Jueyi Sui

Rain-on-snow induced flood events in Southern Germany

Using the long-term observed data at 10 meteorological stations and 17 hydrological gauging stations in a forest region in Germany, the objectives in the present paper were to examine the space-time variability of precipitation and snow (snow depth and snow water equivalent), to evaluate the importance of rainfall for snow melt process, to explore the simultaneous occurrence of snowmelt and rain on snow, to investigate the characteristics of discharges including peak discharges, to calculate runoff from snowmelt and rainfall, and to analyze runoff depths and runoff coefficients. The dependence of these variables on station elevation was studied. The results of the studied forest region were compared with those of other stations in Germany. The major climatic features were a precipitation maximum in summer, and a discharge maximum in winter. For topographical elevations larger than about 400 m above sea level, rain-on-snow events play a much more important role in runoff formation than pure rainfall events.

Sediment load reduction in Chinese rivers

In this paper, the changes in the annual runoff and sediment transport have been assessed by using the long term observation data from 10 gauging stations on 10 large rivers across China from far north to far south. It is found that the annual sediment yield has generally had a decreasing trend in the past half century. According to the changes in annual runoff and the sediment yield per area, rivers in China can be classified into the following three groups: 1) rivers with decreasing annual sediment transport and stable runoff; 2) rivers with both decreasing annual sediment transport and runoff and 3) rivers with greatly reduced annual sediment transport and decreasing annual runoff. The results indicate that, in all southern rivers (to the south of the Huaihe River including the Huaihe River), there has been little change in average annual runoff but a dramatic decrease in annual sediment transport. In the northern rivers, however, both the annual sediment yield and the runoff show significant evidence of reduction. To further investigate the recent changes in annual runoff and sediment transport, the short-term observation data from these 10 gauging stations in the recent 10 years have been assessed. Results show that both the annual sediment transport and the runoff have decreased significantly in the northern rivers in the past 10 years. Using the Yellow River at the Lijin Station as an example, the average annual runoff for the last 10 years is only 1/3 of the long term average value and the average annual sediment yield of the last 10 years is only 1/4 of the long term average value. More unusually, in the Yongding River the annual sediment yield has approached zero and the runoff has decreased significantly. In addition, the impacts of human activities on the changes in both runoff and sediment transport have been discussed.

Changes in runoff and sediment load from major Chinese rivers to the Pacific Ocean over the period 1955–2010

Abstract Changes in runoff and sediment loads to the Pacific Ocean from 10 major Chinese rivers are presented in this paper. To quantitatively assess trends in runoff and sediment loads, a parameter called the “Trend Ratio T” has been defined in this paper. To summarize total runoff and sediment load from these rivers, data from 17 gauging stations for the duration 1955 to 2010 has been standardized, and the missing data have been interpolated by different approaches according to specific conditions. Over the observed 56-year study period, there is a quite stable change in total runoff. Results show that the mean annual runoff flux entering the Pacific Ocean from these rivers is approximately 1,425 billion cubic meters. It is found that all northern rivers within semi-arid and transitional zones including the Songhua, Liaohe, Haihe, Yellow and Huaihe rivers present declining trends in water discharge. Annual runoff in all southern rivers within humid zones including the Yangtze, Qiantang, Minjiang, Pearl and Lancang rivers does not change much, except for the Qiantang River whose annual runoff slightly increases. The annual sediment loads of all rivers show significant declining trends; the exceptions are the Songhua and Lancang rivers whose annual sediment loads have increasing trends. However, the mean annual sediment flux carried into the Pacific Ocean decreased from 2,026 million tonnes to 499 million tonnes over the 56-year period. During this time there were 4 distinct decreasing phases. The decrease in annual sediment flux is due to the integrated effects of human activity and climate change. The reduction in sediment flux makes it easy for reservoir operation; however, the decrease in sediment flux also creates problems, such as channel erosion, river bank collapse and the retreat of the delta area.

Velocity profiles and incipient motion of frazil particles under ice cover.

A series of experiments for the incipient motion of frazil particles under ice cover have been carried out in laboratory under different flow and boundary conditions. Measurements on flow velocities across the measuring cross-section at different water depths have been conducted. Based on these experiments under both ice-covered and open flow conditions, the impacts of solid boundary (such as ice cover and flume sidewall) on the distribution of flow velocity profiles have been discussed. The criteria for the incipient motion of frazil particles under both smooth covered and rough covered conditions have been investigated. The influence of the roughness of ice cover has been assessed. The relation between shear Reynolds number and dimensionless shear stress for incipient motion of frazil particles under ice cover have been established compared with the incipient motion of sediment particles under ice covered condition. A diagram describing the relationship between critical shear Reynolds number and dimensionless shear stress for incipient motion of frazil particle under both smooth covered and rough covered conditions has been established. Additionally, it is found that the steeper the slope of ice cover is, the greater the critical shear stress for incipient motion of frazil particles becomes. However, the steeper the riverbed slope, the less the critical shear stress for incipient motion of river sediment.

Changes in sediment transport in the Kuye River in the Loess Plateau in China.

Abstract In this paper, the changes in sediment transport over 51 years from 1955 to 2006 in the Kuye River in the Loess Plateau in China are assessed. Key factors affecting sediment yield and sediment transport, such as precipitation depth, discharge, and human activities are studied. To investigate the changes in sediment yield in this watershed, a trend analysis on sediment concentration, precipitation depth, and discharge is conducted. Precipitation depths at 2 Climate Stations (CSs), as well as discharge and sediment transport at 3 Gauging Stations (GSs) are used to assess the features of sediment transport in the Kuye River. The runoff modulus (defined as the annual average discharge per unit area, L/(s.km2)) and the sediment transport modulus (defined as the annual suspended sediment transport per unit area, t/(yr km2)) are introduced in this study to assess the changes in runoff and sediment yield for this watershed. The results show that the highest average monthly discharge during the study period in the Kuye River is 66.23 m3/s in August with an average monthly sediment concentration of 88.9 kg/m3. However, the highest average monthly sediment concentration during the study period in the Kuye River is 125.34 kg/m3 and occurs in July, which has an average discharge of 42.6 m3/s that is much less than the average monthly discharge in August. It is found that both the runoff modulus and sediment transport modulus at Wenjiachuan GS on the Kuye River has a clear downward trend. During the summer season from July to August, the sediment transport modulus at Wenjiachuan GS is much higher than those at Toudaoguai and Longmen GSs on the Yellow River. The easily erodible loess in the Kuye River watershed and the sparse vegetation are responsible for the extremely high sediment yield from the Kuye River watershed. The analyses of the grain size distribution of suspended load in the Kuye River are presented. The average monthly median grain size of suspended load in the Kuye River is largest in February and then decreases until June. In July, the average monthly median grain size of suspended load approaches another peak and decreases until September. Then, the median grain size of suspended load starts to increase until February of the following year. However, the average monthly median grain size of suspended load in the Yellow River at Toudaoguai and Longmen GSs is the smallest between early summer and late fall. The median grain size in the Yellow River starts to increase in November and approaches the largest size in January.

FLOW AND HIGH SEDIMENT YIELD FROM THE HUANGFUCHUAN WATERSHED

This study deals with general features of sediment transport in the Huangfuchuan river, an important branch river of the Yellow river, draining the Loess Plateau in China. The long-term precipitation records at 3 climate stations near the watershed have been used to assess the changes in precipitation in this region. By introducing metrics for runoff and sediment transport, long-term observations on discharge and sediment concentration at the Huangfuchuan gauging station on the Huangfuchuan river have been used to assess the changes in runoff and sediment transport in the Huangfuchuan River compared with those at the Toudaoguai and Fugu gauging stations on the Yellow river. It is found that both runoff and sediment metrics have a clearly downward trend. From July to August, the long-term sediment transport modulus at the Huangfuchuan gauging station is much higher than those at the Toudaoguai and Fugu stations on the Yellow river. The long-term average sediment transport modulus at the Huangfuchuan gauging station are high, ranging from 14000. t/(km2 y) but decreasing to 5900 t/(km2 y) over the last 10 years. Additionally, analyses on grain size distribution of suspended load in the Huangfuchuan and Yellow river have been conducted.

Clear-water scour around semi-elliptical abutments with armored beds

Abstract Experimental study has been carried out under a clear-water scour condition to explore the local scour around semi-elliptical model bridge abutments with armor-layer bed, compared with the local scour process around semi-circular abutment. Two types of model bridge abutments, namely semi-elliptical and semi-circular abutments have been used in this experimental study. The model abutments had a ratio of streamwise length of abutment to the length of abutment transverse to the flow of 2 for semi-circular abutments and 3 for semi-elliptical abutments. In total, 50 Experiments have been designed and conducted under different flow conditions such as bed shear velocities, flow depth, and dimensions of bridge abutment model, as well as grain size of the bed material. Based on these experiments, the scour process around bridge abutments has been assessed. The dependence of the equilibrium scour depth of the scour hole on hydraulic variables has been studied. Empirical equation describing the equilibrium scour depth of the scour hole around bridge abutments has been developed.

Investigation of turbulence characteristics in channel with dense vegetation

Abstract In this experimental study, the turbulent flow in a channel with vegetation by using sprouts of wheat on channel bed was investigated. Two different aspect ratios of channel were used. An Acoustic Doppler Velocimetry was used to measure parameters of turbulent flow over submerged sprouts of wheat, such as velocity profiles. The log law and the Reynolds shear stress distribution were applied. Results indicate that the position of the maximum turbulence intensity superposes on the inflection point situated over the top of submerged vegetation cover. Quadrant analysis shows that near the vegetation bed, the sweeps and ejections appear to be the most dominant phenomenon, while far from the vegetated bed, the outward is dominant event. Results also show that the aspect ratio plays an important role on the contribution of the different bursting events for Reynolds stress determination.

Incipient Motion of Non-Cohesive Sediment Under Ice Cover –An Experimental Study*

Based on a series of experiments under both ice-covered and free surface conditions, the present article discusses the role of flow velocity and critical shear Reynolds number for incipient motion of bed material. The influence of the resistance coefficients of both the underside of the ice cover and the channel bed on the location of the maximum velocity has been discussed. In addition, the impacts of ice and composite resistance coefficients on flow velocity for incipient motion of bed material have been assessed. The diagram describing the critical shear Reynolds number and the dimensionless shear stress for the incipient motion of sediment under ice covered conditions with different under cover resistance coefficient has been established. The effects of grain size on densimetric Froude number for incipient motion of bed material have been investigated. A relationship between the densimetric Froude number for incipient motion of bed material and the median grain size of bed material as well as the roughness coefficient of channel bed and roughness coefficient of ice cover has been established.

Forecast of water level and ice jam thickness using the back propagation neural network and support vector machine methods

Ice jams can sometimes occur in high latitude rivers during winter and the resulting water level rise may generate costly and dangerous flooding such as the recent ice jam flooding in the Nechako River in downtown Prince George in Canada. Thus, the forecast of water level and ice jam thickness is of great importance. This study compares three methods to simulate and forecast water level and ice jam thickness based on field observations of river ice jams in the Quyu Reach of the Yellow River in China. More specifically, simulation results generated by the traditional multi-variant regressional method are compared to those of the back propagation neural network and the support vector machine methods. The forecast of ice jam thickness and water level under ice jammed condition have been conducted in two different approaches, 1) simulation of water level and ice jam thickness in the second half of the period of measurement using models developed based on data gained during the first half of the period of measurement, 2) simulation of water level and ice jam thickness at the downstream cross sections using models developed based on data gained at the upstream cross sections. For this reason, as the results of simulation and field observations indicated, the back propagation neural network method and the support vector machine method are superior in terms of accuracy to the multi-variant regressional method.