CN105464040A - Numerical computation method for alluvial river bank collapse process - Google Patents

Abstract

The invention discloses a numerical computation method for an alluvial river bank collapse process. The method comprises the steps that according to measured profile terrain of last year and the measured or computed water level process of the current year, a slope crest and a slope toe of a river bank are determined, and morphological parameters of the river bank are computed; the slope toe scouring width and the bed surface erosion and deposition thickness of the river bank are computed; the change process of the water table in a soil body of the river bank is computed; the buckling safety factor of the river bank is computed, and whether collapse occurs or not is judged; the bank collapse strength is classified. The combined action of various bank collapse influence factors is considered, and the river collapse process, the scouring process of the slope toe of the river bank by water flow and the change process of the water table in the soil body of the river bank can be simulated at the same time; the change process of river bank stability over time can be directly reflected, the occurrence frequency of collapse at all time periods can be estimated, and the collapse width, occurrence time and morphology after collapse of the river bank can be predicted.

Description

The numerical computation method of alluvial channel collapse on the bank process

Technical field

The present invention relates to hydraulic engineering technical field, specifically a kind of numerical computation method of alluvial channel collapse on the bank process.

Background technology

Collapse on the bank is an importance of alluvial stream river-bed deformation process.Extensive collapse on the bank not only can damage built river regulating structure, increase the pressure of flood in littoral city, affect bank paddles the normal operation of facility, and main flow and deep trouth may be caused to swing, and the river gesture destroying section, local is stablized.But, existing collapse on the bank simulation and monitoring technology not perfect, can not predict in time or real-time monitored to the generation of collapse on the bank and evolution.Therefore by mathematical method to collapse on the bank process simulation, become research Bank Failure rule and carry out collapse on the bank prediction a kind of important means.

Existing collapse on the bank analogy method is usually theoretical based on stability of slope in river dynamics or soil mechanics.

The former generally adopts sediment movement theory of mechanics, in conjunction with measured data analysis, determine to affect the husky factor of key water of collapse on the bank, and propose riverbank and collapse the empirical relation moved back between the husky factor of width, speed etc. and these water, as the power function relationship between flat beach river width and inlet flow rate, riverbank collapses the power function relationship etc. between back speed rate and residue current shearing stress.This semi-analytic method can reflect the Bank Failure of long duration usually, but due to the complexity of collapse on the bank mechanism and the difference of the aspect such as bed configuration and riverbank soil strength variation, it is not suitable for the collapse on the bank process in river course in prediction short time interval, cannot determine the moment that collapse on the bank occurs and scale etc. yet.

From the research of soil mechanics angle, then based on the different mechanical mechanisms of bank slope collapse, propose all kinds of collapse on the bank pattern and corresponding bank stability computational methods, and part considers that different alluviation and non-alluviation factor are on the impact of bank stability, as factors such as current scour, river water level lifting and riverbank soil strength variation changes.What these class methods can calculate riverbank in a period of time collapses the process of moving back, and comprises the change of river bank shape, collapses and move back width and number of times etc., but relates to many factors due to collapse on the bank mechanism, the comprehensive effect considering each factor at present still tool acquire a certain degree of difficulty.

Summary of the invention

The object of the present invention is to provide a kind of numerical computation method of alluvial channel collapse on the bank process, the change procedure of the diadactic structure riverbank of certain specific section in the husky process effect stability inferior of certain water and bank slope form can be predicted in time, as collapse on the bank moment, avalanche number of times occurs and collapses to move back width etc., for collapse on the bank prediction and forewarn provide scientific basis and technical support.

Principle of the present invention is: riverbank collapses that to move back the factors such as the phreatic table change that to change with offshore current scour, riverbank soil strength variation, river water level and cause closely related, is necessary the impact considering these factors.Offshore current scour can cause riverbank steepening and highly increase, the major control factors that this normally natural alluvial channel collapse on the bank occurs, and the collapse on the bank as the middle and lower reach of Yangtze River 80% all betides the position, concave bank curved top of meeting the punching of stream top or current violent scour; And river water level lifting and the phreatic table change that causes thereof can change stress condition and the mechanical characteristic of the soil body, especially the collapse on the bank generation tool of water-break phase is had a significant effect.Therefore, the present invention, in conjunction with the calculating of toe scour process, phreatic table change procedure and bank stability three aspects, proposes the numerical computation method that a kind of diadactic structure riverbank collapses the process of moving back.

The numerical computation method of alluvial channel collapse on the bank process of the present invention, comprises the steps:

Step 1, according to the measured profile landform after last year flood and actual measurement then or calculate water level process, determines riverside slope top and toe, calculates the river bank shape parameter such as riverbank height, the initial gradient;

Step 2, calculate riverbank toe and wash away width and bed morphology thickness, concrete steps are as follows:

Step 2.1, calculates the bed morphology thickness in certain period △ t;

Step 2.2, calculates current to the transversal flow width of toe;

Step 2.3, based on step 2.1 and 2.2, amendment river bank shape;

Step 3, calculate phreatic table change procedure in the soil body of riverbank, concrete steps are as follows:

Step 3.1, divides computing node in the soil body of riverbank, determines initially to play phreatic table;

Step 3.2, according to one dimensional fluid flow governing equation, calculates each node phreatic table after a certain time period;

Step 3.3, based on step 3.1-3.2, determines the average of phreatic table in bank slope certain limit;

Step 4, calculates stream bank stability safety factor, judges whether it avalanche occurs, and concrete steps are as follows:

Step 4.1, based on step 2, determines inside soil body failure surface location;

Step 4.2, calculates the stability of potential sliding mass, i.e. stream bank stability safety factor F _s;

Step 4.3, contrast F _swith the size of threshold, judge whether riverbank avalanche occurs; If so, then according to failure mechanics amendment river bank shape, and step 4.4 is entered; If not, step 4.5 is entered;

Step 4.4, the form of toe accumulation body is determined after there is avalanche in riverbank;

Step 4.5, after the stream bank stability of present period has calculated, has entered the calculating of subsequent period, has returned step 2;

Step 5, divide collapse on the bank intensity, concrete steps are as follows:

Step 5.1, records the avalanche times N on riverbank in whole calculation interval _tmoving back width △ W with collapsing, determining to collapse every year and moving back width △ w;

Step 5.2, moving back △ w value according to collapsing every year, by the collapse on the bank severity level division system of setting, determining collapse on the bank strength grade.

The beneficial effect of the numerical computation method of a kind of alluvial channel collapse on the bank of the present invention process is:

(1) consider the acting in conjunction of multiple collapse on the bank influence factor, riverbank can be simulated simultaneously and collapse the process of moving back, current to the change procedure of phreatic table in the scour process of riverbank toe and the riverbank soil body;

(2) directly can reflect bank stability process over time, estimate the frequency that day part collapse on the bank occurs, and can predict that collapsing of riverbank is moved back width, moment and the river bank shape after collapsing are occurred;

(3) can predict the outcome according to collapse on the bank, by grade classification system, determine the collapse on the bank intensity in river course, for the management of Bank Failure dangerous situation provides foundation.

Accompanying drawing explanation

Fig. 1 is the flow chart of computational methods of the present invention.

Fig. 2 is the schematic diagram utilizing low water level and fracture morphology to determine riverbank toe.

Fig. 3 a is the generalization form that first avalanche occurs on riverbank;

Fig. 3 b is the generalization form that riverbank toe is piled up;

Fig. 3 c is the generalization form that secondary avalanche occurs on riverbank.

Fig. 4 is the schematic diagram of collapse on the bank strength grade division system.

Fig. 5 is the change procedure of the stream bank stability safety factor of upper Jingjiang chaste tree 34 section of simulation.

Fig. 6 is the bank slope collapse process of upper Jingjiang chaste tree 34 section of simulation.

Detailed description of the invention

Below in conjunction with accompanying drawing, the present invention will be further described.As shown in figs 1 to 6, a kind of numerical computation method of alluvial channel collapse on the bank process, concrete steps are as follows:

Step 1: according to the measured profile landform after last year flood and actual measurement then or forecast level process, determine riverside slope top and toe, calculate the river bank shape parameter such as riverbank height, the initial gradient.

Collect the landform of last year and calculate the water level prediction in time, determining river bank shape.Consider that the toe on riverbank is usually located under water, be difficult to observe, therefore in the present invention, toe is determined jointly by fracture morphology and the low water level in calculating time.

See Fig. 2, riverside slope top can direct basis section landform be determined, as the S in figure ₁point, and determine that the method for toe is: first calculate extreme low water Z _lunder riverbed area A, water surface width B and corresponding mean depth H (=A/B), now the elevation of toe is then Z _lwith the difference of H; Then according to landform, the horizontal coordinate of toe position is tried to achieve by interpolation calculation, as the S in figure ₂point.

Riverbank height H _bcalculate respectively by following two formulas with gradient S:

H _b＝Z _tp-Z _te；S＝H _b/(X _tp-X _te)(1)

Wherein X _tpand Z _tpbe respectively top horizontal coordinate, slope (m) and elevation (m); X _teand Z _tebe respectively toe horizontal coordinate (m) and elevation (m).

Step 2: riverbank toe washes away calculating, comprises step 2.1-2.3.This step emphasis comprises two aspects, is respectively the transversal flow width of the longitudinal erosion and deposition thickness in riverbed in prediction a period of time and current.

Step 2.1: calculate the bed morphology thickness in certain period

Toe place bed morphology THICKNESS CALCULATION relates to offshore flow condition and riverbed composition situation, and the present invention adopts the river-bed deformation equation at offshore place to calculate, that is:

${\mathrm{\ρ}}^{\′}\frac{{\mathrm{\ΔZ}}_b}{\mathrm{\Δ}t}=\underset{k=1}{\overset{M}{\Σ}}{\mathrm{\α}}_{sk}{\mathrm{\ω}}_{sk}(S_k-S_{*k})---\left(2\right)$

In formula: △ t is the time (s); Δ Z _bfor toe erosion and deposition thickness (m); ρ ' is bed material dry density (kgm ^-3); M is the packet count of nonuniform sediment; S _k, S _{* k}be respectively the sand content (kgm of kth particle diameter group silt ^-3) and sand holding ability (kgm ^-3); α _skfor the restoration & saturation coefficient of kth particle diameter group silt; ω _skthe effectively heavy speed (ms of kth particle diameter group silt ^-1).Wherein River Sediment Carrying Capacity adopts Zhang Ruijin sand holding ability formulae discovery, that is:

$S_{*}=k{\[\frac{U^3}{{\mathrm{gH\ω}}_s}\]}^m---\left(3\right)$

In formula: k is coefficient, m is power exponent; U is mean velocity in section (ms ^-1); ω _sfor the average setting velocity (ms of nonuniform sediment ^-1); H is the depth of water (m); G is acceleration of gravity (ms ^-2).

Step 2.2: calculate current to the transversal flow width of toe.

The transversal flow width at toe place is relevant with the scouring intensity of current and the impact resistance of the soil body, wherein the scouring intensity of current can be expressed as the shearing stress of current, soil body impact resistance is then expressed as the soil body and starts shearing stress, therefore current then can calculate transversal flow width △ B (m) of toe both this, corresponding expression formula is as follows:

ΔB＝k _d·(τ _f-τ _c)·Δt(4)

In formula: Δ t is the time (s); k _dfor coefficient of scouring (m ³(Ns) ^-1); τ _cfor the soil body starts shearing stress (Nm ^-2); τ _ffor current shearing stress (Nm ^-2).τ _f=γ _ωrJ, γ _ωfor the unit weight (Nm of water ^-3); R is hydraulic radius (m), the dark H (m) of approximate water intaking; J is water surface gradient.

To diadactic structure riverbank, the soil body composition of upper and lower layer is different, causes its impact resistance difference to some extent.In this case, when toe punching dark (offshore bed surface is lower than top viscous soil horizon floor elevation) in calculating, cause bottom, riverbank sandy soil layer to expose, when upper and lower layer soil body all suffers current transversal flow, Δ B is similar to and gets the average that this two layer soil body washes away width.

Step 2.3: calculate bed morphology thickness and transversal flow width based on step 2.1 and 2.2, river bank shape is modified.

Step 3: the change procedure calculating phreatic table in the soil body of riverbank, comprises step 3.1-3.2.

Step 3.1: the computer capacity first determining phreatic table, and the spatial mesh size calculated, then divide computing node in the soil body of riverbank.The phreatic table initial value of riverbank inside soil body adopts the extreme low water calculating the time.

Step 3.2: adopt the one dimensional fluid flow governing equation with table, calculate each node phreatic table after a certain time period.Corresponding equation is:

$\mathrm{\μ}\frac{\∂Z_g}{\∂t}=k_c{Z}_g\frac{{\∂}^2{Z}_g}{\∂x^2}---\left(5\right)$

In formula: Z _git is phreatic table (m); k _csoil body osmotic coefficient (ms ^-1); μ is specific yield, t is the time (s); X is horizontal coordinate (m).This governing equation solves and adopts with downstream condition: 1. the phreatic table at edge, riverbank flushes with water level in river course; 2. distance riverbank farthest, edge in computer capacity be 0.Because Analytic Method formula (5) was only applicable to the Seepage problems with Simple Boundary Conditions in the past, therefore implicit difference method in the present invention, is adopted to calculate.

Step 3.3: based on step 3.1-3.2, determines the average of phreatic table in the inside soil body certain limit of riverbank, and calculates corresponding moment inside soil body pore water pressure U (kNm ^-1) and matric suction S (kNm ^-1) size.

Inside soil body pore water pressure is relevant with phreatic table with the size of matric suction, and unsaturation inside soil body more than phreatic table exists matric suction, and following saturated clays inside is then pore water pressure.After obtaining phreatic table based on step 3.2, think that both are all along the linear distribution of failure mechanics.(as Fig. 3 a).

Step 4: bank slope collapse calculates, and be divided into first avalanche and secondary avalanche, the latter, with the mode generation avalanche of parallel retrogressing, comprises step 4.1-4.4.

Step 4.1: the river bank shape predicting gained based on step 2, determines the position of inside soil body failure mechanics.

As shown in Figure 3, if also there is not avalanche in riverbank, then effluent bank overall height H ₁riverbank height H more than (m) and turning point ₂m () calculates relative altitude H ₁/ H ₂, and draw crack depth H _t(m) and riverbank overall height H ₁ratio K, then calculate the failure mechanics angle beta of the first avalanche in riverbank by following formula.

$\mathrm{\β}=\frac{1}{2}\{{\tan }^{-1}\[{\left(\frac{H_1}{H_2}\right)}^2(1.0-K^2){\mathrm{tani}}_0\]+\mathrm{\φ}\}---\left(6\right)$

In formula: φ is soil body angle of internal friction (°); i ₀for the initial gradient (°) of side slope.

If riverbank avalanche occurred, then next avalanche belonged to secondary avalanche, and now riverbank inside soil body failure mechanics angle is the slope of river bank after first avalanche, was namely still β.Then, think that failure mechanics is by riverbank toe, just can determine the position of failure mechanics according to β.

Step 4.2: the bank stability index calculating sliding mass, i.e. stream bank stability safety factor F _s, this index is defined as the ratio of soil body skid resistance and sliding force on failure mechanics.

As shown in Figure 3 a, based on step 4.1 and step 3.2, determine the gravity G of sliding mass, more than phreatic table calculate by natural unit weight and saturated unit weight respectively with the following soil body; According to river water level calculation side to water pressure P; Then (G, P and pore water pressure is comprised according to potential sliding mass stressing conditions u, matric suction S), and soil body mechanics parameters calculates the skid resistance F of potential sliding mass _rand sliding force F _d, finally calculate stream bank stability safety factor F _s=F _r/ F _d.Wherein F _rcalculate by formula (7), that is:

F _R＝c'L+Stanφ ^b+(N _p-U _l)tanφ'(7)

In formula: c ' is effective cohesion intercept (kNm ^-2); φ ' is effective angle of inner friction (°); Tan φ ^bfor shear strength is with the rate of rise of matric suction; L is failure mechanics length (m), L=(H ₁-H _t)/sin β ₁; N _pfor the gross pressure (kNm of failure mechanics normal direction ^-1), N _p=Gcos β ₁+ Pcos α; U _lfor total uplift force (kNm of failure mechanics normal direction ^-1), U _l=U+Vsin β ₁, wherein V is the pore water pressure (kNm acted on draw crack ^-1).

Skid resistance F _dcalculate by following formula, that is:

F _D＝Gsinβ ₁+Vcosβ ₁-Psinα(8)

In formula: α is side direction water pressure P and failure mechanics inter normal angular separation.

Step 4.3: contrast F _swith the size of threshold, judge whether riverbank avalanche occurs.

Example calculation in the present invention gets F _sthreshold is 1.0, works as F _swhen being greater than this threshold, riverbank remains stable; And work as F _swhen being less than this threshold, can there is avalanche in riverbank.If avalanche occurs on riverbank, then according to the volume of failure mechanics determination sliding mass, and revise river bank shape (the broken line a-b-A as in Fig. 3 b).

Step 4.4: the form of toe accumulation body is determined after there is avalanche in riverbank.

A soil body part after avalanche is by current toe, and another part is then deposited in riverbank toe, plays temporary transient shielding effect.According to actual observation, accumulation body is often triangular in shape, and its gradient be approximately sliding mass stablize slope ratio under water.Therefore slope ratio can be stablized under water according to stacking volume (can be similar to and get 50% of avalanche volume) and the soil body, determine to pile up triangle, the c point namely in Fig. 3 and d point.Now river bank shape is broken line a-b-c-d).

In addition, because the river bank shape a-b-c-d after piling up is not suitable for secondary avalanche calculating, therefore in secondary avalanche calculates, approximate form a-b-B is adopted, as shown in Figure 3 b.Wherein B point horizontal coordinate is identical with A point, but elevation is A point height and ulking thickness Δ Z _a(m ²) sum, and Δ Z _a=A _a/ W _a, A _along-pending (the m of single expanded letter for accumulation body ²); W _afor piling up width (m), being similar to and getting N times of riverbank height (getting N=3 ~ 4 in example calculation of the present invention).

Step 4.5: after the stream bank stability of present period has calculated, entered the calculating of subsequent period, return step 2.

Step 5: collapse on the bank intensity divides, and comprises step 5.1-5.2.

Step 5.1: the avalanche times N recording riverbank in whole calculation interval _tmoving back width △ W with collapsing, determining to collapse every year and moving back width △ w.

Step 5.2: moving back width △ w value according to collapsing every year, by collapse on the bank severity level division system, determining bank slope collapse strength grade.

Collapse on the bank intensity is divided into 4 etc.: when collapse every year move back width △ w<20m/a time, collapse for weak; During 20m/a≤△ w<50m/a, for collapsing more by force; During 50m/a≤△ w<80m/a, for collapsing by force; During 80m/a≤△ w, for play is collapsed.

In the present invention, the collapse on the bank severity level division system of example calculation is see Fig. 4.

Claims (1)

1. a numerical computation method for alluvial channel collapse on the bank process, is characterized in that comprising the steps:

Step (1), according to the measured profile landform after last year flood and actual measurement then or calculate water level process, determines riverside slope top and toe, calculates river bank shape parameter;

Step (2), calculate riverbank toe and wash away width and bed morphology thickness, concrete steps are as follows:

Step (2.1), calculates the bed morphology thickness in certain period △ t;

Step (2.2), calculates current to the transversal flow width of toe;

Step (2.3), based on step (2.1) and step (2.2), amendment river bank shape;

Step (3), calculate phreatic table change procedure in the soil body of riverbank, concrete steps are as follows:

Step (3.1), divides computing node in the soil body of riverbank, determines initially to play phreatic table;

Step (3.2), according to one dimensional fluid flow governing equation, calculates each node phreatic table after a certain time period;

Step (3.3), based on step (3.1)-step (3.2), determines the average of phreatic table in bank slope certain limit;

Step (4), calculates stream bank stability safety factor, judges whether it avalanche occurs, and concrete steps are as follows:

Step (4.1), based on step (2), determines inside soil body failure surface location;

Step (4.2), calculates the stability of potential sliding mass, i.e. stream bank stability safety factor F _s;

Step (4.3), contrast F _swith the size of threshold, judge whether riverbank avalanche occurs; If so, then according to failure mechanics amendment river bank shape, and step (4.4) is entered; If not, step (4.5) is entered;

Step (4.4), the form of toe accumulation body is determined after there is avalanche in riverbank;

Step (4.5), after the stream bank stability of present period has calculated, has entered the calculating of subsequent period, has returned step (2);

Step (5), divide collapse on the bank intensity, concrete steps are as follows:

Step (5.1), records the avalanche times N on riverbank in whole calculation interval _tmoving back width △ W with collapsing, determining to collapse every year and moving back width △ w;

Step (5.2), moving back △ w value according to collapsing every year, by the collapse on the bank severity level division system of setting, determining collapse on the bank strength grade.