Differences of the flood losses among households

To simulate the water infiltration and evaporation process, an interception function is adopted in the model, which is set with a value of 1mm/tick for all cells.

Thus the cells all have interception as an attribute to make water reduce in every step. As in real word, the evaporation is ~0.1mm per 15 min (tick) and infiltration reduces about 50% runoff (Ren and Guo, 2006), the model artificially initializes an interception value of 1mm per tick, which is a bit large to save model running time.

If there is a flood warning, the households have an 80% chance to receive the warning. Once received, the household can determine the near future rainfall trend and intensity (rainfall scenario), and therefore can estimate its inundation depth.

Options for warning lead time and interval are available on the model interface and can be set manually before running the model. When flooding occurs, households will change their color from white to red, and then to black, to show visually how deep they are inundated.

In the flood damage section, all residence buildings have the same standard to flood damage. Therefore, the parameter in the loss rate function (Equation 8) is set as Bc1 = 0.06. Variant in-house properties of each household are packed as one property, and the loss rate parameter (equation 9) is set as Pc1 = 0.1. In the flood response section (equation 10), parameter a has the value of 1/(5×10⁹), which is adjusted to the goal of making the Rc curve fitting with existing publications (e.g.

Dutta et al., 2003; Li, 2003). These parameter values can be adjusted once the real value in the case area is available or when the model is applied in other areas.

While it is desirable to have a model that represents the reality to a large degree, currently there is generally insufficient information on the behaviors and responses of individuals and organizations during flood events to parameterize the agent behavior rules (Dawson et al., 2011). Moreover, many of the river channels have been regulated and artificial drainage pipes have been fixed in the research area during the last decades, however information on this has not been available for this research. Thus the flood water flow in this model is not able to exactly reflect the real world flooding event. The purpose of this case study is to experimentally demonstrate the utility and potential of an agent-based model to be used in a flood loss analysis and flood incident management. Once better data become available, the model can be evolved more realistic. At this stage, the model supports exploring the process of flood loss along with various household responses and compares the effectiveness of different flood response strategies and measures.

0 20 40 60 80 100 120

1 6 11 16 21 26 31 36 41 46 51 56 61

Time (tick)

Damages (thousand HK$) HH1

HH2 HH3 HH4 HH5 HH6 HH7 HH8 HH9 HH10

Figure 5-9 Flood Damages of the sample households in RS3 with rain warning of 12h lead time and 3h warning interval.

0 2 4 6 8 10 12 14 16

1 6 11 16 21 26 31 36 41 46 51 56 61

Time (tick)

Response rate (%)

HH1 HH2 HH3 HH4 HH5 HH6 HH7 HH8 HH9 HH10

Figure 5-10 The proportion of responding investments to total capitals, for the sample households in RS3 with rain warning of 12h lead time and 3 h warning interval.

HH1, HH2, HH8 and HH9 suffer the most damages among the sample households (Figure 5-9), incurred by fairly less investments in response measures (Figure 5-10). This is mainly because the four households are the poorest out of the ten, in terms of their total properties. Limited by their scarce economic resources, they have low adaptive capacity, which in turn limits their response investments.

This indicates that the lack of economic resources significantly contributes to the absolutely larger extent of flood damages. This is an evidence that, comparing with the riches, the disadvantaged communities and their people are in a higher risk of

0 5 10 15 20 25

HH1 HH2 HH3 HH4 HH5 HH6 HH7 HH8 HH9 HH10

Household

Loss proportion (%)

Damage losses Response costs

Figure 5-11 The proportion of responding costs and damage losses, for the sample households in RS3 with rain warning of 12h lead time and 3h warning interval.

Figure 5-11 clearly shows that households who invest more in flood response measures suffer less flood damage. Some households (HH3, HH4, HH6 and HH7) even respond so effectively that they don’t suffer damages at all. This indicates that responding measures are quite helpful for reducing flood inundation damages.

5.4.3 Flood loss in different response strategies

Under a certain rainfall scenario, there will be different damages for households receiving different warning information. The model uses the term lead time (LT) to represent the time period from the time of warning release to the time of flood start.

And warning interval (WI) is used to denote the time period from one warning release to the next one. The model has run several times under RS3 with different lead time of warning but the same warning interval (3h). The results are shown in Figure 5-12. It manifests that generally warnings with shorter lead time contribute to higher flood damage whereas longer lead time helps reduce damages. While comparing to the cases with various warnings, the damages are always the highest in the absence of warning release. It is noted that changes of lead time do not always lead to changes of damage for some households (e.g. HH8 and HH10). This indicates that there may be other decisive factors in function as well, which leads to further tests on warning intervals.

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HH1 HH2 HH3 HH4 HH5 HH6 HH7 HH8 HH9 HH10

Household

Damage (Thousand HK$)

No warning Lead time 1 Lead time 2 Lead time 4 Lead time 8 Lead time 12 Lead time 24

Figure 5-12 The difference of damages, for the sample households in RS3 with 3h warning interval but different lead time of warning.

In these tests the warning has been set with lead time of 12h and with various warning intervals ranging from 2h to 12h. A test on the situation of no warning has also been included. Figure 5-13 shows the result of higher damage accompanied by longer warning interval. Further investigations focus on the combined effect of lead time and warning interval changes. The findings, as shown in Figure 5-14, mean that without warning the flood loss rates of households are prone to reach the highest level, comparing with other conditions with warnings. In addition, a longer lead time and shorter interval can obviously alleviate the loss rate. However, this significant impact is not applied to the case of HH2, largely due to its features of poor and very low adaptive capacity. Therefore, I argue that the effectiveness of warning system on alleviating flood loss is strongly correlated with the household’s economic situation.

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HH1 HH2 HH3 HH4 HH5 HH6 HH7 HH8 HH9 HH10

Household

Damages (Thousand HK$)

Interval 1 Interval 3 Interval 6 Interval 9 Interval 12 Interval 18 Interval 24 No Warning

Figure 5-13 The difference of damages, for the sample households in RS3 with 12h warning lead time but different warning intervals.

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HH1 HH2 HH3 HH4 HH5 HH6 HH7 HH8 HH9 HH10

Household

Flood loss rate (%)

No Warning LT2-WI24 LT4-WI12 LT6-WI8 LT8-WI6 LT12-WI4 LT24-WI2

Figure 5-14 The difference of flood loss rate, for the sample households in RS3 with different warning information.

LT: lead time; WI: warning interval.

Comparing the results from Figure 5-12 and Figure 5-13, HH3, HH4 and HH10 are the households whose damages perform much different in both the situations of changing warning lead time and warning interval. Given that HH3, HH4 and HH10 are the richest households in the 10 samples, it indicates that the amount of exposed properties is sensitive to response strategies, such as whether warning information is received.

0 50 100 150 200 250 300 350 400

0 5 10 15 20 25 30 35 40 45 50 55 60

Time (tick)

Damage (Thousand HK$)

HH1 HH2 HH3 HH4 HH5 HH6 HH7 HH8 HH9 HH10

Figure 5-15 The damages for the sample households in RS3 with no warning information released.

In the situation of no warning, households suffer flood damages without taking any response measures and the damages increase along with the growing of the flood water level until the flood recedes (Figure 5-15). Furthermore, the final flood loss rate of all households is lower in the condition of a flood warning (lead time 12h and interval 3h) than that in the condition of no warning at all (Figure 5-16). It’s also very clear that the poor households, HH2 and HH8, have small differences in the two warning scenarios, which further proves that flood warning does not help much if the adaptive capacity of a household is too low.

0 10 20 30 40 50 60

HH1 HH2 HH3 HH4 HH5 HH6 HH7 HH8 HH9 HH10

Household

Flood loss proportion (%)

RS3-No warning RS3-12-3

Figure 5-16 Comparison of the flood loss proportions of the sample households between RS3 and no warning scenarios.