Comparison between 1D and 2D models to analyze the dam break

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Ignacio Valcárcel LencinaTRITA LWR Masters Thesisto get the maximum water level and discharge alongwith the time of travel at different locations of theriver downstream of the dam.Dam break hazard analysis (DBHA) provideinformation on the consequences of a possible dambreak for risk estimation and rescue planningpurposes. To determine the flow through a dambreach and to simulate flood propagation in thedownstream valley numerical models are used inDBHA.Any output of a dam break hazard analysis is related tothe accuracy and amount of input data. We cannotexpect to have accurate results if our data is poor. Therequired data is characterized as (ICOLD 1999):• reservoir data (including catchmenthydrology and river flood conditions)• structure data• topographic data• sediment and debris data.Reservoir data includes stage-area and stage-volumerelationships and the bathymetric data for thereservoir. Catchment hydrology data may for examplebe reservoir inflow and river flow data includingnatural flood observations. Structural data consists ofdata for dams and other structures. Topographic datais needed for the whole area which may be flooded bya dam break flood. Terrain model technology providesexcellent tools for handling of topography data.The results of DBHA should provide information foremergency action planning, land use planning andother purposes in a form that is appropriate to the enduser. A Geographical Information System (GIS)provides efficient tools to transfer the information indigital form and to analyse the results. By using GISflood inundation maps and other related informationcan be produced in different scales depending on thescale of the base map which is used in thepresentation. In a project, maps are produced forflood inundation (max inundation and the inundation0,5 h, 1 h, 2 h and 3 h after the initiation of thefailure), for flow velocity, for damage parameter (waterdepth times flow velocity) and for flow depth. Thescale used for printed maps is generally 1:20000 butthe scale 1:10000 is also used.Water level hydrographs and tables for certainlocations may be important for emergency actionplanning. From hydrographs the arrival time of floodwave and the speed of the rising of water level can beobtained.Hydrological analysisThe watershed model is a semi-distributed model withthe sub-basins. Each sub-basin has separateprecipitation, temperature and potential evaporation asinput. The model has a flood area-model whichsimulates water exchange between river and floodplains. The flood-area-model is simulated with shortertime steps than the main model. At every reach ofriver with embankments and weir, the model calculatesthe water level in the river and discharge through theweir over the embankment into the flood plain. Thispart of the model is important during the floodseason.This simple hydraulic model is verified against theresults from a complete hydraulic model in order tokeep up the accuracy of the simulation. The watershedmodel is based on a conceptual distributed runoffmodel, water balance model for lake, river routingmodel and flood area models.The input variables for the model are dailyprecipitation, temperature and potential evaporation.As input for a dam break simulation three floodsituations should be simulated. Floods with a returnperiod of 20, 100 and 10000 years create or determinewith the operational hydrological catchment model.The method used to determine a 10000 year flood isbased on precipitation with a return period of 10000years.Determination of the breach hydrographsDetermination of flow through a dam breach has lotof uncertainties. A numerical model for erosion of anembankment dam can be used for the determinationof the discharge hydrograph. The results of theerosion model are compared with those of othermethods. In the studies the breach is normallyassumed to happen at three locations with twohydrological conditions which are HQ 1/100 flood andmean flow (MQ).One-dimensional flow modellingA usual 1D model is based on four point implicitdifference scheme and uses the solution algorithmDAMBRK model.The 1D flow model for the DBHA normally covers abig area mainly downstream of the reservoir. We talkabout kilometres to cover the whole domain. Thecross-sections used in the model are taken from aterrain model. We talk about cross-sections, reachesand junctions in the 1D model. Some of the reachesare fictive channels (flood plains, connecting channelsetc).In the 1D model the breach is normally assumed tohappen at three locations with two hydrologicalconditions which are HQ 1/100 flood (with a returnperiod of 100) and mean flow (MQ). The breachhydrographs which are used in the simulations aredefined by using the breach erosion model. A constantroughness coefficient (Manning n = 0,060) could beused in all simulation cases.Two-dimensional flow modellingIn a 2D simulation a finite element model is normallyused. The finite element mesh has thousand ofelements depending on the computer capacity to dealwith it. The mesh size ranges from approximately 5 mto 50 m. The river beds, the bridges, the roads, and therailways have been highly refined to accurately modelflood propagation. In regular areas such as roads,26
Comparison between 1D and 2D models to analyze the dam break wave - Appendixreservoir dykes, and river beds, regular grid applicationis used. For other features such as railways,embankments, and constraint lines are imposed. Allthese features are quite visible on the mesh.Urban areas and floating debrisIt is necessary to develope methods to calculate floodwave propagation in urban area. Some models use thegeometry approach and houses are taken into accountin model geometry. Other models use the porosityapproach.In the first model, the geometry is represented by agrid consisting of rectangles (10x10 metres forinstance). Generally, the calculated water levels arehigher in the cases where buildings where taken in toaccount (maximum difference about 0,5 m). Thepropagation of a flood wave is generally a bit slowerwhen the buildings are taken into account. However,the effect on propagation speed is not very significant.The effect of buildings on damage parameter is notvery significant and can not be seen as very importantfor the planning of emergency actions. However, insome specific cases the effect can be important.Analyses of the resultsThe following analyses can be simulated during theproject, for instance:RUN 1Base flow (HQ 1/100 ) + breach hydrograph for maxreservoir level, roughness varies between Strickler 15(Manning's n = 0,06666) and 40 (Manning's n =0,025).RUN 2Base flow and breach hydrograph as in RUN 1,roughness is constant for the entire modelling areawith Strickler 15 (Manning's n = 0,06666).RUN 3MQ base flow + breach hydrograph, constantManning n varying in the entire modelling areaaccording to land formation and vegetation.RUN 4HQ 1/100 base flow + breach hydrograph, otherconditions as in RUN 3.RUN 5Conditions of RUN 3 modified according to thechoice of modelling buildings (porosity or geometry).RUN 6Conditions of RUN 4 modified according to thechoice of modelling buildings (porosity or geometry).The breach location is assumed to be the same for allthese analyses.The reason behind using different models to simulatethe same case is not to compare the computationalalgorithms. The reason is to get an idea how much theresults differ depending on the models.The modeller uses his own judgement for choosingthe friction factors for different areas. The water levelcomparison is done on different locations downstreamof the dam. The progression of the dam break flood isalso compared on maps.In the case of very complicated topography, the use ofa two-dimensional model seems to be morereasonable. The use of a 1-dimensional model needs alot of experience since the cross-sections have to beput at the right locations.Summary .To determine flow hydrographs through the dambreach opening is crucial to the results of a DBHA.There are a lot of uncertainties in the determination ofa breach hydrograph and sensitivity analyses have tobe used to ensure the results.A one or two-dimensional model can be used in thecalculations. The experience shows that with carefulmodelling and accurate data the results of differentmodelling approaches may be relatively close eachother. However, there are a lot of uncertainties in themodelling.Several methods can be used to model the flow inurban areas (porosity or geometry approach). Theconcrete results of a dam break hazard analysis foremergency action planning consist of inundationmaps, water depth maps, damage parameter (flowvelocity times water depth) maps as well as water leveland velocity hydrographs and tables. The results ofDBHA should be presented in such a way that theycan be used efficiently in the emergency/rescue actionplanning and risk analysis. The use of theGeographical Information System (GIS) is essentialfor that purpose.C) EMERGENCY ACTION PLANNINGIntroductionThe main purpose of the RESCDAM project was todevelop emergency action planning in the event of adam failure. The topics of the emergency actionplanning are approached from the point of view ofrescue authorities.Dam safety is an issue for a few people only, such asdam owners or dam holders, the rescue andenvironmental authorities. Only a few of these peoplehave been faced with a serious dam failure or damemergency situation. In case of an extensive accident,situation management is almost impossible withoutadvance planning. Lack of experience is also aproblem when considering the emergency action planof a dam. It is difficult to prepare an emergency actionplan for an event that can only be imagined.Organizing the warning and evacuation of thepopulation and rescue operations in a dam failuresituation.Dam failure is an extremely demanding accidentsituation to everyone involved. A strong water flow,varying water depth, floating debris, damaged road27
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Comparison between 1D and 2D models to analyze the dam break

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