Drinking Water from Dry Riverbeds 6.6

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 m 3 ) 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

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. Silt Fine Sand Medium Sand Coarse Sand Size (mm) < to to to 5.0 Saturation38%40%41%45% Water extraction 5%19%25%35%

Why storing water in sand? Low evaporation losses No contamination from livestock or other animals No water borne diseases from mosquitoes or reptiles

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

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

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 (>25m) 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

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

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

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

Example: Mwiwe riverbed, Kenya In 2004 in Kenya the Mwiwe riverbed was surveyed. 1)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

Probing Probing rods Measuring width and depth

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

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 M 3 sand Water extraction is 25 %, so water volume is: 557*0.25 = 139

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

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

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 m 3, at elevation of 80 metres above pump

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 m 3 to 2,997 m 3

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

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

Costs of subsurface dam Example

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)

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

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