Pieter Jacobus Wemelsfelder | |
|---|---|
.jpg) Pieter Jacobus Wemelsfelder (1907-1995), pictured in 1943 | |
| Born | (1907-11-18)18 November 1907 Goes, The Netherlands |
| Died | 1 July 1995(1995-07-01) (aged 87) Almen, The Netherlands |
| Nationality | Dutch |
| Citizenship | Netherlands |
| Alma mater | Delft University of Technology |
| Known for | Probabilistic approach to determining flood levels for storm surges |
| Scientific career | |
| Fields | Civil Engineering Hydraulic Engineering Hydrometry |
| Institutions | Rijkswaterstaat Waterloopkundig Laboratorium |
Pieter Jacobus (P.J.) Wemelsfelder (18 November 1907 – 1 July 1995) was a Dutchhydraulic engineer who made significant contributions to the field ofhydrometry in the Netherlands, and in hydraulic engineering internationally. In addition to his involvement in the design and planning of theDelta Works, he published widely and is notable for the first use ofprobability theory in the design offlood levels.
Wemelsfelder introduced a systematic approach to understanding and predicting the occurrence of storm floods, considering both the characteristics of the sea's probable and possible heights and thehuman and economic interests at stake. His methodology involved creatingfrequency curves for storm floods, using a standard frequency curve applicable to differentgauges worldwide, and classifying storm floods based on theirprobability of exceedance. This classification system helped in understanding the variability of maximum storm floods over different time periods.
Wemelsfelder emphasised the importance of considering both the period and risk when determining design levels, advocating for a two-dimensional approach to flood protection. His approach required establishing a frequency curve for each gauge, determining the period during which the risk is present, and choosing anacceptable total risk value for serious damage, generally not exceeding 10%, and as low as 0.1% for critical areas. He noted the need to balance the costs of safety measures with the economic and human values they protect, and recognised the importance of incorporating contingency in design to account for uncertainties.
The body set up by theDutch Government in response to the1953 North Sea Flood, the Delta Commission, adopted Wemelsfelder's probabilistic methods, setting design levels based on a risk of total loss of 1 in 1,000 years for critical areas, ensuring a high level of safety. His contributions have had a lasting impact oncoastal engineering and continue to inform the design and implementation offlood defences in the Netherlands and beyond.
Wemelsfelder was born inGoes, the son of Jacob Abraham Wemelsfelder and Jannigje Verschoor, in 1907.[1][2] After completing his studies at theDelft University of Technology, Wemelsfelder worked at theWaterloopkundig Laboratorium and later atRijkswaterstaat, where he served as the head of the Hydrometric Department of the Water Management and Movement Directorate. One of his major accomplishments was the development of methods and instruments for hydrometry in the Netherlands.[3][2]
Wemelsfelder introduced aprobabilistic approach to determining design flood levels forstorm surges in the Netherlands. Prior to his work,flood protection measures were based on adeterministic approach that relied on the highest previously recorded water levels, along with some estimation. For example, the height of theAfsluitdijk was determined based on the highest observedstorm surge,[4] with the height of the crest determined based on insufficient data aboutwave run-up. This became apparent soon after the first significant storm surge following the completion of thedike in December 1936, when water in theWadden Sea reached to around half a metre below the dike.[5]
In 1938, Wemelsfelder introduced a significant change in the design approach through a brief note on the frequency of storm surges, in which he carried out astatistical analysis of water levels measured between 1888 and 1937 at Hoek van Holland to derive the probability distribution of such events. Wemelsfelder determined a statistical estimate of the cumulative distribution of sea-level heights during high tide, and determined that theexceedance frequencies, where represents the number of times the level was exceeded during years, closely followed a straight line when plotted onlogarithmic paper.[6]
.jpg)
.jpg)
Prior to Wemelsfelder's work, S.H.A. Begemann had applied statistical methods tohydrological aspects such asprecipitation and runoff for irrigation. In theUnited States, publications onstochastic hydrology such as those byAllen Hazen and others had been appearing since the early twentieth century.[7] However, Wemelsfelder's statistical analysis of water levels measured between 1888 and 1937 atHoek van Holland enabled the derivation of aprobability distribution of storm surges. He published his findings in the Dutch journalde Ingenieur in March 1939, which revolutionised the way flood protection measures were designed in the country.[8][9]
By using a frequency curve on alogarithmic scale, Wemelsfelder showed that an effective statistical overview of storm surges could be obtained. His 1939 paper demonstrated that the structure of thedistribution of storm surges over the years, both in terms of strength and frequency, can be accurately represented by a probability law.[8]
In 1939, the establishment of theStormvloedcommissie(Storm Surge Commission) was prompted by concerns regarding the state of manydikes inZeeland. Under the leadership ofJohan van Veen, the commission adopted Wemelsfelder's probabilistic approach as the basis for determining the probability of water level exceedance and the calculation of dike heights. This was a departure from the earlier approach of relying solely on previously recorded high water levels.[10]
Despite the commission's recommendation to raise the levels of the dikes, theGovernment of The Netherlands did not take action, and the dike system remained vulnerable throughoutWorld War II. After the war, attention turned to rebuilding efforts, and this was exacerbated in 1953 further to acatastrophic flood that claimed 1,836 lives in The Netherlands, and caused billions ofguilders in infrastructural damage.[11]
The storm surge associated with the 1953 flood saw water levels reach 3.85 metres aboveNormaal Amsterdams Peil (NAP) atHoek van Holland, higher than the crest height of the dikes which had been determined based on previously recorded highest water levels (3.28 metres above NAP) at the same location.[12][13][14][11]
In response, the Dutch Government formed theDeltacommissie(Delta Commission), which was charged with making recommendations for reducing the risk of such disasters. The commission relied heavily on the analysis and solutions put forth by the Storm Surge Commission, which had already adopted Wemelsfelder's probabilistic approach for determining dike heights.[15][16]
The Delta Commission recommended a target exceedance frequency of 10−4 per year as the basis for design levels in central Holland, and its work led to the enactment of the 1958 Delta Act. Wemelsfelder's work actually considered a factor of safety that increased the exceedance frequency to that which corresponds tom=10−5, wherem is the acceptable risk. This corresponds to atotal loss figure of 1% in a 1000-year period.[17]
Wemelsfelder made significant contributions to the Delta Commission's analysis and recommendations, and was active in research throughout his career, publishing a number of technical papers in Dutch and English.[18][19][20][21][11][22][23] His findings continue to informflood protection measures in the Netherlands today.[24][25][26][27]
In 1946, Wemelsfelder published a 358-pagemonograph focused on topics such ascultural development,societal change, the structure of anorganic society,governance,legislation, andinternational order, entitledPlan voor een redelijke Nederlandse samenleving(English: Plan for a reasonable Dutch society).[28]
The book set out Wemelsfelder's thoughts on how The Netherlands could organise society in political and economic fields in the immediate aftermath of theSecond World War and dealt with issues including combatingunemployment, the establishment of a savings credit bank, reorganisation of banking and industry,pensions, the introduction ofchild allowance and the introduction of a separate income for married women.[29]
Between 1965 and 1972, Wemelsfelder prepared reports on various storm surges which occurred in the Netherlands. All reports from this period are available atTU Delft Repository:Link:
| International | |
|---|---|
| National | |
