Assessment of sand dam potential for ephemeral rivers


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Assessment of sand dam potential for ephemeral rivers

  1. 1. Assessment of Sand Dam Potential for Ephemeral Rivers in Yatta and Mwala Constituencies, Eastern Kenya<br />Alex R. Oduor, Kipruto Cherogony, Douglas Nyolei and Maimbo Mabanga Malesu <br />
  2. 2. A Tribute to The Nobel Peace LaureateProfessor Wangari MaathaiWho commissioned this study via the GBM Organization <br />
  3. 3. Definition of a sand dam<br />A masonry barrier across an ephemeral river that conserves water beneath a throwback of sand<br />
  4. 4. Upstream view of a sand dam<br />
  5. 5. Rationale for dependency on sandy rivers<br />Rainfall unreliability: Range, seasonality, <br />High evaporation and/or siltation of surface water bodies<br />Inadequate runoff capturing on the hillsides<br />Saline/brackish river water <br />Sand conserving water for long periods thus the major dependent source<br />
  6. 6. Fetching water from sandy rivers<br />
  7. 7. Challenges for inhabitants<br />Undulating slopes: water carried either by women or ferried on donkey carts. Labour intensive/time consuming<br />Despite its reliability in terms of availability, quantity is insufficient for inhabitants<br />During dry seasons, the waters salt content increase in some rivers<br />
  8. 8. Objective of the study<br />Develop criteria for identifying sandy rivers.<br />Use GIS to generate watersheds for sandy rivers.<br />Use GIS to locate ideal sand dam sites.<br />Compute runoff generated in each watershed.<br />Determine potential sand dam and water storage capacity.<br />Discuss implications of sand dam water on livelihood enhancement to the inhabitants.<br />
  9. 9. Identifying sand dam rivers<br />A size threshold for stream delineation was set using GIS modeling technique on DEM,.<br />With this approach, streams with <5m or > 20m widths were annexed. <br />Streams <5m widths:<br /> Slightly higher slopes with fairly rugged beds. <br />Bank erosion evident in some sections, <br />Conservation using check dams. <br />Streams >20m widths:<br />Too large <br />Good for sub-surface dams or Sand Galleries. <br />The sand rivers were confirmed through observation of the area in google map:<br />
  10. 10. Identification of sandy rivers using Google image<br />
  11. 11. Identification of sandy rivers using Google image<br />
  12. 12. Incorporation of hydrologic and engineering principles in GIS to model, compute and assess the runoff potential of watersheds<br />Landuse, slope and soil basemaps generated to determine RWH Potentials. <br />The land use/land cover generated from most recent Aster imagery. <br />Slope map generated from a 20m DEM. <br /> The layers pre-processed to ensure compliance with required data format, resolution and projection. <br />Proportional loss model used to determine runoff potential for the sand dams (See Equations 1 and 2). <br />
  13. 13. Proportional loss model to determine watershed potential<br />Where <br /> = the net rainfall rate at time t (mm/h);<br /> = the rainfall rate at time t (mm/h);<br /> = the runoff coefficient (-);<br /> = the runoff volume of the storm (mm);<br /> = the total rainfall volume of the storm (mm);<br />NB: DEM is then used to generate areal maps of watershed whose product <br /> with runoff coefficient and rainfall produces the watershed potential<br />
  14. 14. Determination of sand dam & water storage potentials<br />Where <br />Vsd = Sand dam capacity (m3);<br />Vw = Water abstraction potential (m3);<br />Ai = Area of the lower elevation layer (m2);<br />Ai+1 = Area of the upper elevation layer (m2);<br />h = thickness of elevation (m);<br />
  15. 15. Results & Discussions<br />
  16. 16. Watersheds and river courses with sand dam potential<br />Water courses are denser in Mwala than in Yatta constituency, attributed to the plateau phenomenon<br />Watersheds in Mwala have undulating slopes and higher sand generation capacity than those of Yatta <br />Mwala has 17 watersheds with potential for sand dams. <br />There are a total of 263 sand dams (125 existing, 138 proposed).<br />The BWR per HH during a 6 months dry period is 28800 m3. <br />Total number of families that can be potentially served is 998.<br /> Or an average of 56 families per watershed. <br />
  17. 17. River network for Mwala and Yatta constituencies<br />
  18. 18. Spatial depiction of watersheds with sand dam Potential<br />
  19. 19. Water Harvesting Potential for River CATCHMENTS<br />
  20. 20. Number of sand dams in River CATCHMENTS<br />
  21. 21. Watersheds & sand dam potentials<br />
  22. 22. Mapped depiction of river courses with potential for sand dam technology<br />
  23. 23. Conditions for sand dam water storage capacities<br />Mwala has a topography of gullies developing from stony hills with 25% water storage capacity as cf’d to Yatta with flat rocky topography and a sand dam water storage capacity of 10%.<br />
  24. 24. Only 5% of runoff generated from watersheds is potentially available for use in the sand dams<br />
  25. 25. Conclusions & Recommendations<br />
  26. 26. There is a good potential for sub-surface dams and sand galleries which are yet untapped<br />
  27. 27. <ul><li>Although sand dams can’t be ignored, a higher potential for harnessing runoff water exists in the hillsides
  28. 28. Per-capita storage can be increased by rehabilitating existing sand dams</li></li></ul><li>More accurate runoff estimation can be achieved using SCS-CN method<br />Where:<br />Q = runoff (mm) <br />P = rainfall (mm)<br />S = maximum recharge capacity (Water storage parameter of <br /> rainfall after 5 days antecedent moisture retention<br />NB:<br />The area different land uses is obtained using command tools <br />of Arc/Info or ArcView within GIS environment.<br />
  29. 29. Runoff Curve Numbers<br />
  30. 30. Surface water technologies e.g. Earth dams, Ponds & sub-surface dams are cheaper than sand dams. Their potentials should be explored.<br />River catchments with potentials of > 6M ltrs/yr to be given priority attention during implementation. These include: Mitini, Ikombe and Isivuni. <br />Conclusions & Recommendations<br />
  31. 31. Erokamano<br />for your understanding<br />