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…
Yesterday, Wim Kleermaker graduated at the TU Delft on a research project he conducted on our slurry test circuit at Damen Dredging Equipment. Specifically, he was investigating the wear behaviour in our dredge pumps. The noteworthy aspect of this project, was that Wim was supervised by our colleague Suman Sapkota. As long time readers in the audience might remember Suman was my own pupil some years ago1.
Wear is a very common process in the dredging industry and one of the main cost factors in a project2. It is beneficial to know the amount of wear to expect in a certain condition and be able to predict the budget to reserve for this nuisance. This is only possible when we as a manufacturer will be able to predict the wear rate and pattern can provide the information to the operator for his estimates. We do have historical data that will allow us to provide a ball park figure, but a more analytical approach might assist us in particular unusual cases. Furthermore, it will also provide us insight in the impact of certain design decisions for the wear performance of a certain pump design. For Wim’s graduation, he had to approach this academically: come up with a simulation model and verify this with measurements.
The measurements were done in our slurry pump test circuit. This circuit has been highlighted a couple of posts back3. For Wim’s experiments, he used an impeller under a certain operating condition and mixture properties. Before and after a representative period, the condition of the impeller was measured and the difference is a measure of the wear experienced.
Wear (or scientifically: erosion) is related to the impact of the particles on the material surface. In order to know the kinetic energy of the particles, the flow field has to be known. As the flow fleild cannot be measured directly at the test circuit, we have to resort to Computation Fluid Dynamics. We already know of Suman’s graduation, to look for patterns in the flow lines, but Wim has extended the procedure to also quantitively estimate the related erosion.
Although there is only a limited amount of data available, comparing the results of the CFD estimation and the measured erosion are looking promising. This is certainly a workflow that will provide us the unique tools for engineering better pumps and assisting customers in their specific projects.
Although Wim will not join our ranks in the dredging community and pursue a different career in another interesting industry, we are sure he will be constructive and dedicated colleague at Marin.
Sometimes, explaining a part of the dredging process can be as simple as seeing through the mixture. In this exhibit we can demonstrate what happens beneath the surface of the cargo in the hopper1,2,3. Along the way, we explain some quirky behaviour in other phases of the dredging process, also. The exhibit consists of five tubes in a frame, that can rotate around a horizontal axis. In the tubes are various types of soils. Each with their own settling behaviour. The exhibit was recently added to the Damen Dredging Experience. One more reason to highlight it here.
One major part in the dredging process is the hydraulic transport of particles in a carrier fluid. Pickup and transport have been touched upon in previous posts4,5. Here we concentrate on the end of the process: settling and deposition. This can be either in a hopper or on the discharge area. In both cases you will only see the fluid surface during the process and at best the top of the deposited sediment. How the material came there, was deposited and stacked up can’t be readily seen. As the tubes allow these processes to be observed from the side, we can follow the events.
The exhibit can be started by upending the frame with the tubes. The material that sat in the lower end gets now on the top end. They all are released at the same time and we see immediately see the differences in settling velocity for the different particle sizes6. The gravel falls down within ten seconds. The sand is slower and the clay even has problems getting started. One nice observation is the mixture of soils. Against the height of the tube, the fractions in the sample are released simultaneously. Still, the fractions separate over the fall height and stack up again in their original order. This not only happens in the tube. In the hopper or the discharge area, a widely graded sediment will sort itself to the various fractions.
Although for all the samples the particles are released simultaneously, you can still see a slight difference in settling velocity within each sample. This can be either due to slight variations in size that are possible within each mesh size used for sieving. Another cause for the differences might be the differences in shape. A perfectly spherical particle will have a faster settling velocity than an oddly shaped potato7.
And even then, the initial particles that fall down have a greater velocity than the particles in the bulk of the sample, even when having the same particle size and shape. This is due to the water flowing up around the particle. The upward flow is slowing down an adjacent particle. This interaction is called ‘hindered settling’. At high concentrations this can contribute to the efficiency of pipe line transport8. But for the settling it is really hindering the loading time.
At the very end of the settling, the particle gets deposited at the bottom, or on top of another. The water that is caught in between has to escape. This causes one last puff of fluid to flow upward and take the very find dust present upward. This happens with each particle that settles and causes the layer of dust to lift to the surface of the deposited sediment. So even when loading a cargo of gravel, you will always end up with a layer of dust on top. So, don’t judge the quality of the cargo just by the dust you see on top. Take a deeper sample or base your evaluation on the signals from the sensors from the screening tower.