Drinking Water from Dry Riverbeds

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Drinking Water from Dry Riverbeds
6.6
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Four types of Riverbeds

• 1 Hilly and stony catchments
• - Producing coarse sand
• - Extraction rate of 35 (Meaning 350
  liters of water can
• be extracted from 1 m3)
• 2 Gullies originating from stony hills
• - Consist of medium coarse sand
• - Extraction rate of 25
• 3 Flat farmland
• - Fine textured sand
• - Extraction rate of 10
• 4 Stony riverbeds
• - Bouldered and fractured rocks
• - Low potential due to high seepage

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Water Storage in Sand

• Empty spaces between sand (voids) are filled when
  a dry river bed is flooded.
• Tiny voids get saturated slower and less water
  can be extracted compared to coarse material.
• The courser the sand the higher the volume of
• water that can be extracted.

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Why storing water in sand?

• Low evaporation losses
• No contamination from livestock or other animals
• No water borne diseases from mosquitoes or
  reptiles

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How to find potential riverbeds?

• Draw a sketch of the riverbed
• Walk along the riverbed with community members
• While walking write down
• Location and types of water indicating trees
• Location of waterholes
• Location and types of rocks and boulders
• Location of calcrete (this turns saline in water)
• Coarseness of sand
• Location of hand-dug wells, boreholes and weirs
• Names of houses, schools and road crossings

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After the first sketch

• After creating the riverbed sketch, an agreement
  with
• the local community should be made on
  possibilities for
• improving their water supply.
• A more detailed survey should be carried out to
• - Collect data for drawing designs
• Estimate yields of water
• Cost of construction, operation and maintenance

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Site criteria of sand dam

• Suitable riverbeds must have two high river banks
  to enable wing walls for over flowing
• Do not built on fractured rocks or large boulders
  to prevent seepage. Build on solid bedrocks
  instead or 1 meter in solid and impermeable soil
• Riverbeds with fine textured sand are unsuitable
  because water extraction rate is low
• Wide riverbeds (gt25m) are too expensive for
  building sand dams
• Sand dam should not be build near calcrete due to
  salinity
• Grow water indicating vegetation as proof of
  riverbed capacity
• Catchment areas should contain stones
• Construct sand dams on natural underground dykes

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Design criteria

• Flood water should deposit coarse sand into the
  dam reservoir since sand has a good
• extraction rate
• With a dam wall sand can be harvested
• (sand will always be transported in lowest
• part of the water. Spillway needs to be
• raised every phase)
• Because of the water force, the width
• of walls is 0.75 from the height
• Dam walls must be keyed at least 1
• meter into solid impermeable soil
• Front of dam wall has a gradient of
• 1/8 of the height

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Design criteria (2)

• Spill over apron (where overflowing water
• will fall on must be reinforced onto dam wall
• Apron has same width as dam wall
• Large stones in apron will break
• the water force
• Key under dam wall extends up
• to end of wing wall to prevent
• seepage
• Water extraction for example
• through galvanized pipe

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Maintenance criteria

• Sand dams require careful maintenance and
  immediate repair during flooding
• Committees from community owned sand dams must be
  well organized since they have to meet and
  discuss about a problem. In critical situations
  this might be a problem

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Example Mwiwe riverbed, Kenya

• In 2004 in Kenya the Mwiwe riverbed was surveyed.
  
• Probing procedure
• A probing rod was hammered down in the middle of
  the riverbed until it hit the floor under the
  sand with a dull sound.
• The level of sand was marked on rod to find out
  water depth
• In the notches of the rod coarseness of sand can
  be seen
• Type of floor can be seen from tip of rod
• Height and width of banks are measured with two
  long tape measures
• Presence of water indicating vegetation, roads,
  waterholes etc. was noted
• Probing was done at interval of 20 meters

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Probing
Measuring width and depth
Probing rods
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Profile of riverbed

• After probing a longitudinal profile was drawn
• From different profiles it can be seen that the
  underground at point 18 is the most suitable for
  the sand dam

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Volumes water in sand of Mwiwe

• The volume of a dam reservoir
• Maximum depth maximum width maximum
  throw-back/6 Volume
• (Throw back is the horizontal length of water in
  dam)
• For Mwiwe
• 3.25 (max depth) 25.7 (max width) 40
  (throw-back)/6 557 M3 sand
• Water extraction is 25 , so water volume is
  5570.25 139

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Options

• To increase water volume, the maximum depth or
  width can be increased
• Three options were given in survey report to
  engineers
• Subsurface dam of soil, 30 cm below sand in
  riverbed
• Weir of concrete or rubble stone masonry to 30 cm
  above sand level
• Sand dam or rubble stone masonry to 5 meters
  above sand level

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Considerations

• Sustainability
• Affordability
• Long term demand
• Population increase (as effect of new water
  availability)

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Choice in Mwiwe

• Based on evaluation reports and consideration as
  mentioned before, they conducted in Mwiwe a sand
  dam of rubble stone masonry of 1.5 meter above
  sand surface.
• Additionally the following constructions were
  made
• 72 meters of infiltration pipes
• Infiltration pipes have a gradient towards
  riverban
• Hand dug well with a diameter of 3 meters will be
  sunk in riverbed
• Well is built of curved concrete blocks
• Water extraction is done by surface pump with
  diesel generator
• Pump delivers water to head tank of 100 m3, at
  elevation of 80 metres above pump

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Results

• The sand dam raises the water table from 1.7
  meters below the surface to 1.5 meter above the
  surface
• Storage capacity increases from 139 m3 to 2,997 m3

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Constructions in dry rivers

• Riverbed intakes
• Dig waterhole
• Sink hand dug well next to waterhole
• Hydro-dynamic well head
• Riverbank intakes
• Infiltration pipes drain water into hand dug well
• Sinking wells

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Subsurface dams of soil

• A subsurface dam will
• Block underground flow of water
• Raise water level in the sand to 30 cm below
  surface of riverbed

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Costs of subsurface dam
Example
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Construction guidelines

• Step 1)
• Build subsurface dams (weirs, and sand dams)
  preferably on underground dykes situated
  downstream or underground water reservoirs to get
  maximum water volume
• Use the most clayey soil for construction of the
  dam
• (To find out which soil is best use bottles with
  soil samples, pour water on top and place them up
  side down to see which soil has the slowest
  infiltration rate)

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Construction guidelines (continued)

• Step 2
• After identifying suitable soil, remove all sand
  in riverbed in a 3 meter wide stretch
• To prevent seepage, make a key (trench) of 100 cm
  wide ad 60 cm into solid soil
• Transport clayey soil
• Fill key with 20 cm moisturized clay
• Repeat laying out layers of 20 cm until dam wall
  has reached 30 cm below surface of sand
• Walls have a slope of 45 degrees, smoothened with
  shovels an wooden floats
• Sand is back-filled on both sides and on top

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Source Water From Dry Riverbeds E.
Nissen-Petersen Danida, 2006