LOSS OF LIFE STUDY CAUSED BY DAM FAILURE – A CASE STUDY

Jung Hwa Lee 1, Jaya Kandasamy

2, Amit Chanan

2

1. SMEC Australia, North Sydney, NSW

2. University of Technology, Sydney, Sydney, NSW

ABSTRACT

A method developed by Wayne J. Graham has been widely used to determine the Population at Risk (PAR) and Loss of Life (LoL) caused in the event of a dam failure and its results have been used for developing consequence assessment as part of dam safety management procedures in Australia. The UK Environment Agency (UKEA) and the Department of Flood and Rural Affairs (DEFRA) also developed a method to estimate the risks of death or serious harm to people caused by flooding from reservoir failures. This paper presents a literature review on these methods within the context of evaluating methodologies for estimating the PAR and LoL. A case study dam in NSW was carried out to compare and evaluate the two methodologies. From this case study, the two methods for estimating potential LoL could be compared and the results could have implication on dam safety risk assessment and emergency planning. The case study reveals that the Flood Risk to People could derive more precise estimates of PAR whilst Graham (2010) is more effective for estimating LoL.

INTRODUCTION Loss of Life (LoL) is an index that estimates the severity of a dam failure in terms of the likely loss of people’s life. Population at Risk (PAR) is defined as the population or number of people who are directly exposed within an inundation area. LoL is expressed as the rate or number of the people who could lose their lives in an inundation area and is based on the value of the PAR adjusted for variables including the severity of flooding and flood warning time. The PAR, in turn, can be estimated by analysing a range of floods using the flood inundation zone for that particular flood. Two methods that are widely used to determine PAR and LoL were developed, separately, by Graham (1999) and UKEA and DEFRA (2003). There are many factors which can influence the life of people and PAR, some of which are listed as follows (Smith et al 2009):

Time of dam failure;

Warning time given to the community;

Risk category of area;

People’s awareness of flooding; and

Promptness in evacuating an area.

In 2010, Graham updated the 1999 method for estimating PAR and LoL caused by dam failures. The main difference between these methods lies in the manner census data is applied over the entire area of inundation. According to Graham (2010), LoL resulting from dam collapse has generally been higher for residents in close proximity to the dam compared with those situated further downstream. This is because of the behaviour of floods in the upstream areas in terms of velocity, depth and rate of rise are much more extreme than in downstream areas.

UKEA and DEFRA (2006) has also been updated with more careful considerations of flood severity in relation to the characteristic of catchment, type of housing, seasons, and the community’s readiness for evacuation and their decision to evacuate when given a flood warning caused by dam failure.

BODY OF THE PAPER 1. Assessment Method for LoL and PAR The methods used here for the potential LoL estimation from dam failure are Graham (2010) and UKEA and DEFRA (2006). These two methods were developed using historical information from similar types of flood events albeit in different countries (US and UK). In adopting the two methods, empirical local information such as census data and mechanistic elements such as collapse of building and shelter has been used. Table 1 summarises the fatality rate for variables of flood severity, warning time and warning urgency. The fatality rate is the probability of each combination of the variables occurring. The fatality rate is applied to the PAR to estimate the LOL.

Table 1 Recommended Fatality Rates for Estimating Loss of Life (LoL) Resulting from Dam

Failure, Graham (2010)

2. UKEA and DEFRA (2006) The method given in UKEA and DEFRA (2006) is a form of multi-criteria assessment used to estimate PAR and LoL based on the concept of flood hazard, area vulnerability and people vulnerability. These provide estimates of the fatalities and injuries for flood events arising from dam failure. The assessment method is summarised in figure 2. The method estimates the percent of people at risk based upon hazard rating and area vulnerability out of the total number of people in a defined hazard zone (PAR). The approach includes a people vulnerability score based on the percent of people with long term illness or who are of old age. These parameters can be used to estimate PAR and LoL across dambreak flood events. Uncertainty in this procedure is generally high, particularly in the number of people who will be exposed to a flood and the wide range of site specific factors that affect whether people are injured or killed. 3. A Case Study The Case Study Dam is located in the Hunter River catchment and is situated north of Singleton, NSW (Figure 3). Downstream of the dam, the water course runs though cleared general farming area until it meets the Hunter River approximately 30 km downstream of the dam. After the confluence, the Hunter River continues to flow through major country towns of Singleton and Maitland.

Figure 3 Schematic Map of the Case Study Dam

(State Water 2009)

A conceptual study of potential LoL in the event of a failure of the case study dam was carried out assuming a sunny day failure scenario. This means that the failure is assumed to occur at the full supply level and when there is no concurrent flooding (ANCOLD 2000). 3.1 Graham (2010) The dam failure scenario for sunny day failure was assessed for the following seasonal and day/night categories:

Dam fails during the summer - Day / Night

Dam fails during a non-summer period - Day / Night

3.1.1 Population at Risk (PAR) Summer PAR The number of houses within the inundation area were counted by the use of both the inundation map and an aerial map (Google Map). The Basic Community profile data set from the 2006 census was used to obtain the number of people per one dwelling in each Census Collection District (CCD) which is the smallest unit of measure for the census information (Appendix Table 2). Total dwellings inundated in Singleton were assumed by prorata calculation of the inundation area. Two time categories, day and night, were used to calculate the PAR for each CCD. The night time PAR (Appendix 3) was determined by assuming a constant occupancy rate, which was derived from the population and number of dwellings occupied per CCD (Appendix 2). The night time PAR is assumed to be the base PAR.

Figure 4 Graham (2010) Summer PAR

Estimation of the daytime PAR is more complex. It should be assumed that the PAR is separated between workplace, school and households. An average workplace and school occupancy rate was calculated from the 2006 census data and applied to the base PAR (Appendix 4). The number of beds in the district hospital affected by the flooding was estimated from the NSW Health Services Comparison Data Book 2007/2008 (Appendix 5). The workplace and school occupancy rate assumes 100% attendance of all workers and students. However, 100% attendance is not likely and a multiplier is required to make allowance for

individuals that are sick, on leave, on excursion, etc. The multiplier used, 0.9, was estimated by Assaf et al. (1997) for the development of their mathematical model for B.C Hydro’s Keenleyside dam. The daytime household occupancy rate of 0.2 was adopted (Assaf et al., 1997; SKM, 2000). All these assumptions and factors used for Summer PAR are shown in Appendices 6 and 7. Non-Summer PAR It is assumed that the visitors counted in the CCD data were only present during the summer season. Therefore, the PAR during the non-summer seasons excluded visitors in order to estimate the population per dwelling ratio. Appendix 8 presents the population per dwelling ratio for non-summer night time PAR calculation. Worker and student ratios as well as people in a house ratio shown are for non-summer day time calculation of the PAR. Appendix 9 shows the PAR calculated under non-summer night time category by adopting the population per dwelling ratio summarized. For non-summer day time calculation, the same approach used to calculate the summer PAR was adopted in Appendix 10.

Figure 5 Graham (2010) Non-Summer PAR

Table 2 provides summary of PAR during day and night time for summer and non-summer seasons. A comparison of night and daytime PAR generally shows that PAR for night time is greater that daytime.

Table 2 PAR Summary by season and day/night time category

3.1.2 Flood Severity Flood severity is given in the form of water depth and the depth – velocity product as per Figure 1. Severity of Medium is given for the section 1 to 3 and severity of Low/Medium is given for the section

4 (Appendix 11 and 12). 3.1.3 Warning Time Daylight could provide the opportunity to observe the break and the resulting flood and people have a clear view which would help easier and quicker evacuation whereas night time people might be asleep or would not be able to see due to darkness. Warning time was adopted from Hydro Tasmania (2001). Accordingly, the time taken to warn the PAR in lower section was estimated to be 2 hours for day time and 1.75 hours for night time. For Singleton town area, the time taken to warn the PAR was estimated to be 7.5 hours in day time and 7.0 hours for night time (Appendix 13). 3.1.4 Flood Severity Understanding The flood severity understanding was estimated to be “vague” for the first and second sections downstream of the dam. However, the population near Singleton town area benefits from information of flooding that occurs first at Camberwell by receiving a subsequent “precise” warning of the flood severity (Appendix 14). 3.1.5 Fatality Rates Table 3 provides the suggested fatality rates based on the warning time, flood severity understanding and flood severity. The fatality rates were obtained from Graham’s recommended fatality rates in Table 1.

Table 3 Fatality Rates

3.1.6 Loss of Life LoL estimates determined based on Table 3 are shown in Table 4.

Table 4 Estimated Loss of Life

Table 4 shows the loss of life estimated to rounded numbers. During the non-summer season, loss of life ranges from 17 for daytime up to 95 for night-time. Higher values occur during the summer season due to increased number of visitors. A

failure of the case study dam in summer could result in a loss of life range from 17 for daytime up to 97 for night-time. 3.2 UKEA and DEFRA (2006) 3.2.1 Average Depth and Velocity The average flood depth can be obtained from flood levels which are outputs of dambreak modelling (MIKE11) and catchment topographic and inundation map. Average velocity is estimated based on the consideration that the flooding of the area will be fairly rapid. 3.2.2 Hazard Rating (HR) The hazard rating (HR) was estimated using the equation in Figure 2, adopting the values for average depth, velocity and debris factor (Appendix 15) for each location. The flood depths were more than 0.75 m and/or velocities were more than 2 m/s. The debris factor (DF) was taken as 0.5 for the first three areas which were mainly agricultural land and 1.0 for the last area which includes the Singleton town centre. 3.2.3 Area Vulnerability (AV) The sum of the flood warning, speed of onset and nature of area factors is used to derive the AV value. i. The score for flood warning is on a scale of 1

(good warning system) to 3 (no warning system) Score for flood warning = 3-(P1x (P2+P3))

Where, P1 = % of Warning Coverage Target Met (assumed 80%) P2 = % of Warning Time Target Met (assumed 100%) P3 = % of Effective Action Target Met (assumed 75%)

The targets were assumed as indicated in UKEA and DEFRA (2006) for the case study dam catchment, which has made most progress in achieving the targets and the ‘generic’ score for each location was 1.6

ii. The speed of onset of flooding has been assumed to be ‘rapid’ for the first two areas which attracts a score of 2 while the score of 1 was adopted for the last two areas.

iii. The area of the case study dam catchment under consideration is mainly agricultural area with some residential properties except Camberwell and Singleton town centre. As such, this is considered to be a ‘high’ risk area which attracts a score of 3 while the score of 2 was allocated for Camberwell and Singleton town centre.

3.2.4 People Vulnerability (PV) The estimated population within inundation area was obtained by the following approach. i. The population was obtained from Appendix 3

since the area under consideration is the same to Graham (2010) Summer PAR.

ii. Determining the percent of residents over the

age of 75 and the percent of residents with disability, which were derived from the population and number of occupied dwelling per CCD

iii. Applying the % residents age>75 ratio to the population

iv. Applying the % residents with disability ratio to the population.

v. Substitute the values to the equation in Figure 2. The population, numbers of residents > 75 yrs old and numbers of residents with disability in the inundation area are summarised by four areas in Appendix 16. Table 5 is the summary of the results of PAR and LoL using the UKEA and DEFRA (2006). The number of people exposed to the risk (N (ZE)) is multiplied by 2Y to obtain the number of injuries. The fatality rate (ie the proportion of injured people who die) is assumed to be proportional to the Hazard Rating. The number of injuries is multiplied by 2HR (as a percentage) to obtain the number of deaths. 4. Discussion

4.1 Population at Risk The PAR from dambreak was estimated by two methods. The UKEA and DEFRA (2006) shows the PAR to be around 2,178 which is the lowest average value and it is almost half the lowest figure of PAR (4,048) from the Graham’s methods (Table 6). The reason why Graham’s method has a larger PAR is likely because it counted all people in the inundation area and the varying occupancy type without any application of flood behaviour and people’s vulnerability to the PAR calculation. By doing so, there could be several areas where the accuracy might have been sacrificed by using the Graham’s methods. It could be questioned how many people are in their beds during the night time category. Some might be at work, heading towards home after work, or on holiday. However, this might be balanced by visitors who stay overnight on the floodplain. Therefore, a rational assumption would be that the night time PAR could be base PAR.

Table 6 Summary of PAR derived from three methods with the case of the case study dam

failure

Figure 6 Comparison of average PAR values from

two different methods There is little difference in PAR between Graham (2010) summer and non-summer under the day time (Table 6). This may be because of the small number of visitors which were adopted in the PAR calculation. However, there is notable difference between Graham (2010) for summer and non-summer cases under night time category. The PAR for the later was reduced by 134 from 5,200 to 5,066 (Table 6). By comparing the results from the above two methods, it can be concluded from Table 7 that at the time of dam failure the PAR in the inundation zone may vary from 2,178 to 5,200 (Table 6 and Figure 6). However, 85% of the PAR is located in Singleton, over 70 kilometres downstream of the dam and having minimum of 7.5 hours warning time of impending peak flood levels. Hence the LoL in the lower range of estimates would be expected. 4.2 Loss of Life Table 7 summarises the LoL from potential case study dam failure by adopting the three different methods.

Table 7 Summary of LoL from the two methods

The two methods derived different estimates of LoL. The first method (Graham 2010) suggested using a range of flood severity against the result of DV (form of water depth and the depth – velocity product) in order to find out a more reliable LoL. Thus Graham (2010) gives two different LoL values; 17 for low severity and 51 for medium severity for day-time during both summer and non-summer season (Table 7). Also, a LoL of 54 (low severity) and 97 (medium severity) were estimated for Graham (2010) under the summer night time category. Graham (2010)

under a non-summer night category shows a reduced LoL of 53 (low severity) and 95 (medium severity) (Table 7). From the Graham’s method, the average LoL figures under medium flood severity category were very similar at 74 and 73 and are smaller than the LoL (91) from the UKEA and DEFRA (2006). There is a more distinct difference between the average LoL between Graham (2010) under low flood severity category assessed at 36 or 35 and the UKEA and DEFRA (2006) assessed at 91. However, LoL from the UKEA and DEFRA (2006) is similar to the LoL from Graham (2010) under night time category value estimated as 91 and 97 or 95 respectively (Table 7). By comparing the different methods, it can be concluded that the average (day and night) LoL caused by the case study dam failure varies from 17 to 97 (Table 7). CONCLUSION The PAR estimates from the Graham’s (2010) method are much higher than that obtained from the UKEA and DEFRA (2006) method. However the LoL values with the Graham’s methods are similar to the UKEA and DEFRA (2006) and it even provides lower LoL under the day time category. It should be noted that the Graham’s methods allows for a range of fatality rate accounting for flood severity, warning time and flood severity that could help to reduce uncertainty while the UKEA and DEFRA (2006) suggests only one figure for LoL. The UKEA and DEFRA method could derive more precise estimates of PAR whilst Graham’s method is more effective for estimating LoL. Therefore, it would be effective to use those methods accordingly. If the PAR is important, using the UKEA and DEFRA method is more reliable. On the contrary, if the focus is on LoL, Graham’s method would be suitable and the UKEA and DEFRA method can be used for sensitivity verification purpose.

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Figure 1 Summary of method for assessing LoL, Graham (2010)

Figure 2 UKEA and DEFRA (2006)

Table 5 Numbers of Injuries and Fatalities in Each Location

KM Area Av.Depth Av. Velocity

Hazard

Rating

(HR)

Area

Vulnerability

(AV)

People

Vulnerability

(Y)

X=HR x

AV (%)Population N(ZE)

No.of

Injuries

=

2xYxN(ZE)

Fatality

rate

= 2xHR

(%)

No.of

deaths

19.752 1-Upper Section 4.20 7.44 33.87 6.6 6.42 100.00 252 252.14 32.38 67.73 21.93

39.007 2-Lwr Section 2.69 4.99 15.27 5.6 7.17 85.50 264 225.67 32.36 30.54 9.88

65.335 3-Hunter River Downstream of Confluence 1.94 3.51 8.78 5.6 8.41 49.15 245 120.44 20.27 17.55 3.56

80.484 4-Singleton 1.71 3.44 7.74 4.6 11.47 35.59 4439 1579.86 362.34 15.47 56.07

2178 447 91

The UKEA and DEFRA (2006)

Total (rounded) Note: X is % of people exposed to risk. If the score exceed 100, this is simply taken as 100.

N (ZE) is the number of people exposed to risk in each hazard zone.

APPENDICES 1. Number of Dwellings in inundation area from Google Map

2. Summer CCD Data for the population per dwelling ratio

3. Summer PAR by Night-time Estimation

4. Summer Dwelling Structure

1 Assumption

1. Total dwellings in Singleton are assumed by prorata calculation of the inundation area ie) Number of Buildings in Singleton inundation area is 75% of whole Singleton area. Therefore, number of Buildings at Singleton (2111 dwellings) x 75% =1583 but it simplified as 1555.

2. At 70.928 & 80.484 km, the average population ratio used between Glenridding and Singleton & Lower Belford and Glendon.

ie) (2.60+2.52)/2= 2.56 & (3.03+3.03)/2=3.03

5. Summer PAR at Day-Time

6. Summer PAR - Downstream of Glennies Creek Dam by Night & Daytime During Summer

km Section PAR

(Day-time) PAR

(Night-time)

0 to 19.752 1-Upper Section 209 252

19.752 to 39.007 2-Lwr Section 210 264

39.007 to 65.335 3-hunter River Downstream of Confluence 188 245

65.335 to 80.484 4-Singleton 3445 4439

7. Summer PAR - Downstream of Glennies Creek Dam

8. Non-Summer ABS - Dwelling Structure

2 Assumption

1. Daytime Household occupancy rate adopted was 0.2 (Assaf et al. (1997)) ie)Population in House at 6.826 km = (PAR at Night time) x People in House Ratio x occupancy rate rea

2. Work & Student occupancy rate adopted was 0.9 (Assaf et al. (1997)) ie)Work at 6.826 km = (population x ratio of worker) x occupancy rate = (42x0.51) x 0.9 = 19.

3. Number of Hospital assumed all patients in above area will

use singleton district hospital

9. Non-Summer PAR by Night Time

10. Non-Summer PAR during Day Time

11. Flood Severity

12. Number of PAR exposed to specific flood severity categories

Number of Population at Risk Exposed to Specific Flood Severity

Location

Sunny Day Dam Failure Cause

Summer Season Non-Summer Season

Day Night Day Night

1-Upper Section (to 19.752km)

209 M

252 M

208 M

247 M

2-Lwr Section (19.752km to 39.007km)

210 M

264 M

208 M

252 M

3-Hunter River Downstream of Confluence (39.007km to 65.335km)

188 M

245 M

187 M

239 M

4-Singleton (65.335km to 80.484km)

3445 L to M

4439 L to M

3445 L to M

4328 L to M

3 Assumption

1. Same assumptions from Appendix 2 were applied.

13. Warning Time Available for each Location (hours)

Warning Time (hrs)

Location

Sunny Day Dam Failure Cause

Summer Season Non-Summer Season

Day Night Day Night

1-Upper Section (0 to 19.752km)

0.30 0.00 0.30 0.00

2-Lwr Section (19.752km to 39.007km)

2.00 1.75 2.00 1.75

3-Hunter River Downstream of Confluence (39.007km to 65.335km)

5.00 4.50 5.00 4.50

4-Singleton (65.335km to 80.484km)

7.50 7.00 7.50 7.00

14. Flood Severity Understanding

Flood Severity Understanding

Location

Sunny Day Dam Failure Cause

Summer Season Non-Summer Season

Day Night Day Night

1-Upper Section (0 to 19.752km) Vague Vague Vague Vague

2-Lwr Section (19.752km to 39.007km) Vague Vague Vague Vague

3-Hunter River Downstream of Confluence (39.007km to 65.335km)

Precise Vague Precise Vague

4-Singleton (65.335km to 80.484km) Precise Precise Precise Precise

15. Guidance on debris factors for different flood depths, velocities and dominant land use

(DEFRA 2006)

16. People Vulnerability – Downstream of Glennies Creek Dam

Distance (km)

Area Population >75yrs

% residents aged over 75 or over

Disability

% residents suffering

from long-term

illness

Y

0 to 19.752

1-Upper Section 252 11 4.30% 5 2.12% 6.42%

19.752 to 39.007

2-Lwr Section 264 6 2.38% 13 4.79% 7.17%

39.007 to 65.335

3-Hunter River Downstream of Confluence

245 12 4.89% 9 3.52% 8.41%

65.335 to 80.484

4-Singleton 4439 296 6.67% 213 4.80% 11.47%