Browsing by Author "Nyandwi, Ntahondi"
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Item An Account of Sandbank Depositional Modes, Menai Strait, UK(1992) Nyandwi, NtahondiItem Beach Sand Supply and Transport at Kunduchi in Tanzania and Bamburi in Kenya(2012) Shaghude, Yohanna W.; Mburu, J. W.; Uku, Jacqueline; Arthurton, Russell; Nyandwi, Ntahondi; Onganda, H.; Magori, Charles; Sanga, IgnatioBeach-head erosion of sandy beach plains in eastern Africa threatens tourism-related infrastructure and the livelihoods of beach users. The nature and drivers of physical shoreline change at Kunduchi, Dar es Salaam, and Bamburi, Mombasa, are described with analyses of beach sand transport through the annual monsoon cycle and the provenance and sustainability of the beach sand supply. Time-series records of wind-vectors at Dar es Salaam and Mombasa show similar averaged patterns. Because of the contrasting alignments of these coasts, the net wind-wave driven longshore transport at Kunduchi (trending NNW) is northnorth-westwards, while at Bamburi (trending NNE) there is little net transport. At Bamburi, the beaches are recharged with reef/platform-derived calcium carbonate sand and siliciclastic sand discharged from the hinterland via tidal channels. At Kunduchi, recharge comprises mostly river-borne siliciclastic sand, but riverine sand mining threatens natural replenishment, jeopardising beach maintenance. Eroding beach plain deposits contribute siliciclastic sand at both sites.Item Coastal Tanzania, a New Home to the Living Coelacanth: An Oceanographic Analysis(2006) Nyandwi, NtahondiCoelacanth is no longer a new fish in Tanzania. After it was first discovered in September 2003 off Kilwa Masoko in southern Tanzania and with awareness campaigns, fishermen have reported further landings since, mostly in Tanga, northern Tanzania at Kigombe village. Analysis of habitat conditions suitable for the coelacanth indicates that the oceanography may just be right for the fish to have a permanent residence as opposed to the theory of drifters from a natural home elsewhere. At one time three specimens were captured together in the same net indicating that they formed a group. Oceanographic studies of the habitat in Tanga suggest that there could well be a resident population supported by existence of sharp slopes and steps. Caves in which the fish hides might be present at about 150 m depth where suitable temperatures of about 21°C are found.Item Ecological and Engineering Importance of the Bet el Ras Beach-Sandstone(2006) Nyandwi, Ntahondi; Kangwe, Juma W.A unique sandstone known as the Bet el Ras sandstone, supposedly of Holocene age, occurs profusely along the western coast of Unguja but more extensive at its locality area, Bet el Ras, where it forms a rocky shore. The sandstone has been blasted in the past for construction of the sea wall that fronts the Zanzibar Town, and stands to be blasted further for renovation works. From the results of the survey made, it is clear that the sandstone is ecologically and environmentally important such that its total removal will lead to loss of a habitat as well as enhance coastal erosion and sediment input in the littoral zone thereby impacting on the ecology of the associated flora and fauna. Since none of the fauna species is endemic and due to its engineering importance, selective extraction may be considered such that extraction of the sandstone be done in steps and cavities be refilled as well. A thin veneer may also be left close to the land as protection against erosion. This will also minimize sediment input in the littoral zone that would otherwise have affected the community structureItem The effects of monsoons on the East African Coastal current through the Zanzibar Channel, Tanzania(Fisheries and Marine Institute, Memorial University of Newfoundland, 2013-12) Nyandwi, NtahondiIt is long established that the East African Coastal Current flows northward along the coast of Tanzania throughout the year. However, uncertainty has long existed as to the influence of this current in the Zanzibar Channel. Victims of various capsized vessels within the channel have been picked up both to the south and north of the accident site depending on the monsoon period. Current monitoring and measurements have recently been made in the Zanzibar Channel with the view to understanding the flow structure within the channel. In the first campaign measurements were made during both the Northeast and Southeast monsoon periods along a cross-channel transect using a lowered an Aanderaa RCM9 current meter. In the second campaign an upward-looking acoustic Doppler current profiler (ADCP) was deployed at one station for 2 years to capture variations in the current profile over both monsoon periods. In the central, deeper part of the channel the current was found to flow northwards at all depths during the Southeast monsoon period with surface velocity of 0.5 ms-1. During the Northeast monsoon period, however, the current was found to flow southwards at the surface while a northerly flow continued close to the bottom. Away from the central part of the Channel, the 2 year ADCP data showed that the flow was northeastwards throughout the water column and reversed during the Northeast monsoon months of December, January, February and March. The surface flow direction around coral islets and sandbanks varied depending on the location and the stage of the tide. The findings suggest that the current in the channel may be considered to be a small part of the East African coastal current that flows northwards, but it is reversed by the Northeast monsoon in the upper layers.Item Episodic Atmospheric Changes and Their Impact on the Hydrography of Coastal Waters in Tanzania(2001) Nyandwi, Ntahondi; Dubi, Alfonse M.A shift in the time of occurrence and duration of the rainy and dry seasons has been observed over Tanzania and the East African region in recent years. During 1998, El Niño rains produced substantial fresh water and sediment supply to the coastal zone. During that period, large sand bodies were deposited off river mouths and at stream entrances. Hydrographic measurements showed a persistent decrease in salinity and temperature of inshore waters indicating that coastal waters have been trapped along the shore. Such environmental changes could have negative effects on the biodiversity of the coastal zone. Based on beach profile measurements, the shoreline was predominantly eroding prior to the rains but accreted during the El-Niño rains. The tidal flats received a larger volume of sand. While the beach and tidal flats were receiving this supply of sand, river banks eroded as channels deepened and widened. In many instances, newly cut flood channels have remained semi-permanent serving as tidal inlets leading to tidal inundation of the low lying areas behind the beaches. These observations point to the significance of episodic events in maintaining or shaping the coastal zone.Item The Experience of Tanzania in Coastal Protection. Successes, Failures and New strategies(1999) Dubi, Alfonse M.; Nyandwi, NtahondiItem Geomorphological Potential of Coelacanth Habitat Across Mozambique-Tanzania Border.(2010) Nyandwi, NtahondiThe coelacanth is a new fish in Tanzania. It was first discovered in September 2003 after it was spotted by a tourist in a fish market. It was caught by fishermen in the deep waters off Songo Mnara Island offshore of Kilwa Masoko Town in Southern Tanzania. The find potentially added southern Tanzania to the list of known coelacanth areas of the southern Western Indian Ocean Region. Observed current conditions indicate that the waters are swept by strong (2 m/sec) northward flowing current. This suggests that there must exist hideouts for the fish, such as caves in canyons and overhangs. Carefully analysed single cruise bathymetric data obtained from the region indicate existence of canyons off the Tanzania- Mozambique border region, which are believed to provide habitat to the fish. The data point to the potential of the area as habitat for the elusive coelacanth.Item A Hydraulic Model for the Shore-Normal Energy Gradient in the East Frisian Wadden Sea(1995) Nyandwi, Ntahondi; Flemming, Burghard W.Item Hydrodynamics of Chwaka Bay, a Shallow Mangrove-Fringed Tropical Embayment, Tanzania(Elsevier, 2000) Nyandwi, Ntahondi; Mwaipopo, O. U.Time-series data of currents, sea levels and temperatures from Chwaka Bay,Zanzibar were analysed with the view of understanding the water circulation of the bay. The analyses show that there is a tidal asymmetry in the bay, with peak ebb tidal currents in the deep channels (45 cm s−1) being stronger than flood tidal currents (35 cm s−1), and ebb periods (7 hours) being longer than flood periods (5 hours). The velocity and time asymmetry as well as the asymmetry in the current direction are controlled by the morphological variations of the tidal basin. It was found that, as the water flows from the inner bay during the ebbing tide, it first drains towards the main tidal creek which leads to concentrated but delayed flows. The temperature variations in the inner part of the bay are predominantly diurnal, whereas at the mouth of the bay they are semi-diurnal. There is a general temperature gradient between the inner bay and the mouth, the highest temperatures being recorded in the inner bay (30.14°C). This indicates high residence times of the bay waters, presumably resulting from entrapment.Item Individual problem areas related to man's influence on coastal erosion.(1996-04) Nyandwi, NtahondiItem The Influence of Suspended Sediment Concentration on the Vertical Velocity Profiles in an Intertidal Environment(1996-01) Nyandwi, NtahondiItem Interdisciplinary Survey on the Status and Socio-Economic Impacts of Coastal Erosion along the Tanzanian Coastline and Islands(1997) Francis, Julius; Nyandwi, Ntahondi; Msuya, Flower E.Item Lake Tanganyika Holocene Record on Variability in Precipitation in the Malagarasi Catchment Basin(Springer, 2002) Muzuka, Alfred N.; Nyandwi, NtahondiThe stable isotope composition of organic carbon (OC), abundance of OC and nitrogen, and C/N ratios for core T97-69V are used to document late Holocene variability in the sources of organic matter (OM) in the Malagarasi delta (Lake Tanganyika) and in precipitation in the Malagarasi catchment basin. Core T97-69V, located at latitude 5°12.92’S and longitude 29°40.50’E, was retrieved at a water depth of 60 m. The organic δ13C values for this core ranges from −25.0λ to −19.8λ, and averages −23.0∀1.4λ.. The δ13C values decrease down-core to the base of the core with one spike of depleted δ13C values interrupting this general trend. A similar trend of down-core decrease is also observable for the contents of OC and nitrogen whose abundance averages 3.751.1% and 0.40.1% respectively. In contrast, the C/N ratios with mean value of 11.45∀2.3 do not display any clear down-core trend. Furthermore, there is a sharp shift in the stable isotope compositions of OC to lower values at about 200 cm. An enrichment in 13C in the upper 200 cm associated with higher contents of OC and nitrogen and relatively low C/N ratio values can probably be attributed to complete utilisation of available nutrients (high primary productivity) in conjunction with deposition of large proportion of C4 material derived from the catchment areas. Low isotope values of up to 4λ in the lower 200 cm of the core suggest higher input of C3 type of material derived from phytoplankton and terrestrial plants. Furthermore, a downcore decrease in 13C suggests that the level of precipitation in the Malagarasi Basin has decreased since mid-Holocene, and most likely this trend was associated with a decrease in the lake levels. Two spikes of low isotope values may suggest period of above normal precipitation that resulted into transportation of a significant quantity of terrestrial OM.Item Land Reclamation as a Cause of Fine-Grained Sediment Depletion in Backbarrier Tidal Flats(1994) Flemming, Burghard W.; Nyandwi, NtahondiGrain size distribution patterns along the East Frisian coast (German Wadden Sea) were evafuated with the aim of identifying potential effects of man-made structures (e.g. dikes) on the textural composition and sediment budget of the area. It was found that the distribution of various grain size fractions and settling velocities revealed a well-defined cross-shore energy gradient. Adjacent to the mainland dike the sediment is dominated by the 3.0–3.5 phi sand fraction (>70% by weight). In comparison to the sedimentary sequences normally observed in unobstructed backbarrier depositional environments, the sediments along the land-ward margin are in this case conspicuously depleted in grain sizes <3.5 phi, the corresponding size fractions contributing <20% by weight. This suggests that the dike interrupts the normal energy gradient, thereby inhibiting the deposition of sediments with settling velocities <0.5 cm s−1. This interpretation is confirmed when comparing individual cross-shore transects. The shorter the transect between the barrier island shore and the dike, the greater the fine sediment depletion. Since many faunal assemblages are grain-size specific, it is further suggested that the backbarrier ecosystem must have undergone significant modifications as a result of the substantial reduction in mud flats since the onset of dike construction some 1000 years BP. It is predicted that in the wake of the persisting and possibly accelerating sealevel rise this process of fine sediment depletion will continue and also affect progressively coarser sediments.Item The Major Cause of Observed Erosion Surge on the Beaches North of Dar es Salaam City(2010) Nyandwi, NtahondiSurges in coastal erosion north of Dar es Salaam city have been documented from 1977 to the early 1980s and around 1997/98. Analysis of the wind data shows that the documented increase in coastal erosion coincided with increased wind speeds. Extreme winds in excess of 10-11 m s -1 were experienced during 1976-78, 1984 and after 1996. Their coincidence with extreme high tides produced the high erosion rates. Using beach profile data for the years 1998-2000 the evolution of the beach profile with the changing monsoon winds and sediment input by rivers could be demonstrated. The beach erosion took place during the Southeast Monsoon when the alongshore sediment transport is northwards. During the Northeast Monsoon, the reversed transport is smaller to rebuild the beach. In this study incipient seaward shoreline shift was observed towards the end of the 1999 SE monsoon period and subsequent monitoring revealed a seaward shift by the end of the next NE monsoon. The shift of the shoreline was attributed to a redistribution of the large volumes of fluvial sand that was supplied during the El-Nino rains of April/May 1998. With the drop in annual wind strengths, coastal erosion has since stabilized.Item Morphodynamics of the Manyema Tidal Delta at Kunduchi, Tanzania(WIOMSA, 2013) Nyandwi, Ntahondi; Shaghude, Yohanna W.; Mburu, J. W.; Dubi, Alfonse M.; Sanga, I.; Arthurton, RussellThe prevailing northward longshore drift of beach sand on the northern part of Msasani Bay, north of Dar es Salaam, is interrupted at Kunduchi by the tidal flushing of Manyema Creek, a mangrove ecosystem partially developed for salt production. Shoreline changes around the creek mouth in recent decades have eroded coastal land and destroyed buildings, prompting stakeholders to construct protective groynes and revetments. Sand is transported across the delta by the interaction of two forcing processes – currents that flush the creek in response to tidal variation and, more generally, those generated by monsoonal wind-driven waves. A study of the sand morphology of the creek and delta platform together with time-series satellite imagery permitted demarcation of the respective sand transport pathways and morphodynamic changes in the delta over the last decade. The sand transport regime has promoted erosion of the shore to the south of the creek mouth, and has resulted in intermittent delivery of sand to beaches north of the channel. Attempts to stabilise the shore around the creek mouth are described and their effectiveness evaluated.