3 The dike of West Friesland - seaweed dikes (Fig. 3: 1)
It’s a fairly well known fact that eelgrass was an important element in the construction of sea dikes in the Northern Netherlands. Dikes constructed this way were called ‘seaweed dikes’ (wierdijken) and they were the main line of defence against the sea for the coasts of West Friesland, Friesland and the islands of Wieringen and Schokland (Van der Heide 1992, 78). The first time eelgrass is mentioned in connection with a dike is in a text from AD1319 that deals with apportioning the liability of maintaining the dike that protected West Friesland: the Westfriese Omringdijk (Gottschalk 1971, II, 289; Borger & Bruines 1994, 21). The omission of any kind of technical detail on the use of eelgrass implies that it probably already was a well-known technique at that time: ‘Every man in Frisia will make his [part of the] dike… with earth the earthen dike and with seaweed the seaweed dike.’ (Beenakker 1988, 191).
Many historical seaweed dikes are still preserved under modern dike bodies. There are, however, not many instances in which these dikes have been examined archaeologically and archaeobotanically. Yet on many occasions sections were made trough modern dikes (Van Geel et al. 1983; Danner et al., 1994), and part of a seaweed dike was reconstructed recently in Wieringen. Also, in the recent project for the reinforcement of part of the Westfriese Omringdijk, strict archaeological supervision means that there will be plenty of opportunities for future research.[2]
A seaweed dike consists of an earthen body which is protected on the seaward side by a ‘belt’ of compacted eelgrass. This ‘belt’ is fixed to the earthen body by rows of posts and the base is further protected by bundles of brushwood. The intertwining eelgrass leaves protect the earthen body, that forms the real barrier, from foundering (Blankaart 1698, 275-276). As such seaweed dikes took over the function of the foreshore, as the land available for this purpose was rapidly diminishing since the development of the Zuiderzee in the twelfth century AD (Danner et al. 1994). Large-scale use of eelgrass protection started only in the fifteenth and sixteenth century AD, when there was hardly any land left to be given up as foreshore. Still, even then only those segments of the dikes that were in immediate danger from the sea would be protected by an eelgrass belt. This use of eelgrass continued until 1730, shortly after the advent of the pile worm (Teredo navalis) which destroyed the posts fixating the eelgrass.
According to some sources, the belt of eelgrass could sometimes measure about seven meters high and four to seven meters thick (Schilstra 1974, 26-30). Sections, however, suggest an average of about four by two meters. Every two or three years the top layer of the belt had to be repaired, and once every century or so the entire belt had to be replaced. Also, the eelgrass that was to be used had to be freshly fished up, not gathered on the beach, and it could not be older than 4 days when used for construction.
Taking into consideration that we are speaking of several hundreds of kilometres of dike bodies in the northern Netherlands, and that every six cubic metres of fresh eelgrass yielded only one cubic metre of compressed eelgrass, the amount of eelgrass used must have been staggering, even if not all parts of the dikes were protected by a belt like this. It probably also means that from the 15th century onwards the gathering of eelgrass acquired commercial possibilities because of the rising demand, in combination with the outsourcing of construction work to contractors