Research On The Running Effect Evaluation And Improvement Design Of Grass Swale

Grass swale is one of the ecological drainage system in the LID (Low Impact Development)facilities, it can remove the pollutants in the runoff effectively. The peak flow and the totalvolume of the runoff can both be controlled at the same time. Precipitation and rainfall intensityare all the important factors that affect operation of grass swale, but the current studies haven’tdivided the running effect of grass swale according to factors mentioned above. And comparedwith the traditional drainage system, the grass swale covers a slightly larger area, theaccumulation and leaching of pollutants are both need to be solved. And all the factors abovelimits the application of grass swale in the high density areas of city.To solve the problems mentioned above, the running effect of grass swale in the heavyrains are evaluated, and the running date of grass swale accumulated in InternationalStormwater Best Management Practices Database is analyzed statistically. The transformationrule of the pollutants in the grass swale is analyzed from two aspects: Statistical analysis andfield monitoring. The standard convey is improved through adding perforated pipe anddecreasing the thickness of planting soil, and the effect of weird and vegetated check dam tothe removal efficiency was also analyzed. The accumulation of pollutants in the soil of grassswale in Shen Zhen Niu Shan park was investigated, and the service life of the swale isestimated.The mian research contents and result are listed below:(1) Compared with small rainfall events, the removal efficiency and stability of standardconvey grass swale in the heavy rains decrease significantly, the average removal efficiency ofSS is only10%, and the leaching of nitrate from the soil of grass swale is obvious, the peakflow and toral runoff volume reduction effect decrease significantly too.(2) The results of statistical analysis shows that not all the pollutants removal efficiencyare related to the concentration of the pollutants in the runoff. For example, the removalefficiency of NO3-、TOC、bacteria、As and Ni don’t have a certain relationship with itsconcentration in the runoff. And the removal efficiency of TKN also associates with theprecipitation and rainfall duration. With the increasing of rainfall, the peak flow reduction effectdecreases gradually. When the rainfall is more than45mm, it is difficult for the grass swale toreflect good runoff reduction effect.(3) The relationship of TP、NH4+、COD、SS removal efficiencywith the length of grassswale comply with the exponential attenuation model, but under different influent concentration, the coefficient of the model changes as well, the NO3-removalefficiencydoesn’t complywiththe rule.(4) The pollutant removal efficiency of swale with perforated pipe promoted significantly,and after adding a weird to the grass swale, the runoff retention capacity significantly increased.But still can’t effectivelyprevent the suspendingof small size(40microns)particles; Comparedwith weird, the vegetated check dam can control the suspending of small particles efficiently.(5) Compared with the surrounding soil, the soil characteristics of the grass swale in theNiuShan park have changed significantly, organic matter in the planting soil shows a trend ofdecreasing overall, and the pH of the soil is a little lower than the other section of the swale.The heavy metal in the soil is a little higher than the background, but still in a good condition.By estimating, the grass swale in the NiuShan park can still run effectively for39years withoutexceeding the limitation of the specification.