Muncaster Et Al 2D Hydraulic Modelling

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30th Hydrology and Water Resources Symposium 4 - 7 December 2006 Launceston, TAS Design flood estimation in small catchments using twodimensional hydraulic modelling –A case study Steve Muncaster Associate Water Technology 15 Business Park Drive, Notting Hill VIC 3168 Email: Warwick Bishop, Associate Water Technology 15 Business Park Drive, Notting Hill VIC 3168, Email: Andrew McCowan Director Water Technology 15 Business Park Drive, No
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  30  th  Hydrology and Water Resources Symposium 4 - 7 December 2006 Launceston, TAS    Design flood estimation in small catchments using two-dimensional hydraulic modelling –A case study   Steve MuncasterAssociate Water Technology 15 Business Park Drive, Notting Hill VIC Warwick Bishop,Associate Water Technology 15 Business Park Drive, Notting Hill VIC 3168, Andrew McCowanDirector Water Technology 15 Business Park Drive, Notting Hill VIC 3168,  Abstract: Design flood estimates are often required for small catchments as part ofdevelopment planning and/or infrastructure design. Traditionally, the probabilistic Rational Method hasbeen the principal approach to estimate design peak flow with simple hydraulic calculations employedto size culverts and bridge waterways. The application of the Rational Method, while well ingrained inengineering practice, relies on significant simplifications of the catchment runoff process. Thesesimplifications can lead to uncertainty surrounding design flood estimates. More sophisticatedanalysis is possible through the use of runoff routing models such as RORB, Rafts or Urbs. Suchmodels allow greater detail to be incorporated into the analysis at a subcatchment level, althoughsome simplifications of the runoff process are still necessary. Recent developments in two-dimensional hydraulic models enable the direct application of rainfall excess onto the computationalgrid. Increasingly high resolution topographic data is becoming available, often associated withproposed development of a particular area. These two developments facilitate the application of two-dimensional hydraulic models as a runoff routing model in small catchments. This paper discusses thekey aspects of this application including appropriate topographic data sources, computational gridresolution and effective roughness values. Results from a preliminary application to small ruralcatchments in the Geelong region are presented and used to illustrate key aspects. In particular, theselection of appropriate hydraulic roughness is critical. Better representation of catchment storage isprovided through the use of topographic data in the computational grid. The use of a two-dimensionalhydraulic model integrates the hydrologic and hydraulic aspects into a single model. Furtherinvestigation is required to assess the role of hydraulic roughness in determining surface runoff rates. Keywords: runoff routing, hydraulic roughness, catchment storage, flood estimation   1 INTRODUCTION The flood behaviour analysis of smallcatchments is commonly undertaken as part ofinfrastructure and development conceptualdesign. This flood analysis typically consists oftwo components: hydrologic analysis(determination of peak flows and floodhydrographs) and a hydraulic analysis(determination of flood depths, extents andconceptual design of hydraulic structures).Historically, the common approach employedthe Rational Method and simple culvert routinesas the two principal analysis tools. A morerefined approach may employ a runoff routingmodel (e.g. RORB (Laurenson and Mein1997)).Key catchment characteristics for determiningflood response are runoff production (i.e.catchment losses) and the available catchmentstorage. The Rational Method lumps the losscharacteristic into the runoff co-efficient and thestorage catchment is reflected in the time ofconcentration. Runoff routing models (RORB,(Laurenson and Mein 1997), WBNM (Boyd et al1994), & URBS (Carroll 2003), etc) split thesetwo characteristics with the runoff productiondealt with by the loss model. The catchmentstorage is reflected by the use of a powerrelationship between catchment storage andoutlet. The familiar S = KQ m is found in variousforms in RORB, WBNM and URBS.More rigorous analysis of catchment response,particularly catchment storage, have been inthe past limited by the lack of available datarequired to construct such models.  30  th  Hydrology and Water Resources Symposium 4 - 7 December 2006 Launceston, TAS    Assessment of the variation of availablecatchment storage within small catchments canbe aided by available topographic data. Two-dimensional hydraulic models can utiliseavailable topographic data in the analysis ofavailable catchment storage and its role in floodresponse.Recent developments in the cost effectivecollection of topographic data, such as AirborneLaser Scanning (ALS), has lead to increasedavailability of detailed topographic informationand the potential use of two-dimensionalhydraulic analysis. Further, two-dimensionalhydraulic models have evolved significantlyover recent years (McCowan et al 2002).Generally the use of two-dimensional (2D)hydraulic models has been for thedetermination of flood extents and levels overlarge floodplains. With this approach, ahydrologic model/analysis has provided a floodhydrograph as a boundary condition to thehydraulic model. However, many 2D hydraulicmodels are now capable of modelling the effectof net rainfall on the computational grid.This development has enabled the integrationof the hydrologic and hydraulic components offlood behaviour analysis for small catchments.This paper discusses the critical aspects of theapplication of 2D hydraulic models as a tool forrunoff routing in small catchments. Particularattention is paid to the following: o Analysis of catchment storage o Role of hydraulic roughness (Manning’s “n”)in determining catchment response o Conservation of mass within twodimensional models when applied with directnet rainfall.A case study for several small catchments tothe south of Geelong is provided to illustratethis approach.This paper aims to provide an initial insight tothe application of 2D hydraulic models to runoffrouting and is not intended as a rigorous reviewof this application. 2 APPLICATION OF HYDRAULICMODELLING TO RUNOFF ROUTING ATCATCHMENT SCALE A number of hydraulic based models have beendeveloped for routing of surface runoff at thecatchment scale. These models usetopographic information at various resolutions,and hydraulic solution techniques as the basisto evaluate the connectivity and conveyance ofoverland flow paths. These models include Liuet al (2003), Jain and Singh (2005), and Fortinet al. (2001). A review of similar applications isprovided in Singh and Woolhiser (2002).Typically these models have employed thekinematic wave and/or diffusive wave approachto the solution of the Saint Venant equations.These approaches overcome the computationaldemands of the full dynamic wave hydraulicmodel (Singh 1996). The kinematic waveand/or diffusive wave approach typically uses alower spatial resolution (larger computationalgrid size). This lower spatial resolutionfacilitates the use of more coarse topographicdata as the basis for the computational grid.Further these models typically simulate therunoff generation component as well as therunoff routing component of surface flows.Fully-dynamic wave 2D hydraulic models havegenerally been employed for floodplainhydraulic analysis. Recent developments haveseen dynamic wave hydraulic models, as suchMIKEFlood (DHI 2005) and TUFLOW (WBM2006), able to have direct net rainfall on thecomputational grid. The fully-dynamic wavemodels are generally employed with a highspatial resolution (smaller computational gridsize) than the diffusive wave models discussedabove. This high spatial resolution allows forsmall features which influence catchmentstorage characteristics to be adequatelyschematised and resolved (Horritt and Bates2001). The increased availability of highresolution topographic data has the potential toaid the application of fully-dynamic wavemodels to runoff routing.Various formulations of hydraulic roughnesshave been employed in the 2D hydraulicmodels. Determination of hydraulic roughnessparameters is well developed for floodplainapplications of 2D hydraulic models. For thesefloodplain applications, hydraulic roughnessvalues are ideally determined through modelcalibration against observed flood levels andextents. However, in the absence of observedflood data, many references (e.g. Chow 1959)provide guidance.In typical floodplain applications of 2D hydraulicmodels, flow depths are generally in order oftens of centimetres to several metres. Whenapplied for the purposes of runoff routing, flowdepths away from the main waterways aresmall (< 0.1 m). At such shallow flow depths,the effective roughness may increase due tothe effect and type of ground cover. As a result,the roughness value applied to the same typeof ground cover may vary as the flow depth  30  th  Hydrology and Water Resources Symposium 4 - 7 December 2006 Launceston, TAS    increases. Further investigation of appropriateroughness values away the watercourses isrequired to refine the application of dynamicwave 2D hydraulic models to runoff routing. 3 CASE STUDY – ARMSTRONG CREEK,GEELONG3.1 Study area The case study focuses on 6 small catchmentslocated to the south west of Geelong. The studyarea has been identified as a possible futureurban growth area for Geelong. Figure 1(provided at the end of the paper) shows thegeneral location of the study area andcatchment delineation. The naming conventionfor the catchments was taken as the City ofGreater Geelong specification. The catchmentareas are: C253&C254 – 442 ha, C255C – 324ha , C257 – 2833 ha, C267A - 74 ha, C267C – 45 ha and C267D -32 ha.The study area consists mainly of rural land.The associated drainage is via a number ofdrainage depressions and larger watercourses.Generally the watercourses in the study areaare not well defined with extensive shallowflooding occurring frequently.The seven catchments can be divided into thefollowing two groups o Enclosed catchments – catchmentsentirely contained within the study area. o External inflow catchments – Catchments not fully enclosed in thestudy area and have catchment inflowsfrom outside the study area. 3.2 General model application The 2D hydraulic modelling package employedfor this study was MIKE Flood. The MIKE Floodpackage is a state of the art tool for floodplainmodelling that has been formed by the dynamiccoupling of Danish Hydraulics Institute’s wellproven MIKE 11 river modelling and MIKE 21fully two-dimensional modelling systems. (DHI2005)Two approaches have been used for theapplication of MIKE Flood. These twoapproaches were developed to reflect the twocatchment types as follows: o Enclosed catchments:- MIKE Floodwas applied as an integrated hydrologicand hydraulic model to the entirecatchment. Direct net rainfall wasapplied to the computational grid. o External inflow catchments:- MIKEFlood model was applied as per theenclosed catchments for the portion ofthe catchment within the study area.Runoff contributions from thecatchment external to the study areawere then determined by a RORBmodel.The MIKE Flood models employed a 5 mcomputational grid for all the catchments in thiscase study.Both the MIKE Flood and RORB models wererun for a range of design storm durations inorder to assess the critical durations. 3.3 Available input model data There is no streamflow data available for thecatchments in the study area. As such, formalcalibration of the hydrologic and hydraulicanalyses has not been possible.A photogrammetric survey covered the entirestudy area consisting of a regular grid of spotelevation, contours and breaklines to definelinear features. Further, the details of culverts,bridges and retarding basins were incorporatedinto the model. 3.4 Design rainfall and losses Design rainfall depths and temporal patternswere obtained from Australian Rainfall andRunoff (IEAust 1999). A uniform design rainfallspatial pattern was applied. To determine net100 year design rainfalls for input into the MIKEFlood, an initial loss of 10 mm and continuingloss of 2 mm/hour were adopted for the ruralareas. 3.5 External catchment design inflowhydrographs For the external inflow catchments, the RORBmodels were developed for the entirecatchments. The RORB parameter kc wasdetermined using a regional prediction formula(Pearse et al 2002). The RORB modelsincorporated the existing retarding basins.The design losses, for the RORB models, wereadopted as an initial loss of 10 mm andcontinuing loss of 2 mm/hour. In urban areas, afraction impervious of 0.45 was adopted.  30  th  Hydrology and Water Resources Symposium 4 - 7 December 2006 Launceston, TAS    3.6 Hydraulic roughness As discussed, the estimation of hydraulicroughness is complicated by the shallow flowdepths occurring in catchments away from theformal watercourses. To assess the impact ofroughness on computed peak flow at thecatchment outlet, the following two hydraulicroughness scenarios were tested:1. Uniform roughness across the entirecatchment (Manning’s n = 0.04)2. Varied roughness with higher roughness inareas away from the watercourses.(Manning’s n = 0.04 for watercourses andn = 0.1 for remainder of catchment) 4 RESULTS4.1 100 year peak flow For the enclosed catchments, the sub-catchment peak flows obtained from the MIKEFlood models were compared to peak flowsobtained from the Rational Method. TheRational Method as outlined by VicRoads(1999) was applied with a 10 year runoff co-efficient (C10) of 0.1. The 10 year runoff co-efficient was obtained from Australian Rainfalland Runoff (IEAust 1999). Table 1 Design 100 year peak flowestimates: Enclosed catchments Catchment RationalMethodpeak flow(m 3  /s)MIKEFlood peak flow (m 3  /s)UniformroughnessVariedroughness(floodplain 0.1)C255 C 8.619.6 13.3C267 A 2.94.4 2.6C267 C 2.04.6 3.4C267 D 1.64.8 3.8 For the external inflow catchments, Table 2shows the MIKEFlood 100 year peak flowscompared to the RORB model peaks flows. Table 2 Design 100 year peak flowestimates: External inflow catchments Catchment RORBmodel(m 3  /s)MIKEFlood peak flow (m 3  /s)UniformroughnessVaried roughness(floodplain 0.1)C253&C25412.714.8 13.6 C257 4466 48 4.2 Flood hydrograph shape As RORB models were developed for the twoexternal inflow catchments, design 100 yearflood hydrographs were available forcomparison to the MIKEFlood hydrographs.Figures 2 and 3 shows the RORB model andMIKEFlood 100 year design flood hydrographsfor the C253&C254, and C257 catchments forthe uniform roughness scenario. 02468101214160:00 1:00 2:00 3:00 4:00 5:00 6:00 7:00 8:00 9:00 10:00 11:00 12:00 Time (h)    F   l  o  w   (  m   3   /  s   )   RORB modelMIKEFlood   Figure 2 C253&C254 100 year design floodhydrographs 0102030405060700:00 2:00 4:00 6:00 8:00 10:00 12:00 14:00 16:00 18:00 20:00 22:00 0:00 Time (h)    F   l  o  w   (  m   3   /  s   ) RORBMike Flood   Figure 3 C257 100 year design floodhydrographs   4.3 Mass conservation To ensure the MIKEFlood model wasadequately conserving mass, a comparison ofthe net rainfall volume to the computed runoffvolume was made with the results shown inTable 3. Table 3 Mass conservation comparison   Catchment Mass error (% of total inflow)C255 C 2.7C267 A 1.8C267 C 1.0C267 D 1.0
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