Inventorization and Prioritization of Micro Dams and Design Problems in Data Scarce Areas

Abstract

Sudan has realized the importance of small dams in water harvesting long time ago. The first experience with small dam in Sudan goes back to early 40’s. The prime utilization of the micro dam was store water to meet irrigation and domestic water demands. The number of constructed small dams are growing with time, most of which were constructed on seasonal ungauged wadis where limited design information is available. Due to the inadequate designs there are cases of failure, dam breaks and complete siltation of the live storage in few years. In some cases the dam appurtenants (spillways, gates, etc) were not properly designed due to the limited information and/or improper design procedure. The short –time objectives of the study are to revise design procedures of the existing and proposed micro dams appurtenants and assess them critically to understand their weaknesses. The long –term objective of this study is to evaluate the possible design procedures under the condition of limited information and to develop procedure for designing micro dams in data scarce areas. Simple embankment techniques have been adopted and practiced long ago in the rural Sudan areas. Small dams constructed to provide water supply in rural areas. Investigation of dams requires carefull and detailed studies of hydrology, geology and topography of the site together with the land use and soil conservation aspects. Failure of dams is attributed to improper design and insufficient data. Geneina is located in the west of the Darfur region, near the boarders with Chad and has about 124800 inhabitants. Kerninik is one of its biggest villages with inhabitants of about 12000. The climate is hot and dry during most of the year; the wet season lasts about three months, from June to September. The mean annual rainfall for the last 20 years is about 497.8mm, and the mean annual potential evaporation is 2299.6mm. The Wadi Kaja and Wadi Barray are the main drainage lines in the area. In 1984 total runoff of Wadi Kaja was only 6.1 × 106m3; in 1985 total runoff was 610.8 × 106m3. Most of the people of Kerenik depend on cultivation of Sorghum, millet and the crops from small gardens like mango, Guava, Citrus, vegetables and Fodders. The main water source is the open dug wells. The climate in Darfur is controlled by seasonal movement of the sun and associated inter-tropical convergence zone (ITCZ). The rainfall which is taken into account is from Zalingi metrological station which is just outside the Catchment. The contour map was plotted by surfer. The test pits show that the alluvial deposit is presented (consists of gravel, sands, silts and clay). The type of soil is small grain with straight raw plus crop residue cover with residue about 40% of the surface throughout the year. These soils are in a good hydrologic condition with transmission rate greater than 0.76 cm/h. The maximum probable flood is calculated by SCS-curve number model and Manning formula. The best method which gives a suitable value of maximum probable flood is found to be the Manning method/formula and its value is 124.6 m3/s. The analyses of data show that the mean annual rainfall is found to be 2618.5mm. There is only one rainy season in the region and the peak of the rainfall occurs in the period (July to august). The effective rainy season can be considered from July to September. The peak of rainfall occurs on average three weeks before the peak of the flow and the flow hydrograph starts to pick up in two weeks time after the start of the effective rainy season. A design chart and procedure for design of small dams in the Sudan is needed to achieve proper design and to avoid failures and its problems.