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Wednesday, January 07, 2009

The Instant Wadi Well for Arid Climates

The Instant Wadi Well
by Douglas Barnes

Rainfall in arid regions occurs in large and infrequent events throughout the year. Because the desert environment is a brittle one and because there is so much rain at one time, tremendous erosion occurs. This leads to the creation of scarps and wadis.

Wadi is simply the Arabic word for a riverbed. In an arid region, its water flow is likely to be ephemeral, particularly in headwaters (where permaculturists are most likely to be working). They are also places where a lot of erosion takes place. Looked at from another perspective, wadis are places where a lot of erosion can be stopped.

After teaching a permaculture course in Jordan, designer Geoff Lawton returned to the area a year later and found that someone had built a gabion (an uncemented rock wall, usually held inside a steel cage) across a particular wadi. Although the 8-foot-high gabion was less than a year old, it was already full of settlement and still had water trickling out of the base, despite the fact that there had not been any recent rains.

This natural tendency for gabions in wadis to quickly fill with water-retaining sediment provides us the opportunity to create what I am dubbing the Instant Wadi-Well (for lack of a better name). After determining where the gabion is to be placed, start with a shallow hole about 2 feet (60 cm) deep in a teardrop shape with the tapered end downstream. Place the first row of stones for the well inside the hole and cement them together, leaving the ground unmortared to allow water to seep in. Keep adding and cementing stones until you reach ground level. Once at ground level, continue up but add an extra 2 layers of mortared stone at the tapering end to allow stability in the face of the rushing torrent and sediment that is to come with the first rainfall. Continue this right up the entire height of the well shaft, making the top row at least 3 feet (90 cm) above the height the gabion will be. After all the sediment moves in, it is likely the well casing will only be 2 feet (60 cm) above the sediment.

All that is left is to build the gabion. Wire cages are not absolutely necessary for the gabion to perform, but they are recommended as they make it far less likely that the rock wall will blow out in the face of the torrent of water, sand and silt.

If the well casing is built strong enough, then there will likely be a well within a few major rain event is. The water will have to be tested as deserts tend to have salty soil. Baring salinity problems, this would be a quick and easy way to establish clean wells and combat erosion at the same time!

This technique compliments other systems as well. For example, if there are storage tanks or cisterns at the top of the scarp, a windmill can be employed to pump water from the Instant Wadi Well to this higher storage. From there, it can be gravity fed down to where it is needed.


rich said...

Good idea. Depending on the site (namely, geology and 'flashiness' of the wadi), it could be worth thinking about putting the well downstream of the gabion, somewhere in the splash apron. If the watershed has lots of big rocks in it that come down, the sediment could knock down your well walls.

That would involve digging a hole, though.....


DJEB said...

Yes, it is not going to fit every site. And as you say, if there are large boulders bouncing down, you are likely to get a smashed well casing. But if the site just has pebbles, sand, silt and a few small stones, any damage would likely be minimal and easily repairable - certainly easier than digging a well out of the sediment then lining it. When I find myself in the right situation, I'll test it out and report on how it works.

Scott A. Meister said...

Regarding concerns of having the well walls knocked down by large objects washing down in the "flash," you could build one, (or a series of) small, protective "barier(s)" above the well location to deflect large debris...or at least deflate it's inertia. Establishing a small series of sturdy trees with deep root systems planted in a staggar within an arrow shape might work well (think staggared bowling pins...instead of having them lined up in a row)...this would allow some silt to flow through, while still deflecting large objects. Saline tolerant trees with deep root systems would be ideal...and, if the right species is chosen, they could aid in purifying the water.

DJEB said...

In the kind of spot where a well could be created with the sediment, finding a planting spot would be the challenge, I think. And they would be buried after the sediment settled. Rock barriers might help, though. I just might be headed to semi arid zones in both China and India this year, so there may be an opportunity to try this out.

Scott A. Meister said...

Honestly...I think wadis occur in extremely arid zones with extremely seldom, but extremely huge rain events. But, hey...if you find one...go for it, and let me know! However...having said that...I think the same tactic could be used to slow down erosion and build-up sediment in deep gullies, which happen quite often in many climate zones. Speaking of which, I keep forgetting to send you the pictures of the huge gabeon we have here at Ishiwari shrine in Yamanakako, Japan.

DJEB said...

Denude the land enough and they will pop up just about anywhere. I've seen them in gravel pits here in Ontario - until vegetation establishes and stabilises them, that is. You won't, however, see them cut out of rock here like you do in deserts. From friend's images, I've seen them in semi arid regions. But I think my chances of finding any in the well-terraced regions of China I will be seeing are slim to none.

arv said...

I would suggest a series of such 'traps' along the length of a wadi so as to create a self sustaining ecosystem . In fact i came across one of these constructions near jerusalem where they made a wall with huge boulders almost eight metres high across a wadi . The area behind it and beyond it was lush green because of the water and sediment that the wall trapped . The are was most likely cultivated 60-70 years back but now is left on its own and is self sustaining .

DJEB said...

I agree with your suggestion, Arv. Wadis can have a series of gabions in them to help check erosion and rehabilitate the land. And as they concentrate water, they are good sites to establish greenery on. They are a very useful tool for desert dwellers.