Sand pre-wash

The term sand pre-wash is understood as a coastal protection measure to protect a beach or a bank from storm surges in order to avoid land loss. This type of coastal protection is mainly used in Germany on the North Frisian island of Sylt and on the East Frisian islands of Borkum , Norderney and Langeoog . In the Netherlands, sand pre-wash is used on almost all coastlines, especially on the West Frisian Islands.

Procedure

Principle of sand pick-up and transfer of the hopper excavator

The sand pre-washing can be carried out either as a beach washing-up or as a foreland washing-up. A combination of the two methods is also possible. Both processes have in common that sand is first extracted from the seabed using a suction dredger. With these large suction dredgers , also known as hopper dredgers , sand is removed from the seabed at a distance of approx. 12 km from the coast . In a mixture of 30% sand and 70% water, it is transported into a large ship's tank. The water is then allowed to run off again.

Beach replenishment

Washing up of sand on Norderneyer Nordstrand in 1949

For washing up on land, the ship approaches up to around two kilometers from the coast. There it is connected to a pipeline ( culvert ) that serves as a connection between the ship and the beach. This work is complicated and can only be carried out in shallow seas with less than five wind speeds . The water-sand mixture is pressed onto the beach at a pressure of 7 bar and distributed using a large bulldozer . After the sand is straightened, it can be entered again immediately. If the use of this complex technology is not impaired by bad weather conditions, about 300 meters of beach length can be replenished per day.

Foreland flushing

Sand pre-flushing using the drop method. The fully loaded suction dredger opens its bottom hatches and places the sand on the coastal foundation.

Sand pre-flushing using the Rainbow method. Due to the lack of water depth, the ship cannot open its bottom hatches and pumps the sand-water mixture to the intended location.

Alternatively, the sand can also be used to stabilize the coastal foundation. The aim is to reduce coastal wear by breaking the waves prematurely. For this purpose, the sand extracted from the seabed is unloaded a few hundred meters from the coastline. There are two different methods for flushing the foreland: the rainbow method and the drop method . With the drop process, the sand extracted with suction dredgers is placed in the desired location through the ship's bottom flaps. If it is not possible to open the bottom flaps due to insufficient water depth, the sand is erased as a sand-water mixture with the help of a fountain (in the form of a rainbow). Occasionally, foreland wash-up and beach wash-up are also combined. If it is advisable, for reasons of tourist considerations, to keep the beach flushing and the associated beach closure as short as possible, “sand heaps” are often created in front of the coast in order to later absorb them again and apply them to the beach. Due to the shorter travel times of the suction dredgers, the total duration of the flushing measure is reduced.

Catch fences after the washes

New sand is to be caught by fences on the dunes so that a pre- dune is created. This can be planted with beach grass to give the dune support.

Abrupt edge on the beach of Westerland / Sylt
Preparations for the suction excavator (pipe laying)
Sand is pre-flushed and distributed by the excavator
Sand flushing
off Westerland / Sylt

Alternative procedures

To islands off loss of land to be protected, can also groynes be used. These consist of concrete, metal or wood and act as breakwaters, but achieve the deposition of rather small amounts of sand. A land reclamation by fascines is possible. Since both methods only cause moderate sediment deposition, they do not offer adequate protection in coastal sections with strong currents and correspondingly extensive erosion.

Web links

Commons : Beach Liquid - Collection of Pictures, Videos and Audio Files

Individual evidence

↑ Sand flushing protects Sylt. Lübecker Nachrichten, November 18, 2017, accessed on August 31, 2019 .
↑ Borkum - sand washes make islands safer. Accessed August 31, 2019 .
↑ 280,000 cubic meters of sand from the Robbenplate for Norderney . Lower Saxony State Agency for Water Management, Coastal Protection and Nature Conservation (NLWKN). June 27, 2007. Retrieved October 6, 2012.
Jump up the beach on Norderney. Nds. State Office for Water Management, Coastal Protection and Nature Conservation, July 12, 2019, accessed on August 31, 2019 .
↑ Langeoog: The largest beach wash-up in front of the Pirolatal begins. Nds. State Office for Water Management, Coastal Protection and Nature Conservation, July 12, 2017, accessed on August 31, 2019 .
↑ Information from the Dutch Ministry of Water and Transport on current sand pre-washings (Rijkswaterstaat) (Dutch, accessed on July 10, 2010) ( Memento from January 11, 2011 in the Internet Archive )

[1] Sand flushing protects Sylt. Lübecker Nachrichten, November 18, 2017, accessed on August 31, 2019 .

[2] Borkum - sand washes make islands safer. Accessed August 31, 2019 .

[3] 280,000 cubic meters of sand from the Robbenplate for Norderney . Lower Saxony State Agency for Water Management, Coastal Protection and Nature Conservation (NLWKN). June 27, 2007. Retrieved October 6, 2012.

[4] Jump up the beach on Norderney. Nds. State Office for Water Management, Coastal Protection and Nature Conservation, July 12, 2019, accessed on August 31, 2019 .

[5] Langeoog: The largest beach wash-up in front of the Pirolatal begins. Nds. State Office for Water Management, Coastal Protection and Nature Conservation, July 12, 2017, accessed on August 31, 2019 .

[6] Information from the Dutch Ministry of Water and Transport on current sand pre-washings (Rijkswaterstaat) (Dutch, accessed on July 10, 2010) ( Memento from January 11, 2011 in the Internet Archive )