Why slopes are bad for anchoring

[thinwater]

[This follows a post suggesting that the main effect of a slope is that it changes the rode angle at the bottom (see below image from that thread). Slope does change the rode angle and that does matter, but ... ]

This is an interesting topic, and I'm starting a new thread for it. When I started sailing, as a new engineer, I understood the above calculation (thanks Nolex), found it obvious and reassuring. Most sailors do. But after years of engineering and sailing, I think it sometimes simulates the situation, but I'm going to have to offer my apology for this. I think it misses the real point entirely and is most often not the most important factor. The important affect of slope is its affect on soil strength and characteristics.

The important questions are:

• why the slope exists
• the slope's affect on soil shear strength

I would really love for a real soil expert to weigh in. I have some experience related to engineering foundations from mountain (rock but also horrible moraine fields), to Peidmont (typically good strong clay), to the Mississippi delta (very soft--even the sand flows like water). Mud, clay, and sand can vary by multiples in strength, depending on micro structure, how they were laid down, and chemistry. The topic is way more complicated than anchors.

---

First, unless it is a severe slope (channel edge), the slope is typically quite small. Perhaps 3 feet in 200 feet, or 1.5%. At 5:1 scope this is less than 10% of the angle, and at 10: scope the chain is probably staying on the bottom (zero bottom angle) both ways. Not a zero effect, but minor.

Let's imagine an extreme case. First, a field of level fine gravel. Place an anchor at infinite scope and it digs. Then place the same anchor on the same gravel, but on a pile that is at its angle of repose (won't get steeper without sliding). Pull the anchor down slope with infinite scope, and it holds nothing at all. The gravel just slides. The rode angle did not change. The material did not change. But the down slope holding is nothing, no matter how much scope. For the same reason you cannot climb up the pile. If you then drag the anchor uphill (same rode angle) you get a very different result. It holds. But it was not the rode angle that mattered.

Why is there a steep slope?

• Dredging nearby. Probably a bad place to anchor because of traffic.
• Strong tide. Often this makes the bottom very hard because anything loose has been scoured away. I know some slopes in the Chesapeake like this; good fishing but terrible anchoring. They're not even steep, just mudstone and super hard clay.
• Outflow "pile" from a river. The sediment may be fresh and poorly consolidated. It may also be near its angle of repose.

Angle of repose. For angular (crushed stone) gravel this is pretty steep, maybe 35-45 degrees. Natural gravel, perhaps more like 30-35 degrees. Same with sand. Bank sand maybe 35 degrees, wet bank sand maybe 25 degrees. But what about river sand, such as found in the Mississippi basin and used for proppant in fracing? The grains are round and the angle of repose is less than 10 degrees wet. My first introduction was a construction project in New Orleans. They brought it what is called pump sand. One day it was in a pile, it rained, and it spread as thought it was cake batter, covering the lot. Lesson learned. sand is not all alike.

[Notice how smooth the proppant sand is at top. No sheer strength. Crappy for anchoring, but easy to pump down fraced wells.]

Coral sand. Not great because it is so light.

Angles make it strong. Good for foundations. Fine angular sand is the stuff anchors makers dream of.

So what if the slope has an angle of 1:30 and the angle of repose of such sand is 1:20? The slope is not geometrically important, but the slope is close to landslide geometry and can hold very little without sheering out.

I don't think it is the maths of a slope and rode angle. I think it is

• Why the slope exists. Tidal souring or outflow.
• The type of soil, its angle of repose, and other soil strength factors.

Discuss. Probably way too complex to define. But I think the effect on rode angle is not the main thing in most cases. The math is reassuring (I thought so for years), and it's not wrong, but it's more complicated.

 

[bikedaft]

Really interesting.TBH i usually try to avoid anchoring on slopes, but sometimes have anchored on the edge of a river delta, with the chain hanging off downwind towards the deep part of the loch. Reckoned it would take a lot of force to lift all that chain up off the deeper bottom to lift the anchor off the horizontal, on the shallow part where it was.

(Walking on the beach on Eigg, pre new ferry pier, dad bellowed "what fool left their anchor here?!??" - on the beach. He quickly realised it was his, chain led straight to our boat...)

 

[noelex]

Why is there a steep slope?

• Dredging nearby. Probably a bad place to anchor because of traffic.
• Strong tide. Often this makes the bottom very hard because anything loose has been scoured away. I know some slopes in the Chesapeake like this; good fishing but terrible anchoring. They're not even steep, just mudstone and super hard clay.
• Outflow "pile" from a river. The sediment may be fresh and poorly consolidated. It may also be near its angle of repose.

Interesting thoughts. I think you may be correct about some of the reasons why the holding is worse on a significant downslope.

However, I disagree about "Why is there a steep slope?". In most of the situations where I have encountered a steep seabed slope it has not been related to the factors you mention (although this obviously possible), but rather due to the natural geography is the area. Some parts of the world the coastline becomes deep very quickly and this is also true of the anchorages.

Parts of Italy are an example where we have encountered some of our most pronounced sloping seabeds. Typically there is no dredging, tides or river outflows involved. It can be over a 1,000m deep less than couple of miles from the shoreline in some areas. This rapid change in depth is reflected in the topography of the anchorages. Some other cruising grounds are similar.

First, unless it is a severe slope (channel edge), the slope is typically quite small. Perhaps 3 feet in 200 feet, or 1.5%. At 5:1 scope this is less than 10% of the angle, and at 10: scope the chain is probably staying on the bottom (zero bottom angle) both ways. Not a zero effect, but minor.

Once again, this depends on location. Some coastlines are very flat. A 3 foot drop in 200 feet is quite sedate. The recommended anchorage at the island where I am currently anchored (in the Caribbean where steep sloping anchorages are rare) is nothing remarkable, but the slope is 4.5° (a slope of 13 to 1, or a 15 foot drop over 200 feet) using Navionics data.

I suspect this is one of the factors a why a Catamaran took three attempts to set their anchor and still managed to drag in only light winds just a few days ago. This was despite a good quality anchor and sound technique. On the plus side, it is probably why we have the anchorage to ourselves every night .

The slope of the seabed whether it beneficial (upslope) or harmful (downslope) is often not given the consideration it deserves.

 

[AntarcticPilot]

I am a geologist, but not an expert on slope stability. But my understanding is that any slope steep enough to approach instability would be far too steep to consider anchoring; critical angles tend to be in the 30°-40° range. Very few submarine slopes in soft sediment come anywhere near that sort of slope; they tend to be deposited from flowing water, so they get deposited in nearly horizontal beds. Steep slopes in soft sediment tend to be erosional features, so they also tend to be associated with strong currents.

The stability of loose sand or gravel is a bit of a red herring. Most substrates have at least some clay minerals or biological material, so once the sediment is deposited, the clay minerals aggregate and bind the sediment. Diagenetic changes also happen surprisingly rapidly, resulting in the binding of loose sediments.

Bottom line is that a seabed at a shallow enough angle for anchoring to be sensible is unlikely to be unstable for other reasons.

 

[geem]

My own experience of anchoring on down slopes, is that they tend to be harder to get the anchor to set. It may take a few attempts as power setting pulls the anchor our. More chain assists in this situation. There are a couple of places I knowingly anchor on a downslope but we also see boats dragging there more often possibly due to poor anchors and/or poor setting techniques and short chain lengths

 

[thinwater]

Interesting thoughts. I think you may be correct about some of the reasons why the holding is worse on a significant downslope.

However, I disagree about "Why is there a steep slope?". In most of the situations where I have encountered a steep seabed slope it has not been related to the factors you mention (although this obviously possible), but rather due to the natural geography is the area. Some parts of the world the coastline becomes deep very quickly and this is also true of the anchorages.

Parts of Italy are an example where we have encountered some of our most pronounced sloping seabeds. Typically there is no dredging, tides or river outflows involved. It can be over a 1,000m deep less than couple of miles from the shoreline in some areas. This rapid change in depth is reflected in the topography of the anchorages. Some other cruising grounds are similar.

Once again, this depends on location. Some coastlines are very flat. A 3 foot drop in 200 feet is quite sedate. The recommended anchorage at the island where I am currently anchored (in the Caribbean where steep sloping anchorages are rare) is nothing remarkable, but the slope is 4.5° (a slope of 13 to 1, or a 15 foot drop over 200 feet) using Navionics data.

I suspect this is one of the factors a why a Catamaran took three attempts to set their anchor and still managed to drag in only light winds just a few days ago. This was despite a good quality anchor and sound technique. On the plus side, it is probably why we have the anchorage to ourselves every night .

The slope of the seabed whether it beneficial (upslope) or harmful (downslope) is often not given the consideration it deserves.

Good point on geography. The only places that is true on the US east coast it is primarily because of underlying rock, and thus the ground is rocky and thus unpredictable. Not deep sand or mud. The sand and mud accumulate on the flats.

There are also large areas near my home where rock extends out under the flats, with very shallow sand covering, no mud. Bad holding. These are areas bordered by cliffs. You would think there would be deep mud, but just a little current over smooth rock seems to be enough to keep it scoured to thin sand and wash the mud/clay out.

Obviously it varies a good deal, but in general here the 20 fathom line is 20-40 miles out, and the 100 fathom line (continental shelf) is 50-100 miles out.

Many possible situations.

 

[thinwater]

... The stability of loose sand or gravel is a bit of a red herring. Most substrates have at least some clay minerals or biological material, so once the sediment is deposited, the clay minerals aggregate and bind the sediment. Diagenetic changes also happen surprisingly rapidly, resulting in the binding of loose sediments.

Bottom line is that a seabed at a shallow enough angle for anchoring to be sensible is unlikely to be unstable for other reasons.

Yes, an exaggeration to make a point.

I will add that there are many river systems and even areas in the Chesapeake where the sand deposits in thick layers without clay or biological binders. They call it river pump sand, because you can just pump it from place to place. I sure it has to do with mineral types, the hydrology of the rivers, and the geography of the drainage basins. The Chesapeake, for example, is one of the muddier places in the world, and yet there are large areas with find sand with no clay or significant biological binding. I don't know, I just know what is. And this fine sand is some of the best holding ground anywhere. I've recorded 1000 pounds with a 2.5 pound Guardian, before stopping only because I didn't want to break anything. Other NG anchors hold about 250 times mass without going very deep. A mile away the mud might only hold 15 times mass, with no obvious feature dividing them. One of the fine said areas (many square miles) is only about 10 miles from the terrible soft mud site that Neeves talked about. The fine sand is on the open Bay along a line of cliffs. The soft mud up a very broad river (1-2 miles wide) a short distance. The main channel of the Bay (5-20 miles wide) actually has a relatively firm bottom, perhaps because of the twice-daily tides. The small creeks (they can still be pretty wide--it is called a creek if it is less than ~ 10 miles long, but a creek can be 5 miles wide) are where the goo is.

 

[thinwater]

One particular area that comes to mind starts near shore with cliffs and broad band of fine sand. In the deeper sand areas it is great anchoring, in the shallow areas people tend to blow ashore. Only local knowledge makes it clear which areas are good, or power setting, or digging with a rod.

A few miles out the depth increases from ~ 10-20 feet to 50 feet over ~ 300 yards. But the real factor is that it is also a point of land at one and of a very large cove (miles both ways) and the current scours the sand off. It's naked, jointed mud rock until it flattens out again. No holding except luck. It's not a strong current, maybe about 1.5 knots max, but it's enough.

The reason holding is bad is not the angle, it is tidal scouring.

Some places you can see the bottom, but not the Chesapeake. Visability is about 2-5 feet, and if you dive to inspect the bottom or an anchor, it is often largely by feel.

 

[B27]

My feeling is, I'd probably be fine on a scope of 3:1 with the wind off the shore.
Much preferable to being on the lee shore whatever the scope, even a small swell would be exercising the nylon in the rode in that situation.
I do anchor that way, waiting for the river to fill with water so I can get to my mooring, but if the breeze is building, I feel the need to keep an eye on things.

In the other post someone opined that the depth indicated by the sounder was irrelevant, but actually it ties with the effective scope. Just tilt your diagram so the seabed is level?

People are way over-analysing things. The seabed is varied far beyond worrying about calculating holding power to the nearest 10%.

 

[Stemar]

My own experience of anchoring on down slopes, is that they tend to be harder to get the anchor to set. It may take a few attempts as power setting pulls the anchor our. More chain assists in this situation. There are a couple of places I knowingly anchor on a downslope but we also see boats dragging there more often possibly due to poor anchors and/or poor setting techniques and short chain lengths

Presumably this is because the pull on the rode is at a greater angle is you're pulling down the slope. I could see that being an issue if you get the anchor set uphill, then the tide or wind changes and the anchor needs to reset itself downhill.

My main concern about anchoring on a slope is that we often anchor in shallow water and dry out. It really isn't a good idea to do that on the edge of a channel - they can be quite steep

 

[Neeves]

I wonder how many recognised anchorages are host to a sloping seabed such that it is a hazard. I confess that in the last 40 years, I don't recall beyond that, I cannot remember an achorage where the slope of the seabed was an issue.

Other facets might be an issue - but I simply don't recall warnings that the seabed sloped 'dangerously'.

It would need to be a seriously steep seabed to cause a modern anchor, that buries itself into the seabed with its flukes at 30 degrees - to the seabed, to be impacted. Obviously if the rode length is too short - and you do not notice the extreme slope then there might be an issue. However most modern anchors set, at 30 degrees to the seabed, within shank length, maybe 1 metre, give or take. The increase in-depth is simply not significant.

We would commonly set at 3:1, power set and then deploy more rode based on the amount of room and the expected weather (and always deploy at least 14m snubber/bridle).

Jonathan

 

[Stemar]

I wonder how many recognised anchorages are host to a sloping seabed such that it is a hazard.

Maybe not failing to reset - I suspect that, unless it's really steep, if that's a problem for me, I probably should have had more rode out, but we anchored in Newtown Creek on the IoW once, at the top of Clamerkin Lake. We dried out, and all was well, because there was enough wind to blow us across the channel onto the shallow bit, but the rode went off into deeper water. On the next tide, there was less wind, so we were sitting to the current and, as the tide dropped, we could see that we were going to have one hull on the shallow part and the other over the edge. Not dangerous, but it would have been an uncomfortable night.

 

[Neeves]

Maybe not failing to reset - I suspect that, unless it's really steep, if that's a problem for me, I probably should have had more rode out, but we anchored in Newtown Creek on the IoW once, at the top of Clamerkin Lake. We dried out, and all was well, because there was enough wind to blow us across the channel onto the shallow bit, but the rode went off into deeper water. On the next tide, there was less wind, so we were sitting to the current and, as the tide dropped, we could see that we were going to have one hull on the shallow part and the other over the edge. Not dangerous, but it would have been an uncomfortable night.

If drying out we would always lay at least 2 anchors, the bow anchor and usually two off the stern, one on each transom. We would simply anchor, bow out, in the direction we would leave. Run the engines gently in reverse suffice to keep the stern where we wanted, working 'against' the bow anchor and then deploy 2 anchor by dinghy off each transom, taking the rode in with the sheet winches. If the yacht goes 'off centre' you can remedy by taking on more or less rode for the stern anchors. The stern anchors would be readied prior to dropping the bow anchor, sitting on the transom steps. On leaving we would retrieve the stern anchors by hand and when near floating would start the engines and keep the bow anchor under tension - we'ed then slip off gently and 'follow' the rode, taking in the slack, as the bow anchor would be in 'deeper' water, maybe only a few cm deeper.

Here we are all dried out. We have two stern lines, one behind the cat and one, the red line (dyneema) to the left.

The picture shows an advantage of our primary rode/snubbers doubled back to a waterline pad eye. The scope is improved as the rode runs from the bridle plate, at the waterline (not at the bow roller on deck). So our scope is calculated as water depth, no allowance needed for height of bow roller above the water line.


Jonathan

 

[thinwater]

I wonder how many recognised anchorages are host to a sloping seabed such that it is a hazard. I confess that in the last 40 years, I don't recall beyond that, I cannot remember an achorage where the slope of the seabed was an issue.

Other facets might be an issue - but I simply don't recall warnings that the seabed sloped 'dangerously'.

It would need to be a seriously steep seabed to cause a modern anchor, that buries itself into the seabed with its flukes at 30 degrees - to the seabed, to be impacted. Obviously if the rode length is too short - and you do not notice the extreme slope then there might be an issue. However most modern anchors set, at 30 degrees to the seabed, within shank length, maybe 1 metre, give or take. The increase in-depth is simply not significant.

We would commonly set at 3:1, power set and then deploy more rode based on the amount of room and the expected weather (and always deploy at least 14m snubber/bridle).

Jonathan

For example, if the tide running over the slope scours off the sand and mud, leaving behind something too hard to any anchor to penetrate.

"Recognized anchorage" is a term used to imply it is a safe and user-friendly place to anchor. I wouldn't expect warnings, though poor holding ground in general is often mentioned in Chesapeake anchorages.

 

[noelex]

Here we are all dried out. We have two stern lines, one behind the cat and one, the red line (dyneema) to the left.

That’s not anchoring. That’s parking .

 

[NormanS]

I wonder how many recognised anchorages are host to a sloping seabed such that it is a hazard. I confess that in the last 40 years, I don't recall beyond that, I cannot remember an achorage where the slope of the seabed was an issue.

Other facets might be an issue - but I simply don't recall warnings that the seabed sloped 'dangerously'.

It would need to be a seriously steep seabed to cause a modern anchor, that buries itself into the seabed with its flukes at 30 degrees - to the seabed, to be impacted. Obviously if the rode length is too short - and you do not notice the extreme slope then there might be an issue. However most modern anchors set, at 30 degrees to the seabed, within shank length, maybe 1 metre, give or take. The increase in-depth is simply not significant.

We would commonly set at 3:1, power set and then deploy more rode based on the amount of room and the expected weather (and always deploy at least 14m snubber/bridle).

Jonathan

We anchor in an awful lot of places which are not "recognised anchorages", some more suitable than others.
Re sloping seabeds: Many of the sealochs in the West of Scotland were gouged out by glaciers, and are classed as fiords. Many of them are deep. Usually there are rivers coming in from the hills at the head of the lochs. These rivers bring down a collosal amount of material eroded from the hills, and deposit it in the form of deltas, above and just under the littoral zone. Most of the material is rock flour and stones, so it settles very quickly, and doesn't travel far. This means that a steep bank is formed, just out from the LW mark, falling sharply away to the much deeper depth in the body of the loch. Obviously nobody would attempt to anchor on this steep slope, but when approaching from seaward, can give the unwary quite a surprise. 25m suddenly becomes 2m.

 

[thinwater]

We anchor in an awful lot of places which are not "recognised anchorages", some more suitable than others.
Re sloping seabeds: Many of the sealochs in the West of Scotland were gouged out by glaciers, and are classed as fiords. Many of them are deep. Usually there are rivers coming in from the hills at the head of the lochs. These rivers bring down a collosal amount of material eroded from the hills, and deposit it in the form of deltas, above and just under the littoral zone. Most of the material is rock flour and stones, so it settles very quickly, and doesn't travel far. This means that a steep bank is formed, just out from the LW mark, falling sharply away to the much deeper depth in the body of the loch. Obviously nobody would attempt to anchor on this steep slope, but when approaching from seaward, can give the unwary quite a surprise. 25m suddenly becomes 2m.

"Obviously nobody would attempt to anchor" is sort of like "recognized anchorage." I'm sure you've seen some strange anchoring and I would not draw hard lines! I've seen many try to anchor along channel edges, because they had too much draft for the flats. Sometimes it works, sometimes not. Often it's a traffic hazard. Often they ground when the wind changes. Tidal swings tangle ropes around keels.

 

[Roberto]

I wonder how many recognised anchorages are host to a sloping seabed such that it is a hazard.

Ideally extend the volcano profile into the water and imagine the depth at say 20m from the shore I passed there many years ago, could not make the lead line touch the bottom. The real hazard would be the eruption, but that only occurs along one side of the island.

 

[AntarcticPilot]

Ideally extend the volcano profile into the water and imagine the depth at say 20m from the shore I passed there many years ago, could not make the lead line touch the bottom. The real hazard would be the eruption, but that only occurs along one side of the island.

The interaction of lava and seawater, along with the rapid erosion of softer volcanic deposits such as ashes and volcanic breccia means that there is usually a break in slope at the waterline.

 

[noelex]

I wonder how many recognised anchorages are host to a sloping seabed such that it is a hazard. I confess that in the last 40 years, I don't recall beyond that, I cannot remember an achorage where the slope of the seabed was an issue.

Many of our favourite anchorages have been in locations (usually deserted) where anchoring is difficult for some particular reason. The location may have a poor substrate, a severe downslope, or is deep limiting the available scope that can be deployed etc etc. Invariably, we will have the place such as this to ourselves even in very busy cruising grounds such as the Mediterranean or the Caribbean. Bliss.

Of course when we have been cruising quieter areas such as the west coast of Ireland or the outer Hebrides there is little need to avoid popular locations, but the ability to use more challenging locations that are not "recognised anchorages" opens up many more possibilities, some of which are the most beautiful.

Modern anchoring gear has improved enormously. Not only do we have much better anchors that work well in diverse substrates, but we have powerful and reliable electric anchor winches that enable large anchors to be easily handled by a small crew. These two improvements have opened up many locations that were previously untenable, especially when contemplating anchoring overnight. Strangely, few cruisers have taken advantage of these improvements and will only use "recognised anchorages".

It is a pity they miss out on some fantastic locations.