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)

References

  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

Increase Your Dredging Knowledge At The End Of The Discharge Line

Keeping watch at the end of the discharge pipe line
Keeping watch at the end of the discharge pipe line

Solving something at the end of the pipe is usually a less desired approach. However, in dredging, it is the place where the valuable stuff is delivered, it might be a good place to start monitoring your process. Let me explain this to you by going back to latest discussed exhibit at the Damen Dredging Experience1.

Pump power exhibit at the Damen Dredging Experience
Pump power exhibit at the Damen Dredging Experience

You might have observed in the pictures of the pump power exhibit, that the velocity of the water flow is indicated by the parabolas of the trajectory. The arc of water is bound by gravity and obeys this trajectory always; independent of the density of the mixture. The two equations of motion can be combined, where the time parameter falls away and the height for a certain distance is only depending on the initial horizontal velocity2. As such, it is fairly accurate indication of the pipe flow. The calculation is universally applicable on earth and the results can be presented in a very simple graph to take with you. Every parabola is labelled with the corresponding horizontal velocity.

Nomogram to find end of pipe velocity
Nomogram to find end of pipe velocity

The above example is a straightforward method to measure the mixture velocity. The US Geological Survey even extended this approach as a standard method to measure the production of wells3. The resulting nomogram has a slightly different layout, as it is intended for finding the production instead of the velocity. For production planning, this will be useful. For monitoring your dredging process, the velocity might be more important. Both approaches of this elegant method do have the benefit, that there is no obstruction needed as in the case of an orifice measurement4.

Nomogram to find the end of pipe production
Nomogram to find the end of pipe production

There is an unconfirmed anecdote that my old professor de Koning started his career as a velocity measurer. In the old days, when he was working as a twelve year old boy with the dredging company of his father. He was assigned to keep watch at the end of the pipe and monitor the mixture pouring out. He had a simple beam with a plumb bob. The beam was moved along the top of the pipe, until the plumb bob was touching the arc of mixture. On the beam were two markings. When the beam was moved in and passed the first mark, the mixture velocity was too low and a red warning flag had to be displayed. If the beam had to move out and the mixture velocity was too high at the second mark, a green flag had to be flown. There was also another white flag, in case only water came on the reclamation area. With this very simple setup, the dredge master could check through his binoculars what the state of the dredging process was.

Working principle and explanation of end of pipe meter
Working principle and explanation of end of pipe meter

They were clever in those days. But the physics still apply. So, even today, one might have a situation, where there is no electronic velocity measurement available (broken, not supplied, not (yet) purchased) and you have to push the limits of the operating envelope of the dredging process. Then, there is probably always somebody around that might be appointed volunteer to be head of the velocity measurement crew. Who knows, he might have a bright future in the dredging academia.

Professor de Koning of the dredging chair at the TU Delft (1981-1993)
Professor de Koning of the dredging chair at the TU Delft (1981-1993)

References

  1. Presenting Pump Power Peculiarities, Playing With Pumps And Pipes, Discover Dredging
  2. Projectile motion, Wikipedia
  3. Estimating discharge from a pumped well by use of the trajectory free-fall or jet-flow method, US Geological Survey
  4. ISO 5167 Measurement of fluid flow by means of pressure differential devices inserted in circular cross-section conduits running full, ISO

See also

How A Ship On The Shore Became a Beacon For My Dredging Career

Stranded Yo, us and another vessel to the rescue (Credit: Co Winkelman)
Stranded Yo, us and another vessel to the rescue (Credit: Co Winkelman)

Long ago, we were sailing with my parents and my brother into the Venezuelan archipelago of ‘Los Roques’.1 After clearing customs, we learned that there was another sailing vessel in trouble. It was stranded on the reefs at the east side of the islands. As we had some spare time we decided to lend a hand in getting them afloat. By eyeball navigation through the channel behind the coral reef, we found the stranded English catamaran ‘Yo’, but had to anchor 200m away. First we learned from a Swiss captain on another assisting yacht they had been pulling together with a maxi yacht on a long hawser to pull them off by power and sail. Alas, to no avail.

Location and map of the islands of Los Roques
Location and map of the islands of Los Roques

The next day, the swiss captain had to leave and we undertook the journey by dinghy to visit the crew on ‘Yo’. It turned out to be a couple with their son and two deckhands. They told us that the maxi yacht was ‘Drum’2 and one of the crew was no other than rock star Simon Le Bon3 himself. We just missed them by a day! He did all the best to cheer them up and you can image what a support that visit meant to these people in such a desperate situation.

Salvage plan to float ‘Yo’
Salvage plan to float ‘Yo’

As my dad was a chief engineer from the merchant marine, he surveyed the damage professionally. ‘Yo’ was sitting exactly on top of the reef. One keel was broken of and the other only half, but was sheared below the wreck and stuck between the coral heads, preventing any movement. After evaluating the state of the boat, the equipment and the location, he actually said: ‘We’re gonna science the shit out of this.’4 Together we devised a cunning plan:

  1. Lift/Float: Remove the rest of the keel. Plug the holes left by the bolts and increase buoyancy of the craft.
  2. Dredge: Lower the rock bottom to increase support from the buoyancy and create a channel to freedom.
  3. Move: Assemble all winches and tackle to leverage the pulling forces. The forces would be so high that we feared we would pull the catamaran in half. So we had to distribute the forces all around the hull.

The structural repairs on the hull were performed by my father. My brother and me were in charge of the winches and tackle. Any spare time was dedicated to cutting the rock below the wreck. For sure, that is a nasty job, we tore our clothes and cut ourselves on the sharp edges of the coral. By practice, I learned the different angles to aim the pickaxe for the best results and the lowest effort: an introduction into Specific Cutting Energy!5 Because we did our calculations careful and our assumptions were right, the boat moved exactly the moment we predicted and in the way we wanted. It was a great moment of revelation: you could actually use all this knowledge from physics classes6 to get you out of a nasty position. It set me on a path where I am now and you are reading this story.

The best home schooling: toolbox meeting for a salvage operation. (Credit: Co Winkelman)
The best home schooling: toolbox meeting for a salvage operation. (Credit: Co Winkelman)

Actually, we did not see them completely get off, as we had other obligations and had to leave. We were confident they would come off, but it was a mystery to us where they did end up. Finally after thirty years, I did a Google search and to my surprise I found they did get off indeed and were even reunited with their first rescuer, Simon Le Bon.7

‘Yo’ is away, but still scars are left behind in the coral where it all happened. (Credit: Google)
‘Yo’ is away, but still scars are left behind in the coral where it all happened. (Credit: Google)

That was my own story on dredging and salvage. Currently, there is an interesting exhibition at the National Dredging Museum8 with better documented cases and very interesting displays. Still, the three steps used for ‘Yo’: ‘Lift, Dredge and Move’ can be distinguished for the other cases there, also.

Exhibit of the salvage operation of the ‘Faustus’ from the Rotterdam breakwater
Exhibit of the salvage operation of the ‘Faustus’ from the Rotterdam breakwater

Remarks

Please keep in mind, that these events happened more than thirty years ago and were about saving the lives of five people in immediate danger. Dredging in coral should only be done under very strict conditions with the health of the ecosystem in the first place and in balance with the necessity of the operation.

References

  1. Los Roques archipelago, Wikipedia
  2. Drum (yacht), Wikipedia
  3. Simon Le Bon, Wikipedia
  4. The Martian: Mark Watney Quotes, IMDb
  5. Experiencing The Cutting Edge Of Dredging Technology, Discover Dredging
  6. BINAS, Noordhof
  7. Rescued woman reunited with pop star, BBC
  8. Scheepswrakken bergen of baggeren? National Dredging Museum

See also