltration processes of rain garden were monitored for 11 times,the final infiltration capacity of 2.277 m/d in 2015 was slightly lower than the initial value of 2.346 m/d in 2011.The generally stable infiltration capacity of the rain garden over 5 year operation indicates good sustainability of the rain garden.(3)According to the projected urban landuse and potential storm intensity changes of the study area,we predicted different rain garden constructions scenarios.The results show that,to drain storm runoff from 10%of the total urban area into rain gardens,the regional runoff coefficient can be reduced from current 0.58 to 0.49 under the current landuse;the same rain garden construction scale can reduce the regional runoff coefficient to 0.52 under the future landuse.To drain storm runoff from 20%of the total urban area into rain gardens,the regional runoff coefficient can be reduced from 0.58 to 0.41 under the current landuse,and to 0.44 under the future landuse.To drain storm runoff from 30%of the total urban area into rain gardens,the regional runoff coefficient can be reduced from 0.58 to 0.35 under the current landuse,and to 0.35 under the future landuse pattern.(4)Field monitoring on flow reduction through two drainage type rain gardens show that,the median value of runoff reduction for a sand and loess layered cell was 46.8%and a uniform loess medium cell was 38.5%;the peak flow reduction were 53.95%and 47.4%,respectively.The flow reduction effect of the sand and loess layered cell was better than the uniform loess medium cell.Under low intensity rainfall,the runoff reduction was 78.8%in the sand and loess layered cell and 74.0%in the uniform loess medium cell.To achieve the goal of 80%annual runoff reduction for the sponge city construction,the two rain garden cells can meet the requirement with their reduction rates of 98.5%and 92.5%under the low intensity rainfall,69.0%and 62.3%under the moderate intensity rainfall.Low to moderate intensity rainfall events account for 69.8%of total annual rainfall in the study area,indicating that rain gardens can be highly functional in the study area.(5)Water quality monitoring from the two different medium rain garden cells show that,the average removal rate of TN was 64.16%in the sand and loess layered cell and 40.48%in the uniform loess medium cell;the average removal rate of TSS was 69.54%in the sand and loess layered cell and 26.24%in the uniform loess medium cell.The phosphorous removal rate was not evident in both cells.NO3-N concentration in outflow was greater than that in the inflow,showing leaching effect of the subsurface drainage.The sand and loess layered cell showed better removal effect on TN and TSS.(6)DRAINMOD simulation on the flow concentration and the drainage processes of rain garden show that,the model can be used accurately to predict the subsurface drainage volume,comparing with the observed drainage volume during the monitoring period,the Nash-Sutcliffe modeling efficiency was 0.984,and the correlation coefficient was 0.992 with a average deviation of 0.024 m3 on.Based on 57 year(1951-2007)meteorological data,DRAINMOD simulation show that the average volume reduction of stormwater runoff through the drainage rain garden can be 18.5%,76.1%of total stormwater runoff can be purified through the rain garden medium.Rain gardens can be very effective in storm runoff management and urban non-point source pollution control.(7)Monitoring results from two rain garden cells that receive road runoff show that,there exist preferential flow in the garden cells;the removal rate for particulate pollutants was high,but there was a net output of dissolved pollutants.The load removal rate was 69.9%for the total particulat phosphorous and-5.8%for the total dissolved phosphorous;the load removal rate was 73.8%for the total particulate nitrogen and 22.3%for the total dissolved nitrogen.Groundwater monitoring results show that focused recharge through rain garden infiltration resulted in water table rise near the rain garden,the maximum rise was nearly 1 m,but the mounding of water table lasted briefly after rainfall. |
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