My Very First Swale

There's a county road above our stock pond and where the ground slopes away from the road, there's an expanse of bare sandstone. Surface water runoff flows in sheets across this and finds its way into our pond through various channels, taking sediment with it.

Today I decided to dig my first swale across one of these runoff channels. The idea is to slow the runoff into the pond, capture sediment to build soil depth, and increase water storage in those soils so that I can plant something on the berm that might survive our hot dry summers.

It's a feeble little swale, two shovelfuls wide and perhaps a dozen long, still in dire need of widening and shaping:





However I was delighted by the dirt I was digging; it's much the softest, richest, hummus-heavy soil I've seen on this property. It's not deep; I could feel the tip of my shovel hitting bedrock. But it's thick enough, especially where I've doubled it on my swale berm.

Given how shallow the bedrock is, I don't expect much water storage due to this swale. But it was educational to dig. I'm looking forward to planting things. Since this is a concentrated runoff area, it won't take much summer rain to keep stuff growing.

I know I'm a total noob for just having figured this out, but: it's really amazing how much you learn about your land the first time you shove your spade into it.

Dan, keep an eye on it during the next good rain that you get. Part of the learning is to observe things before and after.

Be sure that the water has a place to go.
If it gets deep and goes over the top it might wash the dirt down stream.
It may also go down to bedrock and move the swale from below?
Throw some seeds on that dirt so you can get some roots growing into it to help hold it in place.

Absolutely! I plant to go out there tomorrow with some winter seed mix.

I think it's engineered so that surplus water will flow around one end. But one thing I learned in my gold mining days is that rising water will surprise you every time.

Excellent! You are ahead of me. Starting small is a wise thing.

Sorry to have to ask this, but is it really a swale? A swale should have a flat LEVEL bottom, and not allow water to run over it or around it. It should collect water and hold it until it is absorbed by the earth below, and also wicked into the soft mound. Maybe it is just the angle of the picture, but it looks like you have erosion. Would you be better served by a gabion to slow down the water?

It's a swale in intent, but I freely admit that my noob execution may have fallen short of the mark.

However, I think the poor quality of my camera phone pictures is not helping.

What I have dug so far is a ditch, perpendicular to the direction of surface water movement. The bottom is flat but as I understand it a swale should be more of a shallow "u" shape rather than a sharp-sided notch. Frankly I think the first hard rain will crumble the edges down and mellow out the cross section, so I haven't worried about that cleanup detail. This is a Zone 4 or Zone 5 location for me, I won't be visiting it all that often and nobody else will ever see it, so I am not worried about cosmetics.

As for erosion, this is an experiment but I don't *think* my swale will permit water flow past it and downslope unless we get a 25-year storm or so (obviously I am guessing). It's in a spot where a broad but small drainage area concentrates and has just started to turn into a rapidly-deepening gully. A few feet downslope, the erosion becomes considerable as the gully deepens sharply. But where I dug, the "gully" is broader and much shallower. I think/hope/desire that the capacity of my ditch-aspiring-to-be-a-swale is sufficient to catch and allow the infiltration of normal amounts of stormwater, with no overland flow through/past/over/around my swale. However, I've left some room at one end (where the vegetation is healthy and the undisturbed soil looks relatively resistant to erosion) so that if I've guessed wrong, some small amount of water can overflow my ditch and escape around the end of the little berm I created when I dug the ditch. Obviously if there's too much of that, it will start eroding my berm; a small gabion or a few flat armor rocks would probably be excellent insurance in that spot.

In a sufficiently massive rain event, I can imagine my swale-and-berm being overwhelmed and washed away. I view this as a gamble. If I can build enough vegetation on the berm to root it in place, and get enough smaller storm events prior to "the big one" to fill my swale and the area behind my berm with sediment, before the big flood, I've succeeded, and doubled the thickness of soil over an area the size of a couple of kitchen tables. But if "the big one" comes first and washes away my efforts, I've swapped the twenty shovels of good dirt that went into my berm for twenty shovels of fresh sediment that filled in my ditch, and moved those twenty shovels of good dirt down into my pond.

Essentially I am conducting a very novice experiment in sediment capture or terracing. The groundwater infiltration effect of my swale is likely to be minimal; the bedrock here is shallow and everything drains into my pond in any event.

Dan, It certainly wasn't my intent to criticize and most likely the picture is not giving me a true analysis of the situation, but I also live in Oklahoma and I know the kind of rain events we have. I think you do have a real problem there and I admire your recognition of the situation and your attempt to rectify it. I don't think it would be hard to fix so why let all of that silt build up in you pond if you don't need to. I think the problem has an easy solution and will bring with it another fix. Since you have a rock surface under that good soil, the water has no where to go except down the hill, so it will go through the soil, hit the rock, and flow toward your pond taking soil with it. You can see from you digging that the sediment building up in the area is creating good soil, so let's think about keeping it where it is useful to you and not in the bottom of your pond which is a real problem.

This is what I would do as an experiment and it should only cost a few dollars and a couple of hours of your time. I would search for a place where I could drive a short t-post on each side of the gully that is washing out. Since there is rock there, that might be the most difficult part. I would drive the t-post and attach a piece of welded wire fencing across the erosion path, getting it as low to the ground as possible, but your post need to be opposite each other and allow you to erect a fence which will be perpendicular to the flow of the water. If you have rock that you can put behind the fencing (on the uphill side), that would be ideal, but you could also use logs if that is all you have. They won't last forever, but they will last a few years and give you a chance to see if that fixes your problem, and time to collect some rock for a long term solution. You will not be creating a dam, just a gabion to slow down the water flow and catch that soil. The soil becomes another bonus, as you noted when you dug there. If you can keep it there then you will have a great place to grow something desirable as it builds up over time.

Now let's talk about swales. Creating a swale on contour, simply means that you have found the level of the land and this is where you are going to put your swale. So think like this, if I plugged the end of a garden hose, filled it full of water, and plugged the other end and laid it around the curve of my hill so that when I removed the end caps, the water stayed in the hose, or just slowly trickled out, then I have found the nearest thing to level on my land. If it gushes out one end, then I have a ditch, not a swale. Of course, this is not the way you determine it, just a way to think about it. You want to hold water in this swale along the entire length of the swale. A swale isn't really shaped like a "U". The bottom of the swale is perfectly flat and in Oklahoma will likely be clay, because you have removed the top soil, and it is part of the mound on the downhill side of the swale. The backside of the swale is not a "U" either, because it needs to be angled gently to let the surface water flow downward into the swale. If not a gradual drop, you will just erode the side of the swale and create a ditch in the process. Forget the mound for a minute and just think about the excavated part. You have a flat bottom ditch, probably with a clay bottom, with a gentle slope on the high side that is going to allow the surface water to gentle flow downward just as it always did. The difference is that you are catching it and holding the full length of the swale. Pretend again that the mound isn't there. The idea is to let the water that is caught in the swale, slowly infiltrate into the downside of the hill through the wall of the swale. So the water is now soaking in, below surface level, not just flowing over the surface. It will move more slowly going through the soil than it did running over the surface, so you have held and slowed the water flow. Since gravity is in charge here, it is always going to go down. Now, back to the mound..you have created a mound withe the dirt you removed from the hole you dug, and piled it on the low side of the swale, you have added back the top soil, and left it in a soft mound on the downhill side. Since it has water flowing UNDER it, not through it except for rainfall, it will wick up water and become a perfect place to plant valuable trees that will be well watered.

You now have a little gabion that is going to catch the soil, but allow the water to flow through at a gentler pace. I hope I have explained this so that it is clear, up if not, just ask and I will try again. I think you can protect the pond, and save the soil that is coming onto you property anyway. A bonus.

Carol, I appreciate you taking the time to help. I think if you were standing on the property with me, we would not have the misunderstanding that we seem to have. Maybe I need to go and take better photos. Don't worry, I knew your first post was fully in the spirit of helpfulness and the same spirit is manifest in your most recent.

let's think about keeping it where it is useful to you and not in the bottom of your pond which is a real problem.

This is precisely my motive. I think some of the confusion comes in because I used the term "swale" when my motive was as much about sediment control and capture than it was about groundwater infiltration. I realize that swales are typically constructed for groundwater infiltration, but I've created a swale-shaped feature that also (if it works!) has the additional benefit of interrupting the sediment transport that happens during overland sheet flow of surface water. (I think all swales do this.)

This is what I would do as an experiment and it should only cost a few dollars and a couple of hours of your time. I would search for a place where I could drive a short t-post on each side of the gully that is washing out. Since there is rock there, that might be the most difficult part. I would drive the t-post and attach a piece of welded wire fencing across the erosion path, getting it as low to the ground as possible, but your post need to be opposite each other and allow you to erect a fence which will be perpendicular to the flow of the water. If you have rock that you can put behind the fencing (on the uphill side), that would be ideal, but you could also use logs if that is all you have. They won't last forever, but they will last a few years and give you a chance to see if that fixes your problem, and time to collect some rock for a long term solution. You will not be creating a dam, just a gabion to slow down the water flow and catch that soil. The soil becomes another bonus, as you noted when you dug there. If you can keep it there then you will have a great place to grow something desirable as it builds up over time.

I have very little experience but from my reading, what you describe would be the perfect (and necessary) approach if I were working further down the gully where it is narrower and deeper and the water has become concentrated into a high-velocity flow. However it would be a significantly more substantial work of engineering. Everything I've seen about gully management has urged that one start as high up the drainage as possible. If you can catch the water during its sheet flow phase, before it becomes a stream and starts increasing in velocity, you can infiltrate it and prevent the stream from ever forming or reaching a velocity that will move larger sediment particles. Plus, you have a chance of catching the smaller soil particles in your infiltration/capture works. At this point, when the overland flow is still diffuse and low velocity, everything I've seen says that swales are an appropriate means of attempting this. Am I wrong about the theory of this, or just implementing it in a way that gives concern?

You want to hold water in this swale along the entire length of the swale. A swale isn't really shaped like a "U". The bottom of the swale is perfectly flat and in Oklahoma will likely be clay, because you have removed the top soil, and it is part of the mound on the downhill side of the swale. The backside of the swale is not a "U" either, because it needs to be angled gently to let the surface water flow downward into the swale. If not a gradual drop, you will just erode the side of the swale and create a ditch in the process. Forget the mound for a minute and just think about the excavated part.

Indeed, yes. The bottom of my swale-like excavation (let's call it a "SWE") is in a layer of clay closely underlain by stones that I suspect are chunked-up broken bedrock. My SWE is indeed flat on the bottom and arranged on contour, perpendicular to the slope. I did leave the upstream edge vertical and I do understand it's going to erode into the ditch at its angle of repose if I don't get back with my shovel to soften the slope. If I allowed the erosion, I'd end up with a shallower SWE due to the sediment that slumps in from the upstream edge.

You have a flat bottom ditch, probably with a clay bottom, with a gentle slope on the high side that is going to allow the surface water to gentle flow downward just as it always did. The difference is that you are catching it and holding the full length of the swale. Pretend again that the mound isn't there. The idea is to let the water that is caught in the swale, slowly infiltrate into the downside of the hill through the wall of the swale. So the water is now soaking in, below surface level, not just flowing over the surface. It will move more slowly going through the soil than it did running over the surface, so you have held and slowed the water flow. Since gravity is in charge here, it is always going to go down.

This is indeed my ambition for this SWE. I am acutely aware that my peril is that the influx of water may be too great for the capacity of the downhill soil to carry away. That's where I'm experimenting and making a judgement (a guess?) about water volumes based on my roughly 10 years wintering on this land, but not actually observing this bit of it.

Failure mode once the SWE is getting too much influx water to drain away though the downhill wall of the SWE is that water rises until it overtops and erodes my berm, which will be functioning as an extremely weak dam by that point. (The longer I've managed to grow things on the berm by that point, the more its weakness will be reduced -- but it will never be a stout dam and is not supposed to function as one.)

However, I don't really want my berm to wash away as it would in a monstrous storm event that totally overwhelms my system. That would put the soil in my pond -- the opposite of my goal. That's why I built in a little bit of a safety valve by reducing the berm height at one end down to ground level. (In essence, about four shovelfuls of dirt got distributed elsewhere along the berm instead of being put downhill of where they were dug out.) The idea is that when the SWE overfills with water, it starts to run away over the land surface, which retains its original vegetative mat. So there will be some amount of "overflow" possible before the volume and velocity get high enough to (a) scour through the vegetative mat and start eroding the soil or (b) erode the end of my berm as the water washes around it (this is where I was thinking an anchor rock might be useful).

Of course, as the thunder keeps thundering and the rain keeps sheeting down, a sufficiently high volume of surface flow would eventually overwhelm the system and wash it all away into my pond. The question then becomes, how likely is that to happen? And the answer depends on whether I've correctly judged the frequency of extreme rainfall events and their likely effect in the limited drainage area and the volume of my SWE and the porosity of the soil. I think I'm made a reasonable estimation of all those things, but I'm acutely aware of the possibility that I might be wrong. However, I also think that the soil erosion in such an event would be so massive, that the extra contributed by my failed works would be minimal. And it's my hope that by capturing sediment, leveling the land, and creating semi-irrigated berms of rich soil to plant things in, I will be reducing the damage caused by that massive stormwater event when it eventually does come. You've described what I hope will happen quite well:

Now, back to the mound..you have created a mound withe the dirt you removed from the hole you dug, and piled it on the low side of the swale, you have added back the top soil, and left it in a soft mound on the downhill side. Since it has water flowing UNDER it, not through it except for rainfall, it will wick up water and become a perfect place to plant valuable trees that will be well watered.

I truly do hope so! Of course I've done this on a very small scale in one place that may or may not have been appropriately chosen. All I can say in my defense is, I tried to pick a spot where the rewards of success would be worthy and the downside of failure small due to the limited scale of my attempt.

Thanks again for taking the time and effort to try and help! I hope I'm not coming across as impossibly stubborn and defensive as I try to explain my reasoning behind the attempt that I made.

Dan - I say go for it. As you have described it could go either way, but the likelyhood is that you are improving the overall water flow in that area.

Just remember to observe it over time and see how it is working out.

Do you have any loose rocks on your soil surface? Low rock walls laid on contour can help slow water flow and trap sediment.

Have you seen this video

terraces and gabions in desert

They use - low stone walls to slow water in sheet flow right at the top of the catchment and then use gabions lower down in the gully. You can see the sediment build up behind the walls.

Michael, that's an awesome video! I'd sure hate to have to work with an area that arid.

I do have some bare sandstone upslope in this drainage, and there's no shortage of stones. I have been thinking of building some little cages to hold enough soil for some prickly pear or something in an effort to revegetate the bare stone, but now I'm thinking little gabion of pebbles may be the way to start building sediment in those locations. In another similar location I've placed some tight bundles of brush (fascines or faggots) but we haven't had enough surface runoff since I did it to know if they will catch sediment as I am hoping.

I am rather looking forward to the next hard thunderstorm in daylight, so I can run out and see how my water/sediment management experiments are working.

It has been raining -- sometimes hard, sometimes gently, but quite continuously -- for the past 12 hours. The water is gently sheeting across the ground in my yard. Everything is saturated. This is the first time we've had serious precip since I dug the swale-like excavation (SWE) so I schlepped my muddy way out to look at it.

I have to say, Carol's concerns were very well-founded. My SWE is functioning precisely as I hoped it would, except that it is completely full and overflowing after just twelve hours of fairly moderate precipitation. I've seen the country around here get this soaked in the first half-hour of a three-hour thunderstorm. Suffice to say, there's a LOT more water in my swale (and impounded by my berm, overflowing the swale and backing up up the drainage) than I expected after this amount of rain.

On the bright side, the surplus water is trickling gently away around the end of the berm, just as I hoped, and without drama or visible erosion. But double the flow volume and it would be a different story I very much fear.

Major phone upgrade since the first picture, so this one should be much easier to see:

24 hours later, about 12-18 hours after the rain stopped falling. My swale-like excavation now looks just as I imagined it would, full of water that's gently seeping away. There seems to have been no substantial erosion caused by yesterday's water surplus:

Nice to see some sucess there. Do you plan on extending your swales now? I'd consider going further back up the catchment and making a longer swale to catch it before it concentrates so much in that gully.

I do, yes. But it's going to be very tricky, because we are looking at vast expanses of mostly-bare sandstone not twenty feet up-gradient.

I think there's room for another short swale about 12 feet up-grade, though it will be shallow. I have also started to seed the heck out of the little pockets of earth that are scattered in depressions in the limestone. I think a network of lined-up stones on contour -- as shown in that video linked above -- is also in order.

Interesting - the bare sandstone does indeed leave you with a problem. I wonder if it is even possible to infiltrate water into the sandstone (which is after all the aim of a swale!). Here our rock is chalk which definitely does absorb water.

Rock walls to slow flow and trap sediment sound like a reasonable plan and may over time let you build some mulch/soil terraces on the stone. I guess you need to think what your aim is for this area - with bare rock so close to the surface it seems unlikely you will ever be growing crops here so perhaps your aim is just soil stabilisation with some water retention as an added benefit?

Are there areas downstream with more topsoil and hence more possibility for allowing infiltration?

Are there any natural dips in the sandstone and does water pool in them or drain away?

Edit - just reread your earlier post. The main aim here is to block sediment from entering the stock pond, not to infiltrate water necessarily. Rockwalls, trapping mulch and sediment, then seeded with grasses or similar sound like your best bet. You could even look into importing some rocks/gravel/soil from elsewhere on your property to help build with, but this is probably out of the question without some kind of digger.

There is a secondary goal, which is to create places (like my swale berm) where water from rare summer rains can be concentrated and brought into proximity and delayed in proximity with sufficient soil to sustain a few plants without artificial irrigation. The more sediment and vegetative matter I can create or capture and retain up-gradient, the longer it will take rainwater to reach and pass through my swale system.

Unfortunately at this time I have zero budget or equipment for large scale digging/transport. To be honest, right now I don't even have a wheelbarrow --- a lack I'm feeling most keenly. Good ones are stupid expensive! I'm hoping to find a garage-sale bargain this spring. Moving dirt in 5-gallon buckets is not cutting it.

Even the "good" ones aren't that good anymore. Good luck finding a wheelbarrow that works for you. I did a lot of work with two 5 gallon buckets and a strong shoulder strap from an old computer bag as a makeshift yoke, but a wheelbarrow is much easier in gentle terrain.

I have come to appreciate digging hoes in my clay. Best money I spent. http://roguehoe.com/firefighters/firefighters.html
Put it on the list for someday when the moneytree blooms.

Nice digging tools!

I grew up without plumbing so I have a ton of experience slinging a five-gallon can of water in each hand. That helps with the buckets, as does being tall enough for them to clear the ground naturally even when my arms are at full extension. But even so, it goes slowly. I hadn't thought of a yoke, makeshift or otherwise; that's a good idea.

Another thing they don't make like they used to is five gallon plastic buckets; my $2.97 Walmart ones aren't holding up near as well as the ones we used to use to haul gold mining gear that were recycled Chevron lube buckets.

Dan Boone wrote:Another thing they don't make like they used to is five gallon plastic buckets; my $2.97 Walmart ones aren't holding up near as well as the ones we used to use to haul gold mining gear that were recycled Chevron lube buckets.

No kidding! The Walmart buckets are actually the better brand you can find locally. That company (encore plastics) makes a better grade, but you have to internet order them and they are EXPENSIVE.

DO NOT buy the white home depot buckets! they are junk.

I saved my elbows once by making a simply yoke for carrying 5 gallon buckets - it helps alot!

Dan, can you post a topo map of your property with an arrow pointing to where this area is? I could ask you questions for hours to make suggestions here but a simple topo map from your county's website would help tremendously.

Amy I am working on that. It's a backward county so there's no county map on the internet. USDA has topo map GIS data, but it's beyond my ability to convert that into a map. I need to go to my local USDA office, where I'm told they can print one for me; but it hasn't happened yet.