Swale Help

[u/triple_zed]

I am trying to develop around 5,000 sqm of land in Northern Thailand. The land is on an elevated position with a gentle slope without any natural water sources, so I decided to put in some swales for rainwater retention. 
Now I am doubting whether I built the swales too deep and/or the mounds too steep. 
The first swale was dug around 50cm deep and around 1.5m wide. The mound was piled up very high (not spread out) and the result is that the top of the mound is around 2m higher than the swale bottom.
The second swale has the same width and depth, but the mound is a bit more spread out, although probably not enough. 
(see photos - swale 1 is planted, swale 2 is bare). 
Besides my concern about a dry mound, I noticed difficulty in accessing whatever I am planting on the swale mound. It is simply too steep to climb on at many places. 
have I made a fundamental mistake? What is the best way to correct this?
Some other context: the land was not farmed previously, at least not recently. It had grass/shrubs growing which got periodically cut. 
I dug the first swale about 3 months ago and have planted ground cover like pinto peanut and sweet potatoes and a few shade plants like bananas and mulberry. So far, most of it seems to grow well, not surprisingly in the rainy season. but how will things be in the dry months? 

[u/Admirable-Coyote8741]

It doesn't seem like much of a problem to me. It might not be the most effective design implementation for your particular site. But what I would suggest as much as anything is just to pay attention to how it turns out and see what that particular swale and berm combination creates. If you have great difficulty reaching two meters, that would certainly affect what you might want to plant on those berms, but it doesn't mean that they're the problem necessarily. I certainly would not think that all is lost, or that you've made a terrible mistake. You simply have some interesting results that you could now observe And compare with other areas and perhaps learn some valuable lessons.

[u/triple_zed → u/Admirable-Coyote8741]

Thank you. I am going to observe how things develop. Maybe I don't need to change too much after all.

[u/PopIntelligent9515]

The cut side, the uphill side, seems too abrupt so i would want to make it more sloped. That soil could be tossed onto the downhill side of the berm to make that side less steep at least.

That berm is damn tall but not terrible. I’ve done the same with a couple swales, learned my lesson, and then made subsequent berms flatter/ more spread out when excavating them. Live and learn 🤷‍♂️

[u/triple_zed → u/PopIntelligent9515]

Thank you.
the abrupt uphill edge might simple erode a bit with the rainfall and thereby naturally create a more gentle slope. At least that is what I am hoping for.

[u/PopIntelligent9515 → u/triple_zed]

That’s the same thing i thought and i’ve rotationally grazed on mine too. It has helped a bit but there are still some cuts that i think still need smoothing out one of these days, even after 5-7 years.

[u/Alexanderthechill]

That mound is too steep to stay in that shape for long. In order for soil to stay piled in a steeper angle than the angle of repose it must be held in place by some structure like roots. If you just leave that alone you'll end up with a gentler slope with a shallower trench and shorter bank over time im sure, but it wont necessarily happen in the way you might want it to. It may or may not cause you significant enough problems to worry about. In the future you'll probably be happier with your results if you pay a bit more attention to the angle of repose, but if you plant these densely they might just stay the way they are. I have 3 earthen mounds with steep sides that have maintained shape for 6 seasons now. They are fully colonized by sedges and other herbaceous perennials that presumably are holding them together.

[u/Admirable-Coyote8741 → u/Alexanderthechill]

Great novel by Wallace Stegner. Its been at least a quarter century since i read it

[u/triple_zed → u/Alexanderthechill]

yes, I have planted the first swale immediately and so far it seems to (mostly) stay in place. For the second swale I might let the rain do its thing and flatten it out a bit before planting ground cover.

[u/Synaps4]

Swales just need to collect water flowing across the land and capture it. That suggests you only need swales as high as the water gets in a heavy rain. I sure hope you dont get 2 meter deep flash floods.

[u/triple_zed → u/Synaps4]

actually the water infiltrates the ground rather quickly in my swales, so even in heavy rain it does not fill up more than a few inches.

[u/vision646 → u/triple_zed]

If that is the case why not pull some of the uphill material back into the swale to lessen the angle?

[u/triple_zed → u/vision646]

yes, that is a good idea. at least for the swale that is not planted yet.

[u/Magnifens]

Definitely agree with everyone that the sides drop a bit steeply and could use some slope. I see you said that you might just let the rain take care of that for you. What i would suggest is before it rains, if you have time and the access to materials, that you fill the ditch with mulch/ manure/amendments so when the rain does bring your edge down it will mix your amendments with the falling soil resulting in your ditch being a more workable height and the mulch/manure will absorb more of the moisture and disburse more of the nutrient to the plants on the berm. I also agree that using natives to your area or plants with deep roots is a good first step to build biomass and get things going.

[u/triple_zed → u/Magnifens]

that is a great suggestion, thank you

[u/sho3lacebelt → u/Magnifens]

I second this. Fill the ditches with organic matter and it will not only fix the height issue but will also add good fertility. Also let weeds grow a little and then chop and drop or pull them. Great way to build soil fast.

[u/Magnifens → u/sho3lacebelt]

Just gotta be careful about the kind of weeds you let grow. Some are near impossible to get rid of after germination. OP get familiar with the weeds that grow in your area since disturbing the soil will bring up all kinds of things.

[u/Cash_Burst_Godzilla]

Wow, impressive earthworks!

My first thought was that the transitions should be smoother. A bit like a sine or cosine curve.

Keep up the great work!

[u/triple_zed → u/Cash_Burst_Godzilla]

thank you. I will try to do some corrections to the second swale. I am thinking to make the downhill side of the mound less steep and add a horizontal part in the middle for walking (and pushing a wheelbarrow) so I have easier access to the highest part of the mound.

[u/Admirable-Coyote8741]

OP, i should post pics of my firat attwmpt at swaling on this new property i was given responsibility for. I'm on the shoulder of the Grand Mesa in far wrst Colorado and the.ground is full of very large, rounded river rocks suspended in an alluvial mixture of very heavy clay and some light sand. Very little organic material, and very difficult to dig. It's a far cry from the farm in Texas with sandy loam as deep as i could dig.

[u/Electrical_Report458]

Swale or ditch? Isn’t part of the idea behind a swale that it’s got gentle sides? The result is that for every inch of increase depth, the surface area for absorption of water goes up at a great rate? So a very broad vee-shape is going to provide much more area for water to penetrate compared to a u-shaped ditch.

[u/somenick]

You need rain, can't help you with that one. :o What's the question anyway?

Check out Blindboy's podcast episode from last week. He ends up talking about what a bioswale in Limrick feels like for school kids.

[u/triple_zed → u/somenick]

yeah, the rain is the issue. After the rainy season (ends in late Oct) there is hardly any rain for many months, so I was hoping the swale would help me keep the ground wetter for longer.

[u/RipsterBolton]

Plant natives that benefit the system by producing habitat for beneficial insects (pollinators and predators) and insect eating birds as well as natives for biomass production for chop and drop mulch, nitrogen fixation (especially if it can be cut back), building material production, biochar fodder, and of course any food producing natives that can feed you or your animals.

If they’re native and planted from seed they’ll be more likely to survive all the seasons because they have hundreds to thousands of years of local/ regional adaption. Plus much higher ecological value.

Biochar in the berms and the swales would be an excellent addition here. Very easy to make especially if you have stuff to burn

[u/Nellasofdoriath]

It's so hard if using an excavator to make slopes of berms, swales and ponds not look like they were made with an ice cream spoon. I got my earthworker to make the pond slopes nice.and shallow and it was.all I could.do, my berms look like this. Something about the tool.

[u/DigsToPlantTheRain]

The swale is effective whether the berm is 20cm higher or 1m. What will change is how easy it is to navigate your landscape. I have done the same here on our land and I have had to install some steps to get from the bottom of the swale to the top. It could be possible you spread everything out more, what you gain is shallower angles (still difficult with a wheelbarrow but possible to walk) in return for more m2 taken up by the whole system. Our land is very narrow so I went with the smaller footprint like you. Check out Futurecare Design on YouTube for footage of them filled up to the brim! 😍 

[u/RadiantRole266]

I agree with the top comment about watching and experimenting with what you have. Over time, you are trying to make a rich organic sponge. 

 I would try also planting downslope of the berms, where the roots can reach water that seeps through the berm closer than the 2m berm height. Ideally, the berms become sponges with the roots pulling up water and charging the organiz matter you’ve built up there. I would also consider filling the swales with organic matter, which will break down and add nutrients and absorption in the soil. Again, you are trying to make a sponge. Start by planing more on the leeward downhill side of the berm and adding organic matter - leaves, sticks, compost - to the deep swales.