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oa Delfland in 1200. Een gereconstrueerde hoogtekaart
- Amsterdam University Press
- Source: Tijdschrift voor Historische Geografie, Volume 6, Issue 1, jan. 2021, p. 21 - 40
Samenvatting
Delfland in 1200. A reconstructed elevation map
In this study, a proxy is presented for the height of the land above sea level in the past. The height is calculated for the area of the Hoogheemraadschap (water board) of Delfland in the year 1200, based on the changes in the water table in the polders over time. Since 1865, this water table is presented in the Waterschapskaart of the Netherlands, which has seen f ive updates and is now replaced by maps of the individual water boards. These six measurements over a period of 155 years are used to calculate the rate of subsidence of the soil. Together with the present height of the water table above sea level, it is possible to extrapolate back in time to the year 1200. Such an extrapolation well outside the measurement area must be regarded with caution and the quantitative results are compared with expectations based on known geophysical processes in peat and clay soils and the history of the area. The calculated relief in 1200 matches with the expectancies based on the geographical survey of the area. As expected, the f irst dykes along the estuary were merely ment to ward off extreme high water in the summertime when wheat had to be protected against salt water. The clay soils are calculated to lay extremely f lat from the river to 20 kilometre inland. This allows for the observed linear flow of the rivers. According to the calculation, the height of the land above sea level is such that storm surges are expected to reach inland on only rare occasions. This is substantiated by the observation that farms are built on platforms of only a few decimetres in height.
The method has proved its worth for an area the size of Delfland, where statistically significant results may be obtained due to the number of measurements. However, for results on a smaller scale the method is not sensitive enough.