Long tradition of landscape reconstruction in the Netherlands

In the Netherlands there is a long tradition of composing palaeogeographic maps. The manuscript map of the Roman period by Arnoldus Buchelius (1565–1641) is the oldest known reconstruction map of the Netherlands. It is based on Roman writers such as Tacitus and Pliny. In 1574, Jacob van der Mersch made a reconstruction map of the drowned and lost lands in the Dollard following similar Flemish maps of the drowned lands along the Westerschelde and Oosterschelde (Knottnerus, Reference Knottnerus and Essink2013). Later historians investigated the courses of former rivers such as the Fivel in Groningen and lateral migrations of the Wadden Islands (van Veen, Reference van Veen1930; Isbary, Reference Isbary1936).

In the 1950s Pons and co-workers began the first map reconstructions of the Dutch coastal area based on the geological data of Noord-Holland and Flevoland, and subsequently of the whole of the Netherlands (Pons & Wiggers, Reference Pons and Wiggers1959, Reference Pons and Wiggers1960; Pons et al., Reference Pons, Jelgersma, Wiggers, de Jong and de Jong1963). In the 1970s and 1980s regional studies of Groningen and Friesland were done (Roeleveld, Reference Roeleveld1974; Griede, Reference Griede1978; Griede & Roeleveld, Reference Griede and Roeleveld1982) and it became clear that before the Middle Ages large parts of the western and northern Netherlands were covered with an extensive peat bed that in many places has almost or completely disappeared. This peat bed covered West Friesland and the lower parts of Friesland and Groningen (Borger, Reference Borger2007; Slofstra, Reference Slofstra, Huisman, Bekkema, Bos, de Jong, Kramer and Salverda2008). This finding changed the palaeogeographic map picture dramatically as compared to the previous palaeogeographic maps of Zagwijn (Reference Zagwijn1986).

The idea of an extensive peat cover on the sandy areas of Oostergo, Westergo and the Groninger Wold, confirmed by several observations below cemeteries, was quickly embraced by archaeologists and historical geographers. It agrees with the fact that there are hardly any archaeological observations in the time span between Late Neolithic and Bronze Age finds on the one hand, and Medieval finds on the other. The medieval colonisations and their consequences for the peat area were the subject of many archaeological and historical studies: Wassermann (Reference Wasserman1985), Mol et al. (Reference Mol, Noomen and van der Vaart1990), Casparie & Molema (Reference Casparie and Molema1990), Groenendijk & Schwarz (Reference Groenendijk, Schwarz and Knottnerus1992), de Langen (Reference de Langen1992), Knol (Reference Knol1993), Fokkens (Reference Fokkens1998), Schwarz (Reference Schwarz2004, Reference Schwarz and Schmid e.a.2005), Ligtendag (Reference Ligtendag1995), Knottnerus (Reference Knottnerus2005, Reference Knottnerus2008), Bakker (Reference Bakker2001, Reference Bakker2002, Reference Bakker2003), Brinkkemper et al. (Reference Brinkkemper, Bijkerk, Brongers, Jager, Kosian, Spek and van der Vaart2006), Spek (Reference Spek, Huisman, Bekkema, Bos, de Jong, Kramer and Salverda2008), Groenendijk & Bärenfänger (Reference Groenendijk and Bärenfänger2008), van den Broek (Reference van den Broek2011) and Kortekaas (Reference Kortekaas2013).

Many subregional reconstructions were made during previous decades, often with an archaeological background, of the IJsselmeer (or former Zuiderzee area; Lenselink & Koopstra, Reference Lenselink, Koopstra, Rapporl and Soonius1994; Lenselink & Menke, Reference Lenselink and Menke1995), the Lauwerszee region (Vos, Reference Vos1992; Groenendijk & Vos, Reference Groenendijk and Vos2002), Westergo (Vos, Reference Vos and Besteman1999; Vos & Gerrets, Reference Vos and Gerrets2004), the northern Dutch coastal area (Vos & Knol; Reference Vos, Knol and Knol e.a.2005; Vos & Bungenstock, Reference Vos, Bungenstock and Kegler2013; Vos & Knol, Reference Vos, Knol and Essink2013), Oostergo (Vos & de Langen, Reference Vos, de Langen and Huisman2008) and the Eems-Dollard (Vos, Reference Vos2011).

Elsewhere, regional palaeogeographical maps were produced of Zeeland (Vos & van Heeringen, Reference Vos and van Heeringen1997), the Oer-IJ (Vos et al., Reference Vos, Bunnik, Cremer and de Vries2010) and the Kop van (northern part of) Noord-Holland (Lambooij, Reference Lambooij1987; Woltering et al., Reference Woltering, Besteman, van Regteren Altena and Hallewas1999). The Nationale Onderzoeksagenda Archeologie (National Research Agenda Archaeology; Vos, Reference Vos2006) was an incentive for constructing new palaeogeographic maps of the Netherlands resulting in the Atlas van Nederland in het Holoceen (Atlas of the Netherlands in the Holocene; Vos et al., Reference Vos2011). An update of these maps (version 2.0) has been made in Vos (Reference Vos2013a). Amongst other features, in this new version a distinction is made between intertidal flats (flooded twice daily) and salt marshes (flooded only during spring tide and/or storm surges), and the reconstructed peat expansion of the 100 and 800 AD maps has been modified significantly based on new data from the area to the northeast of the town of Schagen (Vos, Reference Vos2012). The distinction between mudflats and salt marshes on the new maps makes clear where people could settle (salt marshes) in the tidal area and where they could not (sand and mudflats). It provides an insight into the occupation possibilities in the past and the opportunity of finding archaeological sites.

Coastal reconstructions in Germany

Unlike in the Netherlands, palaeogeographic reconstructions in northwestern Germany have mainly been made by historians and archaeologists, and focus in particular on coastline development. In 1909, Ramaer compiled a series of maps from the late Middle Ages (1250 AD) up to the present in which the evolution of the Eems–Dollard region in both countries was reconstructed. A map of this area in 1000 AD was compiled by Wasserman (Reference Wasserman1985). A detailed study of the mouth of the Eems based on ancient nautical maps is also available (Lang, Reference Lang1955, Reference Lang1958).

In 1943 a series of reconstructions of the Ostfriesische islands from 1623 onwards was made by Backhaus. Sketches of the original locations of Langeoog Island, from the 8th to the 2nd century BC and from the 1st and 13th centuries AD, were compiled by Barckhausen (Reference Barckhausen1969).

Linke's (Reference Linke2001) study of the geology of Neuwerk Island is useful for reconstructing the landscape there. Rough sketch maps of the coastal evolution from 7500 and 6500 BC to 800 AD were published by Flemming & Davis (Reference Flemming and Davis1994). Homeier (Reference Homeier1969, Reference Homeier1977) reconstructed the successive coastlines and made a regional reconstruction of northwestern Germany around 800 AD. He rightly suggested a major expansion of peat areas in the higher Pleistocene sandy areas. Behre (Reference Behre1999) reconstructed the coastlines of the marsh areas in the northwestern German coastal area for 1500 and 1800 AD. For the Reiderland, Behre (Reference Behre1986a) made detailed landscape reconstructions. As part of his sea-level studies, he also proposed palaeo-coastlines. The study of the Jade–Weser area resulted in a number of palaeogeographic maps for the beginning of the Christian era and for the years 800 AD and 1362 AD (Behre, Reference Behre1999; Reference Behre2004, Reference Behre2005). Palaeogeographic maps of the area around the Jadenbusen and Weser were made by Behre (Reference Behre2012) for the time slices around the birth of Christ, and 800, 1300, 1362 and 1520 AD. The peat land extension on these maps was derived from geological map data and did not change in these reconstructions.

Laser altimetry data of the ground level (Actueel Hoogtebestand Nederland (AHN) in the Netherlands, Digitale Geländmodelle (DGM) in Germany, Light Detection And Range (LIDAR in the UK) up to now have hardly been used in geo- and archaeo-landscape research of Niedersachsen. The study of Heinze (Reference Heinze2013) of the area around Bensersiel was one of the first.

700–100 BC (Fig. 8)

In the Early Iron Age (650–0 BC), the Dutch tidal basins were still open to a great extent, and the salt marshes of the German part were located mainly within the current Wadden Sea. In subsequent periods, the northern Dutch tidal basins silted up more and more, and became salt-marsh land, whereas in large parts of the northwest German coast the salt marshes were subject to erosion and retreated because of the exposed position to the sea.

Around 500 BC parts of the ancient tidal basins of the Boorne, Hunze, Fivel and Ur-Jade were still open, and the Zuiderzee had not yet come into existence. Two large Flevo lakes were present in the IJsselmeer region. The Overijsselse Vecht River flowed into the northern Flevo Lake, which was connected with the Wadden Sea. Through this outlet, marine influence was noticeable in the surroundings of Medemblik (Vos et al., Reference Vos, Bazelmans, Weerts and van der Meulen2011).

As a result of ingressions in the northern and southern Flevo lakes around 400 BC, the Zuiderzee came into existence (Vos et al., Reference Vos, Bazelmans, Weerts and van der Meulen2011). In this period the Oer-IJ stopped draining the southern Flevo Lake and the Utrechtse Vecht River started to silt up. Since then the peat area around the Zuiderzee and the Utrechtse and Overijsselse Vecht rivers discharged into the Wadden Sea. Through the enlarged outlet the sea penetrated more and more, resulting in further erosion and flooding of the peat area. Observations at the location Schagen de Nes, northeast of Schagen (Vos, Reference Vos2012), show that already in the Middle Iron Age clay was deposited on the peat along the draining channels. The drowning of the peat in this region continued into the Middle Ages (450–1500 AD). It is likely that humans played a role in this drowning because in the northern part of Noord-Holland many traces of Roman and medieval peat reclamations occur.

Around 500 BC the salt-marsh area in the tidal basins of the Boorne, Hunze and Fivel rivers had expanded strongly as the result of a continued silting-up. With the silting of the Boorne tidal basin the drainage systems of the Marne and Middelzee became the outlet for the hinterlying peat area and they increased in size and became ingression systems. The expansion of the Middelzee in western Oostergo began around 1200–1000 BC. In the first stage this was a natural ingression that continued until Roman times, when marine influence of the expanding channels of the Middelzee became increasingly noticeable in the region around Sneek (Vos, Reference Vos2007; Vos & de Vries, Reference Vos and de Vries2009, Reference Vos and de Vries2011).

In the eastern part of Oostergo, the area between the Boorne and the Hunze basins, large-scale marine flooding took place in the peat area present at that time (Griede, Reference Griede1978). This ingression channel is named after the local Paesens River, which continued until Engwierum. On both banks of this palaeo-channel clay was deposited on the peat. The development of the Paesens system did not have an anthropogenic cause because humans did not yet inhabit this part of the coastal area on a large scale. The ingression of the Paesens system into the peat area of De Kolken is related to the opening of the coastline east of Ameland. In the first half of the Holocene the coastal area there protruded into the sea for a long time. This headland in the coastline was related to the Pleistocene High of Damwoude in the subsurface, a continuation of the Drents Plateau. It is assumed that due to the straightening and retreat of the coastline near Ameland the coastal barrier to the east was eroded. The hinterlying peat area was not protected anymore, and subsequently a self-reinforcing peat drowning process started, resulting in the Paesens ingression system.

The silting-up of the Hunze and Fivel basins continued gradually after 500 BC. The enlargement of the salt marshes in these areas has been archaeologically dated on the basis of the ages of dwelling mounds (terpen), which become younger seaward (Vos & Knol, Reference Vos, Knol and Knol e.a.2005).

Around 500 BC, the main channel in the Eems estuary was located near the current dike between Emden and Greetsiel. In the Iron Age and Roman Period (0–450 AD) this channel migrated towards the area between Bierum and the Punt van de Reide. There, this led to a landward (transgressive) shift of the mudflat/salt-marsh border and clay deposition on the adjacent peat area (Theunissen et al., Reference Theunissen, van Doesburg, Vos and Laarman2005). These deposits are referred to as the Eems Clay (de Smet, Reference de Smet1960, Reference de Smet1962). In contrast, on the German side of the Eems silting-up occurred as a consequence of the diversion of the main channel and the marshes expanded seaward, and tidal channels, such as the Sielmönkener Bucht, gradually silted up (see map reconstructions in Vos & Bungenstock, Reference Vos, Bungenstock and Kegler2013 and Vos & Knol, Reference Vos, Knol and Essink2013).

Northwest of Norden a Pleistocene high, the continuation of the Oldenburger-Ostfriesland Plateau, was oriented in the direction of the island of Juist. This Pleistocene headland, the High of Norden, is comparable to the Damwoude High. Around 500 BC a salt-marsh area called Bant was present at this high. This salt-marsh area gradually disappeared due to continuing erosion and the enlargement of the tidal flat area (Lang, Reference Lang1951; Haarnagel, Reference Haarnagel1979). The tidal-flat/salt-marsh coastline between Esens and Wangerooge was located in the current Wadden Sea (Heinze, Reference Heinze2000; Niederhöfer, Reference Niederhöfer2013), and the inland coastal peatbog had reached its northernmost position. Only small peat-draining rivers like the Benser, Harle and Crildumer debouched into the Wadden Sea. At that time no large ingression had yet developed in this region.

The Ur-Jade (the forerunner of the current Jadebusen) and the Maadebucht together formed a tributary of the Weser estuary with its mouth between Butjadingen and Cuxhaven (Behre, Reference Behre2012). The Ur-Jade and Ur-Maade were drainage rivers of the hinterlying peat area.

In the course of the Iron Age, in both the German and Dutch parts of the Wadden Sea, occupation intensity increased. This is evidenced by the increase in the number of mounds in these areas. Also the border zone of the peat bogs adjacent to the coastal area was more and more intensively occupied, amongst others, between Leeuwarden and Sneek (Lubbers & Osinga, Reference Lubbers and Osinga2007) and along the seaward margin of the former peat bog northwest of Bensersiel (Heinze, Reference Heinze2000). Thus humans played an increasingly important role in shaping the rural landscape from Roman times onwards (Behre, Reference Behre2012). In the salt marshes and the adjacent peat zone, ditches and trenches were dug on a large scale. In all archaeological excavation sites from that period artificial drainage structures have been encountered. The ditches were connected to natural drainage systems, so that drainage of the area was improved artificially.

As a result of the digging of the ditches, tidal water could be transported easier and faster from the Wadden Sea to the hinterland through the dug ditches and canals. The drainage pattern of natural channels and creeks was artificially changed so that channel systems could cannibalise others, and the groundwater level in the drained area was lowered, allowing oxygen to penetrate the soil so that peat material decomposed (also below the clay cover) and disappeared, particularly at the seaward margin of the peat lands. Grey, organic and humus-poor clays were formed because the plant material of the marsh surface was oxidised completely.

Particularly along the seaward margin of the peat the effects of anthropogenic interventions were extensive because artificial drainage there resulted in strong subsidence. Large-scale man-made parcellation patterns from that time occur, amongst others, in Schagerbrug (west of Schagen), Hempens (south of Leeuwarden; Waldus, Reference Waldus1999) and near Kolken (Nicolay, Reference Nicolay2010). The palaeolandscape research at the archaeological site of Arkum (south of Bolsward) showed that the grey clay cover was deposited from the (tidal) ditches that had been dug there, since major tidal creeks were lacking in the area. Those ditches drained the area well and the groundwater level in the marsh was lowered. From about 100 AD onwards no organic matter was retained in the salt-marsh clay anymore and a grey, humus-poor clay was deposited. Also east of Arkum (near Sneek) the seaward margin of the peat area was silted over after the Late Iron Age and a clay layer was deposited. There too, it is plausible that the digging of ditches played an important role in the silting over. It is likely that the Middelzee tidal channel north of Sneek was a ditch or canal initially. Its straight course and the fact that this channel cuts through a Pleistocene ridge in the subsoil support this view. The incision of the channel to a depth of about 10 m –NAP is explained by the fact that this ‘anthropogenic channel’ has taken over part of the discharge of the Marne (Vos & de Vries, Reference Vos and de Vries2011). This expansion of the tidal storage area of the Middelzee resulted in strong tides and thus to an enlargement and deepening of the man-made ditch.

Small-scale dike structures were already made in the Late Iron Age and Roman Period. Remnants of these dike structures have been found during excavations in the dwelling mounds of Wijnaldum, Dongjum, Peins-Oost and Anjum (Bazelmans et al., Reference Bazelmans, Gerrets, Koning and Vos1999; Nicolay, Reference Nicolay2010). They were primarily intended to protect the farmlands around the dwelling mounds (terps).

100 BC–400 AD (Fig. 9)

During the Late Iron Age and Roman times, at increasingly more locations in the northwestern Dutch and German Wadden Sea, marine sedimentation in the marginal peat lands took place. Clay deposits from that time on the marginal peat bed have been found in the northern part of Noord-Holland (Vos, Reference Vos2012), in the region around Sneek (Lubbers & Osinga, Reference Lubbers and Osinga2007) and east of Leeuwarden near Wartena in the central and eastern part of Groningen (Wold area and vicinity of Appingedam; de Smet, Reference de Smet1960, Reference de Smet1962), and in the coastal area between Norden and Esens (Streif, Reference Streif2004). This is striking because in the tidal basins of the rivers Boorne (Westergo) Hunze, Fivel (Vos, Reference Vos and Besteman1999; Vos & van Kesteren, Reference Vos and van Kesteren2000) and east of the Eems estuary between Emden and Greetsiel (Behre, Reference Behre1999) the development of the salt marshes was largely regressive. Subsidence in the border zones of the peat areas – caused by human reclamations – is likely to have played a role all around the northwestern Wadden Sea. However, more factors may have played a role in the drowning of the border zones of peat areas. The continuous sea-level rise will certainly have contributed to the inundation during major floods. The average sea level rose by about 5–10 cm per century (Kiden et al., Reference Kiden, Denys and Johnston2002, Reference Kiden, Makaske and van de Plassche2008). In addition, specific regional factors may have played a role, such as an increase in the tidal inlet system, by which the maximum storm surge levels in the tidal hinterland could rise.

In the northeastern part of Groningen, the southwestward migration of the Eems Channel certainly had an impact on the drowning of the peat in the hinterland during the Late Iron Age and the Roman period.

In this period, in Wangerland, west of the Weser estuary, the sea intruded into the peat area and the Crildumerbucht ingression system developed (Behre, Reference Behre1999, Reference Behre2004, Reference Behre2012). Datings of the top of the peat – below the covering clay layer – indicate, however, that landward of the ingression system peat development continued until the Roman period (Streif, Reference Streif2004). It is not clear whether the ingression of the Crildumerbucht system had a natural cause (erosion of the protecting coastal barrier), an anthropogenic origin or a combination of both. The more southerly Maadebucht system was further enlarged in the Iron Age (‘Duinkerke I deposits’, cf. Behre, Reference Behre1999). At that time the Jadebusen remained free from major flooding and stayed a (high) peat moor area until the Late Middle Ages (1000–1500 AD).

400–1000 AD (Fig. 10)

During the Migration Period (400–500 AD) the occupation intensity in the salt-marsh area of the northwestern Wadden Sea reduced sharply and so did the reclamation activities. In Friesland, at a few locations near Sneek and Wartena, a very thin peat layer developed in the clay cover, the Layer of Tinga (Vos, Reference Vos2001a). The Layer of Tinga is associated with decreasing anthropogenic activity during the Migration Period. As a result, the man-made drainage deteriorated and peat growth recovered locally (Vos & de Langen, Reference Vos, de Langen and Huisman2008).

The occupancy of salt marshes at the seaward margin of the peat area in the northern Netherlands and northwestern Germany increased strongly again during the Early Middle Ages when the large-scale peat reclamations started, leading to full cultivation of the coastal peat bog and the adjacent peat-moor area on the Pleistocene soils. They caused a significant subsidence in the surface of the seaward margin of the peat area. Because of subsidence the seaward margin of the peat area was flooded during storms and as a result the clay cover expanded strongly in all coastal regions of the northwestern Wadden. Concomitantly the tidal currents to the hinterland strengthened, which in turn led to an enlargement of the tidal channels. Consequently, the Middelzee increased further and the drainage of the Marne River was reactivated.

The Lauwerszee reached its maximum extent in the Early Middle Ages. It is likely that this ingression system had grown already in the Roman Period (Vos & de Langen, Reference Vos, de Langen and Nicolay2010). In contrast, the Peasens ingression system silted up even more. The drainage function of the Peasens system was taken over by the man-dug Dokkumer Grootdiep, which discharged into the Lauwerszee system. The subsidence resulting from peat reclamation enabled the tidal system of the Lauwerszee to enlarge. Near Stroobos, in the seaward margin of the system, the first marine influence has been dated around 700 AD (Vos & Groenendijk, Reference Vos and Groenendijk2005). Between 700 and 1000 AD the system expanded further and large parts of the coastal peat area in this region were covered with a grey clay layer. The Lauwerszee also took over the drainage of the Hunze River by means of a new channel between Zoutkamp and Ezinge, and the noticeable bend in the Hunze River, known as Reitdiep, originated. Previously the Hunze flowed to the Wadden Sea through the northern part of Hunsingo. Humans probably contributed to the coming into existence of the Reitdiep by digging a connecting canal between the Lauwerszee system and the Hunze River, a canal comparable with the Dokkumer Grootdiep. Because of the strong current, the Lauwerszee channel started meandering in this area. These meanders are much larger than those in the Dokkumer Grootdiep. The new course of the Hunze – through the Lauwerszee ingression system – caused the old mouth of the Hunze tidal basin to be silted up definitively to salt-marsh level.

In the Early Middle Ages (450–1000 AD), probably before 800 AD, also in the northwestern German coastal area a new ingression system, the Harlebucht, developed in the peat bog in which the Harle River discharged (Behre, Reference Behre1999). The loss of a protective coastal defence due to the migration of the islands of Spiekeroog and Wangerooge, and the emergence of a large tidal inlet are considered to be the main causes (Streif, Reference Streif1989b). This transgressive coastal development is very similar to that of the Paesens ingression system. In the formation of the Harlebucht system – just like in the Lauwerszee area – the anthropogenic lowering of the peat surface also played a role in the drowning process (Fig. 6; Fig. 13). South of the Harlebucht, the Crildumerbucht and Maadebucht ingression systems also increased in size due to flooding of the hinterlying coastal peat area. Anthropogenic subsidence effects in the Late Middle Ages are mentioned in the German literature and are particularly associated with salt-mining activities or ‘selnering’ (Bantelmann, Reference Bantelmann1967; Behre, Reference Behre2004).

Fig. 13. Schematic cross-section reconstructions through the coastal area of the northern Netherlands (north–south orientation) between 1000 BC and 2000 AD.

East of the Eems estuary no major marine ingressions occurred in the Early Medieval period. The protective coastal barrier consisting of salt-marsh deposits and the relatively minor human interventions are explanatory of this. The same reasons apply also to the Dollard and Jade areas, where no major marine ingressions occurred in this period.

Parts of the Dutch and German Wadden Sea area silted up during the Early Middle Ages. This was the result of sediment supply, which enabled the salt-marsh shorelines to expand (e.g. the Peasens system, the old Hunze tidal-basin system, Fivel Bay, the Sielmönkener Bucht and the salt-marsh shorelines near Butjadingen and near Land Wursten). In the northern part of Noord-Holland large-scale erosion occurred in the Early Middle Ages, as is evidenced by the formation of mudflat deposits in the former peat area. The protective coast was breached to a great extent and the remaining parts of the coastal barrier moved landward (Schoorl, Reference Schoorl1999). A big ingression in the Zijpe occurred in the Early Middle Ages.

1000–1500 AD (Fig. 11)

The Zuiderzee enlarged during the Middle Ages (Ente, Reference Ente1986; Ente et al., Reference Ente, Koning and Koopstra1986; Lenselink & Koopstra, Reference Lenselink, Koopstra, Rapporl and Soonius1994), erosion during major storms playing a major role. For example, during the great storm of 1170 AD a large piece of land, seaward of the current dike at Stavoren, was lost (Schoorl, Reference Schoorl1999) and the Marsdiep came into existence (Schoorl, Reference Schoorl1973). As the inland sea increased in size, wave attack on the shorelines became stronger because of the increasing fetch. The Marsdiep tidal inlet became increasingly important during the Middle Ages (Sha & de Boer, Reference Sha and de Boer1991). However, only in the 15th century was the watershed between North Sea and Wadden Sea (on the line Texel-Schagen) broken and a large channel offered a direct connection with the Zuiderzee (Schoorl, Reference Schoorl1999).

The effects of anthropogenic interventions in the coastal landscape became dramatic in the Late Middle Ages, when the major part of the northern Dutch and northwestern German salt-marsh area was embanked. In the course of the 11th century the construction of dikes had become necessary in areas with strong subsidence. In the previous periods the water flowed back to the sea in a natural way after a flood. However, after the artificial subsidence of the land, water remained present and could only be discharged to the sea during low water through a system of dikes and sluices. The Wolddijk around the strongly subsided peat land near Bedum is an example. The dike had already been constructed there for the reclamation of the seaward salt-marsh area. In the following centuries, the entire coastal area of northern Groningen was diked (Knol, Reference Knol, Borger, Breuker and de Jong2010, Reference Knol2013a). In this way almost the entire marsh area of the northwestern Wadden Sea area of the Netherlands and Germany was diked between 1200 and 1300.

As a result of this diking water could no longer be stored in the salt marshes during storms, leading to the impoundment of water against the dikes and an increase in maximum storm water levels in the Wadden Sea. The increase of storm-surge level and the lowering of the soil surface behind the dikes due to drainage created ideal conditions for major catastrophes. When dikes broke, water plunged into the low-lying polders with disastrous consequences for humans and animals.

Inundations were not only the result of extreme high sea-water levels. In areas where the ground level had dropped sharply, the water in these polders could not discharge easily anymore to the sea. Particularly during periods of heavy rainfall during winter, this resulted in stagnant ponds in the lowest areas. Where the surface level of the area dropped below the mean low water (MLW) level, the discharge during low water through a system of sluices did not work anymore and the low-lying polders could only be kept dry by artificial drainage techniques such as mills.

In diked (peat) areas where strong subsidence had taken place, permanent damage could occur in the form of permanent loss of land when it was hit by a flood disaster. Examples are the peat lands in the western and central part of the Jadebusen (Wartenberg & Freund, Reference Wartenberger and Freund2012; Wartenberg et al., Reference Wartenberger, Vött, Freund, Halder, Frechen, Willershäuser, Schnaidt, Fischer and Obrocki2013) and Dollard (Vos & Knol, Reference Vos, Knol and Essink2013). These areas have in common that in the subsurface Pleistocene valley systems are present and in these valleys a very thick package of peat had developed. This peat package in the Jadebucht and Dollard made these areas highly vulnerable to oxidation of the top layer and soil subsidence when they were artificially drained. Before the dikings and reclamations in the Late Middle Ages, the peat surface there was a few metres above sea level and was protected against coastal erosion by a salt-marsh zone, a natural barrier. After the embankments, the peat surface dropped significantly, even to below mean high water (MHW) level. If a dike broke, water flowed into the peat area during every tide and created large and deep channels that could not be closed by humans anymore (Figs 7 and 14). In contrast, in the lateral located embanked salt marsh areas – with a clay-sandy subsoil – the permanent loss of land after a dike breakthrough was very limited. The surface level of these marsh areas was still above the MHW level at that time because the subsidence, caused by artificial drainage, was relatively small. After the breakthrough, the dikes around these areas could easily be repaired because they fell dry again after the storm flood inundation.

Fig. 14. Schematic north–south cross-section reconstructions of the Dollard area between 1000 and 2000 AD.

The disastrous Marcellus flood in 1362 was responsible for the great drowning of the peat lands of the Jadebusen (Behre, Reference Behre1999), and the entire low-lying peat area was permanently lost. Also, open water connections arose between the Jade and Weser River through the Heete and the Lockfleth, which some 150 years later in the 16th century was diked again. These openings did not become large tidal channels because they occurred in the watershed area between the Jadebusen and the Weser estuary. The relatively low current velocities here resulted into a rapid silting-up of these openings.

In the profile reconstruction of Fig. 14 the drowning of the Dollard between 1000 and 2000 AD is shown. The Dollard was lost only in the 15th century (Knottnerus, Reference Knottnerus and Essink2013) when quarrels about maintenance led to a deterioration of the dikes and the 1412 Cecilia flood caused drowning of large parts of the eastern part of the Dollard. An ingression of the west side of the Eems meander in front of the city of Emden turned Nesserland into a peninsula. The western part of the Dollard area was at that time still protected by an emergency dike between Reide and Finsterwolde, but was lost in the great Cosmas and Damianus flood of 1509. This flood also breached the eastern levee of the Eems meander and Nesserland became an island, which, together with the Punt van Reide, forms the remainder of the tidal levees of the Eems (Knol, Reference Knol2008; Knol, Reference Knol2013a,Reference Knol and Essinkb). Because of erosion and drifting away of large lumps of peat after the inundations (floating peat islands), an inland sea arose with the deepest parts below 4–5 m –NAP. The origin of the Dollard and Jade was ascribed especially to the behaviour of the local elite who, in their feuds, mutually destroyed dikes and sluices (Acker Stratingh & Venema, Reference Acker Stratingh and Venema1855; Ramaer, Reference Ramear1909; Breuer, Reference Breuer1965).

The ingression of the Leybucht east of the Eems estuary between Greetsiel and Norden is comparable to that of the Jade and Dollard. A low-lying peat area was also flooded, to be diked again in the course of the subsequent centuries as it silted up for the major part. It is different, however, in that no deep inland sea developed. Probably the Leybucht developed in the 12^‑/13th centuries and the floods of 1362 and 1374 further enlarged the ingression in the peat hinterland (Behre, Reference Behre1999). Also in case of the Leybucht, humans undoubtedly played a major role in the subsidence of the soil resulting from peat reclamations, a low surface level and lakes being the result (Schwarz, Reference Schwarz2004, Reference Schwarz and Schmid e.a.2005). Behre (Reference Behre1999, Reference Behre2004) assumes a link between the origin of the Leybucht and the erosion of the salt-marsh island of Bant in the Wadden Sea, northwest of Norden. This would have caused the loss of the natural coastal defence, making the hinterland vulnerable. However, the former marsh island was located at a certain distance north of the Leybucht system (Figs 10 and 11). More likely it was mainly the low surface of the peat that enabled the sea to penetrate far inland and led to the creation of the Leybucht and a large tidal channel. The development of the large channel of the Leybucht promoted erosion of the Bant. The wave attack on the island was enhanced due to a larger sea opening to the hinterland. In any event, the exposure of Bant to the sea in combination with anthropogenic peat extraction for salt production was harmful (Behre, Reference Behre1999). The last remnant of the Bant was lost permanently to the sea after 1780 (Behre, Reference Behre1999).

Its namesake, the island of Bant in northeast Frisia, was lost permanently after 1500. The cause of the disappearance was the lateral shift of the Lauwersgeul (Vos, Reference Vos1992). Until the 14th century the tidal channel of the Lauwers discharged east of Schiermonnikoog. In the course of the 14th century the channel broke through the mudflat between Schiermonnikoog and the mainland of northeast Friesland. A large new inlet evolved, comprising the Westgat and the Zoutkamperlaag. Thus wave attack from the sea on the northeastern coast of Friesland increased strongly. The dike northeast of Wierum had to be rebuilt more inland as the result of strong coastal erosion and the graveyard, located on the seaside of the old dike, disappeared.

1500–2000 AD (Fig. 12)

After 1500 AD the land loss was more than compensated for by the accretion of new land. The northern part of Noord-Holland silted up strongly and the silted-up salt marshes were diked in stages. The same holds true for the Middelzee, the Fivel system, the Leybucht and the Harlebucht.

After the great floods in the Jadebusen and Dollard rapid sedimentation took place in the newly formed inland seas. The landward areas of these inner seas quickly silted up to salt-marsh levels and were diked in stages. Over last 60 years deepening and dredging of the tidal channels have changed the hydrodynamic conditions in the Eems estuary. The Eems-Dollard mudflats and salt marshes have been silting up over the last few centuries, but since the 1980s – the large Eems fairway deepening – erosion and deposition alternate and no clear net sedimentation occurs anymore (Commissie voor de Milieueffectrapportage, 2014). This intervention in the tidal system has led, amongst other things, to an increase in estuarine turbidity levels, which strongly affect the ecosystem of the estuary (Bos et al., Reference Bos, Büttger, Esselink, Jager, de Jonge, Kruckenberg, van Maren and Schuchardt2012; de Jonge et al., Reference de Jonge, Schuttelaars, van Beusekom, Talke and de Swart2014).

From the 19th century onwards the intertidal coastal zone along the embanked mainland has also been reclaimed using wooden structures along the dike in which silt was trapped. When such reclamation areas had silted up to the level of the salt marsh they were diked.

In the 20th century the Zuiderzee and the Lauwerszee were cut off from the Wadden Sea by large dikes. Thus the Wadden Sea area has been reduced. In particular, the salt-marsh areas are only a fraction of what they once were. After 1500 AD the peat area also decreased considerably. The coastal peat and the peat areas on the higher Pleistocene grounds (Groenendijk, Reference Groenendijk1997; Spek, Reference Spek2004) have largely disappeared due to large-scale peat-cutting for fuel (turf) and salt, and by oxidation of peat due to exploitation.