A method for simulating flood inundation in urban covered river sections based on double-layer two-dimensional grids

Abstract

The present invention belongs to the technical field of flood inundation simulation, and discloses a flood inundation simulation method for urban covered river sections based on double-layer two-dimensional grids. The method is mainly aimed at local covered rivers in cities. Based on double-layer two-dimensional grids, according to the basic principles of hydrodynamics, the method calculates the change process of hydraulic elements such as water level, flow velocity, flow rate, etc. of flood entering and flowing out of the covered river channel from an open channel and in the covered river section, and interacts with the ground flood confluence area and diffusion area outside the river channel in real time, analyzes the overflow and inundation situation after the flood merges into the river channel and exceeds the flow capacity of the river section, and provides support for conducting rainstorm flood risk analysis in urban areas.

Description

Technical Field

[0001] The invention belongs to the technical field of flood inundation simulation, and in particular relates to a flood inundation simulation method for a city covered river section based on a double-layer two-dimensional grid. Background Art

[0002] In the 1970s and 1980s, due to the needs of urban development and construction, part of the river space was occupied. The upper part of the river section originally used for flood discharge was covered with a canopy and used as a road, or residential houses or concentrated commodity market buildings were built above the river and on both sides of the river, which looked no different from normal blocks. However, once a rainstorm and flood disaster occurs in the city, it is difficult for the flood to flow from the road into the river in the covered river section, resulting in waterlogging in the surrounding areas. At the same time, due to the siltation of the covered river section, the discharge capacity is reduced. When the upstream water is large, the drainage is often not smooth, which further causes serious flooding on both sides of the upstream. For example, according to statistics from Jinan in 2013, 30% of the city's rivers are covered with canopies, and the longest river section is covered with canopies for nearly 2 kilometers. Therefore, simulating the impact of the covered river section on the evolution of floods upstream and downstream and on both sides of the river is one of the important contents of the risk analysis of rainstorms and floods in relevant cities.

[0003] Domestic and foreign scholars and mainstream hydraulic commercial software generally simulate such river sections by generalizing them as culverts or coupling them with one-dimensional pipe and canal models. However, when they are treated as culverts, they are restricted by the unified shape parameter setting requirements and it is difficult to reflect the original river section morphology that is long and constantly changes with space. When coupled with a one-dimensional pipe and canal model for simulation, the continuity of the morphological changes of the covered river section and its upstream and downstream channels will be destroyed, and a certain water level-flow relationship needs to be generalized at the coupling point, which is inconsistent with the real-time changes of the actual water flow with the water level difference and flow velocity of the upstream and downstream sections, which affects the scientificity and accuracy of the calculation results to a certain extent.

[0004] Therefore, how to further improve the accuracy of flood inundation simulation in urban covered river sections is an urgent problem that needs to be solved. Summary of the invention

[0005] In order to overcome the problems of the prior art and address the shortcomings of traditional urban covered river section flood inundation simulation, the present invention proposes an urban covered river section flood inundation simulation method based on a double-layer two-dimensional grid, which not only ensures that the simulation is carried out in accordance with the existing terrain of the covered river section, but also makes the simulation process simple and efficient.

[0006] The object of the present invention is achieved in that:

[0007] The present invention provides a method for simulating flood inundation in a river section of an urban canopy based on a double-layer two-dimensional grid, comprising the following steps:

[0008] Step 1: Collect basic information of the study area:

[0009] Collect relevant basic data on the rivers where the covered river sections in the study area are located and their flood-affected areas, including cross-sectional data of the rivers, the specific location of the covered river sections and the distribution of the top and bottom elevations of the covered rivers, embankment elevation data, ground elevation data of the flood-affected areas on both sides of the river, urban road distribution, and drainage network data.

[0010] Step 2: Construct the urban stormwater model based on the normal condition of the river channel without a canopy:

[0011] The urban stormwater model of the study area is constructed normally according to the absence of covered river sections, including two-dimensional non-structural irregular grid division, grid attribute assignment, attribute assignment of water-blocking structures above the ordinary ground in the area, and parameter assignment of underground drainage network.

[0012] Step 3, grid and channel assignment for the covered river section:

[0013] For the grids corresponding to the covered river sections in the study area, additional cover-related parameters are assigned, including the grid cover top elevation, cover bottom elevation, upper grid roughness, lower grid roughness, and river channel elevation corresponding to the lower grid; for the grid edges (i.e., "channels") at the entrance and exit of the covered river sections in the study area that are perpendicular to the water flow direction, the maximum flow rates of the entrance channel group and the exit channel group are additionally assigned.

[0014] Step 4: Calculation of double-layer grid hydraulic elements in the covered river section:

[0015] In each calculation step of the urban stormwater model constructed in step 2, when the simulation loop reaches the grid corresponding to the covered river section, it is automatically converted to an upper and lower two-layer mode. The upper layer uses the grid cover elevation as the ground elevation to calculate the water flow exchange with the surrounding ordinary ground grids; the lower layer uses the ground elevation on the river cross section corresponding to the grid as the standard, and exchanges water flow with other grids in the river below the cover. At the same time, the highest water level of the lower grid is limited not to exceed the cover bottom elevation. When the water level of a grid at the end of the calculated step within a certain step will exceed the cover bottom elevation, the flow velocity and flow value on the channel that constitutes the grid are corrected in real time so that the water level is exactly equal to the cover bottom elevation at the end of the step.

[0016] Step 5, correction of inlet / outlet flow of the covered river section:

[0017] For the channels at the entrance of the Penggai River section in the study area that are perpendicular to the water flow direction, it is judged whether the sum of the flow rates on all channels is greater than the maximum flow rate in each calculation step. If it is greater, the proportion of the flow rate on each channel to the total flow rate is used as the weight, and the flow rates of each channel are reduced as a whole to a sum equal to the maximum flow rate. The grid water level upstream of the channel is updated and calculated according to the corrected smaller flow rate to reflect the water congestion phenomenon.

[0018] Step 6, outlet flow correction of the Penggai River section:

[0019] For the channels at the outlet of the Penggai River section in the study area that are perpendicular to the water flow direction, similar to the entrance channels, in each calculation step, it is judged whether the sum of the flows on all channels is greater than the maximum flow. If it is greater, the proportion of the flow on each channel to the total flow is used as the weight, and the overall flow of each channel is reduced to a sum equal to the maximum flow. The grid water level upstream of the channel is updated and calculated according to the corrected smaller flow to reflect the phenomenon of poor flood discharge in the Penggai River channel.

[0020] Step 7: Calculate the highest water level and maximum water depth of each grid during the simulation period:

[0021] At the end of each step of the urban stormwater model simulation calculation, the water depth and water level data of the upper and lower layers of the grid are recorded, and the maximum water depth and highest water level of the entire simulation period are counted, so that the flood evolution and inundation distribution considering the influence of the covered river section in the study area can be obtained.

[0022] The advantages and beneficial effects of the present invention are:

[0023] (1) The existing simulation method can only generalize the canopy river channel into culverts or underground pipes, which makes it difficult to reflect the continuous changes in the original river cross-section terrain. The present invention uses a two-dimensional grid for integrated simulation, ensuring that the ground above the canopy and the river channel below can be simulated as closely as possible to the existing terrain, making the calculation results more reliable.

[0024] (2) The flood inundation simulation method for urban covered river sections based on double-layer two-dimensional grids proposed in the present invention has no special requirements for the modeling process of urban stormwater models. There is no need to extract relevant parameters of the covered section according to the one-dimensional pipeline, and there is no need to couple the connection points of the one-dimensional model with the two-dimensional model. There is no need to forcibly construct the water level flow relationship at the entrance and exit of the covered river section. It is only necessary to extract the special parameters of the covered section grid and the entrance and exit channel groups into the model input file. Therefore, the modeling process is simple, which provides an effective processing method for cities with such rivers to quickly construct stormwater flood simulation models and conduct risk assessments.

[0025] (3) The method for simulating flood inundation in urban covered river sections based on a double-layer two-dimensional grid described in the present invention takes hydrodynamics as the basic principle and performs integrated calculation of the covered river sections in upper and lower layers. It has a physical mechanism basis and is more consistent with the actual water flow state of flood movement. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0027] Figure 1 It is a flow chart of the method for simulating flood inundation in urban covered river sections based on double-layer two-dimensional grids according to the present invention;

[0028] Figure 2 The location distribution map of the study area of ​​the embodiment;

[0029] Figure 3 Grid and channel distribution map of the study area for the embodiment;

[0030] Figure 4 This is a schematic diagram of a double-layer grid in the Penggai River section of the study area of ​​the embodiment;

[0031] Figure 5 This is a schematic diagram of water volume correction for the lower grid of the Penggai River section in the study area of ​​the embodiment;

[0032] Figure 6 It is the maximum flooding depth distribution map of a certain flood scenario;

[0033] Figure 7 This is a partially enlarged rendering of the lower grid water level in the Penggai River section and a marked map of water depths at typical locations. DETAILED DESCRIPTION

[0034] Embodiment 1:

[0035] like Figure 1 As shown, this embodiment provides a method for simulating flood inundation in a river section of an urban canopy based on a double-layer two-dimensional grid, comprising the following steps:

[0036] Step 1: Collect basic information of the study area:

[0037] like Figure 2 As shown, this embodiment takes the Xiluo River in Jinan City, where there is a partial covered river section, and the affected areas on both banks as an example.

[0038] Specifically, the relevant basic data of the rivers where the covered river sections in the study area are located and their flood-affected areas are collected, including the cross-sectional data of the rivers, the specific location of the covered river sections and the distribution of the top and bottom elevations of the covered rivers, the embankment elevation data, the ground elevation data of the flood-affected areas on both sides of the river, the distribution of urban roads, and the drainage network data.

[0039] Among them, the elevation distribution data of the top and bottom of the cover river section should be equivalent to the accuracy of the normal river cross-section data. For example, when the cross-section data accuracy of the upstream and downstream of the cover river section is 1:500, the elevation data of the cover section should also reach this accuracy. Other data can be collected and determined based on the research objectives, objective conditions and the difficulty of obtaining basic data.

[0040] Step 2: Construct the urban stormwater model based on the normal condition of the river channel without a canopy:

[0041] The urban stormwater model of the study area is constructed normally according to the absence of covered river sections; specifically, it includes two-dimensional non-structured irregular grid division (the area is divided by quadrilaterals, the grid has four sides, and the angles of the four corners can be 90 degrees, or greater or less than 90 degrees), grid elevation, roughness, flow generation parameters and other attribute assignments, attribute assignments of water-blocking structures above the ordinary ground in the area, and parameter assignments of underground drainage pipe networks.

[0042] When dividing the grid in the river channel, the four sides of the grid should be arranged perpendicular to the water flow direction and parallel to the water flow direction as much as possible, so that the simulation calculation can be more consistent with the water flow characteristics.

[0043] The urban stormwater modeling technology and tools used in this embodiment are the FRAS software independently developed by the China Institute of Water Resources and Hydropower Research. This technical tool is relatively mature and has been widely used in the compilation of flood risk maps in my country. Its basic principles will not be described here. However, in order to demonstrate the application of the calculation method after adding the canopy river channel, the basic situation of the constructed regional stormwater model is introduced in the embodiment.

[0044] like Figure 3 As shown, the study area covers an area of ​​7.89km 2 , divided into 55689 irregular quadrilateral grids with an average grid size of 12m×12m. The length of the Penggai River section is 1.07km, involving 1319 grids. The study area is bounded by the Xiaoqing River embankment in the north, the Dongluo River and the Donggongshang River in the east and west, respectively, and the moat and road in the south.

[0045] Step 3, grid and channel assignment for the covered river section:

[0046] For the grids corresponding to the covered river sections in the study area, additional cover-related parameters are assigned, including the grid cover top elevation, cover bottom elevation, upper grid roughness, lower grid roughness, and river channel elevation corresponding to the lower grid; for the grid edges (i.e., "channels") at the inlet and outlet of the covered river sections in the study area that are perpendicular to the water flow direction (or nearly perpendicular), the maximum flow rates of the inlet channel group and the outlet channel group are additionally assigned.

[0047] The elevation of the grid of the covered river section is obtained by spatial interpolation of the collected elevation point data. The elevation of the bottom of the cover is obtained by subtracting the thickness of the cover material from the top elevation. The calculation formula is as follows:

[0048] E boti =E topi -H covi (1)

[0049] Where i is the grid number, E boti is the bottom elevation of grid i, E topi is the roof elevation of grid i, Hcovi is the cover plate thickness at grid i.

[0050] The maximum flow of the channel group at the entrance and exit of the Penggai River section is calculated using the Manning formula. The calculation formula for the entrance is as follows, and the calculation formula for the exit is similar.

[0051] Step 4: Calculation of double-layer grid hydraulic elements in the covered river section:

[0052] In each calculation step of the urban stormwater model constructed in step 2, when the simulation cycle reaches the grid corresponding to the covered river section, it is automatically converted to the upper and lower two-layer mode (such as Figure 4 As shown in the figure, the shapes, sizes and numbers of the two layers of grids at the same position are exactly the same, and the elevation and roughness are assigned separately. The upper layer uses the grid cover elevation as the ground elevation to calculate the water flow exchange with the surrounding ordinary ground grids; the lower layer uses the ground elevation on the river channel cross section corresponding to the grid as the standard to exchange water flow with other grids in the river channel under the shed cover. At the same time, the highest water level of the lower grid is limited not to exceed the cover bottom elevation. When the water level of a grid at the end of the calculated step within a certain step length will exceed the cover bottom elevation, the flow velocity and flow value on the channel that constitutes the grid are corrected in real time so that the water level is exactly equal to the cover bottom elevation at the end of the step length.

[0053] For normal river sections, when the water level of the grids on both sides of the river exceeds the river embankment elevation or the river bank elevation (for sections without embankments), the flood will flow into the river; when the water level in the river exceeds the river embankment elevation or the river bank elevation (for sections without embankments), the flood will overflow to the grids on both sides. For covered river sections, since they are divided into upper and lower layers for separate calculations, the above two situations will not occur.

[0054] For the upper grid, the water flow exchange calculation is performed with the surrounding ordinary ground grids, and the water depth calculation formula is:

[0055]

[0056] Where: T is the starting time of the current calculation step, DT is the time step, is the water depth of the upper layer of grid i at the end of the calculation step T+DT, is the water depth of the upper layer of grid i at time T, A i is the area of ​​grid i, K is the total number of channels that make up grid i, k is the sequence number of a channel that makes up grid i, is the flow of the kth channel on the upper layer of grid i at time T+DT, L ik is the length of the kth channel of grid i, It is the source and sink term of the upper layer of grid i between time T and time T+DT, referring to the effective rainfall intensity and regional drainage intensity.

[0057] The upper water level calculation formula of grid i is:

[0058]

[0059] Where: is the water level of the upper layer of grid i at time T+DT, is the water depth of the upper layer of grid i at time T+DT, E TOPi is the top elevation of grid i.

[0060] For the lower grid, water flow is exchanged with other grids in the river channel below the canopy, and the water depth calculation formula is:

[0061]

[0062] Where: is the water depth of the lower layer of grid i (i.e., river grid) at the end of the calculation step T+DT, is the water depth of the lower layer of grid i at time T, is the flow of the kth channel under the grid i at time T+DT. The lower grid is affected by the shed cover and does not directly receive rainfall runoff and surface drainage, so there is no need to consider the source and sink terms.

[0063] The calculation formula for the lower water level of grid i is:

[0064]

[0065] Where: is the water level of the lower layer of grid i at time T+DT, is the water depth of the lower layer of grid i at time T+DT, E rivi is the river elevation corresponding to grid i.

[0066] when When Assign value to E boti , and at the same time correct the velocity and flow rate values ​​on the channels that make up the grid so that at the end of the step, the water level is equal to the bottom elevation (such as Figure 5 The calculation method is:

[0067]

[0068] Where: is the flow of the kth channel in the lower layer of grid i at time T+DT after correction, is the flow of the kth channel in the lower layer of grid i at time T+DT before correction, K corik is the flow correction coefficient of the kth channel in the lower layer of grid i, E boti is the bottom elevation of grid i, is the water level of the lower layer of grid i at time T, is the water level of the lower layer of grid i at time T+DT.

[0069] Step 5, correction of inlet / outlet flow of the covered river section:

[0070] For the channels at the entrance of the Penggai River section in the study area that are perpendicular to the water flow direction, it is judged whether the sum of the flow rates on all channels is greater than the maximum flow rate in each calculation step. If it is greater, the proportion of the flow rate on each channel to the total flow rate is used as the weight, and the flow rates of each channel are reduced as a whole to a sum equal to the maximum flow rate. The grid water level upstream of the channel is updated and calculated according to the corrected smaller flow rate to reflect the water congestion phenomenon.

[0071] The total flow calculation formula at the entrance of the Penggai River section at the end of each calculation step is:

[0072]

[0073] Where: is the total flow at the entrance of the Penggai River section at the end of the calculation step T+DT, n is the serial number of the entrance channel, M is the total number of channels at the entrance, is the flow rate of the nth channel at the entrance of the Penggai River section at time T+DT, L n is the length of the nth channel;

[0074] when Greater than the maximum flow Q of the entrance channel group of the Penggai River section maxin When the flow rate of each channel is reduced to the maximum flow rate, the proportion of the flow rate of each channel in the total flow rate is used as the weight, and the flow rate of each channel is reduced to the maximum flow rate. The grid water level upstream of the channel is updated and calculated by substituting the corrected smaller flow rate into formula (4). The flow reduction method is:

[0075]

[0076] Where: is the flow of the nth channel at the entrance of the Penggai River section at the time T+DT after correction, is the flow of the nth channel at the entrance of the Penggai River section before correction at time T+DT, K n is the flow correction coefficient of the nth channel at the inlet, Q maxin It is the maximum flow of the entrance channel group of the Penggai River section.

[0077] Step 6, outlet flow correction of the Penggai River section:

[0078] For the channels at the outlet of the Penggai River section in the study area that are perpendicular to the water flow direction, similar to the entrance channels, in each calculation step, it is judged whether the sum of the flows on all channels is greater than the maximum flow. If it is greater, the proportion of the flow on each channel to the total flow is used as the weight, and the overall flow of each channel is reduced to a sum equal to the maximum flow. The grid water level upstream of the channel is updated and calculated according to the corrected smaller flow to reflect the phenomenon of poor flood discharge in the Penggai River channel.

[0079] The specific calculation method is similar to step 5 and will not be repeated here.

[0080] Step 7: Calculate the highest water level and maximum water depth of each grid during the simulation period:

[0081] At the end of each step of the urban stormwater model simulation calculation, the water depth and water level data of the upper and lower layers of the grid are recorded, and the maximum water depth and highest water level of the entire simulation period are counted, so that the flood evolution and inundation distribution considering the influence of the covered river section in the study area can be obtained.

[0082] The maximum flooding depth distribution of the study area - Jinan Xiluo River and its two banks under a certain return period of rainstorm conditions is calculated according to the above steps. Figure 6 The grid water level rendering and water depth annotation diagram of the partial enlargement of the Penggai River section can be found in Figure 7 .

[0083] Finally, it should be noted that the above is only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred arrangement scheme, a person skilled in the art should understand that the technical solution of the present invention (such as the use of various formulas, the sequence of steps, etc.) can be modified or replaced by equivalents without departing from the spirit and scope of the technical solution of the present invention.

Claims

1. A method for simulating flood inundation in urban covered river sections based on a double-layer two-dimensional grid, characterized in that: The method comprises the following steps: Step 1: Collect basic information of the study area: Collect relevant basic data on the rivers where the covered river sections in the study area are located and their flood-affected areas, including cross-sectional data of the rivers, the specific location of the covered river sections and the distribution of the top and bottom elevations of the covered river sections, embankment elevation data, ground elevation data of the flood-affected areas on both sides of the river, urban road distribution, and drainage network data; Step 2: Construct the urban stormwater model based on the normal condition of the river channel without a canopy: According to the river sections without sheds, the urban rainwater model of the study area is constructed normally; specifically, it includes two-dimensional non-structural irregular grid division, grid attribute assignment, attribute assignment of water-blocking structures above the ordinary ground in the area, and parameter assignment of underground drainage pipe network; Step 3, grid and channel assignment for the covered river section: For the grids corresponding to the covered river sections in the study area, additional cover-related parameters are assigned, including the grid cover top elevation, cover bottom elevation, upper grid roughness, lower grid roughness, and river channel elevation corresponding to the lower grid; for the grid edges at the inlet and outlet of the covered river sections in the study area that are perpendicular to the water flow direction, additional maximum flow rates of the inlet channel group and the outlet channel group are assigned; Step 4: Calculation of double-layer grid hydraulic elements in the covered river section: In each calculation step of the urban stormwater model constructed in step 2, when the simulation cycle reaches the grid corresponding to the canopy river section, it is automatically converted to an upper and lower two-layer mode. The upper layer uses the grid cover elevation as the ground elevation to calculate the water flow exchange with the surrounding ordinary ground grids; the lower layer uses the ground elevation on the river channel cross section corresponding to the grid as the standard, and exchanges water flow with other grids in the river channel under the canopy. At the same time, the highest water level of the lower grid is limited to not exceed the canopy bottom elevation. When the water level of a grid calculated at the end of a certain step exceeds the canopy bottom elevation, the flow velocity and flow value on the channel constituting the grid are corrected in real time so that the water level is exactly equal to the canopy bottom elevation at the end of the step; Step 5, correction of inlet / outlet flow of the covered river section: For the channels at the entrance of the Penggai River section in the study area that are perpendicular to the water flow direction, in each calculation step, it is determined whether the sum of the flow rates on all channels is greater than the maximum flow rate. If it is greater, the proportion of the flow rate on each channel to the total flow rate is used as the weight, and the flow rates of each channel are reduced as a whole until the sum is equal to the maximum flow rate. The grid water level upstream of the channel is updated and calculated according to the corrected smaller flow rate to reflect the water congestion phenomenon. Step 6, outlet flow correction of the Penggai River section: For the channels at the outlet of the Penggai River section in the study area that are perpendicular to the water flow direction, similar to the entrance channels, in each calculation step, it is determined whether the sum of the flow rates on all channels is greater than the maximum flow rate. If it is greater, the proportion of the flow rate on each channel to the total flow rate is used as the weight, and the flow rates of each channel are reduced as a whole until the sum is equal to the maximum flow rate. The grid water level upstream of the channel is updated and calculated according to the corrected smaller flow rate to reflect the phenomenon of poor flood discharge in the Penggai River channel. Step 7: Calculate the highest water level and maximum water depth of each grid during the simulation period: At the end of each step of the urban stormwater model simulation calculation, the water depth and water level data of the upper and lower layers of the grid are recorded, and the maximum water depth and highest water level of the entire simulation period are counted, so that the flood evolution and inundation distribution considering the influence of the covered river section in the study area can be obtained.

2. The simulation method according to claim 1, characterized in that: In step 1, the distribution data of the top elevation and bottom elevation of the covered river section are comparable to the accuracy of the cross-sectional data of a normal river.

3. The simulation method according to claim 1, characterized in that: In step 2, quadrilaterals are used to divide the grid in the river channel so that the four sides of the grid are arranged perpendicular to the water flow direction and parallel to the water flow direction respectively.

4. The simulation method according to claim 1, characterized in that: In step 3, the calculation formula of the cover bottom elevation is as follows: E boti =E topi -H covi (1) Where i is the grid number, E boti is the bottom elevation of grid i, E topi is the roof elevation of grid i, H covi is the cover plate thickness at grid i.

5. The simulation method according to claim 1, characterized in that: In step 3, the maximum flow of the inlet and outlet channel groups of the Penggai River section in the study area is calculated using the Manning formula according to the cross-sectional morphology at the inlet or outlet.

6. The simulation method according to claim 1, characterized in that: In step 4, within each calculation step of the urban stormwater model, for normal uncovered river sections, when the water level of the grids on both sides of the river exceeds the river embankment elevation or the river bank elevation, the flood flows into the river; when the water level in the river exceeds the river embankment elevation or the river bank elevation, the flood overflows to the grids on both sides.

7. The simulation method according to claim 1, characterized in that: In step 4, the water flow exchange calculation is performed for the upper grid and the surrounding ordinary ground grids, and the water depth calculation formula is: Where: T is the starting time of the current calculation step, DT is the time step, is the water depth of the upper layer of grid i at the end of the calculation step T+DT, is the water depth of the upper layer of grid i at time T, A i is the area of ​​grid i, K is the total number of channels that make up grid i, k is the sequence number of a channel that makes up grid i, is the flow of the kth channel on the upper layer of grid i at time T+DT, L ik is the length of the kth channel of grid i, is the source-sink term of the upper layer of grid i between time T and time T+DT, which refers to the effective rainfall intensity and regional drainage intensity; The upper water level calculation formula of grid i is: Where: is the water level of the upper layer of grid i at time T+DT, is the water depth of the upper layer of grid i at time T+DT, E TOPi is the roof elevation of grid i; For the lower grid, water flow is exchanged with other grids in the river channel below the canopy, and the water depth calculation formula is: Where: is the water depth of the lower layer of grid i at the end of the calculation step T+DT, is the water depth of the lower layer of grid i at time T, is the flow of the kth channel in the lower layer of grid i at time T+DT; The calculation formula for the lower water level of grid i is: Where: is the water level of the lower layer of grid i at time T+DT, is the water depth of the lower layer of grid i at time T+DT, E rivi is the river elevation corresponding to grid i; when When Assign value to E boti , and at the same time correct the velocity and flow value on the channel that makes up the grid so that at the end of the step, the water level is equal to the bottom elevation. The calculation method is: Where: is the flow of the kth channel in the lower layer of grid i at time T+DT after correction, is the flow of the kth channel in the lower layer of grid i at time T+DT before correction, K corik is the flow correction coefficient of the kth channel in the lower layer of grid i, E boti is the bottom elevation of grid i, is the water level of the lower layer of grid i at time T, is the water level of the lower layer of grid i at time T+DT.

8. The simulation method according to claim 1, characterized in that: In steps 5 and 6, when the grid water level upstream of the channel is corrected, only the row of upstream grids adjacent to the inlet or outlet is corrected.

9. The simulation method according to claim 1, characterized in that: In step 5, the total flow calculation formula at the entrance of the Penggai River section at the end of each calculation step is: Where: is the total flow at the entrance of the Penggai River section at the end of the calculation step T+DT, n is the serial number of the entrance channel, M is the total number of channels at the entrance, is the flow rate of the nth channel at the entrance of the Penggai River section at time T+DT, L n is the length of the nth channel; when Greater than the maximum flow Q of the entrance channel group of the Penggai River section maxin When the flow rate of each channel is reduced to the maximum flow rate, the proportion of the flow rate of each channel to the total flow rate is used as the weight, and the flow rate of each channel is reduced to the maximum flow rate. The grid water level upstream of the channel is updated and calculated by substituting the corrected smaller flow rate into formula (4). The flow reduction method is: Where: is the flow of the nth channel at the entrance of the Penggai River section at the time T+DT after correction, is the flow of the nth channel at the entrance of the Penggai River section before correction at time T+DT, K n is the flow correction coefficient of the nth channel at the inlet, Q maxin It is the maximum flow of the entrance channel group of the Penggai River section.

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