With the development of social economy and the continuous acceleration of urbanization, the urbanized area and the underlying conditions of urban areas have changed greatly. Under the same amount of rainfall conditions, because the impervious area is added, the runoff coefficient is increased, and the output flow is also increased accordingly.The flood peak flow is increased, and the peak time is earlier, which not only brings a great burden to downstream flood prevention and drainage, but also may As a result, the drainage of rain in some areas is not smooth, and the phenomenon of stagnant water has brought great inconvenience to people's lives. In addition, the precious rainwater resources have not been fully used because the rainwater is discharged. Therefore, many cities have adopted rainwater flooding
methods, such as building cisterns, laying permeable bricks
, and choosing concave oasis. In order to make better use of these rain flood methods, it is necessary to discuss them in terms of flood prevention and increased use of rain resources. The author used the Urban Storm and Rainwater Management Model (SWMM) to calculate and analyze the changes in the flood peak flow of the main section of the drainage pipe under the two methods of laying permeable bricks in
a certain area of Beijing and choosing a recessed oasis.
1 SWMM model overview
SWMM is a comprehensive mathematical model developed by the US Environmental Protection Agency in order to plan and handle urban rainstorms. It can mimic a complete urban rainfall runoff process, including the process of rainwater regulation in ground runoff and drainage systems. The SWMM module includes the primary modules such as a runoff module, a transport module, an extended transport module, a storage / processing module, and a receiving water body. The output of the model can show the current and water quality conditions at various points in the system and in the receiving water body.
① Generalization of sub-basin
In the SWMM model, a watershed is generally divided into several sub-watersheds, and the runoff process is calculated separately according to the characteristics of each sub-watershed. Finally, the outflow of each sub-watershed is superimposed through a flow calculation method. In order to reflect different surface characteristics, each drainage area is further divided into three parts: a. Impervious surface A1 with depressions; b. Impervious surface A2 without depressions; c. Permeable surface A3.
② Surface runoff accounting
Rainfall loss on impervious surfaces without depressions is the initial transpiration; rainfall loss on impervious surfaces with depressions is primarily depressions. With regard to the impermeable surface, there is no runoff when the initial damage is not satisfactory, and once the initial damage is satisfied, the full flow will be produced. As for the pervious surface, in addition to the loss of filling, there is also the loss of infiltration. SWMM uses Horton model, Green-Ampt model and SCS model to calculate infiltration loss.
③ Calculation of surface confluence
The converging process refers to collecting the net rain from each sub-region to the exit control section or directly discharging into the river. Non-linear reservoir model is selected for surface confluence simulation, which is solved by successive equations and Manning equations simultaneously. The model needs to input the area of the study area, the width of the drainage area, the Manning roughness of three different types of surface, the stagnant volume of the surface with stagnant volume, and the slope of the entire drainage area.
④ Calculation of Convergence Subsystem of Pipe Network
The calculation of the pipe network sub-system can be calculated in the SWMM model through the transport module or the extended transport module. They all use the method of solving Saint-Venant equations. The role of the cistern in the SWMM model is to store rainwater runoff from rainfall and then reduce the amount of rainwater entering the pipe network. In the discussion, permeable bricks were
laid in some areas. When the rainfall intensity was less than the infiltration rate, the rainfall did not produce runoff; when the rainfall intensity was greater than the infiltration rate, the runoff was calculated using the Manning formula. Regarding the cisterns on different sub-confluence units, the runoff after the rainfall is satisfied and the initial loss first enters the cistern. When the cistern is full of water, the excess part enters the drainage pipe network.
⑤ Use process of SWMM model
a. Carefully study the area where the rainstorm is simulated, and divide the entire drainage area into several independent drainage pieces according to the laying of the pipeline. b. According to the laying of pipelines and the control area of other equipment, each drainage piece is subdivided into several sub-watersheds, and all sub-watersheds are numbered. c. Find the sub-catchment area and determine the parameters of all models. d. Data entry. e. Calculate the flood peak flow, total runoff and flow progress of each planning section, and analyze it reasonably.
2 Use of models
2.1 Discussion on the generalization of regional drainage system
In this model, the west campus of Beijing University of Technology was selected as the research target, with an area of 2.87 × 104m 2. In the meantime, the impervious area is 0.99 × 104m 2, accounting for 34.5% of the total area; the permeable area is 1.88 × 104m 2, accounting for 65.5% of the total area.
The distribution status is shown in Figure 1. According to the use of SWMM, this area needs to be reasonably generalized, and a simple rainwater pipe network is planned to be built, and the rainwater will be separated into the municipal drainage pipe network. The entire imitation community is divided into 17 drainage communities, with 14 nodes, 13 pipes, and 1 water outlet.
2.2 Determination of model parameters
The permeable area of the study area is 1.88 × 104m2, during which the plot S12 is an oasis. It is planned to lay permeable bricks
in the permeable areas of plots S1 and S5. The Horton infiltration model is used to simulate the rainfall infiltration process in the study area, so the model requires input The maximum infiltration rate f0, the minimum infiltration rate f∞, and the attenuation coefficient α, the three separation values are 76.2mm / h, 3.81mm / h, and 2h-1 . The permeability of the permeable brick
is 2. 89mm / s . The confluence calculation uses a non-linear reservoir model for simulation. Refer to the typical values in the SWMM model user manual.
(Except for the oasis) and the depression volume of the impervious surface are taken separately from 12, 2 and the storage capacity is taken to be 100mm. The slope of the ground surface of the study area was 0.0001, and according to the underlying surface conditions of the study area, the Manning coefficients of the permeable surface, impervious surface and pipeline were taken as 0.03, 0.011, and 0.013, respectively. The simulation process uses dynamic waves for flow calculation.
2.3 Accounting for Planning Rainstorms
The mean value, variation coefficient Cv, and skewness coefficient Cs of the rainfall in each period were obtained according to the atlas and formulas in the Beijing Hydrological Manual, and the recurrence periods separated by this calculation were 5 years, 10 years, and 20 years. Plan daily rainfall progress. The highest peak of the planned rain pattern appears at 23:00, and then a longer time for catchment is required.Because the model requires a longer accounting time, it can swap the main and secondary rain peaks in the daily rainfall and change the main rainfall peak. As early as 10:00, it has been verified that it will not affect the results of imitation. The distribution of daily rainfall progress over the three recurring periods is shown in Figure 2.
2.4 Analysis of the results of imitation
In the simulation, the two methods of permeable bricks
and reservoirs are combined, and the regional runoff process of 4 planned under the conditions of 3 planned frequencies and rainfall is separately calculated to obtain its peak flood flow and peak time, and the total runoff and runoff change coefficient. The four types of planning plans are: no permeable bricks in the
residential area, flat oasis (A); no permeable bricks in the
residential area, recessed oasis (B); permeable bricks in the
residential area, flat oasis (C); permeable bricks in the
residential area , Recessed oasis (D). The simulation results when the rainfall frequency is separated by 5%, 10%, and 20% are shown in Table 1.
As can be seen from Table 1, when the rainfall frequency is 5%, the recessed oasis is used to reduce the peak flow of the pipeline outlet section by 2.78%, about 0.02m 3 / s, the runoff coefficient is reduced by 0.02, and the peak time is delayed by 1min. ; The method of laying permeable bricks
has been used to reduce the peak flow at the outlet section of the pipeline by 8.33%, about 0.06 m 3 / s, and the runoff coefficient has been reduced by 0.05; when both methods are selected, the peak flow at the outlet section of the pipeline has been reduced by 11.11 %, About 0.08m 3 / s, the runoff coefficient decreased by 0.07, and the peak time was delayed by 1min. When the rainfall frequency is 10%, the peak flow of the section of the pipe outlet of the concave oasis is reduced by 5.08%, about 0.03m 3 / s, and the runoff coefficient is reduced by 0.02. The peak flood flow was reduced by 8.47%, about 0.05m 3 / s, and the runoff coefficient was reduced by 0.04. When both methods were used, the peak peak flow was reduced by 11.86%, about 0.07m 3 / s, and the runoff coefficient was reduced by 0.06. When the rainfall frequency is 20%, the peak peak flow of the section of the pipe outlet of the recessed oasis is reduced by 4.44%, about 0.02m 3 / s, and the runoff coefficient is reduced by 0.01. The peak flood flow was reduced by 8.69%, about 0.04m 3 / s, and the runoff coefficient was reduced by 0.03. When both methods were used, the peak peak flow was reduced by 13.33%, about 0.06m 3 / s, and the runoff coefficient was reduced by 0.04.
It can be seen that: ① When only the rain flood method of the concave oasis is considered, with the increase of the rainfall frequency, the fluctuation of the runoff coefficient gradually increases, that is, with the increase of the rainfall frequency, the total runoff Reduce gradually. Therefore, when the frequency of rainfall is large, the use of rain floods with concave oasis is better. ② When only the flood method of permeable bricks
is considered, with the increase of rainfall frequency, the fluctuation of its runoff coefficient gradually decreases. Therefore, when the frequency of rainfall is low, the flood method of laying permeable bricks
plays a more significant role.
It is also known from the test results: ① When the rain flood method of laying permeable bricks
is used, its peak flow reduction and fluctuation are basically stable, because the infiltration rate of the permeable bricks
is very large, and the rainwater falling on the permeable bricks
can pass through the permeable bricks.
Infiltrate into the ground; ② When only the rain flood method of the concave oasis is used, the peak flow reduction and fluctuation of the oasis increase with the increase of the rainfall frequency, but the increased fluctuation slows down, even when the rainfall frequency increases to a certain extent However, the undulations of the reduction of the flood peak flow are reduced; ③ When both rain flood methods are selected, the flood peak flow reduction membrane tank, 60 immersion ultrafiltration membranes, 1 electromagnetic flow meter, 1 blower, and 2 suction Water pump composition. LH 3- 0685 immersion alloy PVC ultrafiltration membrane from Hainan Lisheng Co., Ltd. was selected, with a membrane pore size of 0.01 μm, a planned membrane flux of 27.2 L / (m 2 · h), a filtration cycle of 60 min, and a single membrane backwash The amount of water is 2.2m 3 / h, the aeration of a single membrane is 3 ～ 4m 3 / h, and the backwashing time is 2min.
3.3 Project implementation role
The recovery system was completed and put into operation in 2007. After the immersion membrane system recovered the membrane backwash water from the main process system of the demonstration project, the system water production rate was increased from 79.85% to 98.03%, and the waste water discharge could be reduced by 33 × 104m 3 / a, which saved a lot of water resources. The immersion membrane combination process is adopted to recover the membrane backwash water, which improves the recovery power and reduces the recovery risk, especially the risk of organic matter and microorganism accumulation. Repeated sampling tests have shown that the quality of its effluent water meets the requirements of the "Sanitary Standard for Drinking Water of the Day" (GB 5749-2006), and no further need is required.
Purification treatment is carried out once, and after being disinfected, it can directly enter the water supply pipe network.
① The organic matter concentration in the membrane backwash water is relatively high, and the organic matter characterized by DOC is mainly distributed in the range of MW> 30ku and MW <1ku.
② The combined process of immersion membrane, coagulation, and powdered activated carbon adsorption has a good effect on membrane backwashing water, and the uniform effluent turbidity is 0.07NTU; when the dosage of FeCl3 and PAC is 15mg / L, the effluent CODMn is uniform 2. 81mg / L, with a uniform removal rate of 50.7%; the pH and microbial indicators of the effluent are in compliance with the requirements of the "Sanitary Standard for Drinking Water of the Day" (GB 5749-2006).
③ The immersion membrane recovery and backwash water system was used in the membrane treatment demonstration project, which achieved a satisfactory effect. The effluent from the recovery system meets the requirements of the "Sanitary Standard for Drinking Water of the Day" (GB 5749-2006), and can be directly entered into the water supply pipe network after disinfection. After the demonstration project master
The recovery treatment of the membrane backwash water in the process system has increased the water production rate of the system from 79.85% to 98.03%, and can reduce the waste water discharge by 33 × 104m 3 / a, saving a lot of water resources.