1. Assessment of Sand Dam Potential for Ephemeral Rivers in Yatta and Mwala Constituencies, Eastern Kenya Alex R. Oduor, Kipruto Cherogony, Douglas Nyolei and Maimbo Mabanga Malesu

2. A Tribute to The Nobel Peace LaureateProfessor Wangari MaathaiWho commissioned this study via the GBM Organization

3. Definition of a sand dam A masonry barrier across an ephemeral river that conserves water beneath a throwback of sand

4. Upstream view of a sand dam

5. Rationale for dependency on sandy rivers Rainfall unreliability: Range, seasonality, High evaporation and/or siltation of surface water bodies Inadequate runoff capturing on the hillsides Saline/brackish river water Sand conserving water for long periods thus the major dependent source

6. Fetching water from sandy rivers

7. Challenges for inhabitants Undulating slopes: water carried either by women or ferried on donkey carts. Labour intensive/time consuming Despite its reliability in terms of availability, quantity is insufficient for inhabitants During dry seasons, the waters salt content increase in some rivers

8. Objective of the study Develop criteria for identifying sandy rivers. Use GIS to generate watersheds for sandy rivers. Use GIS to locate ideal sand dam sites. Compute runoff generated in each watershed. Determine potential sand dam and water storage capacity. Discuss implications of sand dam water on livelihood enhancement to the inhabitants.

9. Identifying sand dam rivers A size threshold for stream delineation was set using GIS modeling technique on DEM,. With this approach, streams with <5m or > 20m widths were annexed. Streams <5m widths: Slightly higher slopes with fairly rugged beds. Bank erosion evident in some sections, Conservation using check dams. Streams >20m widths: Too large Good for sub-surface dams or Sand Galleries. The sand rivers were confirmed through observation of the area in google map:

10. Identification of sandy rivers using Google image

11. Identification of sandy rivers using Google image

12. Incorporation of hydrologic and engineering principles in GIS to model, compute and assess the runoff potential of watersheds Landuse, slope and soil basemaps generated to determine RWH Potentials. The land use/land cover generated from most recent Aster imagery. Slope map generated from a 20m DEM. The layers pre-processed to ensure compliance with required data format, resolution and projection. Proportional loss model used to determine runoff potential for the sand dams (See Equations 1 and 2).

13. Proportional loss model to determine watershed potential Where = the net rainfall rate at time t (mm/h); = the rainfall rate at time t (mm/h); = the runoff coefficient (-); = the runoff volume of the storm (mm); = the total rainfall volume of the storm (mm); NB: DEM is then used to generate areal maps of watershed whose product with runoff coefficient and rainfall produces the watershed potential

14. Determination of sand dam & water storage potentials Where Vsd = Sand dam capacity (m3); Vw = Water abstraction potential (m3); Ai = Area of the lower elevation layer (m2); Ai+1 = Area of the upper elevation layer (m2); h = thickness of elevation (m);

15. Results & Discussions

16. Watersheds and river courses with sand dam potential Water courses are denser in Mwala than in Yatta constituency, attributed to the plateau phenomenon Watersheds in Mwala have undulating slopes and higher sand generation capacity than those of Yatta Mwala has 17 watersheds with potential for sand dams. There are a total of 263 sand dams (125 existing, 138 proposed). The BWR per HH during a 6 months dry period is 28800 m3. Total number of families that can be potentially served is 998. Or an average of 56 families per watershed.

17. River network for Mwala and Yatta constituencies

18. Spatial depiction of watersheds with sand dam Potential

19. Water Harvesting Potential for River CATCHMENTS

20. Number of sand dams in River CATCHMENTS

21. Watersheds & sand dam potentials

22. Mapped depiction of river courses with potential for sand dam technology

23. Conditions for sand dam water storage capacities 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%.

24. Only 5% of runoff generated from watersheds is potentially available for use in the sand dams

25. Conclusions & Recommendations

26. There is a good potential for sub-surface dams and sand galleries which are yet untapped

29. Runoff Curve Numbers

30. Surface water technologies e.g. Earth dams, Ponds & sub-surface dams are cheaper than sand dams. Their potentials should be explored. River catchments with potentials of > 6M ltrs/yr to be given priority attention during implementation. These include: Mitini, Ikombe and Isivuni. Conclusions & Recommendations

31. Erokamano for your understanding