Editing Afsluitdijk (section)

=== Developments after construction and height increases ===
The height of the crest of the Afsluitdijk was originally determined based on insufficient data about wave run-up. This became apparent soon after the first significant storm surge following the completion of the works, which occurred in December 1936. During the storm, the water in the [[Wadden Sea]] reached to around half a metre below the dike. For the crown height, storm surge heights indicated by the {{lang|nl|Staatscommissie Lorentz (SCL)}} ''(English: Lorentz State Commission)'' were primarily used, based on storm surges that had occurred in the period 1825–1926.<ref name="thijsse" /><ref name="stuvel" />

According to committee's predictions, if the Zuiderzee had been closed during the 1825–1926 period, the highest water level reached would have been 3.1 metres above Amsterdam Ordnance Datum at Den Oever, and about 3.4 metres on the Friesland coast. However, the commission drew attention to the fact that in a very rare and unfavourable set of circumstances, even higher storm surge heights were possible.<ref name= stuvel>{{Cite journal |last=Stuvel |first=H. J. |date=1957 |title=Het eerste offensief: 25 jaar afsluitdijk 1932-1957 |trans-title=The first offensive: 25 years of the Afsluitdijk, 1932-1957 |url=https://repository.tudelft.nl/islandora/object/uuid%3A3cc63b96-c863-40fb-932f-df4be94ee219 |journal=Staatsdrukkerij en Uitgeverijbedrijf |access-date=4 April 2023 |language=nl}}</ref>
[[File:Afsluitdijk predicted and 1894 storm levels.png|thumb|Observed storm surge water levels in 1894 for various locations in the Netherlands, and calculated levels of the same locations with a closed-off Zuiderzee]]
{|class="wikitable"
|+Key to table
|Z||Water level reached during storm in 1894
|-
|Δ||Calculated level increase with new dike
|-
|A||Calculated level with closed-off Zuiderzee
|-
|ΔG||Predicted increase in wave run-up.
|}
All values in centimetres. Z and A are in centimetres above NAP (Normaal Amsterdams Peil/Amsterdam Ordnance Datum).<ref name="thijsse" />

{| class="wikitable sortable"
!Location of the area or tide gauge
!Z
!Δ
!A
!ΔG
|-
|Kaaphoofd
|250
|43
|293
|0
|-
|Den Helder, gauge
|248
|44
|292
|0
|-
|Oostoever
|260
|48
|308
|15
|-
|Westerland (Wieringen)
|247
|79
|326
|15
|-
|Den Oever
|233
|113
|346
|25
|-
|Kornwerderzand
|270
|100
|370
|(15)
|-
|Harlingen
|269
|77
|346
|15
|-
|Nieuw Bildt
|271
|13
|284
|5
|-
|Ameland, Waddenkust
|330
|8
|335
|0
|-
|Terschelling, Waddenkust east
|340
|0
|325
|0
|-
|Terschelling, harbour
|286
|14
|300
|0
|-
|Vlieland, harbour
|287
|16
|303
|0
|-
|Vlieland Waddenkust, Posthuis
|260
|32
|292
|10
|-
|Eierland near Cocksdorp
|280
|30
|310
|0
|-
|Texel, Oostkaap
|245
|79
|324
|10
|-
|Texel, Oudeschild
|254
|65
|319
|10
|-
|Texel, South Point
|240
|50
|290
|0
|}

The lower value was based on the assumption that wave run-up in the shallow area near the Frisian coast would be reduced. However, it became evident as early as 1934 that this assumption was incorrect due to the narrowness of the shallow zone. To address this issue, a section of the dike body was removed. Despite these adjustments, a 1937 storm revealed that they were insufficient, leading to an increase in the crest height to 7.4 metres.

Later research on storm surge flood levels in the Netherlands by [[Pieter Jacobus Wemelsfelder]] and [[Johan van Veen]], along with the devastating effects of the [[North Sea Flood of 1953|storm surge of 1953]], would bring the reality of such extremely rare cases to light.<ref name= "thijsse" /><ref>{{Cite journal |last1=Vreugdenhil |first1=C. B. |last2=Alberts |first2=Gerard |last3=Gelder |first3=Pieter van |date=2001 |title=Waterloopkunde: een eeuw wiskunde en werkelijkheid |trans-title=Hydraulics: A century of mathematics and reality |url=https://research.utwente.nl/en/publications/waterloopkunde-een-eeuw-wiskunde-en-werkelijkheid |journal=Nieuw Archief voor Wiskunde |language=Dutch |volume=S 5, dl 2 |issue=3 |pages=266–276 |issn=0028-9825 |access-date=3 April 2023}}</ref><ref>{{Cite web |last=van Veen |first=J. |date=1939 |title=Te verwachten stormvloedstanden op de benedenrivieren : eerste voorlopige becijfering |trans-title=Expected storm surge levels on the lower rivers: first preliminary calculation |url=https://open.rws.nl/open-overheid/onderzoeksrapporten/@119625/verwachten-stormvloedstanden/ |access-date=4 April 2023 |website=Rijkswaterstaat Publicatie Platform |language=nl}}</ref><ref>{{Cite journal |last=Wemelsfelder |first=P. J. |date=1938 |title=Beknopte Nota over de Frequenties van Stormvloeden |trans-title=Concise note on the frequencies of storm surges |url=https://repository.tudelft.nl/islandora/object/uuid%3A0840f657-17bc-4274-a8a4-49f5c1749e83 |journal=Rijkswaterstaat |language=nl |access-date=4 April 2023}}</ref>

In determining the height for the construction of the inner berm, different considerations were employed compared to those for the main dam crest height. The primary requirement for the crest was to ensure the dike would not be breached during the most severe anticipated storm. To achieve this, the crest was designed to be elevated enough so that, at most, only a single wave could reach it. The IJsselmeer side of the dam was designed with a lower [[factor of safety]] than the Wadden Sea side, as it was anticipated that even under unfavourable conditions, such as a southwest storm over the IJsselmeer with a high average water level, the waves would not exceed about 3.5 metres above Amsterdam Ordnance Datum. Consequently, the berm was constructed to a height of approximately 4 metres.<ref name= "thijsse" />

The inner berm slope toward the future water level of the IJsselmeer (or Amstelmeer) was designed to be steeper than the outer slope, with a ratio of approximately 1:3. The majority of the dam's core consisted of sand, while boulder clay on the exterior provided resistance against water flow, limiting seepage and enhancing dike breach protection. A layer of [[armourstone]], composed of basalt rock, was utilized on the outer slope.<ref name= "thijsse" /><ref name= "stuvel" />

During construction, the slopes were partially fortified with basalt columns and Belgian block stone {{lang|nl|Belgische Bloksteen}}, a block-shaped, hard variety of limestone. Block stone served as a more affordable and widely available alternative to basalt, and the Dutch Government resisted purchasing large quantities of basalt to avoid potential price increases, instead planning to gradually replace the block stone with basalt.<ref name= "thijsse" />