Calculation Based On Hydrology, Water Level Unit Hydrograph And The Pier Water Mark Established In Bridges

As lacking in data of measured runoff,hydrological examination on existent small & middle span bridges usually uses the rainfall to calculate runoff.Reasoning formula method, unit hydrograph method, regional empirical formula methods etc. are the familiar methods,but all those methods need obtaining many parameters,such as watershed area,the length of main river,the average gradient of the river,which cost people tremendous manpower and material resources,thus the difficulty of hydrological examination on existent small & middle span bridges were enlarged.The traditional unit hydrograph method(UHM) is a kind of Black-Box modeling method.According to multiple supposition and addition supposition,the UHM estimates future runoff based on historical data of measured runoff,which is simple and convinent compared with those above-mentioned methods.But many existent small & middle span bridges without hydrological stations usually lack in data of measured runoff. So at this point it is difficult to compute and check hydrology on these bridges with UHM.As corrasion by flood year after year, the piers and abutments of existent bridges reserve the water level records ineluctably, which are called water traces. The piers and abutments of existent bridges have been collecting and conserving the data on the piers and aburments unconsciously since bridges were set up. Compared with the measured water level records, those data are credible.Based on the Black-Box characteristic of the UHM, the unit hydrograph method of water level (UHMWL) is put forward in this paper, which estimates future water level relied on historical data of measured water levels. The historical data of measured water level can be got form the water traces on piers and abutments. Therefore UHMWL steers clear of the difficulty that UHM. Apply in hydrological examination. So as to predigest deductive process, we still think that the UHMWL obeys the multiple supposition and addition supposition. Using existent data of measured rainfall and runoff, the UHMWL solves the unit hydrograph of water level through the method of least squares.Combining the future rainfall, we can use this revised unit hydrograph of water level to calculate the flood water level at the bridge spot, and then to compute the height of water level and the embedded depth of foundation of exist bridges. As is proved by the example of Peijiachuan hydrological station near the Lanyi River and a bridge built on the Beijing-to-Guangzhou railway lines, this method is feasible.The results show that the relative errors of prediction of water level calculated by the unit hydrograph method of water level and by the traditional method are all less than 6%. However, the UHMWL in this paper doesn't need to collect many parameters, such as watershed area, the length of main river, the average gradient of the river, and so on. Also when computing and checking hydrology using UHMWL, it just need the water traces on piers and abutments of the bridge only.Thus it can be a simple method for hydrological examination on existent small & middle span bridges in the area without first-hand data.