from Hacker News

“Sand Dams” Are Transforming African Drylands

by misnamed on 9/7/18, 4:17 AM with 50 comments

• by Meph504 on 9/8/18, 7:09 AM

  These may be greatly helpful for those near where they will be built. But this technique does have a cost, it will kill the downstream deltas By stripping off the alluvial soil, and depositing it at the dams, it never reaches it original destination.

  There is a reason that much of this planets life comes from delta regions, and robbing them of the minerals and fresh water they need to thrive is costing a lot more than providing people the ability to create farms in areas where they shouldn't.

  This isn't terraforming, it's shifting resources to a place that can't sustain themselves naturally, and killing an ecosystem that could without interferences.

  It also seems to not mention that natural waterways including rivers, streams, and tributaries shift and move overtime, largely because once a water way fills up with deposits and debris its no longer the path of least resistance.

• by theothermkn on 9/7/18, 8:01 PM

  One thing I like about this idea is that there doesn't seem to be the impact that, for example, a hydroelectric plant would have. In other words, because the streams are intermittent, and because most of the flow seems to be restored by overflow downstream, you don't interrupt things like migratory fish.

  It's a neat concept. It gets the water beneath a thermal mass, stabilizing its temperature and preventing evaporation. The water can become available to deep-rooted plants and trees, too. I wish I'd thought of it!

• by sj4nz on 9/7/18, 11:51 PM

  Additionally: Digging trenches and planting at the appropriate time have long been known to be useful in these arid places.

  https://permaculturenews.org/2008/11/19/desert-ways/

• by vram22 on 9/7/18, 9:00 PM

  Another related technique is imprinting:

  http://www.imprinting.org/mechanics1.htm

• by tobtoh on 9/8/18, 1:46 AM

  A very interesting concept. IIRC one of the problems of traditional (water) dams is when they silt up since it reduces the water storage capacity.

  However, in this case, the 'silting up' (or sanding up) is the whole intent. So basically they are creating an artificial aquifer. Simple yet effective.

• by kristianp on 9/7/18, 6:14 AM

  Looks great, but it would be nice to see before and after photos, not an animation. Satellite photos would be great.

• by downrightmike on 9/8/18, 1:28 AM

  I've built these on the mountain I'm near after a fire stripped it of trees. Called them check dams, supposed to help raise the water table.

• by TheBill on 9/7/18, 8:52 PM

  IIRC there was a post on something similar being done in the times prior to the British Raj - small valleys would be damed off to capture the monsoon rains & insure that rivers & springs flowed year round.

  https://www.jstor.org/stable/40278311?seq=1#page_scan_tab_co...

• by Jedd on 9/8/18, 1:35 AM

  Seems to be a few comments from people who didn't watch TFV.

  Yes, the idea is that you build a concrete wall that stays there forever. You don't just build one per stream - but place them every so often. Behind the dam the sand builds up after a few rains / years. Most silt, they say, flows over the top, so your downstream nutrient refresh still happens. Before you build the dam you lay down a network of pipes with small holes or slits that then acts as a horizontal spear / well with filtering of larger (sand) particles. In subsequent seasons the sand retains more rainwater for longer after each rain event, and that water can be recovered via that pipe network. Damming also has the benefit of slowing rain water down -- which reduces erosion and improves infiltration down and sideways from the reservoir.

  The idea isn't new - low-head weirs have been popular (though not as popular as I'd like) for a very long time. Placement distances are determined by bed width, height, flow rate, seasonality, available funds, geography, etc -- so it could be every few hundred metres. Obviously concrete and earth moving aren't cheap, but waterways that only flow a month out of the year are much easier (and much more useful) to do this work on.

  People worried about the fish - for non-permanent waterways it's moot. For low-head weirs on waterways that tend to always have flow, there's ways to allow fish movement in both direction.

  Maintenance of the pipework could be an issue, but the nice thing about this approach is you put it in place, and it backfills and gets support during the next rain(s). I've looked at doing this on a low-flow / periodic creek bed before, and it'd involve some major earthworks to excavate down to below the normal water table (a couple of metres) and then the laying down of some very carefully constructed PVC piping - the big risk there is movement and cracking of same as you back-fill.

  PVC pipes are typically used as they're easier to work with, and reasonably safe in that use case. Very small slits - 1 or 2mm - are made along as much of the pipes as possible, that allow water infiltration but should prevent sand. You need to pump-flush for a while to remove the proximal silt, but after that you've got an effective horizontal spear / well that is kind of self-filtering.

  In this case there's nothing stopping you building another sand dam, a few hundred metres away, a decade later and repeating the whole process.

  EDIT: Oh, for non-rendered images of the benefits of even just slowing seasonal water down in arid lands, search for "Geoff Lawton greening the desert" -- he's a permaculturist that's done some spectacular work in Jordan, with proven sustained / sustainable results.

• by DoreenMichele on 9/8/18, 1:36 AM

  Desertification has been going on a long time in Africa. Reversing that seems like a good thing to me.

  This is a brilliant strategy and, also, the "illustrations" (some kind of time lapse stuff) in the article are great.

  (Former environmental science major. Squee!)

• by entee on 9/7/18, 6:35 PM

  Doesn't the "lake" get filled with sand eventually? Do you need to then excavate/move it away to maintain effectiveness?

• by grogers on 9/8/18, 8:59 AM

  Over many years would the sand get compacted down to be less porous, making it less effective at holding water?

• by chungy on 9/8/18, 4:49 AM

  Is there consideration for what impact it might have on the environment?

  At least in the northwest US, damming has been devastating to the natural ecosystem.

• by wodenokoto on 9/7/18, 4:05 PM

  Do they dam up sand, or is the dam made of sand or why is called a sand dam?

• by w0mbat on 9/8/18, 4:32 AM

  In Belgium they've got Jean-Claude Sand Dams.