My site is located in the humid tropical highlands on severely degraded land, the original rainforest having been clear cut by a wood pulp company about 10 years ago. The soil that remains is poorly draining clay. We get over 3000mm annual rainfall, with many intense rain events during the monsoon. Large parts of the land are waterlogged for long periods of the year, except the three month dry period (June-August).
Normal practice around here is to dig lots of drainage ditches straight downhill, with vertical sidewalls. Obviously I want to avoid those kinds of ditches, but I also want to plant coffee and fruit trees that can’t tolerate waterlogged roots. Some kind of earthworks will be necessary. Initially I was thinking of planting on mounds, but what Geoff Lawton says about swales for poor draining soils in this video
makes a lot of sense. I always though of swales as dryland water harvesting tools, but in this case they would function more as long, narrow drainage ponds and sources of building material for raised ridges.
The site is gently sloped (avg. about 5%), stable clay, and no downhill neighbors, so it should be a safe spot for swales. But while there’s tons of info available on building swales for water collection and infiltration, there’s very little on using them for drainage and keeping roots dry. I’m hoping some people on here might have experience with swales for this purpose and can answer some of my questions.
If I install swales, I’m sure they will fill with standing water for most of the year, if not permanently. Will the berms not get waterlogged? Do I need to compact the swale-facing side of the berm to keep it dry? What effect should I expect in the interswale alleys in this pseudo-chinampa setup?
Normally I would plant agroforestry coffee in lines every 3 meters (with medium trees every 6m and emergent trees every 12m), but that's closer than I’ve ever seen swales spaced. On the other hand, advice about not placing swales too close together is usually focused on maximizing efficiency of rainwater collection, which is not an issue for me. Can you have a denser pattern of smaller swales for this kind of application ?
My site is crossed by several drainage ditches dug by the pulp company when they cleared the land directly above me. If I install swales, would you use them as overflow spillways (perhaps with some erosion control modifications like sloping edges, adding some sediment traps, etc.), or fill them in and start from scratch?
There's lots more to say and ask, but it's getting long for a first post, so I'll leave it here for now. Thanks in advance for any advice.
Some pictures to help form an impression of the site:
Gentle 5% slope down to a wetlands and river, no downhill neighbors. You can also see the thin topsoil and soil horizons in the drainage ditch next to the logging road (my only site access).
Looking directly downslope.
Looking upslope, with old growth rainforest directly next to the site. See if you can spot two people on the logging road for scale of the trees. My site goes to the tree line in the back.
The neighboring rainforest has no waterlogging issues, just beautiful humid leaf mulch. My site was like this less than 10 years ago, making me optimistic that it's not crazy to start an agroforestry system in the current waterlogged conditions.
30cm hole naturally fills with standing water. I expect swales would be filled with water semi-permanently.
Typical example of the drainage ditches that cross my site (although this one is located on a neighboring plot).
The new eucalyptus plantation on clear cut forest directly uphill from me, with a drainage ditch that ends up on my land. If I get an excavator to do earthworks, it will be operated by people who only have experience doing this kind of job, so it would be a challenge to get them to dig the kind of swales I have in mind.