Browsing by Author "Malisa, Johnson"

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    Dam safety analysis using physical and numerical models for small dams in Tanzania: the case of Arusha Region
    (University of Dar es Salaam, 2007) Malisa, Johnson
    Different failure modes and causes for the failed earthfill dams in Arusha region have been identified. The resulting extent of embankment damage was observed to be significantly larger where average breach top width was found in most cases more than three times the dam height. Even though, there has not be many small dams that has imposed risk to people`s life in the country to date, the loss resulted from the reported incidents is significant in terms of storage loss, embankment damage, environment and farming system destruction. Most of the existing small earthfill dams in Arusha region have been observed to have significant overtopping potential as a result of inadequate spillway capacity. This has been caused but the inherent uncertainty in the flood estimate method used in ungauged areas where hydrological information is scarce and changes in the catchment characteristics are significant. A method has been proposed and applied toassess and evaluate overtopping potential of a small earthfill dam (s) located in ungauged catchment as well as assessment of damage and danger in the downstream surface erosion resulted into stair-stepped multiple overfalls which finally merged into a single main upstream-migrating headcut. The rate of upstream headcut migration has been evaluated based on measured material properties, hydroulic and dam breach parameters. The important preliminary procedures for dam safety improvement have been highlighted to provide preliminary guidelines in the design, operation and maintenance.
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    Defluoridation of drinking water by adsorption of fishbone media.
    (University of Dar es Salaam, 2001) Malisa, Johnson
    Excess fluoride in drinking water is a problem in some parts of Tanzania, especially the northern regions and a few districts covering the southern highlands especially those in the rift valley. Therefore fluorosis still remains a public health problem in these areas manifesting itself in the form of crippling fluorosis in extreme cases, like in some parts of Arusha region. The prevention of fluorosis by developing simple and inexpensive methods of fluoride removal should be encouraged. The primary objective of this study was to investigate the mechanism and capacity of locally available fishbone char in the removal of excess fluoride from drinking water. Carrying out a literature review, preparation of the media, performing jar test (batch), continuous flow experiments. Chemical analysis of fishbone char and water quality assessments were accomplished in this study. Preparation of the media involves, procurement, charring at different temperatures and duration, sieving and sorting out the media to the different grain size. Furthermore, the results indicate that the decomposition of organic matter of fishbone occurs at temperature between 300°C to 700°C. The appropriate charring temperature range and duration for the fishbone to be used for defluoridation was found to be 400°C and between 3.5 to 4 hours respectively. Fishbone charred at optimum temperature and duration shows a high fluoride removal capacity.