Study And Application Of Small Watershed Characteristics In Hilly Area Based On DEM

Flash flood disaster is one of the most harmful natural disasters in the world.The formation,development and harm degree of flash flood disasters are exposed to the integrated influences of natural conditions such as rainfall,topography and geomorphology,as well as anthropogenic activities.The underlying topography and geomorphology provides disaster inducing environment for the occurrence of flash flood disasters.At the same time,the underlying surface conditions in the mountainous regions are complex,and the non-linear characteristics of the runoff generation and confluence are significant,therefore,it had a great practical significance to statistically analyze the geomorphological characteristics of mountainous catchments and to study the influence of geomorphological characteristics on runoff confluence.And these analysis can also provide important data supports for flood control and drought resistance,agricultural planning,water conservancy proj ect construction and development and other fields.Supported by the national flash flood disaster prevention and control project,the characteristic parameters and runoff confluence characteristics of mountainous catchments in China were analyzed in this paper.The main research work and achievements of this paper include:(1)3030 rivers(less than 10000km2)in mountainous regions were selected for case study.The river source densities under different critical source areas were analyzed,to construct the power function between the critical source area and the density of river source with the form of y = kxa,which was approximated to the inverse function.The spatial distribution of river networks in study area were also analyzed.Moreover,the optimal critical source area in small mountainous catchment is determined as 0.5km2 through the mean variable point method.(2)The variation range,mean value and frequency distribution of the basic characteristic parameters were analyzed for the 480000 small catchments in mountainous regions,China,and the differences and consistencies of each parameter were analyzed among the first grade of flash flood forecasting and early warning zone.The average values of the drainage area,the average slope,the longest confluence path length and its slope and the river slope among all the small catchments were 17.40krTm2?14.26°?10km?0.0296 and 0.0208,respectively.The deviation coefficients of these parameters within the first grade of flash flood forecasting and early warning zone were 0.35?0.85?0.06?35.12?8.34?18.14?0.42?6.22?0.64?27.30 and 0.64?27.30,respectively.The division of the drainage areas was proper,and most of small catchments were located in the alpine and hilly regions,so it is suitable for the calculation and analysis of flash floods.While the distributions of some parameters tended to be discrepant in some zones,including average slope,the longest confluence path length and its slope,and the river slope.(3)Furthermore,the river geomorphological parameters such as river number ratio,river length ratio,area ratio and river slope ratio were calculated for 3030 rivers in mountainous regions,and these parameters were systematically analyzed,including their variation ranges,mean values,frequency distributions,and their differences and consistencies among the first grade of flash flood forecasting and early warning zone.The frequency distributions of the above parameters in the study area tended to be normal distribution,so the statistical results were in accordance with the random distribution law of natural water system.The average values of the above parameters were 4.54,2.33,4.55 and 0.56,respectively.The river networks in the study area were densely distributed,most of which was in their mature stage.The deviation coefficients of these parameters within the first grade of flash flood forecasting and early warning zone were 0?0.11?0?0.19?0?0.14and 0?0.6,respectively.The above parameters showed consistency within each zone.(4)This paper analyzed the relationship among the elevation,the length of confluence path and the drainage area in the small catchments,and explored the hierarchical distribution of the runoff confluences in the small catchments.Two parameters,i.e.,the non-uniform coefficient and the weighted average slope,were proposed to depict the heterogeneous distribution of runoff confluence paths in small catchments.In the study area,the non-uniform coefficients were over one for 80%of the small catchments,which indicated that the centroids of the runoff confluence paths tended to be in the upstream for most of the mountainous catchments.Besides,the weighted average slopes were less than the average slopes for 67.8%of the small catchments,which indicated that the lateral profiles for most small catchments tended to be"convex".(5)The correlation relationship was analyzed between the characteristic parameters of distributed unit hydrographs and the characteristic parameters of small catchments,and the nonlinear empirical fitting formulas were constructed between the parameters using the least square method.The optimal fitting formula was determined as Qm?kFLmJS as for peak flow rate,was T?k(CLm)J as for the confluence time,and was Tm=kLmLa,J'as for the occurrence time of peak flow.