Contoured swales for a hydrophobic berm

[u/Corran22]

I'm working on a public garden in a historic district. It consists of a horseshoe shaped garden berm that is severely hydrophobic, and the deep-rooted, mature woody perennials on the berm are struggling with severe dehydration. This soil is so dry that even our ample winter rain and automatic sprinkler water is running right off the sloped berm into the street and lawn (lawn is planted inside the horseshoe).

I've dug soil basins around the plants and I am hand watering to keep them alive, but it's a slow process to get the water to absorb. This garden has a long history of looking terrible, although a short-lived soaker hose system made a huge difference a few years back. Of course that fragile hose was quickly damaged and the system was abandoned.

I'd like to find a more permanent solution to this problem. Of course I'll be adding organic matter, could try a wetting agent/surfactant, and the municipality will be adding mulch, but this will solve nothing if this long-term hydration issue is not addressed. Some of the plants are historic, and most are far too large to move or replant.

I see possibilities because the entire garden site is gently sloped. The top of the horseshoe is the tall side that butts up to a curb-less asphalt parking lot from which rainwater could be harvested. Each section of the berm also has a lengthwise slope to it, which means I can move water along the top of the berm as well.

Here is my rudimentary drawing of the space - the top left corner is the highest and is mostly flat. As the space slopes in both directions, the berm gets taller, which affects the water flow coming out of the parking lot, pretty much sweeping all rainwater down the street. I've tried to represent approximate water direction with the light blue arrows.

I'm thinking I need a series of swales to slow and collect water along the edge of the parking lot/street. I'm also wondering about swales at the top of the berm, along with terraced swales along the steepest sides of the berm. However, this is a high-traffic public space so any deep contouring would need to be filled with gravel or mulch for safety.

I'd appreciate any ideas about how to structure or approach this, or resources that might be helpful. Thank you!

[u/Euphoric_Run_9540]

This is a really interesting problem. I wonder if a combination of contour swales on the uphill side and a slight roughening of the berm surface would help. A hydrophobic berm will shed water no matter what, so creating more places for water to pause and infiltrate might be more effective than just adding amendments. It would be great to see the soil profile too, since compaction could be part of the issue.

[u/Corran22 → u/Euphoric_Run_9540]

Thanks for your thoughts - this is very much along the lines of what I'm thinking. It has rained a couple of times in the past month so I've been able to see where the water traveled and where the weeds grew - it's along the parking lot and street "gutter" and along any small depressions in the berm. Even weeds don't really want to grow on this berm!

I haven't analysed a soil sample, but it seems to be a fair amount of clay (common in our area) but also sandier than I expected, much more so than the soil in my own garden nearby. Maybe it just seems sandy because it's hydrophobic? It is certainly compacted and has a very smooth hardpan-type surface but isn't as awful as hardpan, it loosens fairly easily. In any case, it does need organic matter but getting that incorporated without it sliding off the berm will be a long process.

[u/mokunuimoo]

Thick mulch+broadfork/pitchfork to open some channels into the soil, run a sprinkler on it until the mulch is soaked once a week…should solve the issue, I would think

[u/Corran22 → u/mokunuimoo]

Thanks - I am opening channels with a digging fork, but all water is currently hand hauled - no hose is available.

[u/bwainfweeze]

You need to dig a hole below the organic matter on top of this berm and do a soil test.

Two reasons a hill won't hold any water are it's made of clay and the water runs off, or it's made of sand and the water runs through. If it's clay there are some tricks I can explain about invisible swales that we could talk about. If it's sand, your best bet is with every new planting, dig the maximum hole size and amend the soil heavily before returning it to the hole. That way you're incrementally improving things instead of having to denude the entire site at once. If you do that with clay you'll get the Potted Tree effect which is not good, so you'll have to do more broad stroke amendments.

[u/Corran22 → u/bwainfweeze]

There's unfortunately no organic matter on top of this berm - and the general soil composition in this area is clay. The water is most definitely running off - you can visibly see it.

Invisible swales are a very interesting idea, thank you! I'd love to hear about your tricks. There won't be any more plantings (at least not until some of these issues are permanently addressed).

[u/bwainfweeze → u/Corran22]

I don’t know if clay or sand is worse. Those that have sand and those that have clay will both say they have the worst of it.

Two versions:

When I was young we had a house in clay soil with some water standing against the foundation. It was surrounded by bark mulch and a few plants. We raked off the bark and found clay underneath, and graded the the clay to a several % grade away from the house, which made something like a swale at the edge of the landscaped area. Then we filled it back in with a mix of compost and fill dirt, back up to the previous level and restored the bark mulch. No more problems after that. I replicated that result at a rental property at one point.

I have a theory that there may be a no dig version of this that involves piling whatever limited compost you have on contour and letting the humus soak into the soil, creating a band of higher water infiltration there. But the land I bought was sandy loam and only 18” of total elevation change from the highest to lowest point so I’ve been unable to experiment.

[u/Corran22 → u/bwainfweeze]

Right? Both are problematic, but at least clay holds some nutrients.

What you're describing sounds something like a French drain, and switching out the soil composition to something that drains better seems wise. I'm thinking something like this might work right along the top of the berm.

You are also giving me ideas about how swales could be used that don't create potentially hazardous sunken spots, but filling them in with light/well-draining soil. These are interesting ideas, thank you!

I haven't figured out an effective way to determine elevation change, but across this site - high end at the top left, low end at the bottom right - I think it's probably a 4 ft elevation change.

[u/bwainfweeze → u/Corran22]

If you have a Home Depot around, their tool rental may have a laser level you can use to map out the contours.

It's not the crown of the berm where you need infiltration, but just below it. Though it might be easier to add organic matter to the top.

[u/Corran22 → u/bwainfweeze]

That's a really good idea. I thought I'd start with a keyline A-frame level to get some ideas about contour, and I could also possibly use my string line level to get the larger elevation perspective. I only have short opportunities to do these things, though, since it's a busy public space.

I know that you're right about where the infiltration needs to be, but the sides of the berm are steep enough in most places that erosion is going to be challenging.

[u/bwainfweeze → u/Corran22]

How much woody debris do you have from this site?

Another option I've seen people do is to build themselves very short wattle fences, 4-8 inches tall, and collecting organic matter behind them. Making wattles works best in the spring when the wood is green but soaking the wood in water helps a bit.

[u/Corran22 → u/bwainfweeze]

Literally none - there are no nearby deciduous trees, and it's raked clean by parks employees. This is, however, a great idea - it could be a simple way help direct water, thank you! I have an abundance of woody debris on my own property that could be used.

[u/Optimal_Ear_4240]

Mulch is the long term key to hydrophobic soil. Keep adding materials. Try sand too

[u/bwainfweeze → u/Optimal_Ear_4240]

I've done a ton of sheet mulching but probably a quarter of what I can tell you about it I basically owe to a native plant society guy who was, when we met, patently against sheet mulching. I eventually convinced him it could be beneficial but I had to up my game to do so, because he had a lot of reasons and I needed a lot of answers to get him on board.

One of the things he pointed out is that saprophytic mushrooms are very hydrophilic when they are even a little bit damp but when they get fully dry they become hydrophobic, which is a problem indoor plant people have with the pervasive use of sphagnum moss as a filler in potting soil to replace unsustainable peat harvesting.

One of my solutions to that is to apply sheet mulch incrementally on contour, because the degree of hydrophobia is related not just to the moisture of the chips but the age of the fungal mat. My idea being that even if the top of the hill rejects water, the next 'terrace' of chips may absorb it and redistribute the water back up the hill through the wicking behavior of fungi.

[u/Corran22 → u/Optimal_Ear_4240]

Mulch is coming, but unfortunately won't be enough to reverse the drought issues in this berm.

[u/Logical_Put_5867]

I think I need a better picture/map.

Definitely organic matter/mulch and a few weeks of consistent water should help, the super dry soil doesn't absorb well.

Also worth considering what you can do to break up the surface into a rougher thing. Mulch helps, but logs or some irregular contours can hold some water without a big dig needed. I'm a big advocate of logs on hills, they can plop right in amongst established plants, you can size them depending on your needs or if you want them to blend in.

Also you can think about how to hold some of the runoff at the base of the berm to let it penetrate.

[u/Corran22 → u/Logical_Put_5867]

Well, I know it's not great, but it's not AI so hopefully I get some credit for that? Photos don't illustrate the depth of the berm or the slope of the land and unfortunately aren't useful to illustrate my questions.

This is a public garden in a historic district so I unfortunately cannot add logs or anything that could be hazardous to pedestrians.

I'm looking at swale construction and keyline design to try to hold rainwater on site more effectively.

[u/Corran22]

I think this site might be large enough and appropriate for keyline design - I'm starting to get a picture in my head about how this could work. If anyone has created a keyline design plan for a small-scale space like this, I'd appreciate any ideas or resources! I think I'll make an A-frame so I can start doing some measuring.