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Earthworks Failure

 
Posts: 62
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We have an 8.5 acre homestead in San Ramon, Nicaragua (12.5° north of the equator). The entire property is on an incline (about 11% on average), with a ravine that runs from the top of the property to a bottom corner, where there is an aquaduct that discharges water under the highway.

During heavy rainfall events, water flows down the ravine, collected from properties above ours, at a substantial clip. I estimate at least 20 gallons per second (I am just guessing). Suffice it to say, the flow is strong enough that I would not venture to cross the ravine for fear of being swept off my feet.

Worried about continued erosion, I dug a pond 2 years ago (12 meters in diameter; 2 meters in depth) near the top of the property during the dry season, with the goal of pacifying the energy of heavy rains and spreading the water via a spillway connected to a series of swales. The pond filled with a few medium intensity rains and I was optimistic that this solution would work. Then, the big rains came. In flow of water from properties above me actually exceeded the capacity of the spillway's ability to discharge it. The water topped the 1 meter freeboard of the wall and eroded it down, carving out a new erosion ditch parallel to the ravine.

Undaunted, the following dry season, I repaired the dam wall, tripled the width of the spillway to nearly 2 meters, and filled the erosion ditch. Again, the pond filled. We even successfully passed through a major rainstorm. Then, recently we had a monster storm and the dam failed again.

Now, I am somewhat at a loss as to what to do next. I do not like the idea of all that water just passing through my property during heavy rains and in the process eroding a progressively deeper ravine. On the other hand, I now realize that the pond was not a good solution in the first place. Along with the water comes significant amount of sediment. I would have had to drain the pond regularly to excavate all of it. In fact, I could simply leave the pond unrepaired and it would fill with sediment within 2-3 years.

Does anyone have advice or ideas about what I can do. In Permaculture, we try to work with nature. I clearly have been working against it
 
pollinator
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Location: Denver, CO
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I hope somebody answers you!

What type of soil do you have?
 
gardener
Posts: 2689
Location: Central Texas zone 8a
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I may have a similar situation. I'm ommitimg the dam, merely digging out a depression. There's no dam to fail.  Because of this, the water direction should not change. It should continue in the same path once it overflows.

As far as sediment, I'm hoping this happens. When I need dirt, I know where to get it. I don't anticipate water in it more than 6 weeks at a time, so I'll have access to the sediment.

Past that,  did you get grass planted on your dam? That will help a lot with erosion,. With enough force this  might not do it. Maybe Having a low spot (spillway) to channel it would help. The wider the better.
 
Angela Aragon
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It is a mixture of volcanic clay, the kind that crumbles in your hand and regular clay. We currently are having another monster rain, so I will have to see what things look like in the morning.
 
pollinator
Posts: 2283
Location: Kent, UK - Zone 8
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I think you need to start further up the water catchment. Once it is flowing in the channel with force there is little you can do to constrain it.

You don't describe your climate. Is it frost free? If so you should look at vetiver grasses. These can be planted in dense hedges along the contour. The roots are incredibly deep and are very effective at stabilising fragile soils like you describe. Water flows through them but is slowed and sediment is retained behind them. Over time the soil depth uphill builds up and you get natural terraces forming which can be worked for agriculture. The vetiver stops sediment and nutrients from washing down the slope.  Start a project like this as high up the water catchment as you can, so that flowing water doesn't have time to build up speed. In addition to preserving the soils on the slopes, and slowing surface flows, more water will soak into the soil and be held up which reduces the massive surge effect in the stream, and should reduce the erosion potential.

Once you have the surface water under control you can start considering how best to restore the eroding channel in an enduring way - bank stabilisation with vetiver might be a partial solution, as might planting hedges perpendicularly across the channel to slow flow and trap sediment - again, starting as high up the hill as you can.

Without being able to see your exact situation I would probably start on your uphill boundary and plant a hedge to immediately intercept and slow surface water flowing from properties above. Perhaps you might talk to your neighbours and see if the are interested in it as well? After a few years you will be able to take slips from your own plants to do more planting.

Vetiver has an enormous root system that stabilises vulnerable soils.


Vetiver is planted as "slips" in rows, about 6 inches apart. As the plants grow the gaps close and it forms a dense hedge. Vetiver is sterile so will not seed and stays where you plant it.


Here is a cross section through a vetiver hedge, showing the old soil boundary (dark layer) and the trapped sediment on the uphill side of the hedge.
 
Angela Aragon
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When we first dug out the pond, we also installed 4 gabions directly in the flow about 8 meters apart up to the border of our property. I was hoping that this would help to clean the water of sediment before it entered the pond. We planted some Vetiver slips around the gabions in an effort to increase their stability. Unfortunately, these either were blown out by the force of the water or the area behind them filled with so much sediment that the water simply found a path around them and continued downward. In the process, the young Vetiver slips either were uprooted or were buried under sediment.
 
Michael Cox
pollinator
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Location: Kent, UK - Zone 8
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That is a shame, but I guess it has at least taught you about your water flows! I do think you are on the right track with vetiver, but you will definitely need to mitigate the force of the flows before young slips can stand up to it.

  • Could you plant rows of vetiver parallel to the channel, but back from the channel edge. These will be easier to establish, and will protect against the channels spreading laterally. Bank stabilisation effectively.


  • Again, I would suggest dealing with the surface flows across your land before trying to address the channel itself. Slow the flow and redirect it before it reaches the channel.


  • This sounds like a big project. You might find a vetiver nursery on site helpful. Somewhere you can tend to young slips and plants for use elsewhere on the land.
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    Posts: 124
    Location: Denton, TX United States Zone 8a
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    Angela, what an exciting prospect! A huge force to contend with, but a huge resource if you can. I hope you won't take exception to me saying so, but please please please be careful. Here in my part of Texas we have a similar situation- massive monsoon season rains falling on heavy clay soils- and I have seen how dangerous that water can become even when someone has a well thought out plan and is being careful.

    It sounds like the water is coming in with such volume and force that no planting would hold it back, certainly not in the first few years. For intermittent flows of that intensity, I would think gabions are your best bet. They would slow the water and catch the sediment. I know you mentioned already placing some, but I was having trouble visualizing. Did you have 4 block gabions, 8 meters apart from each other but in a line? Or 4 rows of continuous gabion perpendicular to the flow of water? I would be shocked if the latter failed to turn the trick- two rows of solid, 2'+ thick gabion should be enough to slow down most anything you can throw at it. Anything with more force than that and I might be more concerned with getting out of the way myself!

    Vetiver and bamboo are good solutions for stabilizing a system once it's established, but I think you have the right idea in looking further uphill to try and slow things down before they get out of hand. More swales, and swales dug above the level of the pond but below the level of the spillway, would cause more water to be gathered but could also hold a large amount of water, as well as pacifying it before it hits the spillway. You could also add a secondary spillway, level with the first, a ways down the swale, to prevent the kind of buildup that would overwhelm the dam wall. gravel in the swales can help prevent blowouts, especially since frost pockets wouldn't be a concern in your climate.

    In lectures I've heard Bill say that, when planning water catchment, estimate the maximum amount you will need to hold then double or triple it. If you're trying to catch the water running down that ravine, at around 20 gallons/second a 12m circular pond to a depth of 2m would fill from bone dry to spillway in under an hour. Like Michael said I would probably try to capture all the water directly falling on your property first, pacify the water coursing down the ravine, and once that's all up and running, think about bringing in a big earth mover for any attempt to tame the river.
     
    Gilbert Fritz
    pollinator
    Posts: 1559
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    What is the catchment area for this flow? How much of the catchment do you control? If I understand it right, the flow is already large and strong when it reaches your upper property line? Is the catchment agricultural land? Wooded?
     
    Angela Aragon
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    I have basically zero control over the catchment. I even tried to buy the property above me so that I would have more control, but to no avail.

    My ravine is an extension of one on the property above me. I am not sure, but I suspect that their ravine is an extension of one on the property above them. It is all agricultural land. Much of it overgrazed. My neighbors above me are uncooperative. They are two brothers that are set in their ways and prefer to just leave things the way that they are.

    The location of the pond is about 16 meters below the property line. As I mentioned, I had 4 rows of gabions built down the ravine from the property line. These all failed. The pond is built in the ravine.

    After reading replies to my post, I am considering revisiting the gabions above the pond. Here is why. I built a pretty substantial loose-rock gabion wall much lower on the property, but blocking the same water flow that fills the pond, moving every rock into place myself. This has worked great. A large quantity of silt has been deposited behind it, which ultimately has caused the water to slow and spread. Additionally, a large quantity of water is stored in the silt.

    The four gabions installed above the pond were not built by me, but by some guys that I had hired. Although they claimed to know what a gabion was, it became painfully evident that they did not have a clue.. Consequently, they did not do the job the way that I wanted. Instead, they basically built 4 rock walls that were doomed to fail. I was preoccupied with some other things on the farm and the rains came before I had a chance to fix them.

    Loose-rock gabions need to be built with a wide base. In fact, height is achieved by expanding the base. This enables them to withstand enormous pressure. The gabion that I built lower on the property worked partly because the incline is much less in that area, which made expanding the base easy. The 16 meters above the pond has a more severe incline, making it more difficult to build the type of loose-rock gabion that I did below. But I am going to look at the physics of it and see if I can work something out.

    Thanks for the replies thus far!
     
    Gilbert Fritz
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    I hope you can get it fixed! Some pictures would be great as the project goes forward.
     
    gardener
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    With that much water flowing down the ravine during large rain events, and with uncooperative upstream neighbors, perhaps your attention might first be given to other areas on your land where water is flowing over the surface.  I know that it probably kills you to see all that wonderful water just gushing down the ravine, but there is a lot of other water elsewhere that is slowly seeping away as well.  

    Perhaps you've already done this, and now you are trying to tackle the big mugguppy.  Understood.  It doesn't have to be one or the other, in fact, you'll want to do both --- water catchment in numerous forms for numerous types of terrain on your property.  But my hunch is that until the guys upstream work cooperatively with you to slow and sink the water that is flowing off their property, the best you can hope for is to contain the damage and keep too much of your soil from washing away.  

     
    pollinator
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    Location: South Central Indiana
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    Hi Angela,  I've built a few dams in clay soils. Large tractors, bulldozers, nothing by hand.  The clay generally makes it easier to build up the key-way (basically the base of the dam that goes into the ground across the ravine.) To state the obvious, your spill-way can't handle the volume of water you experience due to a combination of factors, rain-fall per hour, volume collected by the terrain contour, and vegetation or lack there of.  I didn't see where you mentioned a culvert of any kind. Most dam erosion problems can be stopped by increasing the size of the culvert (over-flow tube) in the spill way and positioning at grade.
    Do you have access to culverts, concrete, steel, or otherwise?
     
    Angela Aragon
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    Interesting. I did not consider using a culvert. I just had a level sill spillway, which, by the way, also eroded. I am sure that there are culverts here. Whether or not I can get one is another thing.

    The pond and keyway were dug by hand; compaction achieved with tamping tools and sledgehammers. Heavy equipment is difficult for regular individuals to access here. It typically involves a lot of red tape, long waiting periods (up to a year or more), and political patronage. Both of us on the farm are autistic (Asperger's Syndrome) and lack the social skills to negotiate through the maze of innuendo, non-verbal  cues, and passive-aggressive communication. Thus, we had to look for alternative solutions.
     
    Marcus Billings
    pollinator
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    Location: South Central Indiana
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    Angela Aragon wrote:Interesting. I did not consider using a culvert. I just had a level sill spillway, which, by the way, also eroded. I am sure that there are culverts here. Whether or not I can get one is another thing.

    The pond and keyway were dug by hand; compaction achieved with tamping tools and sledgehammers. Heavy equipment is difficult for regular individuals to access here. It typically involves a lot of red tape, long waiting periods (up to a year or more), and political patronage. Both of us on the farm are autistic (Asperger's Syndrome) and lack the social skills to negotiate through the maze of innuendo, non-verbal  cues, and passive-aggressive communication. Thus, we had to look for alternative solutions.



    Culverts work well because the water has to go somewhere, and a contained drain pipe works well.  In the small stock ponds I've built, we usually place them to one side or the other of the dam, although I've seen some in the middle that work quite well.
    It sounds like you are no strangers to hard work, so I don't see why you couldn't put one in by hand. I've seen some made out 55 gallon drums with the ends cut out. Maybe a possibility where you are at?  Maybe some other recycled material.  I'll talk to some folks and see what I can find out.

    I think a culvert of some kind is really the way to go because they funnel the excess water through the dam and keep the water from over running and eventually ripping apart the dam itself.
     
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