Technical Studies


Efficiency Evaluation of Choshui River`s Existing Spur Dikes forProtecting Lateral Erosion and Longitudinal Scour/Deposition (1/2)

On river planning of Choshui River, spur-dikes are the common engineering which design to protect the bank of research reaches. Due to the rapidly changing of the flow characteristics and morphology of river, it is difficult to build the principle of spur-dike design. To improve the spur-dike design technology and the efficiency of spur-dike engineering, this research took statistics of the length, distance and angle of spur-dike to analysis the changing process of spur-dikes classification and the cause of destroyed in every research reach. Additionally, according to the surveyed data of the bed-material grain size and the depth of the scour near spur-dike, the hydraulic and sediment transport mechanism were studied in the river reach with spur-dike. The following is the summary of this research. I. River spur-dike construction methods and character Analysis Method Introduction Choshui River’s existing spur-dikes encounters : (1) Riverbed local erosion in front of spur-dikes causing spur-dikes front subsidence and abutment fracture separation. (2)Spur-dikes didn’t connect with river bank , causing river flow pass through the gap between spur-dikes and river bank. (3)Spur-dikes body’s flexibility isn’t enough to adjust spur-dikes body itself to adapt the riverbed scour. Therefore, this study collect spur-dikes design methods of Federal Highway Administration (FHWA), US Army Corps of Engineers (USACE), Japan River Agency, China Water Resources Agency and Taiwan Water Resources Agency. First the general theory of design engineering methods will be explained ,and make use of the parameters which are mentioned in the construction specification such as D/L、L/W as a standard to evaluate the efficiency of spur-dikes. II. Comprehension and analysis of Choshui River drainage basin. In order to comprehension and analysis the drainage basin background data of Choshui River and two main affluent Chenyulan River and Chingshui River. The study collect background data Including management planning project, rivers longitudinal and lateral stability assess, aerial photography, river longitudinal profile comparison and bedload particle size data analysis. By the analysis results, Choshui River after several major typhoon and 921 earthquake, since Chichi weir Sheliao embankment the riverbed erosion situation more obvious and significant change flow pattern that cause riverbed decline and main river deep grooves that river protect construction lead to disaster occur. Chenyoulan river located in the mountains , that the broken loose soil slopes to river and unstable rock collapse occurred, causing the river extremely unstable and trench lines changing rapidly, therefore, riverbed is continue in a high silt trend. Chingshui river is steep and swift , the main channel path change frequency and less stable. Main channel located in a mountainous river and part of the line segment is quite clear, but at the river bend, confluence of tributaries and downstream segments are easy to form a braided stream river channel. III. Movable bed model calibration verification. Collecting the aerial photography and DEM data of Choshui River to examine CCHE2D’s hydrology characteristic and sediment transport parameters to ensure the model’s reliability. The model calibration including : (1) adaptation length,(2) adaptation coefficient,(3) mixing layer thickness. The verify results of this project are similar to the field riverbed change, so the CCHE2D can be use to model the river segment that spur-dikes located. IV. The analysis on source of erosion calamity from spur-dike construction in Choshui River . Selecting the spur-dike construction designing data and research the reason of destruction mechanism. All for promoting the future spur-dike design. Followings are researching places: (1) Choshui River main stream Min-Ho revetment to Bashepu embankment,(2) Choshui River main stream fanzi Dist embankment ,(3)Jiji weir downstream,(4) Choshui River main stream Min-chu bridge to Erhshuitei bridge,(5) Jhuoshuei main stream Tze-Chiang bridge to Xibin bridge,(6) Chenyulan river Shangan embankment and Junkeng embankment,(7) Chenyulan river neimaopu embankment, eykch embankment and Min-der embankment,(8) Chingshui River Liyu dike to Liyu embankment. Later, this study found some contradictions, the ratio of L&W, the ratio of D&L and the property of spur-dike and river. By the way, this study refer to the standard in U.S. and Japan, establish the spur-dike fitting condition based on domestic standard. The characteristic in this study consider the condition of flow and geometry to put to different types of spur-dike in Taiwan. Then, take the common seeing types of rock as accessing example to research the best fitting spur-dike in different place V. The latest construction of spur-disk in Choshui River & investigation of the riverbank erosion mechanism. The source of calamity from the erosion of spur-dike construction is pretty complicated and for now there are no enough information to refer. This study not only summarizes the statement of river cross-section, size of spur-dikes, construction records also complete testing and verifying by CCHE2D. Estimating the flood period the condition of hydraulic sediment transportation protected by spur-dike, finally put the result into the erosion formula to quantification. By the way, this study concentrated on accessing applicability of erosion formula. Analysis found that the prediction of erosion depth all more than reality. As vetment in Choshui River during Sula typhoon, the difference maximum goes to 8.45m, minimum goes to 1.2m. To add up the accurate of estimation. This study assume there’s a safety coefficient between the empirical formula and the real mechanical formula. By doing so not only promote the accuracy also concern the effect of flow space uncertainty. According to the surveying data this year, this study accesses the safety coefficients of FHWA and Neill empirical formula are 5.33 and 6.31, respectively. VI. Research on Choshui River spur-dike construction erosion. In order to know the erosion effect after setting spur-dike, this study survey the spur-dike place erosion and analysis the sediment in riverbank. The researching places in this study are Shueidiliao revement, Jhuoshuei dike, north of Xizhou bridge, Xiaxiqian dike, Min-der dike.