Development of a Digital Terrain Model
Creation of Flood Surfaces for a Series of Flood Events
Creation of a Street Database
Calculation of Risk Ratings

Development of a Digital Terrain Model

Depending on the input data available, the DTMs in FloodAUS are produced using Vertical Mapper® (an add-on program for MapInfo) and/or ANUDEM® (a product from the Centre for Resource and Environmental Studies at the Australian National University, ANU). Terrain input data are generally procured from State Government Departments or Local Government Authorities.

The type of terrain input data varies from study to study. The most common formats are xyz 5m grid data and 2m digital relief contours; 0.5m and 1m contours were available for some studies, but only 5m contours or 25m grid data for others.

Typical digital terrain model

Creation of Flood Surfaces for a Series of Flood Events

Flood surfaces are created in FloodAUS to approximate the actual height and shape of the water surface during a flood. Ideally FloodAUS relies on several flood surfaces, the most appropriate being the ARI 20-year, 50-year, 100-year and Probable Maximum Flood. Flood surfaces are also created using ANUDEM® and MapInfo Vertical Mapper®. Flood surface input data are derived from the most recently available flood studies - usually hydraulic modelling studies performed by consulting engineers.

Of the 24 FloodAUS studies completed to date, 15 include flood surface data to the Probable Maximum Flood or Extreme Flood Event level. For 6 study areas the most severe flood for which data were available is the ARI 100-year event.

Flood surface input data are available in a myriad of formats ranging from surface contours, inundation maps and river profiles. In this respect, virtually every study is different.

Levees have an important impact on flood risk. The influence of ring levees on flood risk has been taken into account in 7 studies. Similarly, flood risk is reduced for addresses protected by levees tying into high ground in a further 10 studies.

FloodAUS flood surface for ARI 50-year flood

Creation of a Street Database

MapInfo®, Streetworks® and MapMarker® are used to create the street address database. Streetworks® contains a database of street segments based on street centre lines, intersections and street numbers at the end of each segment. An in-house linear interpolation program converts the segment data to a list of addresses within each study area. The assumptions made by this program are:

• The Streetworks® database is an accurate listing of segment data
• Addresses are equally spaced along the segment
• Numbering is consistent with addresses at each end of the segment
  • Even at both ends - interpolates only even addresses
  • Odd at both ends - interpolates only odd addresses
  • Mixed even and odd - addresses consecutively numbered
• Addresses are only whole numbers; 15A, for example, does not exist
• Houses are offset 20m from the road centre line

As we do not know whether the parcels of land included in the list are occupied by a house, a commercial/industrial property or are vacant land, they are best referred to as street addresses.

Each street address is spatially located using MapMarker®. Comments are added to the database to identify potential problems such as addresses that are represented by street centroids only and those that could not be geo-coded at all. Generally, over 95% of addresses in each study are successfully geo-coded.

Calculation of Risk Ratings

The FloodAUS Risk Rating is expressed as either an Annual Exceedance Probability [AEP] or an Average Recurrence Interval [ARI] for flood waters reaching a height equal to ground level at the street address, ground level +1m and ground level + 2m. These three options are produced so that the effects of various floor heights can be assessed.

Vertical Mapper® is used to attach ground elevation and flood heights for all flood surfaces to the street address database. Thus, every street address has an elevation and flood heights for the relevant floods.

An in-house program interpolates between the flood surfaces to estimate the Risk Rating for every street address for the three elevations - ground level, ground level + 1m, and ground level + 2m. The result is a database, exported as a text file and MS Excel® file containing, for each street address, the postcode, three Risk Rating values based on AEP and 3 Risk Rating values based on Average Recurrence Interval. A sample database extract is shown below.

Any elevation below the 5% AEP [ARI 20-year] flood height is entered with a 0.0526315 AEP value and an ARI value of 19 [years]. Information for flood surfaces below 5% AEP is not available as the steep slope of the stage vs. AEP curve makes estimation difficult.

Addresses with elevations above the highest flood surface created are allocated a Risk Rating of that ARI + 1. For example if the highest flood surface is ARI 100-year [0.01 AEP] flood, all addresses above that level are given a Risk Rating of ARI 101 years [AEP 0.0099]. If the PMF has an ARI of 10,000 years then addresses above the PMF are allocated Risk Ratings of 10,001.

Sample FloodAUS database