Study on Urban Stormwater Pollutants Transportation Processes
|School||Xi'an University of Architecture and Technology|
|Keywords||stormwater runoff nonpoint pollution best management practice firs flush effect water quality model urban environment|
The rapid urban development experienced in recent decades has led to significant changes in both the volume and quality of rainwater runoff, leading to the eventual eutrophication and the consequent wastage of the water resource. Greater emphasis was now placed on the quantity of storwater runoff, but absence of quality. The theory and technical methods were not available to guide stormwater runoff pollution control, because the wash-off and transportation of pollutants were not investigated deeply in stormwater runoff.With the introduction of the concept of runoff load, a new wash-off model was established, which could apply to simulated stormwater runoff quality, and pollution load was evaluated based on probability distribution model. The transportation regular of stormwater runoff pollution was studied with simulation of quantity and quality, which could be used to establish quantitative evaluation for the first flush effect. Then, the pollutants control parameter formula was proposed based on the transportation regular and first flush effect. In the end, stormwater ecological vegetation was built to removal of stormwater pollutants. The major achievements in this paper were expressed as following:（1） Establishment the wash-off model of stormwater runoffThe runoff load was adopted to substitute rainfall intensity, and stormwater runoff quality model was presented with traditional wash-off model calibrated. The quality model was: C（t）=M0×(0.01+α/(1.1-exp(-Hef/b)×Refα)×exp[(-0.01×Hef+(∫0t-α/(1.1-exp(-Hef/b))×Ref1+αdt)]The new model eliminated the time difference between rainfall and pollutants wash-off, increased the accuracy of simulation. The simulation results of total suspend solid fitted to experiment well, which could be used to investigate the runoff pollution transportation regular. The quality existed direct function relationship with runoff quantity in wash-off model, which was beneficial to study runoff pollution transportation deeply.（2） Presentment the new computational model to pollutants build-upThe vacuuming method usually used to sediment collection, they were not effective for evaluation of the wash-off pollutants because stormwater only removed part of cumulative pollutants. Based on wash-off pollutants in stormwater runoff, the conception of available cumulative wash-off pollutants was presented, and the computational model was established with the wash-off model.It was found that available cumulative wash-off pollutants were far below of vacuuming collection, largely because only a small fraction of pollutants was mobilized to runoff. Available cumulative wash-off pollutants mainly focus on the wash-off pollutants which could applied to wash-off model without calibration. The runoff quality model error would be reduced greatly with using available cumulative wash-off pollutants。（3） Implement the evaluation of runoff pollutant loadsWith pollutant buildup and washoff functions, the cumulative distribution functions （CDFs） of pollutant loads and the expected value of pollutant event mean concentrations （EMCs） were derived. The model can be used to evaluate the stormwater pollutant loads and calculate the pollutant event mean concentrations. The evaluation for pollutant loads indicated that the COD load of Xi’an city was equivalent to the load of 25 million m3 of municipal wastewater, which showed that the pollutants in runoff were major source to urban water environment. The rainfall affected on pollutant event mean concentrations, the EMCs of runoff was negative correlation to rainfall. The concentration of pollutants in initial period was substantially higher than later, so it was effective step to control the runoff in initial period.（4） Simulation to runoff pollutants transportationThe runoff flow was simulated with integrating the rainfall-net rainfall transformation and Overland flow of the Saint-Venant equations. Combined by pollutants wash-off, the stormwater runoff pollutants transportation regular was investigated in different catchments. Then, based on Muskingum flow equation, storm sewer runoff quantity and quality model was presented. The simulation of pollutants transportation could be developed to the first flush effect and runoff pollutants control.（5） Establishment quantitative evaluation model to the first flush effect Integrating runoff quantity and quality, the difference function was created by the difference between the dimensionless cumulative pollutants and the dimensionless cumulative runoff volume. Based the difference function, the strength of first flush was presented: SFF=∫01[f（F）-F]dFThe first flush difference function dominated in positive, and there was peak area, which was effective area to control runoff pollutants, the runoff pollutants should be controlled before peak time. The model implemented the quantitative evaluation to the first flush of stormwater runoff and seasonal flush. The quantitative evaluation model to the first flush effect could help to investigate for runoff pollution transportation and be useful development of best management practice.（6） Development stormwater ecological vegetation technologyWith modification of standard bioretention by adding a submerged anoxic zone, stormwater ecological vegetation （SWEV） provided excellent nutrient removal from rainwater. Denitrification in submerged anoxic zone may play an important role in the nutrition balance. Microbes provided the denitrification process to promote the conversion of accumulated nitrogen species to nitrogen gas, with the off-gas release of nitrogen to the atmosphere. Plant harvesting removed only 20% of capture nitrogen, but nitrogen was not found accumulation obviously in SWEV, demonstrating that denitrification in submerged anoxic zone was important to nitrogen removal. In SWEV, the stormater was harvested in the submerged zone and utilized by capillary action, which improved the plant growth. Not only can SWEV provide efficient pollutant removal, but it can also supply an additional water resource. SWEV can be designed for widespread implementation of runoff pollution control.The runoff pollutants control programs and techniques were presented with the transportation of runoff pollution investigated, which would be helpful development of best management practice for stormwater runoff pollutants.