A method for identifying change trend and cause of flood level of multi-branch river section downstream of dam

Abstract

A method for identifying the change trend and cause of the flood level of a river section with multiple tributaries downstream of a dam, comprising determining the average relationship and amplitude of the water level data and flow data above the n-year flat-plate flow of the target river section before the dam is built, counting the average and extreme situations of the tributary inflow in the main flood season of the main stream, collecting the terrain data of the target river section, establishing a one-dimensional unsteady flow hydrodynamic mathematical model, evaluating the change trend of the river discharge capacity downstream of the dam according to the one-dimensional unsteady flow hydrodynamic mathematical model, and quantitatively analyzing the influence of each factor on the flood level of the river section with multiple tributaries downstream of the dam. The method provided by the present application can accurately identify the change trend of the flood level and effectively separate the influence of single factors by means of statistical analysis and one-dimensional unsteady flow hydrodynamic mathematical model, processing and calculation of observation data, simple process, small numerical calculation workload, and high reliability.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of water conservancy and water transportation engineering, and particularly relates to a method for identifying the change trend and causes of flood level in a river section with multiple tributaries downstream of a dam. BACKGROUND

[0002] Many reservoirs are built on the upstream of rivers to store flood and generate electricity. In the decades or even hundreds of years after the construction of the reservoir, the flood level in the long river section downstream of the dam will be adjusted due to the adjustment of factors such as river geomorphology, river resistance, and the encounter of the main stream and tributaries. Some rivers may even experience the phenomenon of flood level rising after the construction of the reservoir, thereby offsetting the flood control effect of the dam. It is of great significance to timely evaluate the change trend of the flood level downstream of the dam and analyze the main cause of the change of the flood level from various factors for the formulation of the flood control strategy at the river section scale and the adjustment of the reservoir group dispatching strategy at the basin scale.

[0003] Large rivers in nature generally have many tributaries. The flood of the tributaries in the flood season will have a significant backwater effect on the flood level of the main stream. In the alluvial plain river with gentle slope, the influence range of the backwater effect can reach hundreds of kilometers. And this backwater effect has great randomness due to the different water conditions of the main stream and tributaries, which leads to the frequent random fluctuations of the flood level of the river channel under the same flow, bringing difficulties to the evaluation of the flood discharge capacity of the river channel. In the special case after the construction of the dam in the main stream, the flood encounter of the main stream and tributaries is changed, and at the same time, the effects such as deep incision of the thalweg, shrinkage of the flood channel, and increase of resistance in the downstream of the dam appear, which makes the random fluctuations and trend adjustments of the river channel flood discharge capacity interweave with each other and difficult to separate. Under this background, it is difficult to judge whether the flood level has changed in a trend or not, and even if the flood level has changed in a trend, it is also difficult to identify which factor is the dominant one. These are two difficulties faced by the flood disaster situation evaluation of the river section with multiple tributaries downstream of the dam.

[0004] In engineering practice, the simple method commonly used to evaluate the change of the river channel flood discharge capacity in the past is to directly analyze the hydrological data. For example, in the quasi-equilibrium river channel with less backwater effect, the water level-flow relationship curve of the hydrological station is used to describe the discharge capacity of the river section, and in the case after the construction of the dam, the change trend of the river channel flood discharge capacity is judged according to the up and down movement range of the water level-flow relationship curve of the hydrological station in the downstream of the dam. However, in the river section with multiple tributaries, the water level-flow relationship under the condition of backwater presents a complex multi-value rope relationship, and the traditional method is difficult to apply. In addition, there are many factors affecting the flood level in the river section with multiple tributaries downstream of the dam, and the factors are interwoven. How to separate the single factor influence from a large amount of hydrological data observed in the prototype, especially how to separate the effect of tributary backwater, still lacks a set of feasible processing procedures and methods. SUMMARY

[0005] To address the shortcomings of existing technologies, the purpose of this invention is to provide a method for identifying the trends and causes of flood level changes in multiple tributary river sections downstream of a dam.

[0006] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows:

[0007] A method for identifying the trends and causes of flood level changes in multiple tributary river sections downstream of a dam includes the following steps:

[0008] Step 1: Determine the average line and upper and lower envelope lines of the relationship between the water level data above the floodplain flow and the flow data of the target river section over n years before reservoir construction;

[0009] Step 2: Compile statistics on the average and extreme situations of tributary inflows during the main flood season of the main stream;

[0010] Step 3: Collect topographic data of the target river section and establish a one-dimensional unsteady hydrodynamic mathematical model;

[0011] Step 4: Based on the one-dimensional unsteady flow dynamic mathematical model, evaluate the trend of changes in the river channel's discharge capacity after the reservoir is built;

[0012] Step 5: Quantitatively analyze the impact of various factors on the flood level of the downstream tributary confluence section of the dam.

[0013] Furthermore, step 1 specifically includes:

[0014] Step 1.1: For the target river section, collect hydrological observation data for n years prior to reservoir construction, including the average daily flow and corresponding average daily water level at the river inlet and along the river.

[0015] Step 1.2: Combine the beach flow Q at each station b Select stations with values ​​greater than Q b The average daily traffic is denoted as Q. g Q g The corresponding daily average water level is denoted as Z. g ;

[0016] Step 1.3: Fit the Z-axis values ​​of each station before database construction. g and Q g The water level-flow rate function relationship I is Z. g and Q g Relationship Average Line C a The expression;

[0017] Step 1.4: Keeping the curve shape unchanged, based on the water level-discharge function relationship I, change the y-intercept of I to obtain the upper envelope C. u With lower envelope C d This ensures that all Q values ​​at each monitoring station before database construction are met. g and Z gAll are distributed in C u between C d .

[0018] Further, the step 2 specifically includes:

[0019] Step 2.1, for the target river section, collect the daily flow observation data of each measuring station at the inlet and outlet of the target river section before and after the establishment of the database for m years, and determine the main flood period of the main stream;

[0020] Step 2.2, the large tributaries in the target river section with daily flow observation data are recorded as T bi , the small tributaries without daily flow observation data are recorded as T si , collect the daily flow observation data of T bi for m years before and after the establishment of the database, and obtain the interval daily missing water quantity according to the principle of water conservation, so as to obtain the monthly incoming water quantity of T si , the calculation formula of the interval daily missing water quantity is:

[0021] Interval daily missing water quantity = interval outlet daily flow - interval inlet daily flow - T bi daily flow in the interval

[0022] The calculation formula of the monthly incoming water quantity of T si is:

[0023] The monthly incoming water quantity of T si = the average daily missing water quantity per month;

[0024] Step 2.3, use the flow data of all tributaries T bi and T si , respectively accumulate and average the flow of each tributary on the same date for m years, calculate the multi-year average flow Ql ai of each tributary, and record it as the average situation L a of tributary inflow and outflow;

[0025] Step 2.4, combine the main flood period of the main stream, and average the incoming flow of the main stream and each tributary in the main flood period, to obtain the average incoming flow Q fi of each flood source in the flood period, sort Q fi , and select several typical flood years after the establishment of the database and several general flood years before the establishment of the database, record the tributary inflow of several typical flood years after the establishment of the database as the extreme situation L i , record the river inlet flow process as U i , record the tributary inflow of several general flood years before the establishment of the database as L' i , and record the river inlet flow process as U i '.

[0026] Furthermore, step 3 specifically includes:

[0027] Step 3.1: For the target river section, collect topographic data for the corresponding years or nearby years of typical flood years after reservoir construction and general flood years before reservoir construction. The topographic data should be able to fully reflect the topography and resistance adjustment.

[0028] Step 3.2: Establish a one-dimensional unsteady flow mathematical model based on the topographic data, and perform differential discretization using a linearized Preissmann four-point eccentric implicit scheme;

[0029] Step 3.3: Based on the measured mainstream flow U for the corresponding years before and after reservoir construction. i 'and U i tributary inflow L' i and L i In addition, the roughness parameter required for calibrating the one-dimensional unsteady flow hydrodynamic mathematical model is determined using flood hydrological data from stations along the main stream. The water levels at each station calculated by the one-dimensional unsteady flow hydrodynamic mathematical model are denoted as Z. s Let x be the measured water level at each station, and Z be the actual water level recorded at each station. m As y, perform a linear fit with the line y = x, and compare the coefficients of determination R of the fit. 2 Size, R 2 A value close to 1 indicates that a one-dimensional non-steady hydrodynamic mathematical model can better reproduce the flood evolution process.

[0030] Furthermore, step 4 specifically includes:

[0031] Step 4.1: Select several typical flood years after the reservoir's construction, and use the measured main stream inflow U for the corresponding years. i Average inflow of tributaries L a By combining a one-dimensional unsteady hydrodynamic mathematical model, flood evolution calculations were performed to obtain the average daily flow Q at each monitoring station after reservoir construction. c and the corresponding daily average water level Z c ;

[0032] Step 4.2: Calculate the daily average water level Z at each monitoring station. c and daily average traffic Q c The scatter plots are compared with the average line and upper and lower envelope lines of the relationship between the water level and flow data above the floodplain flow of the target river section for n years before the reservoir is built, in order to assess the changing trend of the river channel's discharge capacity after the reservoir is built.

[0033] Furthermore, step 5 specifically includes:

[0034] Step 5.1, for the target river section, in the general flood year before the establishment of the library and in the typical flood year after the establishment of the library, a number of years are selected, first, the water level-discharge relation curve of the outlet station of the river section in the corresponding years before and after the establishment of the library is fitted in the form of a polynomial, and then it is combined with the topographic data, the corresponding parameter roughness, the inlet flow process of the river section in the typical flood year after the establishment of the library, and the tributary inflow process to set a kind of working condition;

[0035] Step 5.2, combining multiple working conditions, the data series corresponding to each working condition is input to each boundary of the model, and the flood routing calculation is carried out by means of the mathematical model and the water level-discharge process of the river section inlet and along the river is output;

[0036] Step 5.3, for the calculation results of a kind of working condition, the water level-discharge function relationship F i is fitted, the characteristic flow level Q x is selected, the fitted water level-discharge function relationship F i is substituted, the corresponding water level Z xi is obtained, 1≤i≤a, and thus the influence of the river terrain, resistance and outlet boundary, and interval inflow on the flood level is obtained;

[0037] Step 5.4, for the selected measured water level-discharge observation data of the river section inlet and along the river in the general flood year before the establishment of the library and in the typical flood year after the establishment of the library, the water level-discharge function relationship II in the general flood year before the establishment of the library and the water level-discharge function relationship III in the typical flood year after the establishment of the library are fitted, Q x is substituted into the water level-discharge function relationship II in the general flood year before the establishment of the library to obtain the corresponding water level Z' x , Q x is substituted into the water level-discharge function relationship III in the typical flood year after the establishment of the library to obtain the corresponding water level Z x , the difference ΔZ between Z' x and Z x is ΔZ, and ΔZ is the total amplitude of the flood level under the action of various factors, and thus the influence of the upstream inflow fluctuation rate on the flood level ΔZ U is obtained.

[0038] Further, in step 5.2, a=9, the water level-discharge relation curves T pre and T post of the outlet station of the river section in the corresponding years before and after the establishment of the library, the topographic data D pre and D post of the target river section in the corresponding years before and after the establishment of the library, the model parameter roughness n pre and n post of the target river section in the corresponding years before and after the establishment of the library, the inlet flow process U1 of the river section in the typical flood year after the establishment of the library, and the tributary inflow process L1 and the multi-year average tributary inflow L a are combined to set 9 kinds of working conditions, and the first kind of working condition is T post, D post , n post , U1, L1, corresponding water level is Z x1 ; the second working condition is T pre , D post , n post , U1, L1, corresponding water level is Z x2 ; the third working condition is T post , D post , n pre , U1, L1, corresponding water level is Z x3 ; the fourth working condition is T pre , D post , n pre , U1, L1, corresponding water level is Z x4 ; the fifth working condition is T post , D pre , n post , U1, L1, corresponding water level is Z x5 ; the sixth working condition is T pre , D pre , n post , U1, L1, corresponding water level is Z x6 ; the seventh working condition is T post , D pre , n pre , U1, L1, corresponding water level is Z x7 ; the eighth working condition is T pre , D pre , n pre , U1, L1, corresponding water level is Z x8 and the ninth working condition is T post , D post , n post , U1, L a , corresponding water level is Z x9 .

[0039] Further, in step 5.3, the influence of river terrain, resistance and outlet boundary, interval inflow on flood level is obtained, Z x1 -Z x2 is the influence of river terrain on flood level ΔZ T , Z x1 -Z x3 is the influence of river roughness on flood level ΔZ n , Z x1 -Z x4 is the superimposed influence of river factors on flood level ΔZ Tn , Z x1 -Z x5 is the influence of river section outlet boundary on flood level ΔZ D , Z x1 -Zx6 is the superimposed influence of river terrain and outlet boundary on flood level ΔZ TD , x1 -Z x7 is the superimposed influence of river roughness and outlet boundary on flood level ΔZ nD , x1 -Z x8 is the superimposed influence of river terrain, roughness and outlet boundary on flood level ΔZ TnD , x1 -Z x9 is the influence of interval inflow on flood level ΔZ L .

[0040] Further, the upstream inflow fluctuation rate influences the flood level ΔZ U , ΔZ U = ΔZ- ΔZ T - ΔZ n - ΔZ D - ΔZ L .

[0041] Further, in step 5.2, the characteristic flow level Q x Select the warning water level corresponding flow of each measuring station along the river section.

[0042] Compared with the prior art, the present application has the following advantages and beneficial effects:

[0043] The present application determines the average line and upper and lower envelope lines of the relationship between the water level data and flow data above the n-year flat flow of the target river section before the library is built, and the average situation and extreme situation of the branch inflow in the main flood season of the main stream are statistically analyzed, and the influence of the extreme interval inflow of the typical flood year after the library is built is separated by using the calibrated one-dimensional unsteady flow hydrodynamic mathematical model, and compared with the average line and upper and lower envelope lines of the relationship between the water level data and flow data above the n-year flat flow of the target river section before the library is built, so as to evaluate the change trend of the river discharge capacity after the library is built, and through the foregoing process, the change trend and cause of the flood level in the river section below the dam can be accurately identified, compared with the prior art in engineering practice, the method utilizes the rapid processing capacity of a computer for a large amount of hydrological information and the rapid simulation calculation capacity of river hydrodynamics, and has small workload and simple operation, and in the complex situation that the branch is numerous, the combination of the main and branch inflows is random, and numerous influencing factors are intertwined, the change trend of the river discharge capacity can be effectively evaluated, and the influence of each factor on the flood level is quantitatively separated. The technology provides a new auxiliary tool for flood disaster situation evaluation, flood control strategy formulation and other engineering practices of the river section below the dam. BRIEF DESCRIPTION OF DRAWINGS

[0044] Figure 1 The method for identifying the flood level change trend and cause of the multi-branch river section below the dam of the present application

[0045] Figure 2 This is a flowchart of a method for identifying the trend and causes of flood level changes in a river section affected by multiple tributaries downstream of a dam, as described in an embodiment of the present invention.

[0046] Figure 3 This is a simplified diagram of a river segment in an embodiment of the present invention.

[0047] Figure 4 This is the multi-year variation and average line of the water level and flow rate above the floodplain flow rate at the inlet station A of the river section before the reservoir was built, as shown in this embodiment of the invention.

[0048] Figure 5 In the embodiment of the present invention, the branch T s The actual variation of the flow rate in the same period of previous years and the multi-year average.

[0049] Figure 6 This is a graph showing the trend of changes in the discharge capacity of the river channel as assessed by monitoring station A at the river inlet in this embodiment of the invention.

[0050] Figure 7 This invention illustrates the influence of river resistance and roughness on water level at river inlet station A in this embodiment of the invention. Detailed Implementation

[0051] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention.

[0052] This invention provides a method for identifying the trends and causes of flood level changes in multiple tributary river sections downstream of a dam, such as... Figure 1 As shown, it includes the following steps:

[0053] Step 1: Determine the average line and upper and lower envelope lines of the relationship between the water level data above the floodplain flow and the flow data of the target river section over n years before reservoir construction;

[0054] Step 2: Compile statistics on the average and extreme situations of tributary inflows during the main flood season of the main stream;

[0055] Step 3: Collect topographic data of the target river section and establish a one-dimensional unsteady hydrodynamic mathematical model;

[0056] Step 4: Based on the one-dimensional unsteady flow dynamic mathematical model, evaluate the trend of changes in the river channel's discharge capacity after the reservoir is built;

[0057] Step 5: Quantitatively analyze the impact of various factors on the flood level of the downstream tributary confluence section of the dam.

[0058] The application determines the average line and the upper and lower envelope lines of the water level data and flow data relationship of the target river section above the n-year flat flow before the reservoir is built, and compares the average relationship of the water level data and flow data of the target river section above the n-year flat flow before the reservoir is built with the upper and lower amplitude by separating the influence of the extreme interval inflow in the typical flood year after the reservoir is built through the calibrated one-dimensional unsteady flow hydrodynamic mathematical model, so as to evaluate the change trend of the river discharge capacity after the reservoir is built. Through the foregoing process, the change trend and causes of the flood level in the river section downstream of the dam affected by the branch buttress can be accurately identified. Compared with the prior art in engineering practice, the method utilizes the rapid processing capability of a computer for a large amount of hydrological information and the rapid simulation calculation capability of river hydrodynamics, has less workload and is simple to operate. In the complex situation that there are many branches, the inflow combination of the main branch and the branch is random, and many influencing factors are intertwined, the change trend of the river discharge capacity can be effectively evaluated, and the influence of each factor on the flood level can be quantitatively separated. The technology provides a new auxiliary tool for flood disaster situation evaluation, flood control strategy development and other engineering practices of the river section downstream of the dam.

[0059] The step 1 specifically comprises:

[0060] Step 1.1, for the target river section, collecting n-year hydrological observation data of the target river section before the reservoir is built, n>=10, including daily average flow and corresponding daily average water level of the river section inlet and each measuring station along the river;

[0061] Step 1.2, combining the flat flow Q b of each measuring station, selecting daily average flow greater than Q b as Q g , and recording the daily average water level corresponding to Q g as Z g ;

[0062] Step 1.3, based on the daily average flow Q g and the daily average water level Z g of each measuring station, fitting the water level flow function relationship I between Z g and Q g before the reservoir is built, the water level flow function relationship I is the expression of the average line C g of the relationship between Z g and Q a ;

[0063] Step 1.4, keeping the function curve linear unchanged, changing the intercept of I on the y-axis according to the water level flow function relationship I, obtaining the upper envelope line C u and the lower envelope line C d , so that all Q g and Z g of each measuring station before the reservoir is built are distributed between C u and C d .

[0064] The step 1 of the present application can collect hydrological observation data of the target river section for n years before the reservoir is built, and determine the average line and the upper and lower envelope lines of the water level-flow relationship above the bankfull discharge before the reservoir is built.

[0065] In the present application, in step 1.3, the water level-flow function relationship I between Q and Z of each station before the reservoir is built can be fitted by Origin, SPSS, MATLAB or Python. g and Z g .

[0066] In the present application, in step 1.3, the water level-flow function relationship I between Q and Z of each station before the reservoir is built is fitted in the form of a polynomial. g and Z g .

[0067] In step 1.3, the water level-flow function relationship I between Q and Z of each station before the reservoir is built is fitted in the form of a polynomial. g and Z g , and the determination coefficient R 2 is determined. a The closer the determination coefficient R g is to 1, the better the average line C g represents the average condition of the relationship between Q and Z of each station before the reservoir is built.

[0068] In the present application, in step 1.4, the water level-flow function relationship I curve is kept unchanged, and the intercept of the average line C a and the y-axis is changed to obtain the upper envelope line C g and the lower envelope line C g of the water level-flow function relationship I between Q and Z of each station before the reservoir is built. u and C d , so that all the Q g and Z g scatter points are distributed between C u and C d .

[0069] The step 2 of the present application specifically comprises:

[0070] Step 2.1, for the target river section, collecting the observation data of the daily average flow of each station at the inlet and outlet of the target river section before and after the reservoir is built, m≥20, and determining the main flood period of the main stream;

[0071] Step 2.2, recording large tributaries with daily average flow observation data in the target river section as T bi , recording small tributaries without daily average flow observation data as T si , and collecting T biBased on the daily average flow observation data for m years before and after reservoir construction, and according to the principle of water conservation, the daily average missing water volume for the interval is obtained, thus yielding T. si The formula for calculating the average monthly water inflow and the average daily water shortage in the interval is:

[0072] Average daily water shortage in the interval = Average daily flow rate at the interval outlet - Average daily flow rate at the interval inlet - T within the interval bi Daily traffic

[0073] T si The formula for calculating the average monthly water inflow is:

[0074] T si The average monthly water inflow = the average daily water shortage per month;

[0075] Step 2.3: Use all tributaries T within the river section bi With T si The flow data is used to sum up the flow data of each tributary on the same date in year m and calculate the average flow to obtain the multi-year average flow Ql of each tributary. ai Let L be the average case of inflows and outflows. a ;

[0076] Step 2.4: Combining the main flood season period of the main stream, average the inflow of the main stream and each tributary during the main flood season to obtain the average inflow Q of each flood source during the flood season. fi , for Q fi The data were sorted and selected from several typical flood years after reservoir construction and general flood years before reservoir construction. The tributary inflow corresponding to the typical flood years after reservoir construction was recorded as the extreme case L. i The inflow process of the river section is denoted as U. i The tributary flow corresponding to several flood years before reservoir construction is denoted as L'. i The inflow process of the river section is denoted as U. i '.

[0077] Step 2 of this invention statistically analyzes the multi-year average of tributary inflows and outflows, and obtains several typical flood years after reservoir construction and general flood years before reservoir construction by sorting the flow during the main flood season.

[0078] In this invention, in step 2.1, the main flood season period of the main stream is determined by first determining the period when the inlet of the river section has a major flood in more than 90% of the years within a year, and then determining the period when the outlet of the river section has a major flood in more than 90% of the years within a year. The common period of the two periods is taken as the main flood season period of the main stream. Here, a major flood refers to a flood that exceeds the multi-year average of the annual maximum flood peak flow.

[0079] In step 2.4, for Q fiThe years were sorted separately according to flood volume from high to low. First, the years were sorted by the inflow from the upper reaches of the river, then by the total inflow from the tributaries, and finally by the inflow from different sections of the river. Several years in which the flood volume of each flood source ranked in the top 30-35% during the flood season after the reservoir was built were selected as typical flood years after the reservoir was built. Several years in which the flood volume of the main stream inflow ranked in the top 30-35% and the flood volume of the tributaries inflow ranked in the top 40-60% during the flood season before the reservoir was built were selected as general flood years before the reservoir was built.

[0080] In this invention, step 3 specifically includes:

[0081] Step 3.1: For the target river section, collect topographic data for the corresponding years or nearby years of typical flood years after reservoir construction and general flood years before reservoir construction. The topographic data should be able to fully reflect the topography and resistance adjustment.

[0082] Step 3.2: Establish a one-dimensional unsteady flow mathematical model based on the topographic data, and perform differential discretization using a linearized Preissmann four-point eccentric implicit scheme;

[0083] Step 3.3: Based on the measured mainstream flow U for the corresponding years before and after reservoir construction. i 'and U i tributary inflow L' i and L i In addition, the roughness parameter required for calibrating the one-dimensional unsteady flow hydrodynamic mathematical model is determined using flood hydrological data from stations along the main stream. The water levels at each station calculated by the one-dimensional unsteady flow hydrodynamic mathematical model are denoted as Z. s Let x be the measured water level at each station, and Z be the actual water level recorded at each station. m As y, perform a linear fit with the line y = x, and compare the coefficients of determination R of the fit. 2 Size, R 2 A value close to 1 indicates that a one-dimensional non-steady hydrodynamic mathematical model can better reproduce the flood evolution process.

[0084] Step 3 of this invention calibrates the roughness parameters of the one-dimensional hydrodynamic mathematical model based on topographic and hydrological data for typical flood years after reservoir construction and general flood years before reservoir construction.

[0085] In this invention, step 4 specifically includes:

[0086] Step 4.1: Select several typical flood years after the reservoir's construction, and use the measured main stream inflow U for the corresponding years. i Average inflow of tributaries L a By combining a one-dimensional unsteady hydrodynamic mathematical model, flood evolution calculations were performed to obtain the average daily flow Q at each monitoring station after reservoir construction. c and the corresponding daily average water level Z c ;

[0087] Step 4.2, the daily average water level Z of each station c and the daily average flow Q c The average line and the upper and lower envelope lines of the relationship between the water level data and the flow data of the target river section above the n-year flat flow before the database is built are compared with the scatter points to evaluate the change trend of the river channel discharge capacity after the reservoir is built.

[0088] In step 4 of the present application, the influence of the extreme interval inflow of a typical flood year after the reservoir is built is separated by a calibrated hydrodynamic mathematical model, and the change trend of the river channel discharge capacity after the reservoir is built is evaluated by comparing the water level-flow function relationship I before the reservoir is built.

[0089] In step 4.2 of the present application, the daily average flow Q of each station c and the corresponding daily average water level Z c are compared with the average line C a , the upper envelope line C u , and the lower envelope line C d before the database is built to evaluate the change trend of the river channel discharge capacity after the reservoir is built.

[0090] In step 4.2 of the present application, the daily average flow Q of each station c and the corresponding daily average water level Z c are compared with the average line C a , the upper envelope line C u , and the lower envelope line C d before the database is built, if the daily average flow Q of each station c and the corresponding daily average water level Z c scatter points can be symmetrically located on both sides of C a , it is considered that the river channel discharge capacity has not changed after the reservoir is built, otherwise it is considered that the trend has changed, if the daily average flow Q of each station c and the corresponding daily average water level Z c scatter points deviate from the upper envelope line C u and the lower envelope line C d , it is considered that there is a significant trend change.

[0091] The step 5 specifically includes:

[0092] Step 5.1, for the target river section, in the general flood year before the reservoir is built and the typical flood year after the reservoir is built, first fit the water level-flow relationship line of the outlet station of the river section before and after the reservoir is built in the corresponding years in the form of a polynomial, and then combine the topographic data, the corresponding parameter roughness, the river section inlet flow process of the typical flood year after the reservoir is built, and the tributary inflow process to set a kind of working condition;

[0093] Step 5.2, in combination with various working conditions, the model inputs the data series corresponding to each working condition of each boundary, and the flood evolution calculation is carried out by means of the mathematical model and the water level and flow process of the river section inlet and each hydrological station along the river are outputted;

[0094] Step 5.3, the water level and flow function relationship F is fitted respectively for the calculation results of a working conditions i , the characteristic flow level Q x is selected, the water level and flow function relationship F i is substituted, the corresponding water level Z xi is obtained, and 1≤i≤a, so that the influence of river terrain, resistance and outlet boundary and interval inflow on the flood level is obtained;

[0095] Step 5.4, the measured water level and flow observation data of the river section inlet and each measuring station along the river in the selected general flood year before the reservoir is built and the typical flood year after the reservoir is built are fitted, the water level and flow function relationship II of the general flood year before the reservoir is built and the water level and flow function relationship III of the typical flood year after the reservoir is built are fitted, Q x is substituted into the water level and flow function relationship II of the general flood year before the reservoir is built to obtain the corresponding water level Z' x , Q x is substituted into the water level and flow function relationship III of the typical flood year after the reservoir is built to obtain the corresponding water level Z x , the difference ΔZ between Z' x and Z x is obtained, ΔZ is the total amplitude of the flood level under the action of various factors, and the influence ΔZ of the upstream inflow fluctuation rate on the flood level is obtained. U

[0096] The step 5 of the present application separates variables and sets multiple working conditions for calculation, so that the influence of river terrain, resistance, interval inflow, upstream inflow fluctuation rate, outlet boundary and other factors on the flood level is quantitatively analyzed.

[0097] In step 5.2, a=9, the water level and flow relationship lines T pre and T post of the river section outlet measuring station in the corresponding years before and after the reservoir is built, the terrain data D pre and D post of the target river section in the corresponding years before and after the reservoir is built, the model parameters roughness n pre and n post of the target river section in the corresponding years before and after the reservoir is built, the river section inlet flow process U1 and the tributary inflow process L1 in the typical flood year after the reservoir is built, and the multi-year average tributary inflow L a are combined to set 9 working conditions, the first working condition is T post , D post , n post , U1, L1, and the corresponding water level is Z x1 ; the second working condition is T pre ​, D post , n post , U1, L1, corresponding water level is Z x2 ; the third working condition is T post , D post , n pre , U1, L1, corresponding water level is Z x3 ; the fourth working condition is T pre , D post , n pre , U1, L1, corresponding water level is Z x4 ; the fifth working condition is T post , D pre , n post , U1, L1, corresponding water level is Z x5 ; the sixth working condition is T pre , D pre , n post , U1, L1, corresponding water level is Z x6 ; the seventh working condition is T post , D pre , n pre , U1, L1, corresponding water level is Z x7 ; the eighth working condition is T pre , D pre , n pre , U1, L1, corresponding water level is Z x8 and the ninth working condition is T post , D post , n post , U1, L a , corresponding water level is Z x9 .

[0098] In step 5.2, the characteristic flow level Q x The warning water level corresponding flow of each measuring station along the river section is selected.

[0099] Preferably, the process method for identifying the change trend and cause of the flood level of the river section where the multiple tributaries converge under the dam provided by the application can also have the following characteristics: in step 5.3, the influence of the river terrain, resistance and outlet boundary, and interval inflow on the flood level is obtained, Z x1 -Z x2 is the influence of the river terrain on the flood level ΔZ T , Z x1 -Z x3 is the influence of the river roughness on the flood level ΔZ n , Z x1 -Z x4 is the superimposed influence of the river factors on the flood level ΔZ Tn , Z x1 -Z x5The influence of the river section outlet boundary on the flood level ΔZ D , Z x1 - Z x6 The superimposed influence of the river section terrain and the outlet boundary on the flood level ΔZ TD , Z x1 - Z x7 The superimposed influence of the river section roughness and the outlet boundary on the flood level ΔZ nD , Z x1 - Z x8 The superimposed influence of the river section terrain, roughness and the outlet boundary on the flood level ΔZ TnD , Z x1 - Z x9 The influence of the interval inflow on the flood level ΔZ L .

[0100] In step 5.4, the upstream inflow fluctuation rate influences the flood level ΔZ U , which is calculated by ΔZ-ΔZ T -ΔZ n -ΔZ D -ΔZ L .

[0101] In an embodiment of the present application, as shown in Figure 3 , the embodiment adopts a certain river section, the schematic diagram of which is shown in Figure 2 , the inlet and outlet stations of the river section are A and B respectively, the internal station of the river section is C, there is an observable tributary, which is recorded as T b , and the tributary without observation data is recorded as T s , and the used water level observation data are the daily average flow observation data and the daily average water level observation data of the river section from 1991 to 2020.

[0102] The method for identifying the change trend and cause of the flood level of the multi-tributary river section below the dam provided by the embodiment comprises the following steps:

[0103] Step 1, determining the average line and the upper and lower envelope lines of the relationship between the water level data and the flow data above the flat-plate flow of the target river section before the establishment of the reservoir.

[0104] Using the 10-year measured water level and flow of each station along the river section, the flat-plate flow Q b of each station is 35000, 35000 and 40000 m 3 / s respectively, and in the Excel software, the daily average flow greater than Q b of each station is selected and recorded as Q b by combining the flat-plate flow Q g of each station, and the corresponding daily average water level of Q g is recorded as Z gAnd by using polynomial fitting to fit the multi-year average water level-discharge relationship I of each station under uniform flow conditions, the Q of each station before reservoir construction was obtained. g and Z g The water level-flow rate function relationship I is given by Q. g and Z g Relationship Average Line C a The expression for the water level-flow rate function I in this embodiment is: Keep the curve shape unchanged, change the average line C a The intercept with the y-axis is used to obtain the Q values ​​of each station before the reservoir was built. g and Z g The upper envelope C of the water level-flow rate function relationship I between them u With lower envelope C d Among them, the Q stations of each station before the database was built at station A. g and Z g The average line C of the water level-flow function relationship I a Upper envelope C u With lower envelope C d See Figure 4 .

[0105] Step 2: Statistically analyze the average and extreme situations of tributary inflows during the main flood season of the main stream.

[0106] The main flood season period of the main stream was determined. In this embodiment, the main flood season period of the main stream is July-August. Using the measured flow and water level data of the river inlet and outlet and the tributary reservoirs before and after m=30 years, the average monthly inflow of the tributary without observation data was obtained. The average inflow and outflow of the tributary was calculated in Excel software. a tributary T with no observational data s For the range of inflow fluctuations and average inflow, please refer to Figure 5 Based on the main flood season period of the main stream, the inflow of each tributary during the flood season is sorted and ranked from high to low according to the flood volume. Several years in which the flood volume of each flood source is in the top 30% in the flood season after the reservoir is built are selected as typical flood years after the reservoir is built. Several years in which the flood volume is in the top 40%-60% before the reservoir is built are selected as general flood years before the reservoir is built. In this embodiment, a typical flood year after the reservoir is built is selected, with a total inflow ranking of 2 and inflow rankings of 1 and 7 in the interval. A general flood year before the reservoir is built is selected, with a total inflow ranking of 13 and inflow rankings of 10 and 22 in the interval.

[0107] Step 3: Collect topographic data of the target river section, establish a one-dimensional unsteady hydrodynamic mathematical model, and calibrate the parameters required for the one-dimensional unsteady hydrodynamic mathematical model.

[0108] The topographic data of the typical flood year after the establishment of the library and the general flood year before the establishment of the library corresponding to the year or the nearby year are collected, the topographic data corresponding to the years before and after the establishment of the library in the embodiment are separated by 15 years, which can fully reflect the topography and resistance adjustment, a one-dimensional unsteady flow mathematical model is established, and the required parameters of the model are calibrated, in the embodiment, the linear fitting coefficient R of the water level calculated by the model and the measured water level is 0.998, 0.994 and 0.995 respectively, and the model can reproduce the flood evolution process. 2

[0109] Step 4, according to the one-dimensional unsteady flow hydrodynamic mathematical model, the change trend of the river discharge capacity after the establishment of the library is evaluated.

[0110] The measured dry stream inflow U i , the average situation of the tributary inflow L a , the flood hydrological data of the dry stream along the measuring station, combined with the one-dimensional unsteady flow hydrodynamic mathematical model, the water level Z c and the flow process Q c of the river section inlet and the internal measuring station are obtained, which are compared with the average line C a , the upper envelope line C u and the lower envelope line C d before the establishment of the library, in the embodiment, the comparison results of the river section inlet measuring station A are shown in Figure 6 , the calculated value can be symmetrically located near C a , and the river discharge capacity does not change after the establishment of the library.

[0111] Step 5, quantitatively analyze the influence of each factor on the flood level of the tributary confluence section downstream of the dam.

[0112] Determine the outlet boundary water level-flow relationship line of the typical flood year after the establishment of the library and the general flood year before the establishment of the library, arrange the topographic data, and inversely calculate the corresponding year roughness by means of the mathematical model. In the embodiment, the outlet boundary water level-flow relationship line does not change obviously before and after the establishment of the library, and the influence of the outlet boundary on the flood level can be ignored. Five working conditions are established to separate the influence of river topography, resistance adjustment, interval inflow and upstream fluctuation rate on the flood level, and the influence of a single factor on the flood level is quantified by means of the mathematical model, Figure 7 The influence of river topography and resistance adjustment on the flood level at measuring station A. The calculated water level-flow function relationship F i , 1≤i≤5 and the water level-flow function relationship II of the general flood year before the establishment of the library and the water level-flow function relationship III of the typical flood year after the establishment of the library are fitted in the form of a polynomial, and the characteristic flow level Q x of the river section is 50000m 3 ​The actual difference of flood level before and after the establishment of the database of the river section inlet measuring station is +1.62m, and the influences of river terrain, resistance, river channel form and resistance adjustment, interval inflow and upstream inflow fluctuation rate on the flood level are-0.75, +0.85, +0.09, +1.37 and +0.16m respectively.

[0113] In summary, the present application aims at the common technical problem that the random fluctuation and the trend adjustment of the flood level in the river section where the dam is located and the tributaries converge, which leads to the difficulty in judging the adjustment trend of the flood level and the difficulty in identifying the influence of each factor. A complete processing flow and method based on hydrological observation data, statistical software and mathematical model of water flow are proposed. The present application can collect the hydrological observation data of the target river section for n years before the establishment of the dam, determine the average line and the upper and lower envelope lines of the water level-flow relationship above the bankfull discharge before the establishment of the dam, statistically analyze the average situation of the tributary inflow, obtain a plurality of typical flood years after the establishment of the dam and general flood years before the establishment of the dam through the flow sorting in the main flood season, calibrate the roughness of the one-dimensional hydrodynamic mathematical model parameters of the typical flood years after the establishment of the dam and the general flood years before the establishment of the dam based on the terrain and hydrological data, separate the influence of the extreme interval inflow of the typical flood years after the establishment of the dam by using the calibrated hydrodynamic mathematical model, compare with the water level-flow function relationship I before the establishment of the dam, evaluate the change trend of the river discharge capacity after the establishment of the dam, and calculate under a plurality of working conditions by separating variables, so as to quantitatively analyze the influence of the river terrain, resistance, interval inflow, upstream inflow fluctuation rate and outlet boundary on the flood level. Through the foregoing flow, the change trend and cause of the flood level in the river section under the dam can be accurately identified. Compared with the prior art in engineering practice, the method uses the rapid processing capacity of the computer for a large amount of hydrological information and the fast simulation calculation capacity of the river water dynamics, has a small workload and is simple to operate. In the complex situation that there are many tributaries, the combination of the inflow of the main river and tributaries is random, and many influencing factors are intertwined, the change trend of the river discharge capacity can be effectively evaluated, and the influence of each factor on the flood level can be quantitatively separated. The technology provides a new auxiliary tool for the flood disaster situation evaluation, flood control strategy formulation and other engineering practices of the river section under the dam.

[0114] Obviously, those skilled in the art can make various modifications and variations to the present application without departing from the spirit and scope of the present application. Thus, if these modifications and variations of the present application belong to the scope of the claims of the present application and the equivalent technologies thereof, the present application also intends to include these modifications and variations.

Claims

1. A method for identifying the trend and cause of flood level change in a river section with multiple tributaries downstream of a dam, characterized in that, The method comprises the following steps: Step 1, determining the average line and the upper and lower envelope lines of the water level data and the flow data relationship of the target river section above the n-year flat flow before the establishment of the library; Step 2, statistics of the average situation and extreme situation of the tributary inflow in the main flood season of the main stream; Step 3, collecting topographic data of the target river section, and establishing a one-dimensional unsteady flow hydrodynamic mathematical model; Step 4, according to the one-dimensional unsteady flow hydrodynamic mathematical model, evaluating the change trend of the river discharge capacity after the establishment of the reservoir; Step 5, quantitatively analyzing the influence of each factor on the flood level of the tributary confluence section downstream of the dam; The step 2 specifically comprises: Step 2.1, collecting the daily average flow observation data of the target river section at the inlet and outlet of the target river section before and after the establishment of the reservoir for m years, and determining the main flood period of the main stream; Step 2.2, the large tributaries with daily flow observation data in the target river section are recorded as , the small tributaries without daily flow observation data are recorded as , the daily flow observation data of m years before and after the database is built are collected , and the monthly inflow is obtained according to the water conservation principle, and the calculation formula of the interval daily missing water quantity is: ​ Interval daily missing water volume = Interval outlet daily average flow - Interval inlet daily average flow - Interval internal daily average flow daily average flow The formula for calculating the monthly average inflow is: Monthly average inflow = average daily inflow for each month; Step 2.3, add up the flow data of all tributaries in the river section and calculate the average flow of each tributary on the same date for m years With , calculate the average flow of each tributary over multiple years , recorded as the average case of tributary inflow ; Step 2.4, combine the dry flow and the flow of each tributary in the main flood period, and then average the flow of each flood source in the main flood period to obtain the average flow of each flood source in the flood period , the is sorted, and several typical post-construction flood years and several pre-construction general flood years are screened out, the tributary inflow corresponding to the several typical post-construction flood years is recorded as an extreme case , the inflow process of the river section is recorded as , the tributary inflow corresponding to the several pre-construction general flood years is recorded as , and the inflow process of the river section is recorded as .

2. The method of identifying the trend and cause of the flood level change of a multi-branch river section downstream of a dam according to claim 1, characterized in that, The step 1 specifically comprises: Step 1.1, collecting the hydrological observation data of the target river section for n years before the establishment of the reservoir, including the daily average flow and the corresponding daily average water level of the inlet and the along-the-way stations of the river section; Step 1.2: Combine the flat-shoal flow at each station Select stations with values ​​greater than The average daily traffic is recorded as ,Will The corresponding daily average water level is recorded as ; Step 1.3, fitting water level-discharge function I between each station before building the database and The expression of water level-discharge function I is and The average line of the relationship ​ Step 1.4, keeping the function curve linear, according to the water level flow function relationship I, change the intercept of I on the y axis, get the upper envelope line and the lower envelope line , so that all the and of each station before the library are distributed between and .

3. The method of claim 1, wherein the method is characterized in that, The step 3 specifically comprises: Step 3.1, collecting the topographic data of the typical flood year after the establishment of the reservoir and the corresponding year or nearby year of the general flood year before the establishment of the reservoir for the target river section, and the topographic data need to fully reflect the topography and resistance adjustment; Step 3.2, establishing a one-dimensional unsteady flow mathematical model according to the topographic data, and using the linearized Preissmann four-point eccentric implicit scheme for difference discretization; Step 3.3: Based on the measured main stream flow for the corresponding years before and after reservoir construction. and tributary flow and In addition, the roughness parameter required for calibrating the one-dimensional unsteady flow hydrodynamic mathematical model is determined using flood hydrological data from stations along the main stream. The water levels at each station calculated by the one-dimensional unsteady flow hydrodynamic mathematical model are denoted as... As x, the measured water level at each station is recorded as . As y, perform a linear fit with the line y=x, and compare the coefficients of determination of the fit. size, A value close to 1 indicates that a one-dimensional non-steady hydrodynamic mathematical model can better reproduce the flood evolution process.

4. The method of claim 1, wherein the method is characterized in that, The step 4 specifically comprises: Step 4.1, select several typical flood years after the reservoir is built, and use the measured inflow of the main stream in the corresponding year , the average case of tributary inflow , combined with one-dimensional unsteady flow hydrodynamic mathematical model, implement flood routing calculation to obtain the daily average flow of each station after the reservoir is built and the corresponding daily average water level ; Step 4.2, the daily average water level of each station and daily average flow The average line and the upper and lower envelope lines of the relationship between the water level data and the flow data of the target river section above the n-year flat flow before the library is built are compared to evaluate the change trend of the river discharge capacity after the library is built.

5. The method of claim 1, wherein the method further comprises: The step 5 specifically comprises: Step 5.1, for the target river section, in the general flood year before the establishment of the reservoir and the typical flood year after the establishment of the reservoir, first fit the water level-flow relationship line of the outlet station of the river section before and after the establishment of the reservoir in the form of a polynomial, and then combine it with the topographic data, the corresponding parameter roughness, the inlet flow process of the river section in the typical flood year after the establishment of the reservoir, and the tributary inflow process to set a kind of working condition; Step 5.2, combining multiple working conditions, inputting the data series corresponding to each working condition to each boundary of the model, and carrying out flood evolution calculation and outputting the water level-flow process of the inlet and the along-the-way hydrological stations by means of the mathematical model; Step 5.3, fitting water level-flow function relationship for each working condition , selecting characteristic flow stage , substituting into the fitted water level-flow function relationship , obtaining corresponding water level , , thus obtaining the influence of river terrain, resistance, and outlet boundary and interval inflow on flood level; Step 5.4, for the selected pre-dam general flood year and the typical post-dam flood year, the measured water level flow observation data of the river section inlet and each measuring station along the way are fitted to obtain the water level flow function relationship II of the pre-dam general flood year and the water level flow function relationship III of the post-dam typical flood year. Substitute the water level flow function relationship II of the pre-dam general flood year to obtain the corresponding water level , Substitute the water level flow function relationship III of the post-dam typical flood year to obtain the corresponding water level , The difference between , is the total amplitude of the flood level under the action of each factor, thereby obtaining the influence of the upstream inflow fluctuation rate on the flood level .​ 6. The method of identifying the trend and cause of the flood level change of a multi-branch river section downstream of a dam according to claim 5, characterized in that: In step 5.2, a=9, the water level-discharge relation of the outlet of the river section is obtained before and after the construction of the reservoir and , the topographic data of the target river section before and after the construction of the reservoir and , the model parameters roughness of the target river section before and after the construction of the reservoir and , the typical flood year river section inlet flow process after the construction of the reservoir and the tributary inflow process , the multi-year average tributary inflow are combined, 9 working conditions are set, the first working condition is , , , , , the corresponding water level is ; the second working condition is , , , , , the corresponding water level is ; the third working condition is , , , , , the corresponding water level is ; the fourth working condition is , , , , , the corresponding water level is ; the fifth working condition is , , , , , the corresponding water level is ; the sixth working condition is , , , , , the corresponding water level is ; The seventh working condition is , , , , , the corresponding water level is ; the eighth working condition is , , , , , the corresponding water level is and the ninth working condition is , , , , , the corresponding water level is .

7. The method of identifying the trend and cause of the flood level change of a multi-branch river section downstream of a dam according to claim 6, characterized in that: In step 5.3, the river channel topography, resistance and outlet boundary, the influence of the reach inflow on the flood level are obtained, Influence of river channel topography on flood level , Influence of river channel roughness on flood level , Influence of river channel factors on the superposition of flood level , Influence of river section outlet boundary on flood level , Influence of river channel topography and outlet boundary on the superposition of flood level , Influence of river channel roughness and outlet boundary on the superposition of flood level , Influence of river channel topography, roughness and outlet boundary on the superposition of flood level , Influence of reach inflow on flood level .

8. The method of claim 7, wherein the method is characterized in that: Influence of upstream flow fluctuation on flood level , by = .

9. The method of claim 5, wherein the method is characterized in that: In step 5.2, the characteristic flow level The corresponding flow of the warning water level of each measuring station along the river section is selected.

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