Reducing uncertainty in flood prediction: the representation of vegetation in hydraulic models

点击翻译按钮获取中文摘要

The summer 2007 flooding in England was the country's largest peacetime emergency sinceWorld War II, with 13 deaths, over 55,000 homes & businesses flooded & an associatedinsurance cost of over £3 billion. Prior to 2007 floods, the UK had experienced a numberof significant flood events over the recent past which have included amongst others; 1)the Easter 1998 floods of Northampton & surrounding towns in the Midlands when 4,200homes were flooded in a 1:50 year event &; 2) the winter 2005 floods of Carlisle, a 1:200year event, when 3 people lost their lives & 1,800 properties were flooded. Following the2007 floods the Government commissioned the Pitt Review to discover the lessons thatneeded to be learnt to manage future flood risk. The key observation reported within thePitt Review relevant to this application is that practices which were undertaken to managethe river corridor; namely dredging, debris removal & notably vegetation clearance, were nolonger being performed as frequently, in order to maintain the ecological diversity of the riverfollowing the Water Framework Directive. This has substantially reduced the capacity of the river channel & has thus increased the potential of flooding. This is set within the contextof the risk of flooding within the UK increasing into the future, with climate change models(UKCIP09) predicting that winters will be ~25% wetter, with an increase in extreme rainfallevents. Flood defences in the UK are managed by the Environment Agency. In order to managethese resources we require knowledge of the capacity of river channels & associatedfloodplains. Aquatic vegetation is present in many UK rivers & this reduces the capacity ofthe channel that causes a reduction in flow velocity, which in turn produces higher waterlevels per unit discharge, thus increasing the risk of flooding. Therefore, there is a needto develop our understanding of how vegetation partitions discharge between changesin velocity & depth & how, in turn, this impacts upon the discharge carrying capacity of achannel, namely conveyance, to better manage flood prediction & prevention within the UK.This proposal argues that we can now measure topography to a high resolution & precision& incorporate it into flood models explicitly. This is not the case for vegetation, & thereremains a lack of understanding of how to represent the influence of vegetation on fluvialsystem function. Indeed, the vast majority of uncertainty in flood model predictions stem fromthe influence of vegetation on conveyance. In order to move away from an empirical basedapproach to the parameterisation of vegetation resistance, a new understanding of theflow & turbulence production is necessary to be able to re-formulated a dynamic vegetationroughness treatment for flood models & thus reduce the uncertainty in flood predictions. Thiswill be achieved by undertaking high resolution experiments in the laboratory in conjunctionwith the development of a new three dimensional model that is capable of predicting boththe flow & the plant movement. The model will be validated using the experimental data& then the two data sets will be combined to enable a new formulation of the drag causedby the vegetation. This new understanding of the influence of vegetation of drag will beincorporated into an industry standard flood prediction model. An existing flood examplewill be used to develop & test the model as this will allow us to; 1) assess how well this newmodeling approach improves model predictions &; 2) disentangle parameterization & dataerror in flood models & enable us to assess what uncertainty needs to be addressed nextgeneration of predictive flood models.