CN103388320B - Washing judging method for river bottom uncovering of high-sandiness flood - Google Patents

Abstract

The invention relates to a hydrologic forecasting method, in particular to a washing judging method for river bottom uncovering of high-sandiness flood, and belongs to the technical field of sediment dynamics. The IPC international patent classification node is E02B1/00. The method comprises the steps of firstly performing bending resistance tests on plaster blocks at a river bottom uncovering river reach; constructing a breaking uncovering model of the river bottom uncovering washing plaster blocks, analyzing various loads acting on the plaster blocks, determining the worst load combination, and establishing a plaster block critical breaking balance equation; and calculating various acting forces on the plaster blocks and resolving the equation to determine the breaking uncovering critical flow speed of the plaster blocks. The method corrects the parameter values of the formula (14) adopted by the traditional forecasting method, is more accurate in calculation, provides decision basis for flood prevention and also provides possibility for collecting the data of the river uncovering washing in real time, so a more accurate forecasting method is provided for judging whether a certain flood leads to the river bottom uncovering phenomenon or not.

Description

A kind of high sand-containing flood " is taken off river bed " and is washed away method of discrimination

Technical field

The present invention relates to a kind of hydrologic forecasting method, be specifically related to the method for discrimination that a kind of high-water-content flood generation " taking off river bed " is washed away, belong to sediment dynamics technical field, IPC international Patent classificating number is E02B1/00.

Background technology

Phenomenon that high sand-containing flood " takes off river bed " is one view on the Yellow River, generally occurs in Little Valleys In The Middle Reaches of The Yellow sediment disposal section and Downstream of The Weihe River section.When high sand-containing flood is by this section, early stage is through the clay block (channel aggradation thing) that density is comparatively large, intensity is higher of certain hour alluvial, started by current and surface, area several square metres even tens square metres, some clods erect vertical with water (flow) direction as wall, then " thump " pour (clod had just is flutterred in water after upright two or three minutes) in water once into, swallowed up by flood very soon.On river surface, a large amount of clod rises one after another, downstream stream seethe and under, bloom in full river, surging, the underwater sound is deafening." taking off river bed " and washing away is that hyper-concentration flow and border, special riverbed interact a kind of phenomenon produced." take off river bed " and wash away and often cause the strong incision in riverbed, sometimes riverbed can be rushed dark several meters by a flood peak, causes riverbed and water level to decline to a great extent, and souring often causes migrating of river course major trough simultaneously, the littoral engineering in river course is done great damage, brings immense pressure to the Yellow River flood control.

Last century the seventies, " taking off river bed " phenomenon just causes showing great attention to of domestic and international water conservancy working person, and has carried out research to this problem.The research method of " taking off river bed " at present and wash away problem divides three classes: the first kind is the field data utilizing prototype hydrometric station, research and analyse the flow and sediment of the Yellow River sediment disposal and section, Weihe River generation " taking off river bed " phenomenon, but because high silt-laden river flow and sediment evolution with distance is larger, therefore use " take off river bed " calculating data that the field data at upstream and downstream hydrometric station, section washes away flow and sediment as " taking off river bed " are inaccurate, and due to " taking off river bed " phenomenon formed condition more special, its randomness occurred is larger, be difficult to capture stronger " taking off river bed " field data of followability in practice, also therefore result in statistics length limited, therefore the flow and sediment that " taking off river bed " phenomenon that the method proposes occurs does not have convincingness.Equations of The Second Kind method is flow and sediment when utilizing the conceptual approach clay block of sediment transport capacity to start, but sediment carrying capacity reflection is transport conditions in silt downstream transport process, the dynamic mechanical relation that can not reflect that clay block is instantaneous when starting, and clay block is taken off to lift and is also had very large difference with the starting of single sand grain, therefore, " taking off river bed " problem can be reflected by the concept of sediment carrying capacity, be worth discussion.3rd class research method is experimental study, the method utilizes flume test simulation " taking off river bed " to wash away phenomenon, and construct two kinds of clay blocks and take off and lift mechanical model: one is by current erosion bottom clay block, when clod occurs upwards to rotate along the continuous portion (fixed end place) with riverbed, clay block front end is in cantilever position; Another kind does not consider water eroding and flooding situation, only consider that the adhesion strength in clay block boundary and riverbed builds mechanics analysis model, but these two kinds of models are all the calculating carried out flow and sediment under clay block full wafer takes off the situation of lifting, and all do not consider the impact of the structural strength of clay block (rupture strength) own.

" take off river bed " at present and wash away the research of problem, due to the difference such as emphasis of research means, method, object and research, therefore unified understanding is still lacked to the criterion, discriminant criterion, mechanism etc. of " taking off river bed ", and due to " taking off river bed " phenomenon formed condition more special, its randomness occurred is larger, be difficult to capture stronger " taking off river bed " field data of followability in practice, the research that result also in " taking off river bed " phenomenon is at present restricted.

Can channel aggradation thing block be started, " taking off river bed " phenomenon can be produced in other words, depend primarily on the synthesis result of various condition, comprising channel aggradation in early stage form and adjustment situation, deposit density and relative roughness, deposit block formational situation and lift height, deposit Block Boundary condition, the mechanical strength of alluvial block itself, peak flood flow and sand content size, peb process duration length, channel geometry parameter, layer vertical pressure fluctuation meter is strong and weak, pressure fluctuation meter is in the phase distribution and Phase Stacking probability etc. of deposit block upper and lower surface.

Wash away the current achievement in research of problem from " taking off river bed ", when derive " taking off river bed " washes away critical criterion, consider the Research Literature that above-mentioned " taking off river bed " wash away influence factor and not yet find.The present invention utilizes " taking off river bed " to wash away clay block mechanical strength experimental data, and considers that " taking off river bed " washes away combined influence factor, the critical discrimination formula that derive " taking off river bed " washes away.

Summary of the invention

The present invention is under in consideration, " taking off river bed " wash away combined influence conditions, builds clay block and fractures to take off and lift mechanics analysis model, and utilize clay block rupture strength experimental data, calculates clay block and to fracture the critical flow condition causing " taking off river bed " phenomenon to occur.The technical scheme that the present invention takes is:

A kind of high sand-containing flood " is taken off river bed " and is washed away method of discrimination, comprises the following steps:

(1), " take off river bed " and wash away an easily section clay block rupture strength test:

Test soil sample washes away for taking from " taking off river bed " the clay block easily sending out section, first test soil sample is reinvented, concrete grammar is: pulverize washing away the clay block that easily a prototype riverbed, section is fetched from " taking off river bed ", put into container to add water stirring, silt is flocculated deposit, by air-dry formation clay block sample, and air-dry time difference is utilized to control the moisture content of clay block sample; Then, the clay block sample secondary to difference group, utilizes Intensity of Breaking Tester to measure rupture strength;

Test method is as follows:

Clay block sample is disposed across in two support cylinder of Intensity of Breaking Tester, test specimen long axis normal is in Intensity of Breaking Tester support cylinder, with the loading cylinder of the speed of 50N/s ± 10N/s by Intensity of Breaking Tester, vertical load is added on the horizontal plane of prism equably, until clay block sample fractures; Formula (1) is utilized to calculate the rupture strength M of clay block sample,

$M=\frac{1.5F_{f}L}{B^3}---\left(1\right)$

In formula, F _fvertically be added in the load in the middle part of its top planes when-test specimen fractures, unit is N;

Distance between L-support cylinder, unit is mm;

The length of side of B-prism square sectional, unit is mm;

Using the average of one group three clay block sample rupture strengths as result of the test, utilize experimental result to set up the relation equation of clay block rupture strength and moisture content, calculate " taking off river bed " by this relation equation and wash away rupture strength M under the clay block saturated aqueous rate easily sending out section _s;

(2) when, structure " taking off river bed " washes away, clay block fractures to take off and lifts stressed Mathematical Modeling:

F _dfor current bulkhead pressure: $F_D=-\frac{{\mathrm{\γ}}_m{C}_D}{2}{V}_b^2\mathrm{bcg}---\left(2\right)$

In formula, V _bfor clay block bottom water flow flow velocity, C _dfor resistance coefficient, γ _mfor muddy water unit weight, b is clay block width, and c is clay block thickness.

for F on plane of rupture _dreaction force: $F_D^{\′}=-\frac{{\mathrm{\γ}}_m{C}_D}{2}{V}_b^2\mathrm{bcg}---\left(3\right)$

τ is the current drag power of clay block upper surface: $\mathrm{\τ}=\frac{{\mathrm{\γ}}_m{C}_D}{2}{V}_b^2\mathrm{lbg}---\left(4\right)$

G _sclay block weight is: G _s=γ _slbcg(5)

In formula, γ _sfor the unit weight of clay block, l is erosion jib-length, and g is acceleration of gravity, and other symbol is the same.

P ₁water pressure: P suffered by clay block ₁=γ _mlbhg(6)

In formula, h is the clay block surface depth of water, and other symbol is the same.

P ₂buoyancy for clay block: P ₂=γ _mlb (h+c) g(7)

Maximum flow of water fluctuating uplift: $F_{d\max }={\mathrm{K\γ}}_m{\mathrm{JV}}_b^2\mathrm{lb}---\left(8\right)$

In formula, K is linear coefficient, units/kg ms, and span: 3 ~ 4.2, J is water-surface gradient.

(3), consider least favorable Load Combination condition under, set up the torque equilibrium equation of each power to y-axis:

$F_{d\max }l+P_2\frac{l}{2}+\mathrm{\τc}-G_s\frac{l}{2}-P_1\frac{l}{2}=M_s\mathrm{bc}---\left(9\right)$

(4), equation solution, draw critical flow velocity V _b:

First to the formula (2) in step (two) and (three)-(9) simultaneous solution, formula (10) is obtained:

$V_b^2=\frac{2M_{s}c+l^2\mathrm{cg}({\mathrm{\γ}}_s-{\mathrm{\γ}}_m)}{2\mathrm{KJ}{l}^2{\mathrm{\γ}}_m+\mathrm{lcg}{\mathrm{\γ}}_m}---\left(10\right)$

Clay block is adopted empirical formula by the erosion jib-length of scour within the Δ t time period:

$l=C_1\mathrm{\Δt}({\mathrm{\τ}}_s-{\mathrm{\τ}}_c)e^{-0.013{\mathrm{\τ}}_c/{\mathrm{\γ}}_c}---\left(11\right)$

In formula, C ₁for bed scour coefficient, τ _sfor the drag force around clay block suffered by sand grain, τ _ccritical incipient motion shearing stress when channel aggradation thing washes away, γ _cfor the unit weight of channel aggradation thing.

${\mathrm{\τ}}_s=0.16D^2{\mathrm{\ρ}}_m{V}_b^2---\left(13\right)$

$\frac{{\mathrm{\τ}}_c}{({\mathrm{\γ}}_c-{\mathrm{\γ}}_m)d_i}=A(1+\mathrm{\ξ}\frac{d_m}{d_i})---\left(14\right)$

In formula, ρ _mfor the density of high sand-containing flood, D is the average grain diameter of movable deposit around clay block, and A is comprehensive effect coefficient, and its value is 0.044, and coefficient ξ value is 1.35, d _mfor the average grain diameter of erosion deposit, d _ifor washed out sand grain particle diameter,

Simultaneous formula (10), (11), (13) and (14) solve, and can obtain V _b, this V _bproduce the critical flow velocity that " taking off river bed " washes away exactly;

(5) " taking off river bed " washes away phenomenon, to judge whether certain flood produces.

When " taking off river bed " washes away easy upstream, section generation high sand-containing flood, according to the observation data of this upstream, section streamflow measuring station to flood flow rate of water flow and sand content, calculate actual riverbed flow velocity, sand content when this flood arrives this section and wash away duration, the critical flow velocity V then utilizing the method for step (four) to calculate generation " to take off river bed " and wash away _b, when this actual riverbed flow velocity is greater than this critical flow velocity V _btime, then can there is " taking off river bed " and wash away phenomenon, otherwise, then can not there is " taking off river bed " and wash away phenomenon.

Method of discrimination of the present invention, whether the high-water-content flood that can comparatively forecast with unerring accuracy can produce " taking off river bed " is washed away phenomenon, for flood control provide decision-making foundation, meanwhile, also for " take off river bed " real-time data acquisition that washes away provides possibility.

Detailed description of the invention

Committed step of the present invention carries out rupture strength test to " taking off river bed " section clay block; Consider " take off river bed " wash away combined influence factor and build clay block and fracture to take off and lift model mathematics, and model is solved.Its concrete implementation step is as follows:

(1) " take off river bed " and wash away an easily section clay block rupture strength test.

Test soil sample is that river bed " is taken off " and wash away the clay block easily sending out section in the Yellow River.First the formation mechenism easily sending out section clay block flocculation deposition is washed away according to " taking off river bed ", reinvent washing away the soil sample that easily a section is fetched from " taking off river bed ", concrete grammar is: pulverized by the clay block fetched from prototype riverbed, put into container to add water stirring, silt is flocculated deposit, by air-dry formation clay block as sample, and air-dry time difference is utilized to control the moisture content of clay block sample.The clay block sample secondary to difference group, utilizes Intensity of Breaking Tester to measure rupture strength.

Test method is as follows:

Clay block sample is disposed across in two support cylinder of Intensity of Breaking Tester, test specimen long axis normal is in Intensity of Breaking Tester support cylinder, with the loading cylinder of the speed of about 50N/s ± 10N/s by Intensity of Breaking Tester, vertical load is added on the horizontal plane of prism equably, until clay block sample fractures.The design formulas of rupture strength M is such as formula (1), and unit is newton/square millimeter (MPa).

$M=\frac{1.5F_{f}L}{B^3}---\left(1\right)$

In formula, F _fvertically be added in the load in the middle part of its top planes when-test specimen fractures, unit is N;

Distance between L-support cylinder, unit is mm;

The length of side of B-prism square sectional, unit is mm.

Using the average of one group three clay block sample rupture strengths as result of the test.Utilize experimental result to set up the relation equation of clay block rupture strength and moisture content, calculate " taking off river bed " by this relation equation and wash away rupture strength M under the clay block saturated aqueous rate easily sending out section _s.

(2) when structure " taking off river bed " washes away, clay block fractures to take off and lifts stressed Mathematical Modeling, calculates clay block load.

F _dcurrent bulkhead pressure: $F_D=-\frac{{\mathrm{\γ}}_m{C}_D}{2}{V}_b^2\mathrm{bcg}---\left(2\right)$

In formula, V _bfor clay block bottom water flow flow velocity, C _dfor resistance coefficient, value is 1, γ _mfor muddy water unit weight, b is clay block width, and c is clay block thickness.

for F _dreaction force: $F_D^{\′}=-\frac{{\mathrm{\γ}}_m{C}_D}{2}{V}_b^2\mathrm{bcg}---\left(3\right)$

τ ' to fracture the vertical shearing of section part for clay block, and it does not produce moment to y-axis, can ignore.

τ is the current drag power of clay block upper surface: $\mathrm{\τ}=\frac{{\mathrm{\γ}}_m{C}_D}{2}{V}_b^2\mathrm{lbg}---\left(4\right)$

τ " be the current drag power of clay block soffit, it does not produce moment to y-axis, can ignore.

Clay block weight G _s: G _s=γ _slbcg(5)

In formula, γ _sfor the unit weight of clay block, l is scour jib-length, and g is acceleration of gravity, and other symbol is the same.

Water pressure P suffered by clay block ₁: P ₁=γ _mlbhg(6)

In formula, h is the clay block surface depth of water, and other symbol is the same.

The buoyancy P of clay block ₂: P ₂=γ _mlb (h+c) g(7)

Maximum flow of water fluctuating uplift: $F_{d\max }={\mathrm{K\γ}}_m{\mathrm{JV}}_b^2\mathrm{lb}---\left(8\right)$

In formula, K is linear coefficient, units/kg ms, and span: 3 ~ 4.2, J is water-surface gradient.

(3), under considering least favorable Load Combination condition, the torque equilibrium equation of each power to y-axis is set up

$F_{d\max }l+P_2\frac{l}{2}+\mathrm{\τc}-G_s\frac{l}{2}-P_1\frac{l}{2}=M_s\mathrm{bc}---\left(9\right)$

(4) equation solution, draws flow velocity V _b, there is the critical flow velocity that " taking off river bed " washes away exactly:

Formula (4), (5), (6), (7), (8) are substituted in formula (9), arrange:

$V_b^2=\frac{2M_{s}c+l^2\mathrm{cg}({\mathrm{\γ}}_s-{\mathrm{\γ}}_m)}{2\mathrm{KJ}{l}^2{\mathrm{\γ}}_m+\mathrm{lcg}{\mathrm{\γ}}_m}---\left(10\right)$

Clay block is adopted empirical formula by the erosion jib-length of scour within the Δ t time period:

$l=C_1\mathrm{\Δt}({\mathrm{\τ}}_s-{\mathrm{\τ}}_c)e^{-0.013{\mathrm{\τ}}_c/{\mathrm{\γ}}_c}---\left(11\right)$

In formula, C ₁for bed scour coefficient, τ _sfor the drag force around clay block suffered by sand grain, τ _ccritical incipient motion shearing stress when channel aggradation thing washes away, γ _cfor the unit weight of channel aggradation thing.

${\mathrm{\τ}}_s=C_D^{\′}{\mathrm{\α}}_D{d}_i^2\frac{{\mathrm{\ρ}}_m{V}_b^2}{2}---\left(12\right)$

In formula, for drag force coefficient, value is 0.4, α _dfor the area coefficient of sand grain, value is ρ _mfor the density of water, d _ifor washed out sand grain particle diameter, other symbol is the same.D is replaced with the average grain diameter D of deposit movable around clay block _i, arrange formula (12) and obtain:

${\mathrm{\τ}}_s=0.16D^2{\mathrm{\ρ}}_m{V}_b^2---\left(13\right)$

Channel aggradation thing critical incipient motion shearing stress τ _c: $\frac{{\mathrm{\τ}}_c}{({\mathrm{\γ}}_c-{\mathrm{\γ}}_m)d_i}=A(1+\mathrm{\ξ}\frac{d_m}{d_i})---\left(14\right)$

In formula, determine through test of many times, the accurate value of comprehensive effect coefficient A is 0.044, and the span of coefficient ξ is 1.25 ~ 1.65, and its accurate value is 1.35, d _mfor the average grain diameter of erosion deposit, d _ifor washed out sand grain particle diameter.

Formula (13), (14) are substituted in formula (11), the design formulas of scour jib-length l can be obtained:

$l=C_1\mathrm{\Δt}[0.16{\mathrm{\ρ}}_m{V}_b^2{D}^2-A{d}_i({\mathrm{\γ}}_c-{\mathrm{\γ}}_m)(1+\mathrm{\ξ}\frac{d_m}{d_i})]e^{-0.013A{d}_i({\mathrm{\γ}}_c-{\mathrm{\γ}}_m)(1+\mathrm{\ξ}\frac{d_m}{d_i})/{\mathrm{\γ}}_c}---\left(15\right)$

To consider around clay block can the particle diameter of erosion sand grain 10 ^-5m () magnitude, the span of A is 10 ^-2magnitude, and therefore, in formula (15) d is replaced with the average grain diameter D of deposit movable around clay block _i, then formula (15) can be expressed as:

$l=C_1\mathrm{\Δt}[0.16D^2{\mathrm{\ρ}}_m{V}_b^2+\mathrm{AD}({\mathrm{\γ}}_c-{\mathrm{\γ}}_m)(1+\mathrm{\ξ})]---\left(16\right)$

Will l is as variable, and simultaneous formula (10), formula (16) solve: formula (10) is substituted into formula (16) and arranges:

$l^3-\frac{C_1\mathrm{\ΔtAD}}{2}(2{\mathrm{\γ}}_c+2\mathrm{\ξ}{\mathrm{\γ}}_c-{\mathrm{\γ}}_m-\mathrm{\ξ}{\mathrm{\γ}}_m)l^2-\frac{C_1\mathrm{\ΔtADcg}}{2\mathrm{KJ}}({\mathrm{\γ}}_c+\mathrm{\ξ}{\mathrm{\γ}}_c-{\mathrm{\γ}}_m-\mathrm{\ξ}{\mathrm{\γ}}_m)l----\left(17\right)$

$C_1\mathrm{\Δt}{D}^2{\mathrm{\ρ}}_{m}c[0.32M_s+0.16g({\mathrm{\γ}}_s-{\mathrm{\γ}}_m)]=0$

Order $a^{,}=-\frac{C_1\mathrm{\ΔtAD}}{2}(2{\mathrm{\γ}}_c+2\mathrm{\ξ}{\mathrm{\γ}}_c-{\mathrm{\γ}}_m-\mathrm{\ξ}{\mathrm{\γ}}_m),b^{,}=-\frac{C_1\mathrm{\ΔtADcg}}{2\mathrm{KJ}}({\mathrm{\γ}}_c+\mathrm{\ξ}{\mathrm{\γ}}_c-{\mathrm{\γ}}_m-\mathrm{\ξ}{\mathrm{\γ}}_m),$

$c^{,}=-C_1\mathrm{\Δt}{D}^2{\mathrm{\ρ}}_{m}c[0.32M_s+0.16({\mathrm{\γ}}_s-{\mathrm{\γ}}_m)],l^{\′}=l-\frac{a^{\′}}{3},$ Then formula (17) is converted into:

${\left(l^{\′}\right)}^3+p{l}^{\′}+q=0---\left(18\right)$

In formula, $p=(\frac{{\left(a^{,}\right)}^2}{3}+b^{,}),q=\frac{2{\left(a^{,}\right)}^3-9a^{,}{b}^{,}+27c^{,}}{27}$

Radical formula according to simple cubic equation obtains:

$l{=\{-\frac{q}{2}+{[{\left(\frac{q}{2}\right)}^2+{\left(\frac{p}{3}\right)}^3]}^{\frac{1}{2}}\}}^{\frac{1}{3}}={\{-\frac{q}{2}-{[{\left(\frac{q}{2}\right)}^2+{\left(\frac{p}{3}\right)}^3]}^{\frac{1}{2}}\}}^{\frac{1}{3}}+\frac{a^{,}}{3}---\left(19\right)$

L is substituted in formula (10), can obtain $V_b^2=f(C_1,\mathrm{\Δt},{\mathrm{\γ}}_c,{\mathrm{\γ}}_s,{\mathrm{\γ}}_m,c,J,K,M_s,D).$

(5) " taking off river bed " washes away phenomenon to judge whether certain flood produces.

When " taking off river bed " washes away easy upstream, section generation high sand-containing flood, according to this upstream, section streamflow measuring station to flood flow rate of water flow and sand content observation data, according to this section hydrological observation data for many years, actual riverbed flow velocity, sand content when this flood arrives this section can be calculated easily and wash away duration, the critical flow velocity V then utilizing step (four) method to calculate generation " to take off river bed " and wash away _b.When this actual riverbed flow velocity is greater than this critical flow velocity V _btime, then can there is " taking off river bed " and wash away phenomenon, otherwise, then can not there is " taking off river bed " and wash away phenomenon.

Claims (1)

1. high sand-containing flood " is taken off river bed " and is washed away a method of discrimination, comprises the following steps:

(1), " take off river bed " and wash away an easily section clay block rupture strength test:

Test soil sample washes away for taking from " taking off river bed " the clay block easily sending out section, first test soil sample is reinvented, concrete grammar is: pulverize washing away the clay block that easily a prototype riverbed, section is fetched from " taking off river bed ", put into container to add water stirring, silt is flocculated deposit, by air-dry formation clay block sample, and air-dry time difference is utilized to control the moisture content of clay block sample; Then, the clay block sample secondary to difference group, utilizes Intensity of Breaking Tester to measure rupture strength;

Test method is as follows:

Clay block sample is disposed across in two support cylinder of Intensity of Breaking Tester, test specimen long axis normal is in Intensity of Breaking Tester support cylinder, with the loading cylinder of the speed of 50N/s ± lON/s by Intensity of Breaking Tester, vertical load is added on the horizontal plane of prism equably, until clay block sample fractures; Formula (1) is utilized to calculate the rupture strength M of clay block sample,

$M=\frac{1.5F_{f}L}{B^3}---\left(1\right)$

In formula, F _fvertically be added in the load in the middle part of its top planes when-test specimen fractures, unit is N;

Distance between L-support cylinder, unit is mm;

The length of side of B-prism square sectional, unit is mm;

Using the average of one group three clay block sample rupture strengths as result of the test, utilize experimental result to set up the relation equation of clay block rupture strength and moisture content, calculate " taking off river bed " by this relation equation and wash away rupture strength M under the clay block saturated aqueous rate easily sending out section _s;

(2) when, structure " taking off river bed " washes away, clay block fractures to take off and lifts stressed Mathematical Modeling:

F _dfor current bulkhead pressure: $F_D=\frac{{\mathrm{\γ}}_m{C}_D}{2}{V}_b^2\mathrm{bcg}---\left(2\right)$

In formula, V _bfor clay block bottom water flow flow velocity, C _dfor resistance coefficient, γ _mfor muddy water unit weight, b is clay block width, and c is clay block thickness,

F' _dfor F on plane of rupture _dreaction force: $F_D^{\′}=-\frac{{\mathrm{\γ}}_m{C}_D}{2}{V}_b^2\mathrm{bcg}---\left(3\right)$

τ is the current drag power of clay block upper surface: $\mathrm{\τ}=\frac{{\mathrm{\γ}}_m{C}_D}{2}{V}_b^2\mathrm{lbg}---\left(4\right)$

G _sclay block weight is: G _s=γ _slbcg (5)

In formula, γ _sfor the unit weight of clay block, l is erosion jib-length, and g is acceleration of gravity;

P ₁water pressure: P suffered by clay block ₁=γ _mlbhg (6)

In formula, h is the clay block surface depth of water;

P ₂buoyancy for clay block: P ₂=γ _mlb (h+c) g (7)

Maximum flow of water fluctuating uplift: $F_{d\max }={\mathrm{K\γ}}_m{\mathrm{JV}}_b^2\mathrm{lb}---\left(8\right)$

In formula, K is linear coefficient, units/kg ms, and span: 3 ~ 4.2, J is water-surface gradient;

(3), consider least favorable Load Combination condition under, set up the torque equilibrium equation of each power to y-axis:

$F_{d\max }l+P_2\frac{l}{2}+\mathrm{\τc}-G_s\frac{l}{2}-P_1\frac{l}{2}=M_s\mathrm{bc}---\left(9\right)$

(4), equation solution, draw critical flow velocity V _b:

First to the formula (2) in step (two) and (three)-(9) simultaneous solution, formula (10) is obtained:

$V_b^2=\frac{{2M}_{s}c+l^2\mathrm{cg}({\mathrm{\γ}}_s-{\mathrm{\γ}}_m)}{{{2\mathrm{KJl}}^2}_{{\mathrm{\γ}}_m}+\mathrm{lcg}{\mathrm{\γ}}_m}---\left(10\right)$

Clay block is adopted empirical formula by the erosion jib-length l of scour within the Δ t time period:

$l=C_1\mathrm{\Δt}({\mathrm{\τ}}_s-{\mathrm{\τ}}_e)e^{-0.013{\mathrm{\τ}}_c/{\mathrm{\γ}}_c}---\left(11\right)$

In formula, C ₁for bed scour coefficient, τ _sfor the drag force around clay block suffered by sand grain, τ _ccritical incipient motion shearing stress when channel aggradation thing washes away, γ _cfor the unit weight of channel aggradation thing;

${\mathrm{\τ}}_s=0.16D^2{\mathrm{\ρ}}_m{V}_b^2---\left(13\right)$

$\frac{{\mathrm{\τ}}_c}{({\mathrm{\γ}}_c-{\mathrm{\γ}}_m)d_i}=A(1+\mathrm{\ξ}\frac{d_m}{d_i})---\left(14\right)$

In formula, ρ _mfor the density of high sand-containing flood, D is the average grain diameter of movable deposit around clay block, and A is comprehensive effect coefficient, and its value is 0.044, and coefficient ξ value is 1.35, d _mfor the average grain diameter of erosion deposit, d _ifor washed out sand grain particle diameter,

Simultaneous formula (10), (11), (13) and (14) solve, and can obtain V _b, this V _bproduce the critical flow velocity that " taking off river bed " washes away exactly;

(5) " taking off river bed " washes away phenomenon, to judge whether certain flood produces:

When " taking off river bed " washes away easy upstream, section generation high sand-containing flood, according to the observation data of this upstream, section streamflow measuring station to flood flow rate of water flow and sand content, calculate actual riverbed flow velocity, sand content when this flood arrives this section and wash away duration, the critical flow velocity V then utilizing the method for step (four) to calculate generation " to take off river bed " and wash away _b, when this actual riverbed flow velocity is greater than this critical flow velocity V _btime, then can there is " taking off river bed " and wash away phenomenon, otherwise, then can not there is " taking off river bed " and wash away phenomenon.