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Fund Project: Project supported by the National Natural Science Foundation of China (50808009); Science and Technology Program Project of the Beijing Municipal Education Commission (KM200810016012); Project funded by the Beijing Municipal Management College Talents Education Program (PHR201008379) It is an important way for non-point source pollution to enter urban water bodies. The existence of sediment in the urban drainage pipeline will directly lead to the reduction of the overflow capacity of the pipeline and thus reduce the drainage capacity of the drainage pipeline. More importantly, the sediment in the pipeline will be washed away by runoff into the water body during heavy rain, forming an impact pollution load, and turning non-point source pollution into point source pollution.
Studies have shown that up to 80% of the water pollution load in developed countries during heavy rain events comes from pipeline sediments. There are many types of pollutants in the sediments in the drainage pipeline, common ones are easily degradable organic substances, nitrogen and phosphorus pollutants that are easy to cause eutrophication of the water body, and oils and fats that are difficult to degrade. Pipe diameter / mm sediment thickness / mm ratio of sediment thickness to pipeline diameter /% project pipeline diameter / mm sediment thickness / mm sediment thickness reinforced concrete pipe.
Substances, toxic petroleum hydrocarbons (PHC) and polycyclic aromatic hydrocarbons (PAHs), and harmful heavy metals and other "triple" substances U, the re-release of these substances may have irreversible effects on water bodies. In addition, the sediments in the pipe network will undergo a series of biochemical reactions under the action of microorganisms, which will change their viscosity and composition, and some toxic and harmful gases will corrode the pipeline.
In summary, it is of great significance to study the sedimentation status and properties of sediments in drainage pipes. In view of the fact that except for a few old urban areas that are combined drainage systems in the urban area of ​​Beijing, the rest are the current situation of the split drainage system, the sedimentation status of some of the split drainage rainwater pipes in Xicheng District was selected for investigation and analysis, with a view to the operation management of the drainage system Provide, provide a basis for further research and control the contribution of sediments to water pollution.
1 Survey area According to the planning of Xicheng District and the distribution of functional areas, select a survey area with a length of approximately 3 km and a width of approximately 1.5 km. The survey area starts from Fuwai Street in the south, Xizhimenwai Street in the north, Xinjiekou South Street in the east, and West Third Ring North Road in the west, and also includes some commercial areas of Financial Street. The scope of the investigation includes typical urban functional areas such as old-fashioned residential areas, newly built residential areas, schools and commercial areas. The drainage system in this area is a diversion system.
Due to the large traffic volume of the main road, it is difficult to investigate the rainwater pipelines located on the main road. Therefore, the drainage pipelines on the pedestrian road and the secondary road within 1m from the pedestrian road were selected as the research objects.
The discarded rainwater pipelines were eliminated, and the deposition status of sediments in 72 rainwater inspection wells and the rainwater pipelines connected to them were recorded in detail.
2 Investigation method For all rainwater pipelines, the downhole measurement method is used to record in detail the thickness of the sediment in the drainage pipeline connected to each rainwater well, the pipe material, the pipe diameter, the pipe intactness, the type of the catchment area and other information. Measure the thickness of the sediment at an interval of about 50cm from the inspection well to the inside of the pipeline within 1 ~ 2m, and measure at least 3 times to take the average as the thickness of the sediment in the investigation pipeline.
3 Results and discussion The survey results show that nearly 80% of the rainwater pipelines in the survey area have different degrees of deposition. Due to long-term deposition and intermittent rainwater immersion, the sediment color becomes black and smelly, and the thickness of the sediment and the corresponding pipe diameter The maximum ratio reached 66.7%, as shown in Table 1.
The analysis believes that the main reason for the formation of sediments is that rainfall erosion causes surface particles to flow into the drainage pipe along with the runoff. Under the action of fluid resistance and its own gravity, some suspended particles in the rainwater runoff are deposited in the drainage pipe.
Among the rainwater pipelines under investigation, there are few pipelines with large deposition thickness (the pipelines with a deposition thickness of> 50% in the diameter of the pipeline account for 4.16%), and the average value of the pavilion is equal to M ss / tt. Outside the pipeline, 41.67% of the thickness of the sediment in the pipeline accounts for 10% to 30% of the pipe diameter. It can be seen that there are generally different degrees of deposition in urban rainwater pipelines, which will affect the overflow capacity of the rainwater pipeline to varying degrees. What is important is that even if the pipeline has a low degree of sedimentation, the pollution caused by the sediment erosion caused by rainfall and runoff still threatens the runoff receiving water body.
After analysis, the distribution of water catchment functions in this research area is relatively similar. The comprehensive runoff coefficients in each water catchment area can be considered to be the same. Through the statistical analysis of the distribution map of drainage pipes in the area, the slopes of drainage pipes in this area are all It is about 0.2% ~ 0.3%, so it is believed that the corresponding catchment area of ​​the pipeline with the same diameter investigated is approximately the same. Compare the deposition status in the DN300, DN400 and DN500 pipelines in the functional areas in the investigation area. See the results. Among them, pipelines with diameters> DN500 are distributed less in each functional area of ​​the survey, so they are not compared. In addition, due to traffic impacts and the abandonment of a few pipelines, the deposition status of sediments in the DN500 and DN300 diameter confluence pipelines could not be obtained on campus and in the business district.
It can be seen that the deposition of pipelines in different functional areas is roughly: old residential area> commercial area> newly built community> campus> business area. In general, the drainage pipeline system in the old residential area is relatively old, with less maintenance, the management of domestic garbage is relatively backward, and there are more particles deposited in the rainwater outlet. In addition, there are many road surface maintenance and renovation projects in old residential areas, which will increase the total amount of particulate matter entering the drainage pipeline. The huge flow of people in the commercial area makes the total amount of garbage generated and the difficulty of standard management difficult, so the amount of sediment in the drainage pipes of these areas is significantly higher than that of other areas. Newly built communities, campuses, and business districts have good basic sanitation facilities, relatively complete garbage management, and relatively small amounts of sediment in drainage pipes. Therefore, to effectively control the sediment in urban rainwater pipelines, it is crucial to the effective management of the environment in the catchment area, especially in areas with poor sanitation. It is necessary to focus on strengthening the management of construction waste in the area and the timely cleaning of rainwater outlets.
A comparative analysis of the thickness of the sediments in the pipelines of the same diameter within the investigation scope shows that, except for the DN300 pipeline, the average change in the thickness of the sediments in the other diameter pipelines conforms to the law of increasing with the increase of the diameter (see). The relatively large amount of deposition in the DN300 pipeline is mainly due to the fact that the DN300 pipelines in the survey area are mostly distributed in old residential areas.
Pipeline diameter / mm The thickness of the deposits in the pipes with different diameters The analysis of the change in the thickness of the sediments in the pipe sections of different diameters on the two rainwater pipelines located in the same catchment area in the investigation area shows that the thickness of the sediments varies with the drainage pipe The diameter increases and increases (see).
The change of sediment thickness in different pipe diameters of the same pipeline was analyzed using SPSS software s to analyze the correlation between the amount of sediment in these two drainage pipes and the pipe diameter. Correlation, the correlation coefficients are 0.989 and 0.983 respectively, and the two-sided test probabilities are 0.097 and 0.017. After analyzing the reasons, it is believed that under small rainfall events, the flow of water into the drainage pipe is small, the degree of disturbance is light, and the water The suspended particulate matter is more likely to deposit to the bottom of the pipeline. Under certain conditions of flow and slope, as the diameter of the drainage pipe increases, the flow rate gradually decreases, and the probability of sediment deposition increases. According to the research results of Pan Guoqing and Liu Cuiyun67, it is analyzed that during a small rainfall period, runoff eroded the ground or the particulate matter of the rainwater inlet at the beginning of the rain to make it enter the pipeline. Before receiving the rainwater from the upstream pipeline under the same flow conditions (investigated The terrain is flat, the slope of the pipeline is basically the same, and the slope has no effect on the flow rate. The downstream pipeline has a lower flow rate than the upstream pipeline. The suspended particles carried by the runoff are likely to deposit on the bottom of the pipeline; as the rainfall process progresses, the rainwater from the upstream pipeline flows into the downstream In the pipeline, the flow of the downstream pipeline increases, but the upstream rainwater that flows in at this time is the initial rainwater, which contains more particulate matter, and will also be deposited in the downstream pipeline; the concentration of pollutants (particulate matter) in the runoff gradually decreases after reaching the peak At this time, the flow in the pipeline also began to decrease, and the sediment washed into the receiving water body by the water flow decreased.
The drainage pipes in the area under investigation involve cast iron and reinforced concrete, mainly reinforced concrete pipes. The different degree of smoothness of the inner surface of the two materials makes the water flow resistance and the ability to intercept substances in the water different. The rougher the pipe, the greater the water flow resistance. Under the same flow rate and slope, the water flow speed is relatively reduced, thereby suspending particles in the water flow. The probability of deposition increases. Three cast iron and reinforced concrete pipes of the same diameter in the commercial area were selected. After comparison, it was found that the amount of deposits in the cast iron pipe was more than 50% less than that in the reinforced concrete pipe. This shows that under the conditions of similar types of catchment areas, the same pipe diameter, and the same catchment area, the deposition amount of sediments in drainage pipes of different materials is significantly different. The use of cast iron pipes can relatively reduce the deposition of sediments.
4 Conclusion By investigating the sediment in some typical drainage pipes in the urban area of ​​Beijing, the following main conclusions are drawn: the sedimentation phenomenon in the drainage pipes in the survey area is widespread, and nearly 80% of the pipes have different degrees of deposition; the drainage pipes in different catchment areas The thickness of the internal sediments is different (old residential area> commercial area> new residential area> campus> business area); the amount of sediment in the drainage pipe increases with the increase of the pipe diameter; the amount of sediment in the pipes of different materials is significantly different, the same Under the conditions, the deposition phenomenon in the cast iron pipe is obviously weaker than that of the reinforced concrete pipe.
The preliminary investigation results of this study effectively control the sediment erosion pollution in urban rainwater pipelines, which is of great significance for improving the water environment quality of urban water bodies. However, the control of pipeline sediments largely depends on the effective implementation of management efforts. Therefore, it is a hot topic in this field to study the law of erosion and pollution of sediments in pipelines, and then to control pollution through engineering measures. Mainly reflected in the following points: â‘ Research on sediment deposition law in typical urban rainwater pipeline; â‘¡Research on the occurrence characteristics of pollutants in pipeline sediment; â‘¢Research on the pollution mechanism of sediment pollutants under the condition of runoff erosion; â‘£Sediments in pipeline Control measures research, etc.
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