Research On Small Watershed Flood Forecasting Based On Double-Excess And GIUH Coupling Model

Shanxi Province is located in the semi-arid and semi-humid region with mountainous terrain.The small watersheds are scattered all over the place.During the flood season,the rising and falling sections of the flash floods are quite steep,causing them to be sudden and harmful.In addition,there is often a lack of hydrological observation data in such areas,so the flood prediction is difficult.Therefore,it is of great scientific and practical value to analyze flash flood forecasting for ungauged basins in semi-dry and semi-humid region.It has been verified by previous studies that the Double-excess runoff generation model has good applicability in semi-dry and semi-humid areas.The geomorphologic instantaneous unit hydrograph(GIUH)is a method for calculating runoff concentration,which is applicable to ungauged basins.At present,the domestic research and applications of this method focus on the humid areas in south China,and most studies build GIUH based on the entire watershed,which was difficult to reflect the spatial differences of geomorphic characteristics in the watershed.The applications of this method in semi-dry and semi-humid region is still rare.By applying the coupling model to the basin with sufficient hydrological data,the applicability of the model is verified,and the sensitivity of parameters is explored,so as to further extend it to the similar areas.This paper takes Wangjiahui watershed in Xinzhou city,Shanxi Province as the research object.The main research contents and conclusions are as follows:(1)Sub-basins division and geomorphic parameters extraction based on DEM.Hydrologic analysis is carried out on DEM,and the critical contributing area is determined to be 3 km²by river network density method.Then,7 sub-basins are divided by the distribution of the river network,and the weight of each rainfall station was obtained through the Thiessen polygon method.Finally,the Strahler method was used to classify the river network and obtain the geomorphologic parameters of each sub-basin.(2)Calculation and analysis of geomorphic unit hydrograph.According to the R-V GIUH theory,the runoff paths of the precipitation particles are determined.The path probabilities and retention time probability density functions are calculated by the geomorphic parameters,and then the GIUH formulas are derived.The instantaneous unit hydrographs are converted into time unit hydrographs by S curves.Based on the comparison and analysis of unit hydrographs by geographical characteristics,the geomorphologic factors affecting the shape of unit hydrographs are summarized into four items:basin area,river network distribution,length of different grade river reach(or area of different grade catchment)and slope.Besides,the dominant factors are different in different basins.(3)Research and analysis of flood forecasting.The coupling model is used to simulate 25 historical representative rainfall events in the Wangjiahui watershed.The results show that the qualified rate of peak flow is 92.0%,the qualified rate of runoff depth is 64.0%,the qualified rate of peak time is 96.0%,and the average qualified rate of the three forecast items is 84.0%,the simulation accuracy of this model is up to grade B.Moreover,the deterministic coefficient greater than0.70 accounted for 72.0%.The results show that the coupling model has good applicability in Wangjihui watershed.In terms of simulation effect,the big floods are better than small floods.The steep floods with single peak is better than the gentle floods.The floods with multiple peaks and no superposition between peaks is better than floods with overlapping peaks.The Sr,b,Ks,₀ and?are the main runoff generation parameters in this model.The Sr,Ks and ₀ are negatively correlated with peak flow and runoff depth.The b and?are positively correlated with peak flow and runoff depth.Due to the difference of physical meaning and action period,each parameter shows different sensitivity to flood peak flow and runoff.From the perspective of peak flow,the parameter sensitivity order is Sr>b>Ks>>?.For peak flow,the Sr is the most sensitive parameter;the b,Ks and ₀ are sensitive parameters;the?is insensitive parameter.From the perspectives of runoff depth,the parameter sensitivity order is Sr>?>>Ks>b.For runoff depth,the Sr is the most sensitive parameter;the ₀ and?are sensitive parameters;the b and Ks are insensitive parameter.The runoff concentration parameter a is positively correlated with the peak flow,but has no effect on the runoff depth.The peak time is advanced with the increase of a.