Difference between revisions of "Riverbed infiltration galleries"

Revision as of 20:00, 7 April 2012

The Omo River in Ethiopia, Africa.

Channels take water from a riverbed to a collector well in the riverbank. They are often screens (slotted or perforated pipes) that are inserted horizontally into a riverbed, but also infiltration galleries can be made from channels with graded gravel as long as sediments are not washed into the collector well. Where screens are used, the screen diameter tends to be larger than that used normally for jetted/driven wells.

Riverbeds having catchments of flat farmland usually contain fine textured sand, that can only yield a maximum of 100 litres of water from 1 m3. of sand, i.e. an extraction rate of 10%. Stony riverbeds containing boulders and fractured rocks have the lowest potential for water extraction due to seepage caused by the boulders and fractured rocks. This seepage is, however, beneficial for replenishment of boreholes situated on riverbanks.

Suitable conditions

• Infiltration galleries are often installed in shallow or fine sediment beds where there is poor permeability or lack of sand depth – in this case, the length proves to be advantageous.
• They can also be sited in areas where riverbanks are too high to allow manual or motorized suction pumps to operate.
• There should be water remaining in the riverbed throughout the dry season.
• Make it in a degrading river section where there is no deposition, so there are coarser grains and no silt deposits blocking flow.

Construction, operations and maintenance

Infiltration gallery Image courtesy of WEDC. © Ken Chatterton. In: Hussey, S.W. (2007) Water from sand rivers: guidelines for abstraction. WEDC, Loughborough University, UK.

Building an infiltration gallery. Infiltration Area: 1080 ft2 (100.3 m2). Cost - $9.33/foot2. Overseas supplier (AUS).

How deep is the water table?

Some rural people and most well-diggers know that water can only be found at certain places in riverbeds. They can also give a rough estimate on how deep they have to dig before reaching the water-table.Their knowledge is based on the fact that certain species of trees and vegetation must have roots reaching down and into the water-table in order to survive droughts. This traditional information (based on Kenyan plants) is compiled in the table below as an example. Make sure to find the unique regional plants specific to your site area.

Since the trees below must have their tap root in the water table, the depth of a water-table can be found by knowing the depth of the tree’s tap root. A rule of thumb states that the tap root of a tree has a depth equal to about 3⁄4 of the height of the tree. The height of a tree can be found by measuring the length of the shadow the tree is casting on the ground and comparing it with the length of the shadow of a stick 100 centimeters long. The two measurements should be taken in the sunshine of early morning or late afternoon when the shadows are longest. For example: If the stick’ shadow is 80 cm long, the ratio is: 80/100 = 0.8. If the tree’s shadow is 12 m long, then the tree is: 12 m x 5 / 4 = 15 m high and the tap root and water level is at: 15 m x 3⁄4 = 11.25 m depth.

Other considerations

• The length of screen required will be greater for an infiltration gallery than for a jetted or driven well – in an infiltration gallery, water flows to the collector well under hydraulic head rather than being pumped out with a suction pump. Diameter of screen used is typically 75 – 300 mm PVC and varies from a few metres up to several hundred metres in length. Layouts vary according to the river widths. Yields are typically 15 litres/min/metre, but depends on depth from river to sump.
• Ensure that galleries are dug deep enough to allow enough flow during the dry season. In practical terms, this means aiming for at least 1 metre depth within the saturated zone. If more than one pipe are installed, distance between pipes should not be less than 3 metres.
• The top of the gallery should be ideally 1.5 metres minimum from the riverbed surface. This is so that:

1. There is at least a minimum of infiltration that occurs from surface water during the wet season when the river is flooded.
2. There is less chance that the well shaft and pipework will be washed away in a flood event. Sand becomes mobile to a certain depth which differs for different rivers but has been recorded to be normally between 0.66 and 2 metres in seasonal riverbeds.

• Graded gravel needs to be placed under and over the pipe to minimize clogging with sediments.

Costs

Below is an example of the costs of labour, materials, and transportation to install a river infiltration system. Currency is in the Kenyan shilling. 100 Ksh = 1.20 USD.

Find out more about installing a riverbed infiltration system, Chapter 4 of Water from Dry Riverbeds. Waterforaridland.com.

Field experiences

Reference manuals, videos, and links

• Designing Intakes for Rivers and Streams. Water for the World. - Lots of design diagrams and instructions on how to build a river intake system.
• Water from Dry Riverbeds. Waterforaridland.com.
• Infiltration Gallery Testing for Integration into Dual Purpose Irrigation Systems: Managed Aquifer Recharge in Walla Walla Basin. Robert Bower, 2011.

Acknowledgements

• CARE Nederland, Desk Study Resilient WASH systems in drought prone areas. October 2010.
• Nissen-Petersen, Erik, Water from Dry Riverbeds. Danish International Development Assistance (Danida) 2006. Waterforaridland.com.