Based On The Isochrones Law Pier Water Mark Of The Bridge Water Damage Warning

In view of the drawbacks of existing methods of water hazard prediction of small and medium bridges, such as inaccuracy of the predicting results, intensive physical labour of out work, high difficulty lack of observational data and hydrographic information, more attention paid to result than process and so on, A new method to deduce the generalized conflux curve of the river basin by the water mark of the pier is presented in the paper, which greatly improves the situation that it is hard to predict when the observational data and hydrographic information of the river basin is deficient. In addition, this method can predict not only the peak discharge of the flood, but also the specific time when the flood reach the bridge. The process of water hazard prewarning of bridge in the paper is as follows:(1) Combining the height of the water mark of pier in history that can be easily observed and have been recorded on the pier with the section shape of the river-bed under the bridge, the corresponding peak discharge of the flood in history can be got on the basis of Xiecai-Manning formula. Then the area of transformed basin can be calculated according to the rational formula.(2) Taking the generalized flood hydrograph as the real flood hydrograph under the bridge, the relationship between flood hydrograph and conflux curve can be discussed from three cases based on the characteristic of the river basin and rainstorm.(3) According to the selected period of time, time interval distribution of the generalized conflux curve can be done, and actually measured process of rainfall can be got by observing the rainstorm fallen in the river basin. Then the flood hydrograph caused by the rainstorm fallen under the bridge can be got by analyzing the conflux process with the isochronal method. In order to improve the isochronal method, area precipitation process of the lower reaches is used to speculate the flood hydrograph caused by the rainstorm fallen under the bridge, in addition to this, peak discharge reduction coefficient and time latency coefficient are put forward to take account of the influence of the adjusting storage by river channel.(4) Checking the water level and scour depth of the bridge caused by the peak discharge calculated to figure out whether there will be water hazard of the bridge and releasing the early-warning information.In flood forecasting of small and medium bridges, this method proposed by this paper has been proved of high degree of accuracy by a practical example and can meet actual production needs. When applied in practical work, this method can greatly reduce the amount of field work; simplify the flood-prediction process so as to improve work efficiency. In particular, this method is more applicable for small and medium bridges whose hydrologic data is insufficient.