Research On Flood Forecasting And Analysis Of Effects Of Water And Soil Conservation On Floods For Yanan Water Basin

Because there are high mountains and steep slops with poor plant cover in conjunction with paroxysmal rain shower or rainstorm, flood in Yanhe River basin occurs frequently and produces disasters that bring tremendous losses to the people in Yanan city. It is significant to establish the flood forecast scheme for preventing floods and abating disasters in Yanan city and downstream towns. Yanhe River is a river with hyper-concentration load. Stream channel erosion and siltation happens quite often during the flood season, which makes the shape of flood hydrograph protean. Therefore, flood forecasting is a difficult work for the area. The thesis mainly establishes a suitable flood forecast scheme for Yanan city.Besides, the large-scale water and soil conservation works have been conducted for several decades in Yanhe River basin, and resulted in the change of transformation from rainfall to runoff on underlaying surface. Nevertheless, it is hard to see the analyses work concerning the effect of water and soil conservation on flood. In fact, these kinds of work redounds to understanding the flood characteristics under new circumstance and promoting more reasonable flood defence measures so as to reduce the loss in flood disasters. This thesis, hence, takes the response of water soil conservation to flood as another research point.The main contents and results of this thesis obtained are shown as follows:1. Analyze the characteristics of rainfall and runoff in the Yanhe Riverbasin from Yanan gauging station upward. The results show that the rainfall of this basin is unevenly distributed both in space and time. Rainfall duration is short, but its intensity is large. And some tendencies appear in the storm center locations. The flood rises and falls rapidly with large peak flow, large sediment concentration and small volume.2. Based on the available observed data, System Model of Genetics (in short SMG) is adopted to be as the flood prediction model at the Yanan station. The SMG is simple for applying, and can reflect the characteristic of uneven distribution of rainfall, thus has the high forecast accuracy especially for peak flow. The resulting flood forecast scheme evaluated according to the National Regulation on Flood Situation reaches the first class and can be applied to practice.3. SMG is required to have the lead-time of one hour for practical situation. Because the lead-time is four times of computing time interval of the model, the occurrence time of flood peak isn't forecasted well. Although the model structure and calculation method were adjusted, the forecast accuracy for the peak time was still under the required standard. Then BP network is taken to modify the forecast, which leads the flood, forecasting scheme to achieve the third class and can supply references for practical flood forecasting.4. From studying the effect of water and soil conservation on the flood in the Yanhe River by the contrast analysis, those are known that the value of annual occurrence frequency of medium and small floods at the Yanan station is becoming small. And the runoff reduces in the wet season, and increases in dry. Under the similar precipitation, flood peak flow, flood volume and sediment load at the Yanan station are all smaller in the 1990s than in 1980s. Flood duration becomes longer; the flood hydrograph is characterized by the rapid rise and slow recession. These changes are favorable to control flood, and develop and utilize water resource in the studying region.