Climate change drives flooding risk increases in the Yellow River Basin

The Yellow River Basin(YRB)has experienced severe floods and continuous riverbed elevation throughout history.Global climate change has been suggested to be driving a worldwide increase in flooding risk.However,owing to insufficient evidence,the quantitative correlation between flooding and climate change remains illdefined.We present a long time series of maximum flood discharge in the YRB dating back to 1843 compiled from historical documents and instrument measurements.Variations in yearly maximum flood discharge show distinct periods:a dramatic decreasing period from 1843 to 1950,and an oscillating gentle decreasing from 1950 to 2021,with the latter period also showing increasing more extreme floods.A Mann-Kendall test analysis suggests that the latter period can be further split into two distinct sub-periods:an oscillating gentle decreasing period from 1950 to 2000,and a clear recent increasing period from 2000 to 2021.We further predict that climate change will cause an ongoing remarkable increase in future flooding risk and an∼44.4 billion US dollars loss of floods in the YRB in 2100.

Prototype observation and influencing factors of environmental vibrationinduced by flood discharge

Due to a wide range of field vibration problems caused by flood discharge at the Xiangjiaba Hydropower Station, vibration characteristics and influencing factors were investigated based on prototype observation. The results indicate that field vibrations caused by flood discharge have distinctive characteristics of constancy, low frequency, small amplitude, and randomness with impact, which significantly differ from the common high-frequency vibration characteristics. Field vibrations have a main frequency of about 0.5-3.0 Hz and the characteristics of long propagation distance and large-scale impact. The vibration of a stilling basin slab runs mainly in the vertical direction. The vibration response of the guide wall perpendicular to the flow is significantly stronger than it is in other directions and decreases linearly downstream along the guide wall. The vibration response of the underground turbine floor is mainly caused by the load of unit operation. Urban environmental vibration has particular distribution characteristics and change patterns, and is greatly affected by discharge, scheduling modes, and geological conditions. Along with the increase of the height of residential buildings, vibration responses show a significant amplification effect. The horizontal and vertical vibrations of the 7th floor are, respectively, about 6 times and 1.5 times stronger than the corresponding vibrations of the 1st floor. The vibration of a large-scale chemical plant presents the combined action of flood discharge and working machines. Meanwhile, it is very difficult to reduce the low-frequency environmental vibrations. Optimization of the discharge scheduling mode is one of the effective measures of reducing the flow impact loads at present. Choosing reasonable dam sites is crucial.

Similarity criterion of flood discharge atomization

By combining the results of prototype observation of flood discharge atomization at the Wujiangdu Hydropower Station, and by adopting the serial model test method, the model scale effect was examined, the influences of the Reynolds and Weber numbers of water flow on the rain intensity of flood discharge atomization were analyzed and a rain intensity conversion relation was established. It is demonstrated that the level of atomization follows the geometric similarity relations and it is possible to ignore the influence of the surface tension of the flow when the Weber number is greater than 500. Despite limitations such as incomplete data sets, it is undoubtedly helpful to study the scale effect of atomization flow, and it is beneficial to identify the rules of the model test results in order to extrapolate to prototype prediction.

Probabilistic analysis for the response of nonlinear base isolation system under the ground excitation induced by high dam flood discharge

According to theoretical analysis, a general characteristic of the ground vibration induced by high dam flood discharge is that the dominant frequency ranges over several narrow frequency bands, which is verified by observations from the Xiangjiaba Hydropower Station. Nonlinear base isolation is used to reduce the structure vibration under ground excitation and the advantage of the isolation application is that the low-frequency resonance problem does not need to be considered due to its excitation characteristics, which significantly facilitate the isolation design. In order to obtain the response probabilistic distribution of a nonlinear system, the state space split technique is modified. As only a few degrees of freedom are subjected to the random noise, the probabilistic distribution of the response without involving stochastic excitation is represented by the δ function. Then, the sampling property of the δ function is employed to reduce the dimension of the Fokker-Planck-Kolmogorov （FPK） equation and the low-dimensional FPK equation is solvable with existing methods. Numerical results indicate that the proposed approach is effective and accurate. Moreover, the response probabilistic distributions are more reasonable and scientific than the peak responses calculated by conventional time and frequency domain methods.

An improved numerical model of ski-jump flood discharge atomization

Atomizing rainfall caused by flood discharge of high dams poses a great threat to the safety of powerhouse and ecological environment.As an indispensable means,numerical calculation is widely used in the safety design of discharge structures.The distribution of rainfall intensity is closely related to the trajectory nappe shape,jet trajectory distances,the splashed water droplet diameter and its velocity,and the spatial distribution of downstream nappe wind.In this paper,an experimental result is used to verify the improved stochastic splash mathematical model under different bucket types and discharge conditions,and the sensitivity of downstream rainfall intensity distribution to the shape of trajectory nappe,discharge flow,spatial distribution of downstream nappe wind,and the corresponding relationship between the droplet diameter and its splashing velocity is analyzed.The results show that the calculation accuracy of downstream rainfall intensity distribution is significantly improved when the above factors are taken into consideration.It is found that the bucket type and flood discharge rate play the greatest role in the rainfall intensity distribution,followed by the downstream nappe wind distribution,and finally the corresponding relationship between the diameter and velocity of splash droplets.Therefore,these factors should be considered comprehensively when the rainfall intensity distribution of flood discharge atomization is calculated.This study can help us to understand the influence factors of flood discharge atomization more deeply and predict the distribution of flood discharge atomization rainfall intensity more accurately.

Estimation of Unit Flood Discharge for Various Land Use Types with a Focus on Urbanization

We proposed unit flood discharge model that defined as the discharge into end-order (smallest) drainage canals. The discharge acts an important role for estimating regional flooding by big rainfall events which leading roughly estimation of flood discharge associated with land use changes as urbanization. In some areas of Japan, increased urbanization with insufficient drainage canal capacity has led to increasingly frequent flooding and flood damage. The aim of this study was to investigate the effect of urbanization on unit flood discharge using a runoff model for the Tedori River alluvial fan area, Japan. The discharge was studied as collecting runoff from paddy fields, upland crop fields, and residential lots. A runoff model for various land use types in the study area was developed using actual and physical properties of the runoff sites, and parameters for paddy fields. The model was tested using 54 big events and inputted those. The maximum total runoff ratio among different land use types was observed for residential lots, and the ratio remained relatively constant across different flood events. The minimum total runoff ratio was observed for irrigated paddy fields. There was a positive relationship between the total runoff ratio and total precipitation for all land use types. Whereas, the relationship between the peak runoff ratio and peak precipitation was variable. The runoff analysis was carried out using 60-min and 10-min precipitation data. For agricultural land, data for both intervals produced similar results.

Response characteristics of plunge pool slabs of Xiaxiluodu Hydropower deformation conditions

The key problem of the energy dissipation scheme of the arch dam body flood discharge and plunge pool below the dam is the stability problem of the plunge pool slab.As the protection structure of the underwater bed,the plunge pool slab bears the continuous impact of high-speed water flow.The hourly average dynamic water pressure on the slab is one of the main loads directly affecting the stability of the slab and is the main factor causing its erosion destruction.After the impoundment of the Xiluodu Hydropower Station,the measuring line of valley width in the plunge pool area has been continuously shrinking.By 2020,the cumulative shrinking value is about 80 mm.In light of the general background condition of valley shrinkage,daily inspection,annual detailed inspection,underwater inspection and drainage inspection of the plunge pool found that the plunge pool has experienced different degrees of damage,which greatly influences the long-term safety stability of the plunge pool.In this paper,the prototype observation data of flood discharge is used as the input load of pulsatingpressure,and the stress and displacement distribution of the plunge pool structure under the vibration load of flood discharge is analyzed under the condition that the stress and strain state of the plunge pool is changed under the influence of valley displacement.The results show that the stress,strain,and displacement distribution of the plunge pool are mainly caused by valley deformation,the vibration caused by flood discharge is little in influence,and the impact effect of deep hole flood discharge tongue on the plunge pool slab is weak.

FLOOD DISCHARGE AND ENERGY DISSIPATION BY JETS FROM OUTLETS IN HIGH ARCH DAM

In this thesis, the scale effect by an aerated water jet diffusing in the water upon the hydrodynamic pressure, the local riverbed scour by multiple layered jets and by those colliding in the vertical and the transverse directions, and the stability of the apron slab both in the inverted arch cushion pool and the flat bottom one by the large discharge and the high water head with 300m level are detailedly researched by means of model test and numerical simulation. A mathematic model simulating the destabilization of the flat bottomed slab is established to open out the mechanism of its stability. Both experimental researches both on the hydrodynamic pressure acted inside joints between the bedrock and the apron slab, and on forces at arch abutments in inverted arch cushion pool are carried out by using an advanced measurement and the imitation means to acquire the mechanism of the inverted arch pool to keep stability and its stability condition.

Adjustment of flood discharge capacity with varying boundary conditions in a braided reach of the Lower Yellow River

It is of necessity to investigate the adjustment of flood discharge capacity in the Lower Yellow River(LYR)because of its profound importance in sediment transport and flood control decision-making,and additionally its magnitude is influenced by the channel and upstream boundary conditions,which have significantly varied with the ongoing implementation of soil and water conservation measures in the Loess Plateau and the operation of the Xiaolangdi Reservoir.The braided reach between two hydrometric stations of Huayuankou and Gaocun in the LYR was selected as the study area.Different parameters in the study reach during the period 1986-2015 were calculated,covering bankfull discharge(the indicator of flood discharge capacity),the pre-flood geomorphic coefficient(the indicator of channel boundary condition),and the previous five-year average fluvial erosion intensity during flood seasons(the indicator of incoming flow and sediment regime).Functional linkages at scales of section and reach were then developed respectively to quantitatively demonstrate the integrated effects of channel and upstream boundary conditions on the flood discharge capacity.Results show that:(1)the reach-scale bankfull discharge in the pre-dam stage(1986-1999)decreased rapidly by 50%,accompanied with severe channel aggradation and main-channel shrinkage.It recovered gradually as the geometry of main channel became narrower and deeper in the post-dam stage,with the geomorphic coefficient continuously reducing to less than 15 m-12.(2)The response of bankfull discharge to the channel and upstream boundary conditions varied at scales of section and reach,and consequently the determination coefficients differed for the comprehensive equations,with a smallest value at the Jiahetan station and a highest value(0.91)at reach scale.Generally,the verified results calculated using the comprehensive equations agreed well with the corresponding measured values in 2014-2015.(3)The effect of channel boundary condition was more prominent than that of upstream boundary condition on the adjustment of bankfull discharge at the Jiahetan station and the braided reach,which was proved by a larger improvement in determination coefficients for the comprehensive equations and a better performance of geomorphic coefficient on the increase of bankfull discharge.

Role of flood discharge in shaping stream geometry:Analysis of a small modern stream in the Uinta Basin,USA

This small modern river system is located on a retativety flat （about 1°-2°）, unconsotidated sandy pediment surface in the Uinta Basin of Utah, USA, and it is with a scare of about 30 m tong and 0.4-0.8 m wide, simitar as a natural flume experiment modet. The smart stream is informatty divided into upstream, midstream and downstream. The anatysis shows that flood discharge influences channet sinuosity and morphology to produce an initiat meandering pattern which is tater changed to a braided and then a straisht pattern in the downflow direction. The upstream segment has a hish sinuous geometry dominated by both erosion （cutbanks） and deposition （point bars）. In the resistance of sporadic vegetation rooting in banks, the upstream flood deviates its original direction, which resutts in the powerful flood intensively eroding the cutbank and accreting clastics to build point bars, and thus producing a high sinuous channet. The midstream is dominated by deposits （many smart bars） with a moderate to tow sinuosity. Due to the bad drainage of the high sinuous channel in the upstream, the strong flood can cut off the point bar comptetely or even surmount the tevee in the fast meandering upstream, which widens the channel suddenty with a quick decreasing current power. Then, the ctastics from the upstream are untoaded in the midstream and form many smart bars. Untoaded sediments protect the bank, and the tow-power current brings a moderate erosion to the bank, which forms a moderate to tow sinuous channel in the midstream. The downstream shows multistage erosionat terraces in its retativety straight channets. After the midstream water drops its toad, it becomes ＂dear＂ and reaches downstream, the tower current power is hetptess to reform channet 8eometry. Thus, the downstream channel sesment keeps a tower sinuous geometry, evenstraight partially. Small amounts of fine clastics are deposited, and simultaneously multistage terraces are formed due to regressive flood erosion. This stream example demonstrates the subtleties of stream flow and the importance of flood discharge in shaping the channel geometry. Although it is difficult to scale up this example to a large river system that carves geomorphic landscape, this case shows how river geometries vary from the traditional patterns due to different gradient.

THEORETICAL AND EXPERIMENTAL STUDIES OF THE FLARING GATE PIER ON THE SURFACE SPILLWAY IN A HIGH-ARCH DAM

The experimental studies of the flaring gate pier applied on the surface spillway in a high-arch dam show that a shock-wave will appear when the pattern of the flow is kept as super-critical. Meanwhile, the water depth at the outlet increases significantly, the flow moves downward in different directions, and the plunging jet is in a narrow and long shape, with a full longitudinal diffusion. In addition, the variation of the flaring gate pier design parameters affects little the discharge capacity of the surface spillway, these parameters including the contraction ratio fl, the contraction angle c~ and the spillway chute angle O. The pressure on the bottom of the spillway increases along the way and reaches the maximum before the outlet, and then decreases rapidly. Due to the flow impacting, the pressure on both sidewalls increases abruptly at the turning line of the flaring gate pier. To see the characteristics of the flow in the flaring gate pier, a simple calculation method is suggested based on the conversation of energy and mass, and the calculation methods for the jet trajectory and the horizontal length in air are also proposed. The results are found in good agreement with experimental data.

HYBRID SIMULATION OF THE HYDRAULIC CHARACTERISTICS AT RIVER AND LAKE CONFLUENCE

The hydraulic characteristics at the confluence reach of river and lake are influenced by multiple factors such as inflow,topography and vegetation resistance,and are very complicated.In this article,the confluence reach of Yangtze River and Dong-ting Lake is selected as a special example and a hybrid model is built to study the flow at this confluence,with the consideration of the interactions between aquatic vegetation and flow.Validation tests and calculations show that the model is effective and highly accurate.The simulations show that the separation levee at the confluence reach may change the discharge capacity in the flood plain,which would be enhanced in the upper reach of the levee,hardly changed in the middle reach and reduced at the lower place.Moreover,the separation levee also limits the water exchange between the Yangtze River and Dong-ting Lake.