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

References

  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

Don’t rock the boat, don’t tip the boat over

DOP Dredge ‘Roanoke’, Long Island, USA

We were quietly enjoying our dinner on a relaxed evening in our vacation. Suddenly, we were rudely disturbed by rumble and clatter from across the valley. For our eyes developed a rock slide. Just as sudden as it started it was already over. Perplexed, we were too slow to capture the event and put it on social media. Afterwards, I took some pictures of the rubble. As you can see, it was not even a proper rock slide, more the collapse of a retaining wall.

Retaining wall collapse, Sóller, Mallorca, Spain, June 21, 2018

Come to think about it, it was not the first collapse I witnessed. Back in 2006, I was visiting our DOP dredge at Roanoke on Long Island, NY in the USA. I had to do some measurements and general inspection. I was below decks connecting the data recorder to the drive system and had to check something with the dredge master. Just when I climbed on deck, he yelled at me to hold on. Immediately a torrent of water and sand was flung horizontally over the dredge. Some stones cracked a window in the control cabin. Within seconds a tsunami lifted the dredge for about a meter and we kept rocking until the reflecting waves in the pit eventually subsided.

DOP Dredge Roanoke with pit bank in the foreground, before it collapsed. Older bank collapses in the background.

That was one big bank collapse to me. A bank collapse is a known, although undesirable phenomenon in dredging1. It is a result of dredging methods, relying on the development of an active bank to produce a heavy slurry, that is sucked up. However, the sediment does not consist of a uniform block of sand. Usually, the sediment is deposited in different layers, each with their own geo technical properties. These result in varying propagation velocities of the active banks. When a ‘faster’ sand is under a ‘slower’ sand, the upper layer is not supported anymore and collapses. As the bank slumps down, it displaces an enormous volume of water and this often causes a tidal wave of its own. At Roanoke, the effects were aggravated by the fact, that the upper bank ran all the way to above water level.

Progression of an active bank and bank collapse

As this bank collapse can be expected when dredging with active banks and different sand layers, dredging companies are very keen on predicting these nasty consequences. Not only for the safe working condition of the crew, but also to prevent material damage and eventually for a stable and reliable delivered profile. Exactly this is what is being investigated by dr. Askarinejad in the Laboratory of Geo-Engineering at the Technical University Delft2. He has a beautiful rig, where exactly those conditions can be simulated and measured. With a neat trick he tips the whole test facility to form an instable bank. This makes the bank collapse on demand3.

Static liquefaction tank TU Delft (Credit: dr. A. Askarinejad)

Basically, this is exactly what we can demonstrate with the ‘breaching exhibit ‘ in our dredging experience4. Of course you are welcome to come over. For those who are not in the circumstance to visit us, you can also visit the National Dredging Museum as they now have a breaching exhibit of their own5.

Handover of our old breaching exhibit to the National Dredging Museum

References

  1. Breaching Process OE 4626, van Rhee, TU Delft
  2. Amin Askarinejad, TU Delft
  3. Statische liquefactietank , Delft Integraal
  4. Loose sand, how hard can it be?
  5. Baggermuseum krijgt model van Damen Dredging, Binnenvaartkrant

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

References

  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