“A probabilistic flood risk assessment and the impact of compartmentation ” Graduation committee: Prof. drs. ir. J.K. Vrijling (DUT, chairman) Dr.ir M. Kok (DUT, HKV) Prof.ir. A.W.C.M. Vrouwenvelder (DUT) Drs. A. Roos (RWS) Information student: Student: R.P.G.J. Theunissen Address: Voorstraat 95-II 2611 JM, Delft Phone: 0641436940 Email: [email protected] Student no: 1005820 This work has been supported by: Directorate-General Public Works Delft University of Technology and Water Management Faculty of Civil Engineering and Road and Hydraulic engineering Province of South Holland Geosciences Institute IV Preface This thesis entitled ‘a probabilistic flood risk assessment and the impact of compartmentation’ marks the end of my study at the Faculty of Civil Engineering and Geosciences at Delft University of Technology and has been executed at the ‘Road and Hydraulic Engineering division of the Directorate General of Public Works and Water Management’. The study was supported by the Province of South Holland. I would like to thank the members of my committee for their interest and constructive criticisms during the process. R.P.G.J. Theunissen January 2006 V VI Summary The current safety philosophy against flooding in the Netherlands as recorded in the Flood Defence Act was introduced in 1960. At the time a policy was chosen based on pragmatic considerations that focused on strengthening and maintaining the primary defences. By defining safety standards, periodical adjustments of hydraulic loads and a five yearly assessment of the quality of the primary defences it was ensured the protection against flooding grows along with physical developments. However besides physical developments like sea level rise and increasing river discharges since 1960 economical and social developments have occurred. The population density and the protected value have increased significantly and furthermore society seems to accept less risk of flooding, as flood disasters are not seen as acceptable natural phenomena’s anymore. In terms of modern risk analyses the probability of many casualties is larger than all other external safety risks together. The current safety philosophy hardly takes economical and social developments into account. Recently the National Institute of Public Health and Environment (RIVM) concluded the current safety philosophy does not lead to a safe and livable Netherlands as was intended. Contrary to the current safety philosophy the flood risk approach takes economical and social developments into account by including the consequences in the analysis. The consequences consist of the expected economical damage and the expected number of casualties. The first objective in this thesis is to apply the flood risk approach at dike ring area IJsselmonde. Therefore flood scenario probabilities have been determined and their corresponding flood simulations have been carried out, multiplying them results in the flood risk. The second objective was to investigate the impact of compartmentation on the flood risk. This summary first describes the determination of the flood risk at IJsselmonde and continues with a description of the impact of compartmentation. Flood risk assessment IJsselmonde IJsselmonde is a relatively small dike ring area surrounded by primary defences that were designed on a safety standard with an exceedance frequency of 1/10000 per year. These primary defences were designed before the implementation of the Maeslantkering and the Hartelkering in the tidal river area. These storm surge barriers significantly reduced the hydraulic loads and therefore the probability of flooding in the tidal river area. After the construction of the storm surge barriers it was decided to reduce the safety standard at IJsselmonde to an exceedance frequency of 1/4000 per year. In this thesis the failure mechanisms overtopping, wave overtopping and damage to the revetment and erosion of the dike body are included in the dike section failure probability analysis. The failure mechanism instability of the inside slope and all mechanisms concerning civil engineering structures have not been analysed for reasons of time and lack of data, furthermore the results of the failure mechanism piping were judged unreliable and are not included in the end results. The primary defences at IJsselmonde consist of 73 dike sections. As the different states of the storm surge barriers have to be included in the failure probability analysis a dike section failure probability at IJsselmonde can be written as follows: PF = P (Failure ∩ hQ ≥ Closure Condition ∩ Correctly Closed) + P (Failure ∩ hQ ≥ Closure Condition ∩ Incorrectly Open) + P (Failure ∩ hQ < Closure Condition ∩ Incorrectly Closed) + P (Failure ∩ hQ < Closure Condition ∩ Correctly Open) VII Dike section failure probabilities are insufficient for the calculation of the flood risk and need to be processed to flood scenario probabilities, which are coupled to a partial dike ring part. A partial dike ring part is defined as a part of the dike ring in which the consequences of a flood vary little if the position of the breach is changed. This definition ensures one flood simulation per partial dike ring part is sufficient. The division in partial dike ring parts as used in this thesis is presented in Figure A and is based on the division in compartments by the secondary defences which also are indicated in Figure A. Furthermore the breach locations that are applied in the flood simulations are indicated this figure, these breaches are situated at the dike sections with the greatest failure probability in the partial dike ring part. 1.0E+00 1.0E-01 1.0E-02 1.0E-03 1.0E-04 1.0E-05 1.0E-06 1.0E-07 1.0E-08 1.0E-09 Probability of exceedance 1-FN(x) 100 1000 10000 100000 Number of casualties (N) FN-curve dike ring IJsselmonde Figure A: Schematization IJsselmonde Figure B: FN-curve The flood risk is approximated by the dominant flood scenarios that contribute for more than 99% to the dike ring failure probability (approximately 1 / 334000 per year). The list of these dominant flood scenarios is presented in Figure C. In this thesis a flood scenario refers to one breach location and one state of the storm surge barriers. As can be seen the correctly closed state and the incorrectly open state are dominant in the failure probability analysis. Flood Breach Failing partial State Scenario Economical dama- Number of Economical Risk of Scenario Location dike ring areas Barriers Probability ge (Billion euro) Casualties Risk (Euro) Casualties 1 A 6 incorrect open 1.18E-06 4.1 600 4854 0.000710 2 A 6 correctly closed 3.10E-07 4 504 1241 0.000156 3 B 7 correctly closed 2.90E-07 2.9 364 840 0.000105 4 A and C 5 and 6 incorrect open 2.58E-07 4.1 600 1057 0.000155 5 A and B 6 and 7 incorrect open 2.20E-07 6.6 1045 1453 0.000230 6 A, B and C 5, 6 and 7 incorrect open 1.30E-07 6.6 1045 858 0.000136 7 A and B 6 and 7 correctly closed 6.18E-08 6.9 868 426 0.000054 8 A, B, C and D 4, 5, 6 and 7 incorrect open 4.87E-08 7.6 1194 370 0.000058 9 E 8 correctly closed 3.96E-08 4.4 789 174 0.000031 10 B and E 7 and 8 correctly closed 2.33E-08 7.3 1153 170 0.000027 11 A and E 6 and 8 correctly closed 1.95E-08 8.4 1293 163 0.000025 12 C 5 incorrect open 1.75E-08 0.005 0 0 0.000000 13 F 1 correctly closed 1.67E-08 0.4 46 7 0.000001 14 B and F 1 and 7 correctly closed 9.58E-09 3.3 410 32 0.000004 15 F 7 incorrect open 8.25E-09 2.5 445 21 0.000004 16 A and F 1 and 6 correctly closed 7.43E-09 4.4 550 33 0.000004 17 A, C and D 4, 5 and 6 incorrect open 7.39E-09 5.1 749 38 0.000006 18 E and F 1 and 8 correctly closed 7.24E-09 4.8 835 35 0.000006 19 6 and 8 incorrect open 5.22E-09 6.3 1124 33 0.000006 20 A, C and E 5, 6 and 8 incorrect open 4.93E-09 6.3 1124 31 0.000006 Total Risk: 11836 0.001723 Figure C: List of dominant flood scenarios Based on the design points of the dominant flood scenarios flood simulations have been made. The hydraulic development of these simulations is described in the main report. The hydraulic consequences of a flood scenario have been translated to an expected economical damage and an expected number of casualties, which is indicated in Figure C. Multiplication of flood scenario probabilities with their consequences results in the risk of a flood scenario. In Figure B the risk is presented as a FN-curve, which indicates the probability of exceedance of a certain number of casualties. The presence of the Maeslantkering and the Hartelkering results in extremely low flood scenario probabilities. IJsselmonde therefore is a VIII very special dike ring and is not representative for other dike ring areas, where the flood scenario probabilities are expected to be significantly higher. Due to the great population density and the enormous protected value at IJsselmonde the consequences immediately are very extreme if a breach occurs. However at IJsselmonde even the extreme consequences are dominated by the flood scenario probabilities resulting in an extremely low flood risk. Impact of compartmentation Compartmentation can be defined as the division of the overall system in compartments by secondary defences (see Figure B) resulting in isolation per compartment and therefore providing protection to the overall system by reducing the consequences of initial failure.