Aid Projects, Community Projects, Earth Banks, Gabions, Irrigation, Land, Regional Water Cycle, Rehabilitation, Storm Water, Water Harvesting — by Salah Hammad January 9, 2013
Shibam: UNESCO World Heritage site
I was recently privileged to be part of the team that accompanied Geoff and Nadia Lawton along with Mr. Tashi Dawa in a very interesting consultancy in the Southern Yemen, specifically The Hadhramaut Valley, or Wadi Hadhramaut.
Geoff was invited by the “Reconstruction Fund of Hadhramaut and Al-Mahra” to give his opinion on what could be done in the valley in terms of flood mitigation and water harvesting from a permaculture point of view.
Wadi Hadhramaut, located only 16° North of the equator, and 200kms inland from the nearest ocean, the Indian Ocean. It is considered an Arid Tropical climate zone, with almost no similar climate zones or topography in the world. The Wadi is more than 200kms long and ranges in width between 500m and 13kms at some points. The average annual precipitation in Seiyun (the biggest town in the Wadi) is 65.2mm.
Its wells and controlled flash floods provided much needed water for cultivation and its rich soil yielded 3-4 crops a year where irrigation was successfully managed. Throughout history, this fertile land gave the economic basis for people to settle and build mud brick towns throughout the Wadi — such as Seiyun, Tarim and Shibam, the UNESCO World Heritage site pictured at top.
Topography, Rainfall and Flooding
Wadi Hadhramout is basically a network of valleys carved into the Eastern Plateau. The main Wadi runs from west to east, bisecting the plateau into two parallel ranges — the Northern and the Southern ranges.
The highlands of Hadhramaut are a rather broad, barren and pebbly plateau, averaging at about 1,300m to 1,400m above sea level, intersected by a few valleys; some deep and some shallow; some narrow and some wide. The uplands are very sparsely populated by a few Bedouins; while the valleys, like Wadi Hadhramaut and Wadi Do’an, are densely peopled.
A distinctive feature of the valleys of the Wadi Hadhramaut network is the flat-topped cliffs that drop vertically to the level wadi bottoms. This causes the runoff to be extremely rapid, with a surface that is either bare or covered in rock debris and minimal vegetation.
It’s mainly when the rain falls heavily on these highlands (March-April and June-September are considered monsoon periods) that the valleys below get flooded. These rainfall events can be very good for agriculture, but often the population in the valleys below do not know when it has rained heavily up on the Plateau, and so, when the water comes gushing down, many are caught unaware. At times the floods can cause enormous devastation to crops and buildings; and many people get killed.
Mud brick buildings built straight under the cliffs but away from the flood path
The floods that affect the Wadi come from tributary valleys that start in the Northern and Southern ranges, bringing runoff from a catchment area of approximately 34,000 square kilometres. Precipitation is estimated at 2040mcm p/a with runoff of about 500mcm.
In Oct. 2008, the tropical storm 03-B caused flooding throughout the Wadi. Thousands of families fled and 100 were declared dead or missing. 7000 people were made homeless. This was a result of thirty hours of heavy rain on the upper plateau.
You can see in the video how flat and massively huge the catchment area is, with thousands of small valleys that form networks of Wadis which drain all the runoff into the floodplains. Also, the main channel can be identified by the amount of Prosopis juliflora growing on its banks.
Sorry about the loud sound, but that’s what a 35 year old Soviet-era helicopter sounds like!
What Could Be Done
Now, what could be done is huge, in terms of flood mitigation, through slowing down the water and harvesting every drop to sustainably accommodate all this water in the valley to benefit the people.
Whatever rainfall comes down, it needs to be infiltrated and retained in the system, if it’s lost through runoff or excessive evaporation rates, then it’s going to gradually degrade the system, and if it’s going on for 100s and 100s of years , then the ultimate output is collapse.– John D Lui
The basic concept for the design for the whole Wadi is to stop and slowly spread and soak the floodwater — in effect to greatly extend the retention time. To do that we need to start at the top and work our way down, with the potential of completely mitigating the damaging effects of flooding.
Palm Farms in the Flood Plains
The effect of this will be a very large increase in soil fertility, giving us the possibility to design and implement a productive, perennial biological system (agroforestry) that will further moderate the effects of large flood events.
This system in its stable, mature form will convert the destructive events of the major floods to creative events that will increase soil water storage and continuously recharge the aquifers with fresh rainwater, which will in effect reduce the salinity of the aquifers. Surface flows of water in major flood events will be greatly moderated in both volume and speed with appropriate beneficial flows ideal for agricultural irrigation. This agricultural irrigation will be greatly expanded over time.
A Bit More Detail
Contour banks on top of the plateau will gently direct the runoff water towards the wadi top head cut. Since no machinery can reach the steep top slopes, a succession of gabions will be built by hand to a height of 1-1.5 m as still traps with spillways to release water in large events.
Top Head Cut as seen from the Plateau
As we go down the slope, accessibility to machinery is going to be easier, with a gentler slope and flatter base. Gabions at this profile will be higher, 1.5-3 m with larger spillways and repeated wherever the landscape profile suits.
In the lower wadi, where there’s an incised channel, gabions with contour swales will stop, spread and soak the water into the floodplain. This will be an ideal situation in which biodiversity can be increased through planting a diverse mixture of drought-hardy pioneer nitrogen-fixing leguminous trees and productive perennial fruit trees. These will grow with minimum inputs of water and fertilizer since the floodwaters will provide the needed nutrition for the soil and the trees. A very detailed illustration of this proposal can be seen in Bill Mollison’s Designers Manual, figure 11.9 and 11.5.
Scour Holes on the plateau, just like described in
Bill Mollison’s Designers’ Manual, Figure 11.6!
During the 3-day workshop Geoff Lawton held, the organisers invited experts from the community to attend. Enthusiasm and the willingness to hear and learn from what was discussed was very evident.
A lot of community work and training is involved in the sustainability of the project. The involvement of community leaders and Muslim clerics in the collective community-awareness strategies are as important as specific training and development courses and activities for the overall community stewardship of the land and the structures.
The establishment of an independent Permaculture Research Institute (PRI Yemen) in the Wadi will be of great importance in the success of the project and the development of a community that actively takes care and sustains the potential outcomes of such as huge project.
During a field trip
10 Comments »
No comments yet.