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Flood damage analysis using GIS

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Flood damage analysis using GIS

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									                  Flood damage analysis using GIS
                      at Gold Coast City Council
Introduction                                                                                      that the survey sample is fairly repre-
The Gold Coast City has extensive flood-                                                          sentative of the whole.
                                                       by Haydn Betts FIE Australia,
plains, most notably the Nerang River                                                               The database is geocoded as points
                                                       Coordinator Flood Strategies,
floodplain, which is 90 sq km in area and                                                         using property centroids.
contains nearly 60, 000 dwellings, 40% of                 Gold Coast City Council                   The database has two purposes:
which are flood prone.                            This paper is a modified version of a paper     • to establish the relationship between
   Many of the residential areas were filled           presented to the NSW Floodplain              estimated property level (from topo-
to the 1:100 year flood level—however,              Management Authorities Conference at            graphic DEM) and floor, garage and
once this level is exceeded many thou-            Wentworth May 2001 co-authored by Don             road levels as surveyed
sands of dwellings become affected by            Carroll and Haydn Betts titled Integrating GIS   • to update the flood damage database
above floor flooding. Therefore, it is                    and Flood Damage Analysis.                with surveyed floor, garage and road
critical to Council that no increases in                                                            levels, replacing those levels estimated
flood levels occur through new floodplain                                                           by statistical relationships.
development.                                                                                        The database is updated periodically
   A GIS based Flood Damage model was                                                             through field survey.
developed to assist the Council with           • stage damage relationships for differing
                                                 dwelling types.                                  Topographic DEM
assessment of the economic benefits of
                                                 These components are combined to                 This consists of a grid of the X,Y and Z
proposed flood mitigation options and
                                               form a property flood damage database.             values for the area under investigation.
secondly to assess proposed floodplain
                                               Once formulated, standard SQL tech-                The X and Y values represent the coordi-
developments. This paper describes the
                                               niques using MapInfo’s MapBasic are                nates of a grid point on plan, whereas the
development of the model and how it is
                                               used to perform statistical analysis by            Z value represents ground level at that
used within a GIS environment.
                                               suburb or by electoral division. Each              point. Grid location points are equally
   A prerequisite of the model is that it
                                               component is discussed, in turn, below.            spaced in the X (east) and Y(north)
could be run quickly and the results could
                                                                                                  directions.
be integrated with Council’s GIS based         Property database                                    For the Gold Coast area fine resolution
data. Therefore it was also decided from       A series of 14 property databases were             grids are used—typically a 5m grid is
the outset that MapInfo/Vertical Mapper        created using information from the                 preferred as coarser grids resulted in
would be used as the model platform. This      Digital Cadastral Data Base (DCDB) and             interpolation errors for properties close
facilitated direct access to Council’s         Citipac (Council’s property rating data-           to canal estates where sudden changes in
property database and visual interpre-         base). Each database represents a catch-           land profile sometimes caused misleading
tation of results.                             ment. Each property entry in the database          results. The grid information is also used
   The overall philosophical approach was      is geocoded as using the property cen-             to establish a relationship between esti-
to estimate flood damage for each              troid. (Properties over 1500 square                mated property levels (Z value at property
property and then accumulate property          metres are excluded from the analysis as           centroid) and surveyed floor levels.
flood damage over a prescribed region.         the centroid of the property may not pass
This approach requires that for each           through the dwelling).                             Flood surface DEMs
property, the floor and garage levels for         The location of this centroid is used to        These are grid surfaces generated by
each dwelling on the property are known.       ‘inspect’ flood and topographic surfaces           hydraulic models, for example MIKE21
   The dwelling type is also required to be    described later in this section. The               (DHI), MIKE11 (DHI) or USF (Qld State
determined i.e. whether it is detached/        Citipac database fields contain infor-             Govt). The flood surface grid is used to
semi-detached, a town house, multiplex         mation on fire, town planning and land             determine the flood level at each property.
unit etc.                                      use codes. These codes, when combined              This is done by point ‘inspection’ of the
   The dwelling type is then used to assign    with lot plan numbers are used to deter-           flood surface using the property centroid.
flood damage costs based on the esti-          mine the number of ground floor dwel-              Mike11 and USF are 1 dimensional
mated depth of inundation above floor          lings/units per property and whether the           models.
level.                                         dwellings are part of a single or multi               The generation of flood surface grids
                                               storey complex.                                    using the results of these models is
Model inputs                                                                                      described by Khan & Betts (2001) in a
The model requires five basic input            Survey database                                    paper presented at the NSW conference.
components. These are:                         This is a MapInfo database containing              This method takes the peak flood level at
• a property database                          fields detailing floor, garage and road            the end and ‘mid points’ of each hydraulic
• a floor level survey database                levels for each surveyed dwelling. It also         model cross-section, triangulates them,
• a topographic Digital Elevation Model        contains descriptions of the dwelling such         deletes spurious triangles, and then
  (DEM)                                        as age and building material type. These           develops the rectilinear grid using Vertical
• flood surface DEMs                           descriptions are for explicit checking             Mapper (a MapInfo ‘plug-in’).

Autumn 2002                                                                                                                                      33
     Residential stage damage
     relationships
     The residential stage damage relation-
     ships were extracted from a Water Studies
     report titled ‘Assessment of Flood Damage
     across the Nerang River Floodplain’ dated
     1997 and the March 2001 extension of that
     study. These reports contain tables of
     internal, external, structural damages
     related to flooding depth. Clean-up and
     financial costs are also included. The latest
     unit costs include inflation rates and an
                                                     Figure 1: MapInfo flood damage model menu.
     allowance for the impacts of GST.
     Model process                                   for each dwelling within each property.            suburb or division, e.g. number of
     There are 4 basic steps to undertake flood      Where dwelling survey data are available,          dwellings flooded above property, garage
     damage analysis. These are:                     the estimated levels are replaced with             and floor level, the damage cost per
     • prepare the property database                 surveyed levels. Where a dwelling is found         suburb/division etc.
     • calculate the property damage on a            affected by flooding, the damage is also
                                                                                                        View results
        property by property basis                   calculated.
                                                                                                        The purpose of this step is to view the
     • analyse the data by suburb or by division        Once the property, garage and floor
                                                                                                        results prepared in the previous step.
     • view the results.                             levels have been established, the damage
                                                                                                        Various degrees of flooding can be viewed
        These steps are incorporated into the        statistics can be calculated for each
                                                                                                        through selecting the depth of flooding -
     flood damage MapInfo menu as shown              dwelling. These include the number of
                                                                                                        as shown in Figure 3.
     in Figure 1.                                    dwellings per property; whether the
                                                                                                          An example of the results is shown in
        Broad outlines of each step are shown        dwelling(s) are flooded above property,
                                                                                                        Figure 4. Stars indicate potentially affec-
     in Figure 2 (opposite) and a brief discus-      garage and floor levels; and if so, the
                                                                                                        ted properties.
     sion on each step follows.                      calculated dollar flood damage for that
                                                     property.
     Prepare the property database                                                                      Model limitations
                                                        Typically damage estimation for a
     The purpose of this step is to create the                                                          In its current form the model has several
                                                     60,000 property sized database using 7
     property damage database by defining the                                                           limitations. These are:
                                                     floods takes approximately 30 minutes to
     area or catchment to be investigated and                                                           • Unless a property is surveyed, it cannot
                                                     complete using a Pentium 600 machine.
     adding topographic and flood surface                                                                 be said with any confidence that the
     levels to each selected property in the         Analyse results                                      property will be actually flooded for a
     database. The property polygon centroid         The purpose of this step is to undertake             given event. This is because the dwelling
     is used to determine these levels. The          statistical flood damage analysis by suburb          floor level is derived from a statistical
     property database is a subset of the            or by Council division (electoral boun-              relationship between the estimated
     catchment-wide property database.               daries). The analysis involves estimating            topographic level for the property
                                                     the number of properties affected in each            centroid and surveyed floor level. This
     Calculate property damage                       suburb or division and calculating the               relationship, while providing reliable
     The purpose of this step is to estimate         total dollar cost by suburb or by division.          average results, is not correct on a
     the number of ground floor dwellings per          The results for each flood surface are             property by property basis.
     property, and calculate floor and garage        stored in a MapInfo table. Each table              • This model does not calculate commer-
     levels using pre-determined relationships       contains damage statistics for each                  cial or industrial damage. However,




                                                                                   Figure 4: Typical results from flood damage model—note also the flood
     Figure 3: Menu to view results.                                               surface grid.


34                                                                                                Australian Journal of Emergency Management
     Step 1: create property database

  Access MapInfo flood damage                              Select property database                                     Append topo and flood surface
  application                                                                                                           levels for each property


    Citywide Base Cadastre/Citipac                                                                                                                         topo &
                                                                          Property details                                    Property details
          Property Database                                                                                                                              flood info



                                                           by catchment/userdefined area/area                           use M21 results and Vertical Mapper
                                                           specified by Mike21


     Step 2: calculate property damage

  Calculate garage and flood levels                Update with survey info                                Calculate damage stastics

                                                                           update with                                          estimated & damage statistics:
                        estimated                                                                                                            • no. dwellings per property
   Property topo &       garage &                      Property   topo &     surveyed                       Property   topo &     surveyed • % damage per property
    details flood info floor levels                     details flood info garage &                          details flood info   garage & • garage flooded?
                                                                           floor levels                                         floor levels • floor flooded?
                                                                                                                                                           • etc.


  use statistical relationship between             access survey database                                 access survey database
  estimated property levels and floor/
  garage level


     Step 3: flood damage analysis

                                                                                                                   Flood 1
                                                                                                                   Suburb 1                No. of properties
                                           damage statistics:
                                           • no. dwellings per property            analysis                        Suburb 2                Flooded above property, garage, floor ...
   Property topo &       garage &          • % damage per property                by division                      Suburb 3                damage per suburb
    details flood info floor levels        • garage flooded?                      or suburb
                                           • floor flooded?
                                           • etc.                                                                  Flood 2
                                                                                                                   Suburb 1                No. of properties
                                                                                                                   Suburb 2                Flooded above property, garage, floor ...
                                                                                                                   Suburb 3                damage per suburb

                                                                                                                   Flood 3
                                                                                                                   Suburb 1                No. of properties
                                                                                                                   Suburb 2                Flooded above property, garage, floor ...
                                                                                                                   Suburb 3                damage per suburb


                                                                                                                   Summary file (by flood)
                                                                                                                   Flood 1 cost        no. dwellings
                                                                                                                   Flood 2 cost        no. dwellings
                                                                                                                   Flood 3 cost        no. dwellings



     Step 4: view results



                                                                                             suburb 2
                                                                                                                                     flood affected
                                                                                                                                       properties
                                    table of results

                                                                                 suburb 1
                                                                                                        suburb 3




Figure 2: Formation of the flood damage property database.


Autumn 2002                                                                                                                                                                            35
       these can easily be incorporated using
       dollar damage–square metre relation-
       ships predominantly used for commer-
       cial and industrial properties.
     • The model does not take explicit
       account of the duration of flooding that
       a dwelling may experience. This is
       currently an area of research and has
       not been widely applied in practical
       applications throughout Australia.
     • Finally, the stage-damage parameters,
       and parameters that define the relation-
       ship between topographic DEM and
       floor level are ‘hardwired’ in the model.
       However, changes are being made to
       allow the user to define these parameter
       values. This will be done by creation of
       a special initialisation file that the
       model can read and assign parameter
       values accordingly.

     Model testing/calibration
                                                    Figure 5: Currumbin Creek —residential property damage
     The results of the model were tested against
     those results prepared by the Water Studies
     1997 report ‘Flood Damage across the           Creeks in the southern part of the City,              that an amount of 270 mm has been
     Nerang Floodplain’ (Water Studies, 1997).      conducted under the auspices of the                   added to the storm surge levels as an
       The Water Studies approach used              Natural Disasters Risk Management                     allowance for future sea level rise under
     distributions of flooding depth derived        Studies Program. Both creeks are subject              ‘Greenhouse’ climate change conditions.
     from surveyed properties, and applied this     to the coincident events of storm surge               Note that the damage is for residential
     distribution on a suburb by suburb basis       and flooding and in this study, it was              property only. The area below Line C and
     to all flood affected dwellings.               found that storm surge is a significant             to the left of the 1:100 year flood proba-
       The flood damage results of both models      problem. A series of rainfall and coinci-           bility represents the likely future risk to
     were found to be within 20% of each other,     dent storm surge events were routed                 existing development, and provides an
     after allowance was made for use of            through hydrologic and hydraulic models             inundation envelope below which future
     differing surveyed dwelling data sets.         and the flood damage model.                         development should not be permitted.
                                                      It was postulated that:                             Further analysis is required to examine
     Flood damage model use                         • For any given tidal surge, there is a flood       the likely impacts of higher rainfall
     The flood damage model has now been              of sufficient magnitude to force the              intensities and storm surge impacts when
     used several times in different ways.            saltwater prism from an estuary. A                global warning scenarios are further
                                                      larger surge requires a larger flood. The         advanced. (The same conditions are being
     Development testing
                                                      form of a plot of the envelope of flood           recommended for Tallebudgera Creek).
     Testing the relative impacts of develop-
                                                      and surge probabilities will be different
     ment by comparing the flood damage
                                                      for each riverine system depending on             Conclusion and future directions
     statistics pre- and post-development. This
                                                      the shape and hydraulic capacities of             A flood damage model has been deve-
     not only provides the number of proper-
                                                      the waterways and floodplains, as well            loped that integrates the Gold Coast City
     ties/dwellings affected by a particular
                                                      as the relative locations of the flood-           Council’s property and rating databases,
     development but the financial impacts
                                                      plains.                                           flood model results (via grids) and
     and individual identification of proper-
                                                    • In fully developed floodplains, flood             topographic DEMs. The model, while
     ties that would be disadvantage by the
                                                      damage is a coarse indicator that could           sophisticated in its database access,
     development.
                                                      suggest the interdependence of storm              management and analysis, is very simple
     Flood mitigation works                           surge and flood magnitude and the                 at the elemental level i.e. the calculation
     Flood mitigation projects that would             shape of the envelope.                            of flood damage for each property.
     reduce the impacts of flooding can be            Plots of flood damage vs. inundation                The model has been successful because
     similarly assessed. The results provide the    probability are shown in Figure 5 (a                the flooding impact of development pro-
     financial benefits of mitigation projects      reprint of Figure 2 in the NSRMSP report            posals and flood mitigation options can
     that can be used in benefit-cost studies.      – GCCC 2001). A series of dotted lines (A,          now be routinely assessed in dollar terms.
     Such projects currently being examined         B & C) have also been drawn on the graph.             Furthermore model outputs can be
     include dredging, bridge replacement or        • Line B is a line drawn between the 1:100          viewed visually, so that risk assessors can
     modification, weir modification, and             year surge/1:20 year flood and 1:100 year         assess where key damage areas are. The
     dam raising.                                     flood/1:50 year surge points. (This line          results can be also analysed by electoral
                                                      most closely represents the likely                division so that elected representatives
     Floodplain planning                              coincidence of flooding and storm                 can get a better sense of flooding prob-
     Council has recently completed a study           surge as now perceived).                          lems within their division.
     of the Tallebudgera and Currumbin              • Line C is a repetition of Line B except             The model when used in combination

36                                                                                               Australian Journal of Emergency Management
with other GIS/Hydrological model
routines provides Council the ability to
assess the city-wide flood damage impact          Disaster Management for Public Health
due to proposed changes in land use/town
                                                  18-23 August, 2000, Sheridan Plaza Resort, Cairns Queensland (5 day course)
planning zones.
   It is anticipated that as more dwelling         The course aims to provide information and skills required to assist in managing the
floor levels become surveyed, the model                                  public health implications of disasters.
will be used in real time to assist Emer-
                                               Topics: Potential Public Health Disasters,     Management, Vector Control, Psychological
gency Services personnel prioritise
                                               Planning and Preparedness Phase, Public        Aspects of Disasters
evacuation strategies in the event of the      Health Disaster Plans, Roles and Respon-
major flood.                                   sibilities, Response and Recovery Phase,       Would benefit: Environmental Health
   At some time in the future it would be      Case studies, Bio-terrorism, Environmental     Officers, Indigenous Environmental Health
                                                                                              Workers, Elected members of Local Govern-
desirable for the model to be extended to      Contamination, Communicating with the
                                               Public, Media Communication, Provision of      ment, Public Health Nurses and other health
include commercial properties but that                                                        professionals, Local Government Disaster
will mean the development of appropriate       Safe and Adequate Water, Safe Food
                                               Supplies, Community Vulnerability Waste        Officials.
stage-damage curves.
                                                     Contact: Paul Endres, Senior Environmental Health Officer, Queensland Health,
References                                                  Tropical Public Health Unit, PO Box 1103, Cairns Qld Aust 4870,
Carroll D. & Betts H. 2001, ‘Integrating GIS         phone: 07 4050 3616; fax: 07 4031 1440, email: Paul_Endres@health.qld.gov.au
and Flood Damage Analysis’, NSW Annual
Floodplain Management Authorities
Conference Papers, Wentworth, 8–11th May,
2001.
  Gold Coast City Council 2001, Tallebudgera
and Currumbin Creeks Flood Risk Manage-
ment Study— Risk Assessment Report, Gold
Coast City Council, September, 2001.
  Khan S. & Betts H, 2001,‘Real Time Flood
level forecasting for the Nerang River
System using integrated Hydrology, Hyd-
raulic and GIS Models’, NSW Annual
Floodplain Management Authorities
Conference Papers, Wentworth, 8–11th May,
2001.
About the Author
Haydn Betts heads the Gold Coast City
Council’s Flood Strategies Section within
the Strategic and Environmental Planning
Branch, where he acts as Council’s
hydraulics expert with respect to hy-
drology, hydraulic modelling, flooding
implications for town planning and flood
mitigation projects.
  In the last 6 years, Haydn has been
involved in several contentious develop-
ment applications and provided advice
in a number of Planning and Environment
Court Appeals.
  Prior to moving to Gold Coast in
January 1996, he was employed by Laun-
ceston City Council on a number of flood
mitigation projects, was the City’s Flood
Warning Officer and was Executive Offi-
cer for the Tamar River Project Improve-
ment Committee. Haydn has also worked
for State Government and in private
industry as a consultant.
  Haydn is a Fellow of the Institution of
Engineers, Australia and is currently
undertaking part-time study towards a
PhD in floodplain management.

   This article has been refereed

Autumn 2002                                                                                                                                 37

								
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