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单击此处编辑母版标题样式,编辑母版文本样式,第二级,第三级,第四级,第五级,2017/3/16,#,The effects of,river restorationon,catchment,scale flood,risk and flood hydrology,Lian Sun 3/16/2017,Copyrights: School of Environment, Beijing,Normal Unveisity,Published on,EARTH SURFACE PROCESSES AND LANDFORMS,In 2016,Interpretation of High Quality Literarure:,Abstract,Background:,In,the face of,challenge of flood risk,a continued reliance,on,engineered,at-a-point flood,defenses,is seen as both unrealistic and undesirable,.,Research advances:,The,contribution of,soft engineering solutions (,e.g.,riparian forests, wood in rivers,) to integrated, catchment scale flood risk management has been demonstrated at,small,scales but,not larger,ones.,In this,study:,we use reduced complexity hydrological modelling to,analyze,the effects of land use and,channel changes,resulting from river,restoration,upon flood flows at the catchment scale,.,Results:,1,U,sing,engineered,logjams(,溪流倒木,) have,highly,variable impacts,on,catchment-scale flood peak,.,2,The,most promising restoration scenarios,for flood risk management are for riparian forest restoration at,the sub-catchment,scale, representing,20 40% of the total catchment area, where reductions in peak magnitude of up to 19% are,observed,through de-synchronization of the timings of sub-catchment flood waves.,Floodplain forest and,Engineered logjams,Managers,r,estore floodplain morphology and river,channel,to,modify the,flood hydrograph(,洪水,过程线,),Floodplain,forest:,a land cover restoration,Engineered Logjams (ELJs): a river channel restoration,Principle:,the,geomorphological,configuration of a drainage basin,determines the,time that it takes for,a,runoff,unit to travel,to a point where there is a flood risk.,Relative fine in smaller basin,but not clear in larger basin.,Require a spatially,explicit evaluation of where each intervention,is added,Floodplain forest and,Logjams,Floodplain,forest:,a land cover restoration,Floodplain forest efficacy:,interception, infiltration, slowing conveyance, attenuating,runoff,and,efficacy increases,as the forest,ages.,However, there,are few projects that have tried to,assess,how different extents and locations of floodplain,forest contribute,to flood,risk changes at different spatial scales.,Floodplain forest and,Logjams,Engineered logjams:,a river channel restoration,Engineered Logjams (ELJs) efficacy,:,I,ncrease,hydraulic roughness,increase water levels;,S,low,flow,velocities and thus increase flood wave travel time.,Increases,in water level may also,increase the,connectivity between a river and its,floodplain.,Efficacy varies, depending on their,type and spatial,frequency.,Floodplain forest and,Logjams,Fewer,attempts,to identify,catchment-scale,impacts,Reasons:,Effects that,are apparent at the local scale do not translate to the larger,F,urther downstream,impact,depends,upon the relative timing of the,contributions,from all,sub-catchments,Reducing,flood,risk in,one,sub-catchment may,actually,increase flood,at the,catchment-scale.,Catchment,scale flood,wave synchronization (,洪水波时间同步效应,),are not well,understood.,Numerical modelling,approaches:,OVERFLOW model,The specific aim of the work,is to evaluate the,impacts of,various restoration scenarios in multiple spatial,configurations impact,on downstream peak flood flows.,OVERFLOW,model,：,spatially distributed,unit hydrograph,models,(,单位线模型,),It explores general relationships between flood,hydrology,and the extent, location and magnitude of land-use,change at,the catchment,scale.,Data,:,98,km,2,Lymington,River catchment in southern UK.,A,rainfall and,subsequent,moderate flood event,of 3% exceedance 30 31,March 2006,was used for,calibration,.,Parameters,(average,channel values):,channel depth = 1.19 m,channel,width = 4.98 m,D,84,= 19,mm,Mannings,n,=,0.048 0.003,Numerical modelling,approaches:,OVERFLOW model,The specific aim of the work,is to evaluate the,impacts of,various restoration scenarios in multiple spatial,configurations impact,on downstream peak flood flows.,OVERFLOW,model,：,spatially distributed,unit hydrograph,models,(,单位线模型,),It explores general relationships between flood,hydrology,and the extent, location and magnitude of land-use,change at,the catchment,scale,Data,:,98,km,2,Lymington,River catchment in southern UK.,A,rainfall and,subsequent,moderate flood event,of 3% exceedance 30 31,March 2006,was used for,calibration,.,Parameters,(average,channel values):,channel depth = 1.19 m,channel,width = 4.98 m,D,84,= 19,mm,Mannings,n,=,0.048 0.003,Calibration results,N-S index 0.98,Scenarios,Use,reach,or,segment,scale,(,水系级别划分尺度,) to reflect restoration.,For each scenario,435 model runs,were conducted on scales,ranging from,the segment to the entire catchment,.,These,4,35 runs can,be classified,as,5,types,: (1) each,segment individually,(2) sets,of five sequential,segments,(3) each,individual,sub-catchment, (4) combinations,of sub-catchments up to and including the entire catchment, and (5),all,segments of a given,Shreve reach order,(,施里夫水系分级,) had restoration scenarios up to order 1-10.,Shreve reach order,Engineered logjams restoration,Floodplain forest restoration,Uncertainty analysis,The greatest source of uncertainty:,Mannings n,Analysis method:,four levels,(two,higher,and two,lower,values, 50%,model runs are either,located,in the bottom left quadrant (both magnitude and,time over,peak reduced) or upper right (both magnitude and time,of peak increased).,Results:,Peak,magnitude changes,and Shape of hydrograph,25 year forest growth,50 years,forest growth,100 years,forest,growth,forest scenarios lead to a broadening of the flood hydrograph,(i.e.,few shorter duration, higher peak),meaning: percentage,of the catchment altered,Scenario,5,6,7 (change forest and channel logjams),meaning:,change in time,over peak,Results:,Peak,magnitude changes,and Shape of,hydrograph,Scenario,5,6,7 (change forest and channel logjams),3 stages: 0 22%; 22 50%, 50%,model runs are either,located,in the bottom left quadrant (both magnitude and,time over,peak reduced) or upper right (both magnitude and time,of peak increased).,forest scenarios lead to a broadening of the flood hydrograph,(i.e.,few shorter duration, higher peak),Results:,Peak,magnitude changes,and Shape of,hydrograph,Results:,Spatial patterns of response,Results:,Spatial patterns of response,Hydrologically,distal reaches,(,末梢河段,/,低级水系,),large,reductions in peak discharge,Hydrologically proximal reaches(,近流域出口河段,/,高级水系,),increase in peak,discharge,Discussion,1.,Local attenuation,effects,(,河段,削峰效果,),General,spatial picture for individual reaches by,only applying ELJs,to individual reaches,The magnitude of change to peak,discharge with,ELJ insertion at the reach scale is proportional to,neither reach,length nor catchment area restored,S,uggesting,that,location,within the catchment and the characteristics of the,reach are,more important than the number or extent of dams,:,T,he signal,associated with geomorpological dispersion is dominant,.,For,forest,regeneration,a,decrease in peak magnitude,tend,to show a,larger decrease,with older modelled forest age,.,(,林龄越老，削峰效果越明显,),Discussion,2. De-synchronizing,sub-catchment flood,waves,（子流域洪水波时间同步效应分析,/,峰现时间分析）,The effect of extending or translating a contributing hydrograph from a reach or sub-catchment will,vary depending upon where,in the catchment network it is,located,.,This mechanism,explains,many of the model runs in which the,restoration,treatment was found to increase peak,discharge.,The,relationship between,time for flood wave arriving,at the catchment,outflow and discharge,For forest restoration applied on a,sub-catchment,basis,and for,22% to 50,%,of the channel,network,there are,large decreases,in peak discharge up to 19,%,.,Suggesting,sufficient extent to de-synchronize the sub-catchment runoff,contribution from the main catchment flood wave.,Note,:,when,catchment,area increases, it is likely that the,de-synchronizing effects will be more,complicate,d,because they will also influence the relative synchroneity of individual sub-catchments,.,Discussion,2. De-synchronizing,sub-catchment flood,waves,（子流域洪水波时间同步效应分析,/,峰现时间分析）,Discussion,3. Implications,for river and catchment restoration,(,1). The,most promising river restoration scenarios,for,mitigating flood risk:,applying,forest restoration,at the,sub-catchment scale,(22 47% of channel network restored) in distal,headwater locations,this,was found to result in a mean reduction,to peak,discharge of 10% after 25 years of forest growth and,no in,stream large wood management/removal,.,(2).,Conversely,ELJs,were,not found to be a predictable flood mitigation measure, particularly,at the 1 5 km reach based scale,Conclusions,Both floodplain forest and engineered logjams are,capable of attenuating local flood waves,however the,effect of this,local,attenuation at the catchment scale is highly,variable.,Engineered,logjams,alone,cannot be counted on to reduce flood risk at the,catchment scale.,Restoring floodplain forests and allowing these to naturally,increase,the quantity of large wood in the channel over time,represent,the most promising catchment restoration,scenarios.,Thank you !,
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