Modern Uses And Legendary Excuses For Manual Depth Sounding

Depth sounding lead and rope
Depth sounding lead and rope

Never waste a moment to tell a good story. Usually, you’ll find informative or educational stories on this platform. This time, I literally found an opportunity to tell you a fun story. All it took, was this nifty little classic navigational instrument. The crew on the dredge used to calibrate their modern survey system1 or checked the delivered depth with this ancient tool. Ever seen one like this? It is a depth sounding lead2. Well, I doubt this one was made from lead, based on the estimated weight and appearance, but it does have all the other characteristics of a normal depth sounding lead.

Evolving from a stone on a rope, the depth sounding lead was used to sound the depth. The plummet was made from lead. The rope was marked at regular intervals according to the shoe size of the current king. Cast overboard, the lead sank and keeping the rope tight, the depth at that location could be read from the markings on the vertical rope. It involved some nimble dexterity to stand at the lee side of a fast moving vessel in a choppy sea to handle the lead, a bundle of coiling rope and accurately reading the depth at the right moment. Hands down to all those seafarers that explored the world in old times and managed to navigate the globe on this instrument.

Sounding the depth manually with rope and lead (Credit: Wikipedia)
Sounding the depth manually with rope and lead (Credit: Wikipedia)

The depth was not the only information gained from this action. When you look closely, there is a hole at the bottom of the lead. On the picture above it is empty, but it ought to be filled with grease or wax. When the lead touched the bottom, some of the dirt was caught in the grease. When the lead was retrieved, the cling-ons were inspected. These could be either: sand, mud, gravel, peat, silt or even shells and other biological detritus. The material was reported on the charts also. This made navigation in charted waters easy: compare the sample with the indicated bottom condition. And that brings me to my fun story.

Before the Dutch reclaimed their land, there was a large water body in the Netherlands, called the ‘Zuiderzee’3. Or, South Sea as opposed to the North Sea, which most of you might know. This Zuiderzee, was extensively used for fishing. The skippers did not have charts, but they relied on oral tradition handed down through the ages of where what kind of soil would be available. Near Urk, you might find rocks. Near Pampus, there will be a lot of mud and around Stavoren, there is the famous ‘Vrouwenzand’ (Sand Bank of the Lady of Stavoren4). So, when the fishermen cast their depth sounding leads out, they knew the location of their vessel and the depth beneath it.

Map of the ‘Zuiderzee’ (Credit: Wikipedia)
Map of the ‘Zuiderzee’ (Credit: Wikipedia)

One of those skippers boasted he did not even have to see and feel the sample, but just by tasting it, he could pinpoint his location within a hundred yards. Hard to believe, right? The cabin boy on board thought likewise. So, he devised a cunning plan. After lunch, the skipper went down to the cabin for a short nap and instructed the cabin boy to bring him the lead to taste the sample. But, our clever cabin boy sank the lead in the crate with potato’s. The bottom of the crate was covered with clay from the potato’s. Carefully bringing the sample to the skipper, the cabin boy woke him up and awaited his reaction. The skipper woke up groggily and grappled for the lead with half closed eyes. He stuck his finger in the sample hole and tasted the material inside. Suddenly, his eyes went wide open and he exclaimed: Oh, disaster! The dikes have broken again! The land is flooded and we are sailing over farmer John’s potato patch!

You never know what you dredge from the bottom of a potato crate
You never know what you dredge from the bottom of a potato crate

References

  1. Positioning and survey system, Damen
  2. Depth sounding, Wikipedia
  3. Zuiderzee, Wikipedia
  4. Lady of Stavoren

See also

Deposition Of Dredged Material At Reclamation Areas In Ancient Chinese And Modern Times

Hills of Jingshan Park Beijing
Hills of Jingshan Park Beijing

As promised, I still have several stories for you and this is another one. As you may remember, we’ve visited China for attending the WODCON in Shanghai1 and afterwards travelled to Beijing for sightseeing. A must see destination in Beijing is the Forbidden City. The epicentre of ancient Chinese power, the seat of the emperor. Once the exclusive domain of the supreme ruler, now a tourist attraction for the general public. The Forbidden City was mainly build in the Yongle era of the Ming dynasty2 between 1407 and 1420. It comprises numerous courtyards and halls and temples. All the buildings are surrounded by thick walls and a moat.

Moat around the Forbidden City
Moat around the Forbidden City

This moat is an impressive 6 meters deep and 52 meters wide. That is a big moat. But remember it is long: 3.5km around3. So, it is an impressive moat. Now consider this moat is dug in the fifteenth century. It has been dug by hand! Imagine, thousands of labourers digging, carrying and removing the soil from the moat. That is quite an operation.

To put this in perspective. The moat has a volume of 6x52x3,532m=1,101,984m³. Yes, that is over a million cubic meters. Even for a modern dredging project it is a serious volume. And digging a hole at one place is the first step. Where do you dispose it? At a dredging project, there is a reclamation area. As this was dry land, there was no reclamation area. So, what do you do with such a volume? If you pile it up, you can store a volume of V=1/3 pi r² h in a cone. Assume a slope of one third of the height to the radius, the height of the pile can be calculated and will be around 49 meter. And that is exactly what the ancient engineers did: they created the hill of Jingshan Park4. With its five peaks, it is not exactly a cone, but the estimated height was quite close!

Height marker at the top of the hill in Jingshan Park
Height marker at the top of the hill in Jingshan Park

The engineers had probably carefully planned how they constructed this hill and planned the delivery of the material accordingly. Nowadays, with the much higher production rates and shorter project delivery times, it is highly inadvisable to build a reclamation area with this height. There are several reasons why not to do it like that. First, it would take time to drain the pore water away from the core of the hill. Loading more on top quickly would make it very instable. Sometimes with disastrous results5. Another is when you create high banks, it will be easier for shear planes to form and collapse the structure that way. Lastly, a lower reclamation area will also have a larger surface area and more choice to select multiple locations to evenly distribute the material in volume and composition. A well designed reclamation area requires good knowledge of the deposited material and a skillful team that operates the equipment to manage the deposition.

Explanations of issues with depositing sand at reclamation areas
Explanations of issues with depositing sand at reclamation areas

Based on the exposed rocks sometimes seen on the sides of the Jingshang Park hill, the core is probably consisting of bigger rocks as a kind of backbone. But not every rock found in the moat ended up in the hill throughout the area. Several decorative rocks can be found that have a typical size that could just be handled by manual labour. Just another tribute to the perseverance of those classic engineers.

Decorative stone in Bei Hai Park west of Jingshan Park
Decorative stone in Bei Hai Park west of Jingshan Park

References

  1. WODCON XXII, EADA
  2. Yongle Emperor, Wikipedia
  3. Forbidden City, Wikipedia
  4. Jingshan Park, Wikipedia
  5. Aberfan disaster, Wikipedia

See also

CEDA DMC Visits the Anse du Portier Project in Monaco

CEDA Dredging Management Commission at a site visit in Monaco. (Credit: CEDA)
CEDA Dredging Management Commission at a site visit in Monaco. (Credit: CEDA)

In the past intermezzo, a lot of blog ideas past my mind. In due time, I will share some of them with you. Others already arrive by themselves naturally. e.g. Lately we’ve had another CEDA Dredging Management Commission meeting1. In preparation for the upcoming CEDA Dredging Days2, we discussed some publications that will be presented there3. Next to the meeting, we also did a site visit to a prestigious project. The Monaco extension project ‘Anse du Portier’ certainly demanded some serious management skills for the dredging works.

Overview of the Anse du Portier project
Overview of the Anse du Portier project

The extension project had already received a lot of attention in the press and in the dredging community. It certainly is a remarkable project, where a lot of disciplines are coming together. I would like to refer you from the excellent video on the Anse du Portier project itself4:

Extension en mer de Monaco – Techniques de construction (Credit: Anse du Portier Project)
Or, if you can hold your breath, to the presentation of Camille Kapella at the CEDA Dredging Days5, where she will elaborate on all the difficult challenges in the project. At the moment of our visit, the last caisson had just been placed in the construction6.

Last caisson in the constructed sea wall at Monaco
Last caisson in the constructed sea wall at Monaco

Caissons are a demanding construction in terms of dredging. Of course, there are examples, where location and placement were not so important, but usually the requirements are much stricter and the conditions much harsher. Caissons have to be placed next to each other in the first place. And joining them all together might end up in a big deviation as errors propagate through each misplacement. This has been recognised already for a long time. Even one of the first tunnels built of sunken caissons, the Maastunnel7, had specifications that are still in use today. So, how did they do this? There is a nice historic video available from the old Polygoon Journaal.

Building of the Maas Tunnel (Credit Polygoon Journaal)
In the case of the Maastunnel, they employed wires driving huge dials and sight line beacons. Under perfect conditions, enough time and an enormous amount of manpower, the objectives can be achieved. Nowadays, this approach would be too costly or can’t be used as the local circumstances prevent them. Waves, tides, difficult location or other factors are the edges of the envelope for modern caisson placement and all were present here in Monaco. Specifically, the challenges at the Anse du Portier site were the steep bedrock, the open coastline vulnerable to waves and environmental concerns. For each, of the challenges, appropriate solutions were chosen to manage the project.
The construction of the caissons to withstand the wave action during the lifetime is remarkable. The top of the caissons are equipped with so called patented Jarlan chambers8. This is a design concept known in the offshore construction to temper the wave action. Waves enter the construction through slots in the walls and enter a chamber with more columns for further dissipation of the wave energy. A similar approach is already discovered by nature itself: coastal mangrove forests.

Last caisson at the Anse du Portier with Jarlan chamber slots covered for transport
Last caisson at the Anse du Portier with Jarlan chamber slots covered for transport

References

  1. Dredging Management Commission discusses papers on contract-type selection and soil investigations, CEDA
  2. CEDA Dredging Days 2019, CEDA
  3. Effective contract selection: CEDA’s guide to optimised contracting methods, CEDA
  4. Anse Du Portier, Youtube
  5. Dredging in Monaco: challenges and solutions, CEDA
  6. Monaco Land Extension Project Reaches Milestone, Caissons Belt Completed, DredgingToday
  7. Maastunnel, Wikipedia
  8. Jarlan Chamber, Espacenet

See also