Dellewal Terschelling: Stunning Example Ancient Building With Nature

Panorama of the port of West Terschelling and constructed mudflat
Panorama of the port of West Terschelling and constructed mudflat

For most of us, the summer holiday is already long gone. So for me also. Still there is an observation I made that I want to share with you. We have been sailing on the Waddenzee1 this summer. Sailing, anchoring, mudflat hiking2, counting seals and other animals. One of the highlights was a visit to the island of Terschelling. A lot has changed since I was there last time with our boat. It must have been at least 30 years ago. At that time, we had to moor against the quay wall wherever one could find a spot. Now, there is a modern marine in the back of the port, specially made for yachts. Although stacking the boats next to each other is still the standard.

Port of West Terschelling and the new marina
Port of West Terschelling and the new marina

The new marina is more or less in the same area where we used to moor. From there you have an excellent view on a mudflat, that has been there been for ages3. When I was young, I couldn’t understand what its purpose was. Only that old fashioned Dutch flat bottomed boats were still allowed to anchor and stand dry. For us kids, it was an excellent place to muck about with our little dinghy and get dirty walking on the mudflat. We just enjoyed it was there. I still don’t know the exact name of the mudflat. I’ve seen it called ‘Dellewal’, although that seems to be just the quay side over there. Other names that I found on charts are ‘Oostelijk Ras’ and just ‘De Plaat’. Enjoying a nice sundowner and contemplating life and dredging in particular, I suddenly saw the purpose of the area: it works like a ‘Spuikom’4. I really don’t know how to translate this in English. So, let me explain what it is intended for.

Explanation on the working principle of a ‘Spuikom’
Explanation on the working principle of a ‘Spuikom’

Basically it is a part of the mudflat that is located at the south of Terschelling. About 85 ha in surface area, it is separated from the Waddenzee by a low dam. Just high enough to the high water level in the neap tides. The dam has an opening at the back of the harbour, near the marina. I Noticed that the water outside the marina was rushing by and the water inside was practically standing still. I figured out that the rushing tide was used to flush the old port. The huge surface area stores a lot of water that has to pass the quay in the port. Effectively increasing the flow velocity there and reducing sedimentation. This certainly helps in maintaining a navigable depth for marine traffic. Moreover, as each tide the area is filled from all around the dam and mostly emptied through the port, there is a resulting nett transport out of the port.

Hjulström diagram for particle transport in streams (Credit: Wikipedia)
Hjulström diagram for particle transport in streams (Credit: Wikipedia)

Voila, that is why the old islanders build those dams! Any other person would be satisfied with this plausible answer. Have pity on me, I can’t stop solving the riddles of the sands. Wouldn’t this be easier with a dredge? Apparently, near Terschelling, there is a sedimentation rate of 0.5 to 1 mm per year5, or about 600 m³ annually. With the dam, this has to be kept out of the port with the volume behind the dam. The average increase in flow is about 0.5 knots. According to the Hjuström diagram6, this will transport particles smaller than 10 mm out of the harbour. The stored volume has a potential energy as in a power dam of about 6.25 GJ. This is released twice each tide, resulting in a delivered power of 280 kW. Combined, this results in a specific transport power consumption of 4000 kW/m³/h. No contractor in his right mind will ever use a machine with such a performance. BUT: the energy is free and working flawlessly for at least 200 years. I still have to see a machine doing that. OK. We can step up the analysis even further. Drawing the 280kW continuously from the tide is eventually slowing down the rotation of the Earth. Just for those worried: each year, one day will be in the order of 10-19 seconds longer…

The moon decelerating earth’s rotation by tugging on the tides (Credit: NASA)
The moon decelerating earth’s rotation by tugging on the tides (Credit: NASA)


  1. Wadden Sea, Wikipedia
  2. Mudflat hiking, Wikipedia
  3. Landschapsvisie Baai Dellewal Terschelling, Feddes Olthof
  4. Spuikom, Wikipedia (Dutch))
  5. Slibsedimentatie in de kwelders van de Waddenzee, Arcadis
  6. Hjulström curve, Wikipedia

See also

HYDRO 2018 Gdansk: Selecting A Dredge For Your Reservoir Maintenance

Barrage du Ksob, M’Sila, Algeria with a DOP dredge 350

This week, I am here in Gdansk for a presentation on the HYDRO 2018 Conference1 and assist at the Damen booth at the corresponding exhibition. The paper and the presentation are already prepared and I am very excited to do the presentation, but I can’t wait till tomorrow and I like to share the story now, already. So, you, as my favourite audience, will have my personal spoiler after so many teasers have been floating around2,3,4.

General modes of siltation at the usual location in a reservoir

The thing is, dam maintenance and reservoir restoration is something already long on my attention list. Back already in 2008, I wrote a paper on this subject for the CEDA Dredging Days5. Over and over we’ve conveyed the message on various platforms, that dredging might be a viable solution for sedimentation problems in reservoirs. Usually, the solution by dam owners and operators is to flush, sluice or store the sediment. This looks horrible from a dredging perspective, but it is also to the environment. You either smother or starve the downstream river with sediment. As a right minded dredge enthusiast, you see many possibilities to dredge such a project. Immediately we can identify what dredge to use on which location for which purpose.

Selection of applicable dredges for reservoir dredging

If you are very close to the dam and the length of the discharge line allows it, you might even not need a dredge pump. (No wear parts!) It is a so called siphon dredge. But as soon as there is some further transport involved, either distance or uphill, you need a dredge like a cutter suction dredge or a DOP dredge. For even further discharge, you might employ a booster for increased discharge pressure. If the distance becomes very far, you might have to resort to grabs and barges.

Water injection dredging principle and example (this example would be too big for a common reservoir)

As an intermediate solution you might even consider using a water injection dredge. Usually the reservoir is in the mountains and a bottom gradient will be present, enabling the required gravity flow. The actual dredge should have created a silt trap where it can collect the inflowing material from the water injection dredge. Than it can handle the material as usual.

Alternative uses for the dredged sediment a) silt farming as fertile additive b) gravel extraction for concrete

Off course, the dredged sediment belongs to the river and the best thing would be to gradually release the sediment after the dam. But there might be conditions, where it is beneficial to extract the valuable fraction of the sediment and use it for agriculture or as aggregate in the construction industry.

Dredge selection diagram for reservoirs

We noticed, that it is often difficult to convey to dam owners and operators which dredge to select for which job. Sediment is seen as a liability and not as an asset and they rather neglect issues associated with the sediment. So, I made an attempt to have a plain and simple selection diagram. That is the core of my manuscript. But my objective is, that we will see many beautiful dredges contributing to a sustainable and viable operation of hydropower dams and reservoirs.

New DOP dredge family


  1. HYDRO 2018: Progress through partnerships, Hydropower and Dams
  2. LinkedIn Teaser, Saskia den Herder
  3. Damen: Spotlight on Hydro Power Dam Maintenance
  4. LinkedIn Teaser, Olivier Marcus
  5. Multi Functional Small Dredging Solution For Maintenance Of Deep Irrigation Reservoirs And Hydro Power Dams, CEDA

See also

DOP Dredges for Power Dams and Reservoir Maintenance

New DOP dredge family

Today, our product group director, Olivier Marcus, gave a presentation1 on the maintenance of irrigation reservoirs and power dams with a range of specially developed DOP dredges. Here, I am happy to elaborate on his message and share with you highlights from his presentation. As you may have noticed from my previous posts, reservoirs are a long standing interest of mine. It was one of my first commissioning jobs at my previous employer. It made me aware, that reservoirs and power dams are not always the clean energy and sustainable solution that they are usually presented for.

Don’t get me wrong, we need these kind of structures to pass on our planet in a better state to the next generation, than we received it ourselves. As with everything: we have to take care of these responsibly.

Barrage du Ksob, M’Sila, Algeria

Dams and reservoirs have mainly two problems. First, if the bottom of the reservoir hasn’t been cleared before filling with water, there is a lot of biomass available. Especially in warm and shallow lakes, the decomposing material can generate a lot of carbon dioxide. The CO2 footprint of the generated electricity might even be higher than from diesel driven power plants2. So much for the green image of hydro power. Second, the interrupted sediment transport will cause the reservoir to fill up. Eventually the storage capacity isn’t enough anymore and the operation has to be abandoned. So much for the sustainable part of the image.

The first problem should have been tackled during the building phase of the power dam. Maintenance during operation for the second problem. The simplest method is to flush the reservoir through the bypass. But you can’t flush all the sediment. The heavy particles up stream will remain in place. And you smother the life of the river downstream. A more effective method is to use dredges. Over the years, we have supplied several of these vessels, each specifically engineered and built for their own location.

Special design reservoir dredges (ul: Cosider, ur:La Mahuna, bl: Djebel Debagh, br: Gross Glockner)

In an initial survey of these dredging opportunities, we found some common features and could develop a range of DOP dredges specific for reservoir maintenance. The transport capacity of the river should be matched by the capacity of the dredge. Often resulting in a range suitable for a DOP pump. Their flexibility makes the dredges adaptable to work on all the different areas in a reservoir.

Various areas in a reservoir with their corresponding DOP dredge solutions

It’s been more than ten years after our first introduction of these nifty little dredges3 and with the experience gained, it was time to present you a next generation that could assist in achieving a sustainable power generation and a more environmental friendly operation. Main innovation is that this new range is also available in an all-electric version. Which should make sense, if it is going to work at a power dam. Always discuss this with the power dam operator, as they are not very fond of plugging in a dredge. They fear distortions and fluctuations on their ‘product’. Than consider alternative clean energy from solar or wind and lastly the old reliable diesel option. With a lot of reservoirs already losing their capacity, there is a lot of opportunity for these type of dredges.

Brave ambitious dredge Djebel Debagh has a lot of work ahead


  1. Hydropower & Dams Asia, Damen
  2. Hydroelectric power’s dirty secret revealed, New Scientist
  3. Multi functional small dredging solution for maintanance of deep irrigation reservoirs and hydro power dams, CEDA

See also