COMPARISON OF COASTAL FLOODING PROBABILITY CALCULATION MODELS FOR FLOOD DEFENCES

Transcription

1 COMPARISON OF COASTAL FLOODING PROBABILITY CALCULATION MODELS FOR FLOOD DEFENCES Elisabet de Boer 1, Andreas Kortenhaus 2 and Pieter van Gelder 3 Reliability alulations for oastal flood defene systems have been widely used for many ase studies worldwide. In this researh two different reliability tools (both aiming to alulate the overall flooding probability of a flood defene system) are ompared using the same data. The main objetive of this paper is to ompare the different tools and the outomes of both models applied to (i) a simple dike and (ii) a pilot site. INTRODUCTION Floods are a threat to millions of people who live in lowlands. Reliability alulations for oastal flood defene systems have been widely used for many ase studies worldwide. They provide the prinipal basis for risk analysis and management investigations as oneptually shown e.g. in Oumerai (2004). Within the European FLOODsite projet (GOCECT ) these methods have also been used for pilot sites in Europe to further develop flood risk management methodologies. The objetive of this researh is (i) to investigate two existing probabilisti design tools for flood defenes and (ii) to find the most interesting parts from these tools for a flexible and widely appliable software tool. For this purpose preliminary investigations have been performed using two different reliability tools, both aiming to alulate the overall flooding probability of a flood defene system. In this paper, these systems are being ompared using the same pilot site data. Hene, the prinipal objetive of this paper is to ompare the different tools and the outomes of both models applied to (i) a simple dike and (ii) a pilot site. RELIABILITY TOOLS PC-Ring PC-Ring is developed in The Netherlands for the projet Veiligheid van Nederland in Kaart (VNK). The objetive of this projet is to assess the safety of the dike ring areas in The Netherlands. PC-Ring alulates the failure probability of a flood defene system (dike ring) and the ontribution of eah element (dike setion) to it. The dike ring is divided in dike setions with equal properties. The alulation requires a model desribing the mehanism, strength data and loading data. PC-Ring onsist of two separate programs of whih one 1 Civil Engineering, Delft University of Tehnology, Stevinweg 1, 2628 CN, Delft, The Netherlands 2 Leihtweiß-Institut für Wasserbau, TU Braunshweig, Beethovenstraße 51a, 38106, Braunshweig, Germany 3 Civil Engineering, Delft University of Tehnology, Stevinweg 1, 2628 CN, Delft, The Netherlands 1

2 2 alulates the failure probability of one dike setion and one failure mehanism and the other one ombines the setion and mehanisms into an overall failure probability. The methods onsiders dikes, dunes and strutures. The failure mehanism in PC/Ring are overtopping/overflow, sliding, heave/piping and damage and erosion of the outer slope. ProDeih ProDeih (Probabilisti design methods for sea dikes) has been developed within a German researh projet. The objetives were to find and disuss tools to assess the overall failure probabilities for a range of sea dikes in Germany and to develop a model for reliability based design of dikes. ProDeih onsist of two parts. A program performs the deterministi and probabilisti alulations of the separate setions and mehanisms and in an Exel appliation the results are ombined to get the overall failure probability. The method only onsiders dikes. 25 failure mehanisms are defined and divided into four groups: global failure mehanisms, failure mehanisms on the seaward slope, failure mehanisms on the shoreward slope, failure mehanisms in the dike ore. Some of the failure mehanisms are ombined in a senario, whih will lead to failure of the dike. CASE STUDIES The desribed reliability tools are applied to two ase studies: a single not existing model dike and a pilot site in Germany. Model dike Figure 1 shows the ross setion of the single dike that is used for the firste omparison between PC-Ring and ProDeih. B d do BB di Clay layer h Hs u hb h i 0.0 m Homogeneous Sand Figure 1. Cross setion of the model dike, whih is used in the alulations.

3 Pilot site in Germany St. Peter-Ording at the German North Sea oast was seleted as one of the FLOODsite pilot sites. The territory of the ommunity amounts to 2800 ha with about 6300 inhabitants. Presently, flood protetion is provided by dikes (about 14.0 km long, about 8.0 m high) as well as dunes (about 1.0 km long, up to 18 m high), surrounding the ommunity on three sides over a length of more than 15 km in total. The reason to hose this site was beause a preliminary reliability study of the pilot site was already performed using the German ProDeih model (Kortenhaus (2003)). The overall flood defene system was split up into 13 dike setions, whih were regarded independent from eah other. RESULTS The omparison of the two models has shown that there are signifiant differenes in the struture of the program, input data and presentation of the results. Table 1 summarises some general results. Table 1. Results of the omparison of the reliability tools PC-Ring and ProDeih PC-Ring ProDeih Coastal systems at any loation Appliation area Coastal systems, lakes and rivers in the Netherlands, although the hydrauli input an be altered for other loations Flood defenes Dikes, dunes, strutures Dikes Input Required hydrauli input data very detailed, geotehnial and geometrial data in both tools is omparable 3 Simple hydrauli input data required, geotehnial and geometrial data in both tools is omparable Fault tree Simple fault tree Two extensive fault trees Failure Only failure mehanisms that diretly mehanisms lead to failure 25 mehanisms, partly subdivided into 13 senarios Handling of the input data are a main soure of differenes between PC-Ring and ProDeih: 1. Geometri input for ProDeih is provided in parametri form whereas PC- Ring uses full ross setions, espeially for dunes. 2. ProDeih onsiders a large range of failure mehanisms for sea dikes, but annot alulate dunes whereas PC-Ring had a more simple approah for dikes and onsiders dune failures. 3. ProDeih uses the wave parameters at the toe of the struture not taking into aount different angles of wave attak and any wind. PC-Ring starts its alulation using offshore (joint) probabilities of wind and wave diretions. 4. In PC-Ring the water level in front of a dike is determined by interpolation of the water level statistis at different stations. A onditional Weibull distribution is used to desribe the water level. In ProDeih a Lognormal distribution is used. The water depth depends on the water level and the level of the toe of the dike.

4 4 5. Wind veloity is determined on basis of the water level (for western wind diretions) in PC-Ring while in ProDeih the wind veloity is not diretly onsidered. 6. In ProDeih wave height and wave period both are normally distributed. The wave height is limited by the water level (breaker riterion). The wave period is limited by the wave steepness (max. 6%). In PC-Ring the atual wave height, period and diretion are derived from the wind diretion, wind veloity and water level. These parameters are indiret stohasti parameters. Exept for these differenes in the input parameters results for the model dike showed differenes due to the model fators and limit state funtions used and the way both models read the wind and wave statistis. This aused diffiulties for the atual alloation of auses for these differenes. An example of this is de limit state funtion of overtopping: PC-Ring (Steenbergen (2003)): h h Z = ln( q = + ) ln( moqo ) fb ln H sξγ mo 0.06 ProDeih (Kortenhaus (2003)): Z = R mr,min = h mh sξγ h + ln q gh 3 s tanα / s q 3 2gH s ξγ b Input parameters, like water level and wave height, are derived differently in both models. Also the fators and loation of several parameters (like the model fator m) ause different results. Table 2 presents an indiation of the ause and the values of differenes between both reliability tools whih have onsequenes for the results. Table 2. Influene of the different aspets of the reliability tools. Aspet Differene fator Water level 1 Wave height ± 10 Model fator 1 to > 100 Limit state funtion Variable, for example a fator 3 for piping Calulation method Variable CONCLUSIONS An advantage of PC-Ring is the more advaned stage of development. Some aspets that are more elaborated and the availability of the extensive hydrauli boundary onditions for The Netherlands give the program a head start on op (1) (2)

5 ProDeih. There are however also omponents of ProDeih that are of interest for a flexible and widely appliable reliability method. PC-Ring only onsiders the relevant failure mehanisms, whih simplifies the fault tree. The required input for the ombination program is in PC-Ring generated automatially. In ProDeih the failure probabilities have to be transferred manually. The stohasti and hydrauli input shows most differenes. The hydrauli boundary onditions are implemented very elaborately in PC-Ring. In PC-Ring more alulations are performed to obtain the failure probability. In PC-Ring failure mehanisms are mutually dependent. The amount of limit state funtions and the arrangement is totally different in both programs: The referene whih is used for a limit state funtion and the way this funtion is desribed already aused onsiderable differene in the results. The output from both programs onsists of the same parameters, but the presentation of the output is different. Reommendations onerning the software tool The new software tool an use aspets from both PC-Ring and ProDeih. Flexibility in the use on different types of areas, loations and flood defenes. Possibility to hange distribution types. Availability of the Duth hydrauli boundary onditions and the possibility to add relations whih are used in other ountries. Possibility to hoose the simple approah whih is available in ProDeih. Provide different types of alulation methods. Visualisation of the results in fault trees. DISCLAIMER This paper reflets the authors' views and not those of the European Community. Neither the European Community nor any member of the FLOODsite Consortium is liable for any use of the information in this paper. REFERENCES Kortenhaus, A., Probabilistishe Methoden für Nordseedeihe. Ph.D. thesis, Dissertation, Fahbereih Bauingenieurwesen, Leihtweiß-Institut für Wasserbau, Tehnishe Universität Braunshweig, Braunshweig, Germany, 154 S. Oumerai, H., Sustainable oastal flood defenes: sientifi and modelling hallenges towards an integrated riskbased design onept. Keynote leture, First IMA International Conferene on Flood Risk Assessment, Session 1, IMA- Institute of Mathematis and its Appliations, Bath, UK, pp Steenbergen, H.M.G.M., Vrouwenvelder, A.C.W.M., Gebruikershandleiding PC-Ring versie 4.0. TNO Rapport (2003-CI-R0023), TNO Bouw Delft 5

6 6 Steenbergen, H.M.G.M.; Lassing, B.L.; Vrouwenvelder, A.C.W.M.; Waarts, P.H., Reliability analysis of flood defene systems. Delft, The Netherlands: Delft University Press, Heron, vol. 49, no. 1, pp

7 KEYWORDS CSt COMPARISON OF COASTAL FLOODING PROBABILITY CALCULATION MODELS FOR FLOOD DEFENCES 1 st Author: de Boer, Elisabet 2 nd Author: Kortenhaus, Andreas 3 rd Author: van Gelder, Pieter, P.H.A.J.M. Flood risk Flood defene Reliability tools ProDeih PC-Ring 1