Flow Influence Line Method And Its Application In Bridge Hydrology Of Medium And Small Watersheds

Bridges play an important part in transport network. So bridge failures caused by floods always lead to traffic interruption and serious loss to railway transport and highway transport. What'a more, bridge failures always affect the development of the rescue operations and pose serious threats to people's lives, property and personal security.There are plenty of bridges in medium and small watersheds, and the hydrologic calculation of these bridges is rather troublesome. Therefore, seeking out a simple, practical and effective hydrologic calculation method is of significance in actual projects as well as in consideration of the hydrological situation and the characteristics of medium and small watersheds. Taking the randomness, complexity and uncertainty of floods into account, watershed hydrology model based on flood data, is the most reasonable model. On this occasion, flow influence line method, which integrates instantaneous unit hydrograph method and influence line method in structural mechanics, is put forward in this dissertation. Flow influence line method is a hydrology method based on water marks in history, and it can be used for hydrologic calculation of bridges in medium and small watersheds. The main contributions of the dissertation are as follows:(1) Flow influence line method is put forward, which is an innovation and integration for the theories of instantaneous unit hydrograph and section stress calculation of simple beam through stress influence line method. Under flow influence line model, the flow computation process is turned to the rainfall series loading process on flow influence line.(2) The empirical calculation formula of rising duration t1, a model parameter of flow influence line, is proposed. The formula is t1=0.278L/V, where L is the length of the main river course(km); V is the average flow rate (m/s).For watersheds without actual average flow rate, the rate at water level of flood land line can be adopt. In some cases, for instance, the lag time of flood-peak or water levels and corresponding rainfall series of several big floods are obtained, the rising duration t1 can be calculated directly.(3) The selection standard of rising duration t1 and falling duration t2 of flow influence line in medium and small watersheds is established. The ratio criteria are:t1:t2 is 1:2 at general conditions; for the small size watersheds in mountainous areas with steep gradient, fast speed and poor vegetations, t1:t2 is 1:3; for the larger watersheds in plain areas with gentle gradient, slow speed and good vegetations, t1:t2 is 1:1.6. The criteria are built on simplification analysis of instantaneous unit hydrograph and Guangdong comprehensive unit line. Also hydrological characteristics of medium and small watersheds are taken into consideration, while actual hydrology observation data in Guangxi Zhuang Autonomous Region are taken as reference.(4) The method used for determining peak value of flow influence line is proposed. It is based on the conclusion that the flow obtained by the rainfall series loads at the most unfavorable position of flow influence line equals, is equal to the corresponding flow under water level marks. The peak value of flow influence line, which is concluded by calibration on the larger flood in recent period, is more reasonable in theory. Because the impact of watershed confluence, for example, hydrology, topography, vegetation and other factors, has been hidden in the calibration process.(5) Test and verification of flow influence line method on seven railway bridges and eight highway bridges are established. The results show that the calculated value coincides well with the measured one, which means the flow influence line method is reliable.(6) The sensitivities of some correlative parameters on flow influence line method are analyzed. The impact parameters chose for analysis include roughness coefficient, rising duration, falling duration, flood magnitude chose for calibration, time interval of rainfall series, rainfall loss and so on. Through sensibility analysis, some useful conclusions are made, and flow influence line method is demonstrated and improved.(7) The hydrology design, flood-resistant ability checking, real-time flood forecast on five bridges by flow influence line method is done. The five bridges are Qingbaijiang No.949 Bridge in Baoji-Chengdu Railway, No.19 Bridge in Baotou-Baiyunebo Railway, Jiefang Bridge in Meizhou Guangdong Province, Daguan Bridge in Beijing-Guangzhou Railway and Jiaotongjianli Bridge in Yangdong Guangdong Province. The calculated results show that flow influence line method is convenient in application, and the accuracy meets the requirements in practical projects. Compared with traditional hydrologic calculation methods, flow influence line method has obvious advantages. Especially in real-time flood forecast area, along with the rainfall process, the real-time estimated results of flood characteristic values, such as peak discharge, flood-peak stage, flood-peak time, duration of high water level, etc, could be provided by flow influence line method. So the possibility and the magnitude of flood disaster could be forecasted ahead of time. The timely alert of flood is of great significance for reasonable arrangements for flood prevention and relief work.The flow influence line method, which is calibrated by water level marks caused by flood in history, is rational in theory, convenient in practical application and reliable in computational accuracy. It has obvious advantages. This method can deal with the key problems in hydrologic calculation of bridges in medium and small watersheds. Flow influence line method could provide scientific criterion at the fields of hydrology design, flood-resistant ability checking, and real-time flood forecast. This method mainly focuses on bridges in medium and small watersheds, especially in watersheds without hydrology observation records of floods.