Question: What event in 1170 may have caused the creation of 5,000 km2 large shallow bay of the North Sea in the northwest of the Netherlands?

full_context: Document 1: Southern Bight: Southern Bight is the southern bight of the North Sea bounded by the coasts of the Netherlands, Belgium, France and Great Britain. The Southern Bight is south west of the German Bight and the Wadden Sea. The Southern Bight is roughly delimited in the north by the line The Wash-West Frisian Islands, south of the Dogger Bank, a large shallow part in the North Sea, and the Outer Silver Pit, a deep water channel south of the Dogger Bank.
Document 2: Flevoland: Flevoland (] ) is the twelfth province of the Netherlands. It is located in the centre of the country, where the former Zuiderzee was.
Document 3: 2011 Souris River flood: The 2011 Souris River flood was greater than the hundred-year flooding event for the Souris. The US Army Corps of Engineers estimated the flood to have a recurrence interval between 200 and 500 years. The Souris River is a tributary of the Assiniboine River, which it meets near Treesbank, Manitoba. The Assiniboine meets the Red River of the North in Winnipeg, Manitoba. The flooding has affected Saskatchewan and North Dakota, and overtopped levees in Minot, North Dakota causing the evacuation of about 11,000 residents. The flooding in Minot was worse than the 1969 flood and 1881 flood. Many other towns along the river were affected and many acres of farmland were inundated.
Document 4: Friesland: Friesland (] ; official, West Frisian: Fryslân ] ) or Frisia is a province in the northwest of the Netherlands. It is situated west of Groningen, northwest of Drenthe and Overijssel, north of Flevoland, northeast of North Holland, and south of the North Sea. In 2010, the province had a population of 646,000 and a total area of 5,100 km2 .
Document 5: Black Sea deluge hypothesis: The Black Sea deluge is a hypothesized catastrophic rise in the level of the Black Sea circa 5600 BC from waters from the Mediterranean Sea breaching a sill in the Bosphorus strait. The hypothesis was headlined when "The New York Times" published it in December 1996. It was later published in an academic journal in April 1997. While it is agreed that the sequence of events described by the hypothesis occurred, there is significant debate over the suddenness, dating and magnitude of the events. Over geological eras, water has flowed in and out of the Black Sea basin. This hypothesis concerns the occurrence of the last inflow and the primary point of controversy is whether the event was gradual or catastrophic.
Document 6: Zuiderzee: The Zuiderzee (] ; old spelling "Zuyderzee") was a shallow bay of the North Sea in the northwest of the Netherlands, extending about 100 km (60 miles) inland and at most 50 km (30 miles) wide, with an overall depth of about 4 to 5 metres (13–16 feet) and a coastline of about 300 km (200 miles). It covered 5,000 km2 . Its name means "southern sea" in Dutch, indicating that the name originates in Friesland, to the north of the Zuiderzee (cf. North Sea). In the 20th century the majority of the Zuiderzee was closed off from the North Sea by the construction of the Afsluitdijk, leaving the mouth of the inlet to become part of the Wadden Sea. The salt water inlet changed into a fresh water lake now called the IJsselmeer (IJssel Lake) after the river that drains into it, and by means of drainage and polders, an area of some 1500 km2 was reclaimed as land. This land eventually became the province of Flevoland, with a population of nearly 400,000 (2011).
Document 7: All Saints' Flood (1170): The All Saints' Flood of 1170 (Allerheiligenvloed) was a catastrophic flood in the Netherlands that took place in 1170. Large parts of the Northern Netherlands, and Holland territories were overflowed. This is the flood that created the Zuiderzee. "Lake Flevo" was once a fresh water lake, but after this flood a sea channel opened a connection from the North Sea into the lake through Creil Woods, and turned the lake into the salt-water sea known as the Zuiderzee. The Creiler Forest vanished forever under the waves.
Document 8: 1997 Red River flood: The Red River flood of 1997 was a major flood that occurred in April and May 1997 along the Red River of the North in Minnesota, North Dakota, and Southern Manitoba. It was the most severe flood of the river since 1826. The flood reached throughout the Red River Valley, affecting the cities of Fargo and Winnipeg, but none so greatly as Grand Forks and East Grand Forks, where floodwaters reached more than 3 mi inland. They inundated virtually everything in the twin communities. Total damages for the Red River region were US$3.5 billion. The flood was the result of abundant snowfall and extreme temperatures.
Document 9: Delta Works: The Delta Works (Dutch: "Deltawerken" ) is a series of construction projects in the southwest of the Netherlands to protect a large area of land around the Rhine-Meuse-Scheldt delta from the sea. The works consist of dams, sluices, locks, dykes, levees, and storm surge barriers. The aim of the dams, sluices, and storm surge barriers was to shorten the Dutch coastline, thus reducing the number of dikes that had to be raised.
Document 10: Limfjord: The Limfjord (common Danish: "Limfjorden" ] , in north Jutlandish dialect: "Æ Limfjord") is a shallow part of the sea, located in Denmark where it is regarded as a fjord ever since the Vikings. However it has inlets both from the North Sea and Kattegat and hence separates the island of North Jutlandic Island (Danish: "Nørrejyske Ø", which includes the old provinces "Vendsyssel", "Han herred" and "Thy") from the rest of the Jutland Peninsula. It extends from Thyborøn Channel on the North Sea to Hals on the Kattegat. It is approximately 180 kilometres long and of an irregular shape with numerous bays, narrowings, and islands, most notably Mors, and the smaller ones Fur, Venø, Jegindø, Egholm and Livø. It is deepest at Hvalpsund (24 metres). The Limfjord is not a fjord in the English geological sense — in Danish, as in Norwegian, the term "fjord" can refer to a wide range of waterways.
Document 11: 2011 Thailand floods: Severe flooding occurred during the 2011 monsoon season in Thailand. The flooding began at the end of July triggered by the landfall of Tropical Storm Nock-ten. These floods soon spread through the provinces of northern, northeastern, and central Thailand along the Mekong and Chao Phraya river basins. In October floodwaters reached the mouth of the Chao Phraya and inundated parts of the capital city of Bangkok. Flooding persisted in some areas until mid-January 2012, and resulted in a total of 815 deaths (with 3 missing) and 13.6 million people affected. Sixty-five of Thailand's 77 provinces were declared flood disaster zones, and over 20000 sqkm of farmland was damaged. The disaster has been described as "the worst flooding yet in terms of the amount of water and people affected."
Document 12: Upper Thames River Conservation Authority: The Upper Thames River Conservation Authority is a body based in London, Ontario, Canada. It was created in 1947. It was responsible for the construction of the Fanshawe Dam, completed in the 1950s, to control flooding from the Thames River, which runs through London. During the last ice age, the site of London was the terminus of a large glacier. When the region warmed at the end of the ice age, the glacier melted and receded North, leaving behind a drainage ditch and features such as Sifton Bog. As such, the Thames is a watershed for most of Western Ontario, and is therefore highly susceptible to seasonal flooding. In 1937, such flooding had devastating consequences, destroying over 1,000 homes, and causing millions of dollars in damage.
Document 13: Rhine–Meuse–Scheldt delta: The Rhine–Meuse–Scheldt delta or Helinium is a river delta in the Netherlands formed by the confluence of the Rhine, the Meuse and the Scheldt rivers. The result is a multitude of islands, branches and branch names that may at first sight look bewildering, especially as a waterway that appears to be one continuous stream may change names as many as seven times, e.g. Rhine → Bijlands Kanaal → Pannerdens Kanaal → Nederrijn → Lek → Nieuwe Maas → Het Scheur → Nieuwe Waterweg (→ North Sea). Since the Rhine contributes most of the water, the shorter term "Rhine Delta" is commonly used. However, this name is also used for the river delta where the Rhine flows into Lake Constance, so it is clearer to call the larger one Rhine-Meuse delta, or even "Rhine-Meuse-Scheldt Delta", as the Scheldt ends in the same delta. By some calculations, the delta covers 25,347 km2 , making it the largest in Europe.
Document 14: North Holland: North Holland (Dutch: "Noord-Holland" ] , West Frisian: "Noard-Holland") is a province in the northwest of the Netherlands. It is situated on the North Sea, north of South Holland and Utrecht, and west of Friesland and Flevoland. In 2015, it had a population of 2,762,163 and a total area of 2,670 km2 .
Document 15: Storm of 1897: The storm of 1897 was a severe storm that struck the lower to central North Island of New Zealand on 16 April 1897. It caused the ship "Zuleika" to run aground near Cape Palliser, with the loss of 12 lives, and severe flooding. At Clive the flooding caused the loss of a further 12 lives and one person was drowned near Kapiti. There were six further unconfirmed reports of drowning, bringing the total loss of life directly related to the storm event to between 25 and 31. Based on descriptions of the storm, particularly that of Captain Marten of the "Waiapu", it may have been an extratropical cyclone.
ground_truth: All Saints' Flood