The dissertation focussed on the kind of closure methods used, in the past and at present, to repair breached dikes and to close tidal channels by simple means. By simple means one must think about the application by (mainly) manual labour of local materials and small-scale tools and equipment. The closure of tidal channels is also required in reclamation schemes and to construct dams between islands or from an island to the mainland.
This research was in the first place aimed at Dutch closure methods as practised in the Netherlands, Germany (Schleswig Holstein) and Ghana/Bangladesh (20th century). But it was also considered useful to check the closure methods used in other countries than the ones just mentioned.
Typical topics addressed have been hydraulic features, materials and equipment applied, the importance of manual labour in these closures and their output.
Related topics addressed in this dissertation are the development in time of closure methods and the reasons why sometimes closures failed.
Though flood embankments and sea defences as part of reclamation works have been constructed from the 11th/12th century onward, limited technical information about their construction as well as possible subsequent repairs after breaching, could only be traced for six projects carried out during a period of more than 400 years (1263 – 1675).
More information could be traced about the methods and constructions used in general but not for specific projects.
The closure methods used during the period 1200 – 1700, nearly always started by dumping a horizontal sill of clay clods up to the level of Low tide. Subsequently, the gap was narrowed by dumping clay clods, and the actual closure was, finally, effected by lowering constructions made up of sink-fascines (fascines imprisoning a gravel core or clay clods) and clay clods.. Alternatively, a cofferdam could be constructed across the flow gap.
In case of low tidal ranges, a row of sheet piles or piles, closely placed to each other, was driven across the gap. Sometimes, ships were ballasted and sunk on top of the sill.
At the end of the 17th century floating fascine mattresses were introduced in closure works. They were ballasted and immersed in the breach or tidal channel and functioned as bed protection of the easily erodible soil. Moreover, the aforementioned sill could now be constructed by stacking fascine mattresses on top of each other, again up to the level of Low tide. Subsequently, the closure was performed gradually in a horizontal direction by constructing a bund in sections, comprising fascines, clay clods, stones and wattle work.
From the end of the 19th century up to the middle of the 20th century jetties were constructed across the gap to be closed. A narrow gauge railway track on the jetty enabled the dumping of clay clods or stones in the gap from side tip wagons. Until the Second World war the side tip wagons were loaded by hand and in most cases pushed by manual labour.
During past centuries in many cases simple means were not adequate to close dike breaches and, consequently, the flooded polders could not be reclaimed and had to be given back to the sea.
In the period 1969 – 1985 Dutch closure techniques based on the principles described above were introduced for tidal closures in Ghana and Bangladesh. This required the adaptation of the said closure techniques to locally available construction materials, equipment and skills. Also here, a significant input of manual labour was part of the closure operation.
Closure techniques applied in China, East-Pakistan (which, later became independent as Bangladesh), England and Japan were also studied. It was surprizing to learn that the sink-fascine (having in each country another name, other dimensions and composition) had a major role in closure projects in China and East-Pakistan and, to a lesser extent, in Japan. Moreover, the application of ropes and hawsers (as in the Dutch fascine constructions) turned out to be of vital importance. In China and Japan the closures were required to repair dike breaches along non-tidal rivers. In East-Pakistan and England the closures studied were in the tidal zone.
Finally, a tidal closure performed in Bangladesh (at the Nalian River) by manual labour and simple means in the year 2020, has been studied.
The 75 case histories presented in this dissertation reflect the art of river and tidal closures by simple means over a period of nearly 760 years (1263 – 2020) in six countries. In most cases the area of the tidal basin which could be closed by simple means was less than 2 km2 while the tidal volumes were less than 2 million m3. Production rates in tidal closures varied between 0.26 and 0.62 m3 of fill placed per manhour.
But most of the simple means described can be considered as history. There are, however, countries where cheap manual labour is abundantly available and tidal closures could still be constructed by simple means. Such closures by simple means have been described for the year 1969 (Ghana) and the period 1979 – 2020 (Bangladesh).
The insights from this dissertation will therefore also be useful for future closures constructed by using simple means.