The Variation of Sediment Discharge During Flood Period

Transcription

1 The Variation of Sediment Discharge During Flood Period Yu-Chieh LIN 1, Yu-Min WANG 2*, and Shaing-Min CHEN 3 1 PHD student, Dept. of Civil Engineering, Pingtung University of Science and Technology p @mail.npust.edu.tw 2* Professor, Dept. of Civil Engineering, Pingtung University of Science and Technology wangym@mail.npust.edu.tw (1, Shuefu Road, Neipu, Pingtung 91201, Taiwan) 3 Master student, Dept. of Civil Engineering, Pingtung University of Science and Technology m @mail.npust.edu.tw Suspended sediment concentration in rivers is not only associated with soil and water conservation, but also with changes in the natural environment in a variety of river flow patterns. A rating curve of flow and suspended sediment can help to determine sediment discharge, which is useful in disaster prevention, land conservation and ecological protection. The purpose of this study is to build the relationship between flow and sediment discharge during flood period. In the study, the price meter and depth-integrating suspended-sediment sampler, model DH-59, are used to measure flow velocity and suspended sediment concentration respectively. Data for each was collected once every two hours between July and August 28-31, In the period mentioned, three torrential rains resulted to 68 measurements for flow velocity and suspended sediment concentration, were measured. The sediment concentration was used to calculate the sediment discharge, and regression analysis employed to estimate the flow-sediment discharge relationship. The results showed that each torrential rain had a positive correlation with sediment discharge. The coefficient of correlation (R 2 ) for the torrential rain (July 18-20, 2011), (July 20-21, 2011) and (August 28-31, 2011) was 0.73, 0.61 and 0.93, respectively. Key Words : Sediment discharge, Flow, Flood, DH-59, Sediment concentration. 1. INTRODUCTION Rivers in Taiwan are typically short with small drainage basins, steep slopes and the geology condition of hillside is so weakness thus when typhoon or heavy rainfall occurred, it always caused hillside failure. Therefore, the hillside will fall down with the rainfall, the sediment discharge in Taiwan is too large. Huang (2010) indicated the surface land in Taiwan is accounting for 0.024% of global produce, but the sediment discharge is accounting 1.9% of the global produce in his study of the Characteristics of sediment discharge in Zhuoshui River. After the heavy stream land often loss sediment deposited in river bed result river migration and serious floods. There are severe engineering and environmental problems associated with an imbalance in the transport, erosion and deposition of sediment (Julien 1998). The financial cost associated with sediment has grown over the years due to human influences. Therefore, the ability to quantify sediment loads or discharge is essential for the management of our water bodies and land for the future. Over the years, techniques have been developed to calculate the total load within the river environment. Total load is determined based on the mode of transport, measurement techniques, and sediment source. Hans Albert Einstein, one of the pioneers of sediment transport,

2 developed a sediment transport equation based on the modes of transport. His bed load transport equation is based on the probability that a given particle found in the bed will be entrained into the flow (Einstein 1942). Then in 1950, Einstein developed a method to calculate total load, based on evaluating the bed load transport and integrating the suspended sediment discharge equation. Because streamflow is high or low, the suspended sediment will get large or small, it will make the suspended sediment concentration is different. Zhu and Hsu (2007) conducted there has a complex relationship between streamflow and suspended sediment concentration. In the river has same streamflow, but the suspended sediment concentration can be a difference to power of multiples of ten in the study of the relationship between river flow and suspended sediment concentration in the Yufeng catchment area on typhoon events. In torrential rains events, the sediment discharge is more pronounced. The Soil and Water Resources and Disaster Prevention Science and Technology Research Center of National Yunlin University of Science and Technology (2004) show that river sand content is very important for a variety of hydraulic structures design work, such as irrigation, drainage, water and flood control channels such as improved planning and design of the reservoir, the port channel maintenance, public water purification in the study of theoretical basis of GSTARS mode. Thus, measure suspended sediment concentration is important in hydrological observation. Suspended sediment discharge are usually derived from data collected with sufficient frequency to obtain reliable estimates for the computational interval and period. Most suspended sediment discharge records are computed at daily or annual intervals based on periodically collected data, although some partial records represent discrete or seasonal intervals such as those for flood periods. The method used to calculate suspended sediment discharge is dependent on the types and frequency of available data which always contain a lot of uncertainty. Thus, it is essential to have enough investigation records during high flow period caused by torrential event for a river basin that the reliability for estimating annually sediment discharge could be increased. In this study, Data for each torrential event was collected once every two hours between July and August 28-31, 2011, in Shi-Wen River, Sourthern Taiwan. In the torrential events, there are three, the streamflow and water sample for suspended sediment concentration were collected. Then, The collected samples were analyzed for suspended sediment concentration and sediment discharge, and regression analysis was carried for obtaining the relationship between the streamflow and sediment discharge. The factors which affect suspended sediment concentration and sediment discharge are streamflow, suspended, and slop, etc.. The suspended sediment concentration surveyed by depth-integrating suspended-sediment sampler, DH-59. The measuring point is selected based on the width of the river. And the time as a variable to measure the velocity and area, then get the streamflow. 2. LITERATURE REVIEW (1) Sediment types Cheng (2009) in the river sand transport investigation and measurement technology discussed about river sand transport mechanism and explained: generally the river sand, here represent sedimentation. The definition of sediment is based on USGS and ASTM, through water flow transport, suspended load or accumulation of particles derived from rocks or biological materials or chemical deposite particles. The study also described general river mud sand movement can be divided into four types, they are sliding, rolling, saltating/hoping and the suspended the particle motion.when the river bed shear velocity is greater than the initial critical value of sand particle, the bed material will slide or roll on the river bed connected continuous moving. If river bed shear velocity ncreased steadily, the particle moved alog river bed will produce the hoping phenomenon, and this hoping phenomen is generally enlivened. But when the river bed shear level is greater than sand hoping particle's settling speed, call saltating sand particle will be uplifted to velocity that the sand particles will enter into the suspended load pattern. Usually, three modes of particle motion are distinguished: (1) Rolling and sliding motionor both; (2) saltation motion; and (3) suspended particle motion.when the value of the bed-shear velocity just exceeds the critical value for initiation of motion, the particles will be rolling and sliding or both, in continuous contact with the bed. For increasing values of the bedshear velocity, the particles will be moving along the bed by more or less regular jumps, which are called saltations. When the value of the bed-shear velocity exceeds the fall velocity of the particles, the sediment particles can be lifted to a level at which the upward turbulent forces will be comparable with or of higher order than the submerged weight of the particles and as a result the particles

3 may go in suspension.( Van Raijn,1984) There are three sediment transport modes (1) Wash load: very fine particles which are transported by the water, but these particles do not exist on the bed. Therefore the knowledge of bed material composition does not permit any prediction of wash load transport. (2) Bed-load: the part of the total load which has more or less continuous contact with the bed. Thus the bed load must be determined in relation to the effective shear stress which acts directly on the grain surface. (3) Suspended load : the part of the total load which is moving without continuous contact with the bed as the result of the agitation of the fluid turbulence. The appearance of ripples will increase the flow resistance. On the other hand, more grains will be suspended due to the flow separation on the lee side of the ripples. Thus the suspended load is related to the total bed shear stress, The basic idea of splitting the total sediment load into bed-load and suspended load is that, as described above, two different mechanisms are effective during the transport.( Zhou Liu,2001) In addition, based on ISO 4363 the relationship of these 3 modes can be described by the following chart as shown in Fig.1. (2) Suspended Sediment Discharge Fig.1 The classification of sediment transport Suspended-sediment-discharge records are derived from analytical results of sediment samples and water discharge. Most are computed as daily time-series records. Some are computed on an annual basis, and some are computed for fractions of a day that can be summed to derive daily-value data. The fundamental methods used by the U.S. Geological Survey (USGS) for the collection and computation of daily suspended-sediment-discharge records have not changed since the 1940s. The most commonly used method is based on the derivation of a temporal relation by interpolating between measured suspended-sediment concentration and using measured and estimated concentration values with time-weighted water-discharge values to calculate suspended sediment discharges (Porterfi eld 1972). Consequently, concentration values are merged with discharge values representing a selected time interval and summed to obtain daily suspended-sediment discharges using the equation described as below: where Q s : suspended-sediment discharge, in tons per day or metric tonnes per day; Q w : water discharge, in cubic feet per second or cubic meters per second; Q s : mean concentration of suspended sediment in the cross-section in milligrams/liter; and k : a coefficient based on the unit of measurement of water discharge that assumes a specific weight of 2.65 for sediment, and equals in English units, or in SI units. Reliable suspended-sediment records cannot be obtained unless all concentration values used in the computation are representative of the mean cross-sectional value.it is seldom possible to collect a single cross-sectional sample in the length of time that it takes to obtain a sample with a pumping sampler, or to collect a single-vertical sample. The relation of suspended sediment and water discharges can also developed from logarithmically transformed data with water discharge as the independent variable and either sediment

4 concentration or sediment discharge as the dependent variable. The relations can be expressed mathematically as the following: where Q s : suspended-sediment discharge, in tons per day or tonnes per day; Q w : water discharge, in cubic feet per second or cubic meters per second; a : the intercept; and b : the slope. With the above mentioned methods, the relation can be formulated as either a linear or nonlinear model to find the solution for estimating sediment discharge. (3) The Coefficient of Determination Chu and Hsu (2007) studied the relationship between suspended sediment concentration and streamflow by suspend sediment-streamflow hydrograph and concentration-streamflow hydrograph. There were three data all of the data are a positive correlation. R 2 for the typhoon Remason, Nakeli and Sinlaku was 095, 0.87 and 0.12, respectively. Cheng (2003) mentioned that the most widely used empirical formula presently, where Q is the flow, and Qs can be as the quality of suspended sediment, suspended sediment or capacity of sediment discharge. Fleming, (1969) have used the above formula to analysis the world's 250 basin, and then he found that in the different hydrological and geological the resulting k had a varied considerably. Thus k and n value should be given by the characteristics of each river. However, the suspended sediment quality and flow data record of a torrential rain. And use of this formula usually used the coefficient (R 2 ) to assess its applicability. There are empirical formulae in Taiwan, Zhao (1990) for West Donggang River Harbour pumping station and Laonung River Bridge to regress, the R 2 was above Yeung (1997), got the R 2 to 0.99 by capacity of sediment discharge and flow. Ministry of Economic Affairs Water Resources Agency, Water Resources Planning (2002) in Zhuoshui River analysis the observation data of flow and sediment discharge for Dachiang Bridge and Changyun Bridge, the R 2 was above 0.8. Lee (2008) indicated that was very complex changes between suspended sediment concentration and flow in stream. Even in the same flow the suspended sediment discharge cloud be a difference of more than ten several times square times (Walling and Webb, 1982). Especially, in the period of torrential rain the change is more significant. Regression analysis the suspended sediment discharge and average daily flow for the Neimautsao water station. Before 1990 the R 2 was 0.75 and after 1990 was 0.73, there had high applicability and show there had a significantly segment, the rating curve of after 1990 was upon the rating curve of before 1990, there explained the suspended sediment discharge of after 1990 was higher than before In the same flow, when the range of was 10~100 (cms), the value of sediment discharge after 1990 and before 1990 was 3~5 times. For the typhoon event the variation of flow has got rising. 3. THE STUDY METHOD In the study, the price meter and depth-integrating suspended-sediment sampler, Model DH-59, are used to measure flow velocity and suspended sediment concentration respectively. Suspended sediment concentration and discharge are then analyzed by using regression analysis. The flow of the study is illustrated in Fig. 2. The observation of this study was carried out at Shi-Wen river in the sourthern part of Taiwan. The Shi-Wen River basin is located at southern part of Taiwan at North latitude and East longitude. The length of the main stream is about 22.3km; the area of basin is km 2 ; the average slop is about 0.03, and the flood of planning is 1,300m 3 /s. In the Shi-Wen river, there were 82% in mountainous area. The basin of the Shi-Wen River in the study area is shown in Fig

5 Flow Velocity and Suspended Sediment Concentration Investigation Streamflow Calculation Sediment Discharge Analysis Regression Analysis Results and Discussion Conclusions Fig.2 Research flowchart of study Fig.3 Shi-Wen River Basin 4. RESULTS AND DISCUSSION In this study measured streamflow and collected the sample for suspended sediment concentration for once two hours between July and August 28-31, 2011 at the Shi-Wen Bridge. And the relationship between sediment discharge and stream flow by regression analysis has been established. The followings are the results obtained during the study. (1) Flow and suspended sediment concentration Relationship The Flow and sediment concentration during the observation period are plotted as in Fig. 4 to Fig. 6. Reading Fig.4 that the peak flow was occurred at 3:30 on July 19, 2011with 71.8cms and the peak of sediment concentration peak was found at 5:30 on July 19, 2011 with 16546ppm. Although the peak flow arrived earlier than the sediment concentration, the lag was only 2 hours. By observing Fig. 5, one can learn that, the peak sediment concentration arrived earlier than the peak flow arrival time of 9:30 on July 20th, 2011, and its value is 51cms, the sediment concentration is 12044ppm which occurred at 5:30 on July 20th, Additionally, from Fig 6, the peak flow of the last torrential event was 352cms at 11, 2011 August 29 and the peak suspended concentration was at 5, August 29, 2011with a quantity of 50014ppm. Therefore, under the circumstances of this investigated river basin, the peak suspended sediment may occur earlier than the peak flow

6 Fig.4 The flow and suspended concentration of storm event I Fig.5 The flow and suspended concentration of storm event II Fig.6 The flow and suspended concentration of storm event III

7 (2) Flow and sediment discharge relationship The sediment discharge and flow are analyzed by using regression analysis and the results are shown in Fig. 7 to Fig. 9. Fig. 7 shows that the scatterness of flow and sediment discharge for storm event I, the coefficient of determination (R 2 ) is Secondly, the scatterness of flow and sediment discharge for storm event II is shown in Fig.8 which has a coefficient of determination of Thirdly, the highest R 2 of is found in Fig.9 that may be due to the fact that there were more data points collected in this torrential event. Fig.7 The flow and sediment discharage of storm event I Fig.8 The flow and sediment discharage of storm event II

8 Fig.9 The flow and sediment discharage of storm event III 5. CONCLUSIONS AND SUGGESTIONS From the analysis of suspended concentration, it is observed that the trend of sediment concentration does go with the flow variations which indicates when flow is higher there is opportunity to bring large amount of sediment. However, the results also showed that the peak suspended sediment concentration usually occurs earlier than the peak flow that may be due to the erosion in the basin occurred and deposited in the previous rainfall event. Moreover, the regression analysis results show that Qs = Q , Qs = Q , and Qs = 16.92Q , for storm event I, II, and III respectively. By assuming flow Q = 100 cms and applying to the above three regressed formulaes, that Q s1 = 1911kg / s Q s2 = 1105kg / s, Q s3 = 1859kg / s are obtained, these results suggest that, although the first torrential rains is smaller than the third torrential rains but the deposited sediment was washed out unexpectively. Thus, the third events has the highest performance of R 2 among the three torrential rainfall events. Flow and sediment discharge relationship is affected by the time of occurrence of soil erosion, antecedent rainfall, rainfall intensity, and other unstudied factors. However, based on the above results of correlation between flow and sediment discharge, the observed data is still limited. Thus, in the future through long-term investigation and development of Taiwan's local flow-sediment discharge formula; water resources planning and water related infrastructure design will be improved. REFERENCES 1) Wang, Y. M., Juang, S. C. and Lai, G. J. : Study on Suspended Sediment of River for Rainstorm, USTB-NPUST Conference, C-E045, pp1~6 Beijin China, ) Integrated Water Resources Planning Committee. : Study of Taiwan's western river sediment discharge estimated, ) Chu, Y. T. and Hsu, M. L. : The Relationship vetween Discharge and Suspended Sediment Concentration at Typhoon Events in Yu-Feng Catchment, Journal of Geographical Science, Vol.49 pp.1-22, ) Lee, L. T. : Variability Analysis on Rainfall, Runoff and Sediment Discharge in the Chenyoulan-Stream Watershed, Master Thesis Department of Hydraulics and Ocean Engineering, National Cheng-Kung University, ) Lin, M. L. and Lin, J. C. : The Influences of Typhoons and the Characteristics of Suspended Sediment Yields in the Upper Catchment of Lan-Yan Chi. Journal of Geographical Science, Vol.33, pp39-53, ) Chen, H. L. and Chang, J. C. : The Discharge and Sediment Load of Heavy Rainfall Events in the Tsengwen River Basin, Southern Taiwan. Journal of Geographical Science, Vol.48 pp , ) Research Center for Soil and Water Tesources and Natural Disaster Prevention of National Yunlin University of Science and Technology : The theoretical basis of GSTARS mode, ) Water Resources Agency, Ministry of Economic Affairs : Experimental Study of Sediment Transport Relationships for the lower Cho-Shui River after the 921 Chi-Chi Earthquake (2/2), ) Water Resources Agency, Ministry of Economic Affairs : Feasibility Study of Shi-Wen Reservoir (6),

9 10) Cheng, S. J. : A Study on the Suspended Load Transport Rate for Jia-Xian Diversion Weir, Department of Civil Engineering, National Pingtung University of Science and Technology, ) Yun Da, Cheng.: River sand transport surveys and measurement technology, Water Resources Agency, Ministry of Economic Affairs, ) Van Rijn, L. C. : Sediment Transport, Part I : Bed Load Transport, Journal of Hydraulic Engineering, Vol.110 pp , ) Liu Z. : Sediment Transport, Instituttet for Vand, Jordog Miljøteknik Aalborg Universitet,