CEDA has always been an advocate of sharing and disseminating knowledge, with an attention to personal contacts1. Of course, there are the high profile Dredging Days2 and the WODCON3 level events, but once in a while there are also smaller events. As part of the smaller program, Young CEDA4 regularly organizes visits at projects and companies to bring together the new generation of our dredging community. This time, a group of young professionals visited our company Damen Dredging Equipment here in Nijkerk5 to get familiar where we are and what we do. Rick Bekkers, Jorgen Groeneveld, Suman Sapkota and Reinier de Vries where hosts of the event and organised, with assistance of the ladies of our reception and back-office, a nice program with presentations a yard tour and of course drinks and the proverbial ‘bitterballen’.
After Rick and Reinier gave a presentation on the premises and the products, there was a yard tour and a demonstration of our dredging simulator by Wouter Beekman. And I am always happy, that the visit to our Damen Dredging Experience was a great success again. We do have our own little museum to experience the difficult issues in the dredging process hands-on6. Literally! Specially for this occasion, the exhibits have been expanded with new informative screens, made by Judith Korver. In a few slides there is an explanation on what the visitor experiences by operating the exhibit.
Part of the experience is the staircase leading up to the third floor with the exhibits. In the staircase, there is a collection of wall covering pictures on the history of Damen Dredging Equipment. Personally, I think learning from history is essential to get inspired7. Back in the old days, people had to be very innovative to come up with solutions within their limited technological level. And Mr. de Groot, the founder of our local Damen company was very innovative8. He was the inventor of the double walled dredge pump9 and the soil pumping station10.
A lesser known fact by now may be, that originally Mr. C de Groot was a contractor. He was a true descendant from a Giessendam dredging family and tried his luck at the IJsselmeer werken of the 1930’s. As a basecamp, he leased a land lot next to the sea locks of Nijkerk. Later on, he had more fun on building his own equipment and eventually started a manufacturing facility more inland along the approach canal. You can still recognize the original footprint in the current urban landscape.
At retirement, Mr. de Groot had no successors and had to sell the company. After an intermediate period, De Groot Nijkerk was acquired by Damen Shipyards8 in 1988. Eventually in 2004 our company was rebranded to Damen Dredging Equipment and we proudly fly the blue colours of the Damen family.
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.
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.
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.
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…
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.
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.
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.
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.
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.