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earthworks

Pond or swales to help with erosion?

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posted 9 years ago

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I have a steep hillside that drops about 800 feet, levels out in a plateau, I guess I would call it, then drops another 800 feet. Water gathers from the top half of the hillside when it rains hard, which it does a few times every winter, collects in a specific area in the plateau, then erodes the edge of the plateau before it runs down the last 800 feet. It will shoot out of gopher holes in a heavy rain once the soil is saturated. That hasn't caused any problem, but there's quite a rush of water.

I am thinking of digging out a pond on the plateau to catch the runoff, slow it down on its way, and shore up the eroding edge with drain pipes and cement chunks, like a State highway dept fix. Most of the year it would be empty as it doesn't rain here in the summer. But I don't do anything with that area, so it's easy to experiment with.

There are chunks of bedrock the gophers are kicking up, so that must start at about 18 inches. It can't be a deep pond, but the water might be usable.

Or would it be better to swale across the plateau in a couple of parallel swales to just slow down the runoff as it continues on down the hill?

My priority is to stop erosion, and maybe get something else out of it. Any ideas?

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Michael Newby

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How well vegetated are the hillsides? I would look at ways of slowing and infiltrating the water as high up as feasible then work your way down the property. If the hills are too steep for swales or terraces then you can still plant shrubs and trees on contour which will create a type of swale on contour as the plants collect debris that's making it's way down the hill.

Have you read Brad Lancaster's Rainwater Harvesting Vol. 1 & 2? In them he describes multiple ways to increase water infiltration and decrease erosion.

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Joseph Lofthouse

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Cristo: If you are "in the woods" I wonder how much erosion is really going on... At my place in the desert, any blockage that I make across any gully is filled to overflowing with gravel during any significant rain event. My ravines can get 6 feet deeper or shallower during a single storm. Significant rains might only happen every few years... The depth of the bedrock is the limiting factor at my place about how deep a ravine can become...

I have had the most success with the smallest earthworks... Sticks placed horizontally on contour. Single layer deep rock check dams. Branches thrown into gullies. Bunds 12" tall that direct water to bedrock spillways, or to flat ground where it's energy can dissipate. It's much easier to dig the sediment out of a 12" bund after a storm than to dig it out of a pond...

The big earthworks are hard to engineer successfully. Sometimes they hold, sometimes they fail. Gabions and mesh-fencing strung across ravines have been my most successful big earthworks.

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Michael Cox

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I'd be quite concerned about a catastrophic failure of a pond in the layout your describe. If the water on that plateau saturates the soil you may get a landslip onto the steep slope below. I'd look to get more water infiltrated into the soil higher up on the slope first to control the peak water flow onto the plateau.

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allen lumley

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Cristo Balete ; Lots of good information given here, I especially like the reference to Brad Lancasters Rainwater Harvesting 1 &2 though I have not
looked at it in couple of years!

Do a Google search for : 'Rip Rap with pictures' ! This is a surface treatment for where you don't have adequate vegetation/shrubbery !

This can also work piled in eroded gullies as check dams to slow down the force of the rushing waters, gullies with little pitch can often be well protected by
Fascine Bundles or densely packed brush piles, this should work to slow the flow and trap sediment .

And finally think small, observe, make a plan, add inputs and then observe again! For the Good of the Crafts ! Big AL

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Cristo Balete

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Location: In the woods, West Coast USA

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Thanks, guys. Yeah, I have been observing this hillside for 20 years and am guiltily having to do something about it now. The hillsides are very vegetated with shrubs, poison oak and pine trees because there is ground water coming down it, underground, all year, which is why when there is a heavy rain it doesn't take a lot to have rain running on top of the soil.

I've thought about the saturation, but there is a 35 year old, acre pond about 400 feet away from this site, on the plateau, and if something were going to saturate and slide it would have happened. It was dug deeper, no dam built to hold the water. The gophers are kicking up bedrock at this site, which is also a good sign that it won't move.

I do have runoff going sideways below the lip of the plateau, so I've seen that that works. So stopping that erosion at the lip of the plateau with the rip rap will need to happen. So I'll check out rip rap designs, that does seem to be the way the highway department handles fast running water.

The site I'm needing to work on is lower than the pond, otherwise I would drain it all off into the pond because it has a great overflow with no problems. This is more water in random heavy storms than small bundles or hay bales can handle. It will just dig out underneath them or go around, tried that already.

I will check out Lancaster, thanks.

Joseph, yeah, I've seen those flash floods you guys get. They are hard to believe unless you live there. I couldn't believe those cement swales in the roads and the signs warning drivers. You look around, no rain in site, and crash! There it comes!! We don't get anything like that. Well, at least not yet.

Dan Grubbs

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Let's go back to understanding the cross section of the landscape. You mentioned that it drops 800 feet. Do you mean it drops 800 feet in elevation? Or is the distance between each plateau 800 feet? What I want to understand is the actual degree of slope. Swales fail once you get past a certain degree of slope. What I'd want to understand is: what is the drop in elevation over what distance.

My first step would be to ensure dense and diverse ground cover vegetation. Once you increase the soil's infiltration capacity (by building healthy soil) it will hold a lot of water. So, phase one is ensuring your have really good bio mass (live and decomposing) on the slope. I have a long slope (about 60 feet in drop over 800 feet). Once I stopped haying it, and encouraged a polyculture of cover, the soil's infiltration improved even more than what my swales were doing on the same slope.

I agree with Michael that the general principle of holding as much water at the highest points you can is a good rule of thumb. But, without understanding the actual degree of your slope, any recommendations would be guesses.

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Cristo Balete

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Here are some riprap guidelines that might help somebody from this site: http://water.epa.gov/polwaste/npdes/swbmp/Riprap.cfm. The picture on that website is not even close to what I'm talking about *L*

Gradation. Use a well-graded mixture of rock sizes instead of one uniform size.

Quality of stone. Use riprap material that is durable so that freeze and thaw cycles do not decompose it in a short time; most igneous stones, such as granite, have suitable durability.

Riprap depth. Make the riprap layer at least two times as thick as the maximum stone diameter.

Filter material. Apply a filter material--usually a synthetic cloth or a layer of gravel--before applying the riprap. This prevents the underlying soil from moving through the riprap.

Riprap Limits. Place riprap so it extends to the maximum flow depth, or to a point where vegetation will be satisfactory to control erosion.

Curves. Ensure that riprap extends to five times the bottom width upstream and downstream of the beginning and ending of the curve and the entire curved section.

Riprap Size. The size of the riprap material depends on the shear stress of the flows the riprap will be subject to, but it ranges from an average size of 2 inches to 24 inches in diameter (Idaho Department of Environmental Quality, no date).

Wire Riprap Enclosures. Consider using chain link fencing or wire mesh to secure riprap installations, especially on steep slopes or in high flow areas.

Cristo Balete

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Here's a variation on a fix for the edge I hadn't thought of. This involves fabric, but it maybe it doesn't have to.[br] [br]

UpperTrenchLip.jpg

Cristo Balete

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This way of burying a pipe to dump it over riprap I think concentrates it too much for another 800 foot run downhill. That's why I want to spread it sideways with swales, or stall it in a catchment pond, let it find its way downhill slowly.

BuriedPipeOverRipRap.gif

Cristo Balete

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Dan, this is quick and dirty with some rough dimensions.

ErodingHillside.JPG

Dan Grubbs

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Wow, Cristo ... a 500-foot fall over a 3,000-foot run ... (1 foot of drop every 6 feet of run) that's very steep! If you have a swale on the slope, your back cut would have to be really big and go back up hill quite a ways. The berm would experience a lot of water pressure since a swale berm is NOT compacted, I think it would fail in a large rain event.

So, my recommendation is the same as Michael Newby suggested. I'd plant a diverse row of trees/shrubs on contour lines (one on the uphill edge of the plateau and one on the downhill edge of the plateau) so there wouldn't be a chance of earthworks failure due to water saturation and kinetic energy of water movement perpendicular to contour. This would also give you a protected alley of flat space for a variety of things to do in that sheltered space. These rows of trees would accumulate materials that would build up and begin to serve as a "swale-like" structure to slow water down with much less chance of failure. You might also enhance you slope's ability to keep water up higher if you "ripped" contour lines with a Yeoman's plow or subsoiler plow. This would allow water to drop down into the soil as it made its way down slope.

An outside-the-box idea might be a gabion. Gabions dramatically slow water down and collect material behind them. A gabion wall on contour as shown in the photo might be a solution. Yes, it would require a lot of material, I totally get that, but it is a solution if you live in an area where rock is a primary resource.

Wow, steep slope!

gabion-wall.jpg

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Cristo Balete

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I was totally wrong about how many feet are in a mile, so ignore the mile thing!

Anyway, I may not have made it clear. The steep parts of the hillside are stable. It is already covered with shrubs and trees. It only rains hard a couple times of year so it's not an ongoing thing.

I just want to hold the water on that plateau so that it slowly seeps down through the lower steep section, rather than running straight down. For my purposes I think I will plant a small vineyard on the plateau in swales with berms, so any runoff that comes onto that plateau will go sideways and find its way down slowly, rather than concentrating at that one spot at the lip of the plateau.

I read about ground-cover salvias in a Mediterranean climate for erosion control using low-growing sages like creeping sage or Humbingbird sage that can cover a slope very fast.

I don't think I'll do much contouring until I know where the ground hornets are!

Thanks, Dan, for your suggestions.

Matt Baker

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Are animals an option? Could you use animals to employ a holistic management style grazing regime? Improve water infiltration; increase soil litter, and increase soil organic matter. If you have overland flow the soil is well below its water infiltration and retention potential.

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Dave Dahlsrud

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It sounds like you have a pretty good plan Cristo. I like the swale idea on the plateau. I imagine that there is some slope to that as you mentioned the pond was uphill from your property. You might want to consider something like a tiered swale-ish thing where it meanders from one side of your property to the next. If your soil type could handle it you could even grade the land to have a slight pitch into the hill side (similar to what I've read Holzer does in Austria) and then a bit of a lip on the front of the plateau as well. It doesn't sound like you have any stability issues, so something like that could work as well. It might be easier to get harvesting/maintenance machinery up there this way as opposed to the swales. Just something to consider.

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Cristo Balete

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Matt, I'm not sure I understand this sentence:

If you have overland flow the soil is well below its water infiltration and retention potential.

Dave, it's going to be a small group of grapevines, maybe 25-30, just enough for a hobby, so I am the harvesting "machinery"! Access to that plateau comes from the side, the same altitude as the plateau, as there is already tons of maintenance on the pond, and a well-established road going across there. The road will fill up with water as it is kind of cut back into the hillside, but during the few really heavy downpours then it just jumps the road and takes off downhill.

I once read some chapters from a book about road building in North Africa during World War I, they had awful conditions to try to keep the tanks and trucks moving through tough terrain with lots of rain and mud. Some very good information there, which ought to be online somewhere.

Below that plateau are terraces/trenches that send any run-off water sideways, and those have done well. It's heavy clay, so hand-dug trenches stay put. There are some still from 1998 that get resurrected every once in a while.

Matt Baker

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posted 9 years ago

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Sorry, "overland flow" (hydrology term) happens when the precipitation rate exceeds the infiltration rate, i.e. water is not going into the ground fast enough so it runs over the soil surface taking all the good stuff with it.

If the problem is water erosion the solution is to hold onto that water as long as possible. The best place for water is in the ground - and the best time to stop the water is when it hits the ground. Swales/ponds slow erosion after its happened; It sounds like you already have a swale system in place or planned. I'm wondering how you can reduce the initial erosion from happening by increasing water infiltration.

The two strategies to best accomplish this are to break the soil surface and cover the soil with mulch. Breaking up the soil surface can be done with animals, or machines with a harrow, disc or key-line plow. Mulching can be done with animals if there's already standing vegetation to mash down to the ground, by importing organic stuff like round bales of hay or wood chips, or by planting ground covers.

You mentioned berms too which could be hugelkultur if you have extra woody stuff to get rid of. An added bonus is these is they slow down the wind and stop it from drying out your plants.

Just food for thought. Sorry if it seems pedantic - not sure of your existing knowledge set/experience.

Sounds like a fun project!

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Michael Cox

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Cristo - what is your climate? Do you get frosts?

The ideal biological solution for this is vetiver grass, planted in hedges on contour. It doesn't tolerate frosts unfortunately.

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Cristo Balete

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Matt, yeah, I knew what you meant by overland flow, but the other part I wasn't sure. I've got clay, so it absorbs well....until it doesn't. That doesn't happen very often, but this is just for the occasional extreme storm. the swales around the grape vines ought to catch the runoff, help them out, slow everything down. At least that's the plan!

Michael, I was reading about vetiver grass. It's interesting. Might be an experiment to try. We don't get summer rain, so I usually stick with drought tolerant things, which is why the erosion control sages seem good, and would help the bees and hummingbirds.

C. Letellier

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Number of acres drained and peak rainfall rate? Wondering if you can add an overflow drainage pipe down the hill to handle peak flow. If so then likely a pond could be managed on the terrace. Your local geology will determine that one. But you have provide a path for most of the water that won't erode if you hit overflow condition.

Then wondering how permeable the soil is? Would seeding the area with plants that improve long term permeability help? Forage radishes because of their long root leave organic pipelines into the ground for example. Minimize soil disturbance and plant things that long term improve soil permeability? And anything you can grow on top that increase the mulch layer over the soil will help too. A line of trees and shrubery properly planted might let you grow a berm over time.

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Cristo Balete

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C., it's not so much about acreage as it is a concave area on a hillside that the water collects and goes straight down. I've decided not to do a pond, just too much maintenance. It's too big an area to plant. The native stuff is thick and relentless, and I'm not going to maintain it for 800 feet down a hill. The rainfall that causes fast-running water happens maybe two times a winter, so it's not an ongoing thing, just a sporadic, but possibly destructive thing.

But I did line the 75-foot eroded gully just below the plateau with rip-rap and it is controlling the flow of the runoff nicely. The top 1/3 I angled the rock so it would send the water to the center. Then about every 6 feet I built a large step with large pieces of concrete, then filled in between the steps with smaller rip-rap to slow things down. There are a couple of gopher holes emptying onto the rock gully in addition to what is coming down the hillside, and it seems to be handling it.

At the bottom it goes to the side at a 45 degree angle across the hillside, taking it away from the concave problem area. The gopher holes are all spouting water, not much to be done about that, but about every 100 feet downhill I've trenched more 45-degree angle trenches to send the water off sideways. The rain hasn't been too bad, just enough to test it. Mother Nature can undo anything I've managed to do, but at least it feels like I've got my foot in the door.

Matt Richards

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Necrobumping this thread as I've got a somewhat similar situation to Cristo's here on a property we just purchased, also in California (North/central coast). Cristo I am wondering how your setup faired during this epic rain season we are having this year? Hope all faired well...

Michael Newby wrote:How well vegetated are the hillsides? I would look at ways of slowing and infiltrating the water as high up as feasible then work your way down the property. If the hills are too steep for swales or terraces then you can still plant shrubs and trees on contour which will create a type of swale on contour as the plants collect debris that's making it's way down the hill. .

Michael - Are you suggesting a small contour line (keyline?) which is then planted? Or are you saying simply plant without digging any lines?

Our home is at the top of a bluff with about a 30-40% grade grassed hillside for about 70 feet before a forested treeline. As it is a severe slope I am hesitant to even attempt terraces for fear of waterlogging the hillside and losing it (and subsequently part of the house) during a big rain event like we had many times this year. I was thinking to simple plant out polycultures on the hillside without any water diversion to ensure no increase in chance of a slide. I've ordered the water harvesting books - hoping that might provide some insight, but welcome thoughts from folks here. Thanks.

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