ENGR10006 Chap.12 Flood Damage Economics and Real Flood Mapping
Flood Damage Economics and Real Flood Mapping
Workshops 7 to 10 put a price on flooding and then test the models against a real event. This chapter compares two ways of estimating loss: counting flooded buildings and multiplying by average insurance claims of $34,000 for a dwelling and $71,000 for a business, and the FLFArs depth-damage function d = (h/H)^(1/r) × Dmax with a 3 m storey height and a parameter table for four building types.
It shows how QGIS samples flood depth at building centroids, computes a loss field and draws a proportional symbol map, and why the function's riverine origin is a limitation. It then follows the Rocklea major project: the 2011 Brisbane flood, EPSG:7856, georeferencing an aerial photo, digitising the real extent and comparing it with the model. A worked example compares both loss methods on one block.
What this chapter covers
- 01
Insurance-based loss estimates
- 02
Depth-damage function and parameters
- 03
Reading the damage curves
- 04
Loss fields and proportional symbol maps
- 05
Money saved by protection
- 06
Rocklea and the 2011 flood
- 07
Georeferencing and digitising
- 08
Comparing modelled and real extents
Worked example · free
Comparing two loss estimates for one block
- +1Insurance method: 10 × $34,000 + 2 × $71,000 = $340,000 + $142,000 = $482,000.
- +1Depth ratio h/H = 0.6 / 3 = 0.2, and 1/r = 1/2.3 = 0.4348.
- +10.2 to the power 0.4348 = 0.4966; times Dmax 0.42 gives a damage fraction of 0.2086.
- +1Loss = 0.2086 × 1,500,000 ≈ $312,900, so one house by the depth method approaches two thirds of the block total by the claim method.
Key terms
- Average insurance claim
- The per-building loss used in the simple method, $34,000 residential and $71,000 commercial.
- Rate parameter
- The value r in the depth-damage function that controls how quickly damage rises with depth.
- Maximum damage
- The value Dmax, the damage fraction reached when water fills one storey.
- Proportional symbol map
- A map in which the size of each dot shows the value of a quantity, such as loss.
- Georeferencing
- Placing an image in map coordinates by matching known points to their eastings and northings.
- Digitising
- Tracing features from an image as new vector polygons, such as the flooded area.
- Centroid
- The central point of a building footprint, where the flood depth is sampled for the loss calculation.
Flood Damage Economics and Real Flood Mapping FAQ
Why are two-storey damage fractions so much lower?
The parameter table for two-storey buildings covers the lower storey only, and its maximum damage values, 0.42 for timber and 0.34 for brick, are well below the one-storey values of 0.70 and 0.60. The upper storey is assumed to survive a flood that fills the ground floor.
Is the FLFArs function right for coastal floods?
The workshop notes it was developed at the University of Melbourne for riverine flooding, where water moves slowly, and is applied to the coastal scenario only to demonstrate the method. State that limitation whenever you report losses from it.
What does the Rocklea map assessment require?
The Workshop 10 assessment asks for a map comparing the modelled and the real 2011 flood extents, with all BOLTSS elements and the chosen water level written and justified on the layout, followed by verbal discussion questions with a demonstrator.
How does QGIS turn a flood map into a dollar loss?
Building footprints are converted to centroids, the flood depth is sampled at each centroid, and the field calculator applies the depth-damage expression with the chosen r, Dmax and assumed value of $1.5 million. Summing the new loss field with basic statistics gives the predicted total for the scenario.
Assessment move
Work the depth-damage function by hand for each building type at two or three depths until the order of operations is automatic: ratio, power, maximum damage, value. Plot or sketch the four curves so you can explain why shallow water already causes much of the damage and why two-storey buildings lose less.
Then do the insurance count for your own flood map before and after sandbagging, and express the difference as money saved. In QGIS, follow the centroid, sampling and field-calculator steps carefully, check a few loss values by hand, and build the proportional symbol map with a clear legend.
For Rocklea, set EPSG:7856 before loading data, georeference the aerial photo with four well-spread points, digitise the flooded area, and compare it with your model at the chosen level. Write the differences down as discussion points, together with the modelling limits that explain them. Keep both loss totals in one table with their methods named, ready for the report discussion.
