8th Iiilenmtional Symposium on Lowland Technolog)' September 11-13, 2012 in Bali, Indonesia ACT OF DESPAIR OR FULL-FLEDGED EXPERIMENT: RETROSPECTIVE RESEARCH ON THE 1945 WIEREVGERMEER FLOOD O.A.C. Hoes R.W. Hut' and M. Boomgaard^ ABSTRACT: The present state-of-the-art in flood risk assessment focuses on breach models, flood propagation models, and economic modelling of flood damage. However, models need to be validated with real data to avoid erroneous conclusions. This reference data can either be historic data, or can be obtained fi'om controlled experiments. The inundation of the Wieringermeer polder in the Netherlands in April 1945 is one of the few examples for which sufficient historical information is available. The objective ofthis article is to compare the flood simulation behaviour using flood data from 1945. The context, the breach gi'owth process and the flood propagation are explained. Key fmdings for current flood risk management addresses the importance of the drainage canal network during the inundation of a polder, and the uncertainty that follows from not knowing the breach gi'owth parameters. This case study shows that historical floods provide valuable data for the validation of models and reveals lessons that are applicable in current day flood risk management. Keywords: Floods, Simulation, Polder, Breach Growth. INTRODUCTION geological records to reconstruct surge heights of pre­ historic storms. Gottschalk (1977) used wiitten records Predicting how an embankment fails, how the and artwork to consfruct an overview of all floods from adjacent area fills up with fiood water and estimating the the 14th until 18th century in the Netherlands. More damage involved are essential parts of undertaking a recently. Baart et al. (2011) used 18th century paintings flood risk assessment. The importance of understanding and drawings to estimate storm surge levels. Lumbroso these three issues can be illustrated by the recent fioods & Vinet (2011) compared the February 1953 flood in in Pakistan in 2010 and 2011. The present state-of-the- England with coastal flooding in France in 2010. art focuses on breach models, flood propagation models, However, none of these reconstructed floods were and economic modelling of flood damage (Apel et al. sufficient to validate a flood model. 2004), (Manen and Brinldiuis 2004), (Apel et al. 2006), In our search for a proper dataset to validate a polder (Bouwer et al. 2009), (Apel et al. 2009). However, flood, we found one case for which the necessary models need to be validated with real data to avoid information is as complete as one would like. In 1945 erroneous conclusions and the implementation of the Wieringermeer polder area was flooded deliberately unsatisfactory measures. Such real data can either be by the German armed forces about three weeks before historical data, or can be obtained from controlled the end of World War II. The 118000 German occupying experiments. Full-scale experiments are difficult and troops in the Netherlands controlled the western expensive to accomplish. Experiments on a small scale provinces in 'Festung Holland', but were enclosed by have the problem that results camiot be extrapolated to Allied Forces. To prevent the Allied Forces to enter large areas. Accurate historical data for validation are Festung Holland through the Afsluitdijk fi-om the north, hard to get, even in the Netherlands with its long history the Germans blew up the embanlonent at two locations of floods. During large disasters - as the February 1953 that separated the Wieringermeer polder from Lake flood in England, Belgium and the Netherlands - IJssel. Actually, as the Allied Forces did not even everyone is busy saving his or her loved ones and attempt taking this route, one may quafify the belongings, instead of collecting data. Wieringermeer inundation as an act of despair instead of Sometimes, historical data can be obtained by a military-strategic necessity. Anyway, the water level at retrospective analysis. Jelgerma et al. (1995) used Lake IJssel was 0.12 meter above mean sea level (MSL), ' lALT member, Delfl University of Technology, PO box 5048, 2600 GA, Delft, THE NETHERLANDS 2 Delft University of Technology, PO box 5048, 2600 GA, Delft, THE NETHERLANDS ^ Hoogheemraadschap Hollands Noorderkwartier, PO box 250, 1700 AG, Heerhugowaard,THE NETHERLANDS 31 Hoes, el al. and the surface level adjacent to the location of the blast was 3.5 meter below MSL. At the start of the prospective 48 hours, the water seeped thi-ough the two grooves formed by the explosions, but these gi'ooves developed flu'ther and the polder raged full with a wall of water. Historic information fi-om this inundation is properly recorded and still available in different archives in the Netherlands, as the archive fi-om the Dutch Ministry of Infi-asti-ucture and the Environment, the Noord Hollands Ai-chive in Haarlem, Regional Ai-chive in Alkmaar, the Nieuwland Erfgoed Centre in Lelystad, and the NIOD Institute in Amsterdam. Available are the water levels from the adjacent Lake Ussel during World War II, the time of the blast, the size of the initial breach, the final extent of the breach, and information about times and locations where the water arrived and how fast the water level rose in the polder. These water levels were collected by those who refused to evacuate (e.g. Fig.1). By collecting and combining the information fi-om all Fig. 1 Office building in Wieringerwerf during the these sources to a full-fledged dataset, we could inundation. Ai-ound 30 people sheltered 3 weeks in this building until the capitulation on the 5th of May 1945 accurately reconstruct the Wieringerrneer flood. Such reconstructions are valuable for current flood enlarge the agi-icultural land in the Netherlands. It risk management. Especially in the Netherlands, it is measures 13 km fi-om west to east and 16 lan fi-om south inevitable for water authorities to be prepared for floods, to north (20.000 hectares). The surface level in the because of our low elevation compared to mean sea level. Wieringermeer polder descends fi-om 1 meter below All authorities practice how they will act in case of a MSL in the West towards 5 meters below MSL in the calamity and formulate evacuation plans with the East. The interface between Pleistocene sand and intention that these will lunit the damage and victims in Holocene clay slopes fi-om 4 meter below MSL in the case of a real flood. The starting point is that during this North to 13 meter below MSL in the South. The preparation all necessary knowledge is available thi-ough combination of these two slopes results in a thin clay flood simulations with detailed elevation data. However, layer in the Northeast, causing brackish seepage in the these models are run with default parameters based on canals in this area. theoretical exercises, and the results camiot be validated Two large pumping stations were built to discharge with data fi-om a real situation. The 1945 Wieringermeer water: pumping station Leemans (52°55'16"N, flood is the exception to the rule. 5°02'18"E) with two diesel engine driven pumps of 250 This paper presents the results of a retrospective m3/min at a head of 5.5 m and pumping station Lely analysis of this flood. We have collected available (52°46'35"N, 5°06'12"E) with three electric engine historical material, and built a combined 1D-2D flood driven pumps of 400 m3/min at a head of 6 m. propagation model that accurately reproduces the flood. The construction of the Wieringermeer was totally First we provide a description on the history of this event. organized by the national government, who created an In the second part we discuss the breach growth process ultramodern polder with - for those days - large parcels and inundation of the Wieringermeer polder, hi the last of 20 hectares and an extensive network of navigable part we discuss the reconstruction works and canals and sluices to transport agricultural products by reconstruction costs. In each part we will try to link the ship. Three villages were build: Wieringerwerf, flood in the Wieringermeer polder to the present flood Middenmeer and Slootdorp, of which the village modelling approach used in flood risk analysis. Wieringerwerf (located in the centre of the polder) was planned to become the main village. In retrospect, the three villages were built too close to each other and too WIERINGERMEERPOLDER far away fi-om the so called old land. Next, Middenmeer was build before Wieringerwerf and was also closer to the old land then Wieringerwerf As a resufi The Wieringermeer polder was reclaimed fi-om the Middenmeer developed naturally as a main village next sea between 1927 and 1930. This polder was the first of four large polders reclaimed between 1927 and 1980 to 32 Act of despair or full-fledged experiment : Retrospective research on the 1945 Wieringermeer Flood and September the German army inundated around 120 000 hectares by blocking the discharge of excess water (See also Fig. 2). A telegram fi-om the resistance on the 16th of February 1944 for the Dutch government in exile in England stated that the German army wanted to raise the water level of Lake IJssel (Waalewijn 1985). A high water level was required to have a necessary buffer of water for inundations. Starting fi-om the 27th of February the maintained water level became -0.13 m MSL. The pre-war target level was -0.2 m MSL in the summer and -0.4 m MSL in the winter. Starting from October the water level in Lake IJssel was further raised tin it reached +0.36 m MSL on the 18th of November 1944 (Fig.