Research On The Formation Mechanism And Early Warning Of Rainstorm Flood Risk In Small Watershed Under Changing Environment

Due to the change of underlying surface conditions and rapid runoff yield and concentration after heavy precipitation or continuous precipitation,surface runoff exceeds the drainage capacity of pipe network at development area in loess gully region.The storm flood through drainage network flows into natural channel and superpose with channel flood,waterlogging and mountain flood disasters are prone to occur at the same time.In this paper,Daqinggou and Shengou watershed after land development and utilization in Anning District of Lanzhou City are taken as the typical research areas.Based on the analysis of temporal and spatial distribution characteristics of precipitation and its variation trend by means of hydrological statistics,Kriging interpolation,IPTA method and pole symmetry mode decomposition method,the formula of rainstorm intensity is proposed and the design of rainstorm pattern is deduced by using Chicago rain pattern method.SWMM model was constructed to simulate and analyze the load capacity of drainage systems in land development and utilization areas in different rainstorm recurrence periods,and the waterlogging risk degree is analyzed based on Info Works ICM model.The variation of the designed flood in gully before and after land development and utilization is also described,and the corresponding critical rainfall and early warning index of rainfall for disaster prevention objects are calculated by using the backward regression method of water level and discharge.The main research conclusions are as follows:(1)Light rain was the main precipitation in the study area from 1982 to 2020,followed by moderate rain,and the probability of rainstorm was low.The annual average precipitation in spring,summer,autumn and winter accounted for 22.0%,55.9%,20.2%and 1.8%of the annual precipitation,showing the significant differences respectively.The PCI value of annual precipitation ranged from 17.0 to 79.8,and it fluctuated significantly from 1982 to 2001,while fluctuated steadily from 2002 to 2020.The average annual precipitation increased from 252.9 mm at middle section to 554.3mm and 388.8 mm at east and west side.The mean value of monthly precipitation and the maximum trend length of standard deviation were 32.9mm and 20.1mm respectively,and the maximum variation of monthly precipitation occurred in August and September.The annual precipitation increased from 1982 to 1996,decreased from 1997 to 2008,and increased from 2009 to 2020.(2)The i-t-P data table and fitting curve show that the exponential frequency distribution curve can be used as the base for ascertaining the rainstorm intensity formula.The absolute and relative root mean square error of rainstorm intensity calculated by the corresponding rainstorm intensity formulas of three frequency distribution curves are less than 0.05 mm/min and 5%,and the rainstorm intensity formula of Gengber frequency distribution curve with the smallest error is selected to calculate the rainstorm intensity.(3)The simulation results of SWMM model show that the precipitation,infiltration,runoff and runoff coefficient of different rainfall recurrence periods increase with the raise of rainfall intensity.At 3,5,10,30,50 and 100 years return period,the node overload rate is 69.5%～94.7%,and the overflow flux varies from 0.001 to 3.07 L/s.Pipeline overload rate is 66.9%～91.0%.The peak time range of PFK1,PFK2,PFK3and PFK4 discharge ports is 1.25～3.25 h.The peak discharge at 5,10,30,50 and 100years return period increase by 29.8%～577.8%compared with that at 3 years.The peak time of flood is forward by 12.5%to 38.4%.The submerged area of water depth at different rainstorm recurrence periods is 327.57×10~4～935.87×10~4m~2.The submerged area(h<0.15 m,0.15 m≤h<0.3 m,h≥0.3 m)accounted respectively for 97.24%～98.23%,0.47%～1.13%and 1.21%～1.64%of the total submerged area.(4)The designed peak flood discharge before and after land development and utilization increase respectively by 32.0%～1345.4%,6.4%～1124.3%and 8.5%～418.3%in Daqinggou,Yanchigou and Shengou basins under different design frequencies.The disaster discharge of the second community in western area and No.nine community in eastern area is 99.9 m~3/s,93.6 m~3/s and 203 m~3/s.The critical rainfall at 1,3,6,12 and 24 hours is 49.3～85.5 mm,58.1～80.0 mm and 40.4～72.5 mm.The rainfall indexes of immediate transfer warning are 49.3～85.5 mm,58.1～80.0 mm,and 40.4～72.5 mm.The rainfall indexes of ready transfer warning are 44.4～77.0 mm,52.3～72.0 mm,and 36.4～65.3 mm.