Flash Flood Forecasting Based On Land-atmospheric Coupling

Flash floods happened frequently in the UK.The characteristics of flash floods are great amount of water,short time period and great destructive power.Flash floods almost happened annually in southwest peninsula of the England: Valency basin is siuated in southwest peninsula of England and surrounded by sea from three directions.The outlet cross section of Valency basin is Boscastle flashing port.Heavy thunderstorm and flash floods usually happened in summer.The occurance of flash floods is influenced by the elements of hydrometeorology,the causes of flash floods are comparatively complex.Flash floods are difficult to forecast precisely.Using land-atmospheric coupling to conduct flash flood forecast is meaningfull for flash flood control and reasonable allocation of water resoureces.This thesis first summarizes the causes of flash floods and analyses its corresponding flood response engineering solutions;establishes WRF-PDM coupled model from the perspective of land-atmospheric coupling;reappears Boscastle flash flood;provides vital theritical and pratical foundations on ungauged small basins.The achievements of this thesis are as follows:(1)The causes of Boscastle floods are mainly:1)climate change(sea level rises)2)weather(mesoscalse convective system)and thunderstorm(heavy convective rain,high rainfall frequency,great intensity)3)land surface(vegetation is mostly herbaceous with low interception capability,soil are almost cultivated soil with low natural fertility and water permeability)4)basin influence(valency bain is long and narrow,small bain area,high river density,high channel slope,topography changes quickly,the process of rainfall-runoff is fast)5)human factor(witout complete flood control engineering measures)(2)A large number of dumping of water sand fluid accompanied by land slide and debris flow was observed after the outburst of Boscastle flood.The original channel and slope were destroyed consequently.The main influence of mainstream and its tributaries are the erosion of river banks,bed cutting,lateral movement and avulsion.The flood control engineering measures are: widening and deepening the river channel,stabilizing the side slope,increasing the number of flood protection culverts,installing undergound drainage pipeline,increasing the ground level etc.(3)In terms of rainall simulation,WRF well simulated rainfall distribution over valency basin,small divergence exists in peak flow.Grell-Freitas cumulus convective scheme is better than Zhang-Mc Farlane cumulus convective scheme no matter in total catchment rain or in rainfall distribution.Therefore,Grell-Freitas cumulus convective scheme is more suitable than Zhang-Mc Farlane cumulus convective scheme.(4)In terms of PDM calibration and validation,the value of Sutcliffe Efficiency(NSE)achieves at 0.88 and 0.9 for calibration and validation separately which shows the good results of parameters calibration and paremeters validation.The parameters of of Kenwyn basin can be used for Valency basin(5)In terms of one-way coupling of WRF-PDM,Grell-Freitas cumulus convective scheme performs better than Zhang-Mc Farlane in simuating peak flow.WRF-PDM coupled model can be used for flash flood forecasting over Valency basin.