River sediment supply, sedimentation and transport of the highly turbid sediment plume in Malindi Bay, Kenya

doi:10.1007/s11442-013-1022-x

Abstract

Abstract:

The paper presents results of a study on the sediment supply and movement of highly turbid sediment plume within Malindi Bay in the Northern region of the Kenya coast. The current velocities, tidal elevation, salinity and suspended sediment concentrations (TSSC) were measured in stations located within the bay using Aanderaa Recording Current Meter (RCM-9), Turbidity Sensor mounted on RCM-9, Divers Gauges and Aanderaa Temperature-Salinity Meter. The study established that Malindi Bay receives a high terrigenous sediment load amounting to 5.7×10⁶ ton·yr^-1. The river freshwater supply into the bay is highly variable ranging from 7 to 680 m³·s^-1. The high flows that are > 150 m³·s^-1 occurred in May during the South East Monsoon (SEM). Relatively low peak flows occurred in November during the North East Monsoon (NEM) but these were usually <70 m³·s^-1. The discharge of highly turbidity river water into the bay in April and May occurs in a period of high intensity SEM winds that generate strong north flowing current that transports the river sediment plume northward. However, during the NEM, the river supply of turbid water is relatively low occurring in a period of relatively low intensity NEM winds that result in relatively weaker south flowing current that transports the sediment plume southward. The mechanism of advection of the sediment plume north or south of the estuary is mainly thought to be due to the Ekman transport generated by the onshore monsoon winds. Limited movement of the river sediment plume southward towards Ras Vasco Da Gama during NEM has ensured that the coral reef ecosystem in the northern parts of Malindi Marine National Park has not been completely destroyed by the influx of terrigenous sediments. However, to the north there is no coral reef ecosystem. The high sediment discharge into Malindi Bay can be attributed to land use change in the Athi-Sabaki River Basin in addition to rapid population increase which has led to clearance of forests to open land for agriculture, livestock grazing and settlement. The problems of heavy siltation in the bay can be addressed by implementing effective soil conservation programmes in the Athi-Sabaki Basin. However, the soil conservation programmes in the basin are yet to succeed due to widespread poverty among the inhabitants and the complications brought about by climate change.

Key words: sediment plume, monsoon winds, longshore currents, Athi-Sabaki River, Malindi Bay, Kenya

JOHNSON U. Kitheka. River sediment supply, sedimentation and transport of the highly turbid sediment plume in Malindi Bay, Kenya[J].Journal of Geographical Sciences, 2013, 23(3): 465-489.

References

Abuodha J O Z, 1998. Geology, geomorphology, oceanography and meteorology of Malindi Bay. In: Hoorweg J ed. Dunes, Groundwater, Mangroves and Birdlife in Coastal Kenya, Coastal Ecology Series No.4, Moi University, 17-39.
Abuodha J O Z, Nyambok I O, 1991. Petrology, geochemistry and evaluation of heavy mineral deposits, coastal Malindi and Ungama Bay, Kenya. Marine Mining, 10: 337-351.
APHA, 1992. Standard methods for the examination of water and wastewater. 18th ed. Greenber A, Clescer L S, Eaton A D eds. American Public Health Association, American Water Works Association, Water Environment.
Arthurton R S, 1992. Beach erosion: Case studies on the East African coast. BORDOMER 92 Workshop, International Convention on Rational use of the Coastal Zone, Bordeaux, France. UNESCO/IOC REPORT.
Arthurton R S, Kremer H H, Odada E et al., 2002. African basins: LOICZ global change assessment and synthesis of river catchment-coastal sea interaction and human dimensions. LOICZ Reports and Studies No.25: ii + 344 pages, LOICZ, Texel, the Netherlands.
Batram J B, Balance R, 1996. Water quality monitoring: A practical guide to the design and implementation of freshwater quality studies and monitoring programmes. Published on behalf of UNESCO, WHO and UNEP by E$FN SPON, London, 383p.
Blom J, Hagen H van der, Hove E van et al., 1985. Decline of Malindi-Watamu Reef complex. Laboratory of Aquatic Ecology, Catholic University, 195p.
Brakel W H, 1984. Seasonal dynamics of suspended sediment plumes from the Tana and Sabaki rivers, Kenya: Analysis of landsat imagery. Remote Sensing Environment, 16: 165-173.
Cai Q G, 1998. Research of rill initiation condition on loess hillslopes. Journal of Sediment Research, 1: 52-59. (in Chinese)
Chant R J, Wilkin J, Zhang W et al., 2008. Dispersal of the Hudson River Plume in the New York Bight: Synthesis of observational and numerical studies during LaTTE. Oceanography, 21(4): 148-161.
Chapman, D. 1996. Water quality assessments; a guide to the use of biota, sediments and water in environmental monitoring. 2nd ed. Published on behalf of UNESCO, WHO and UNEP by E$FN SPON, London, 626p.
Delft Hydraulics, 1970. Malindi Bay Pollution II: Delft Hydraulic Laboratory, Report. R611.
Denga F O, Ongwenyi G S, Kitheka J U, 2000. Erosion and sedimentation problems in arid and semi-arid lands of Kenya. In: Gichuki F N, Mungai D N, Gachene C K et al. (eds). Land and Water Management in Kenya: Towards Sustainable Land Use--Proceedings of the Fourth National Workshop (Kikuyu, February 15-19, 1993): 61-63.
Edwards K A, 1979. Regional contrasts in the rates of soil erosion and their significance to agricultural development in Kenya. In: Lal R, Greenland D J (eds): Soil Physical Properties and Crop Production in the Tropics. London: Johns Wiley and Sons.
FAO, 1981. Offshore trawling survey in Kenya-project findings and recommendations. FAO, Rome.
Fleitmann D, Dunbar R B, McCulloch M et al., 2007. East African soil erosion recorded in a 300 year old coral colony from Kenya. Geophysical Research Letters, 34: 5.
Fong D A, Geyer W R, 2001. Response of a river plume during an upwelling favourable wind event. Journal of Geophysical Research, 106: 1067-1084.
Gohil, Pandey, 1995. Wind speed and directions extracted from the Atlas of Surface Wind Vectors in the Seas around India. ISRI, Ahmedabad.
GOK, 1994. The Kenya National Environment Action Plan (NEAP) Report. Ministry of Environment and Natural Resources, Nairobi, Kenya, 203p.
GOK, 1999. Sessional Paper No.1 of 1999 on the National Policy on Water Resources Management and Development, Ministry of Water Resources, Nairobi, Kenya, 64p.
GOK, 2002. 2002-2008 National Development Plan. Government Press, Nairobi, Kenya, 157p.
GOK, 2002. Country Strategy on Integrated Water Resources Management. Ministry of Environment and Natural Resources, Department of Water Development, Nairobi, Kenya, 51p.
GOK-TARDA, 1981a. Athi River Basin: Pre-investment Study: Preliminary Report. Agrar-UND Hydrotechnik Gmbh, Essen. Watermeyer, Legge, Piesold & Uhlmann, London Consulting Engineers, UK.
GOK-TARDA, 1981b. Athi River Basin: Pre-investment Study: Annexes 1-11. Agrar-UND Hydrotechnik Gmbh, Essen; Watermeyer, Legge, Piesold & Uhlmann, London Consulting Engineers, UK.
GOK-TARDA, 1981c. Athi River Basin: Pre-investment Study: Annexes 12-21. Agrar-UND Hydrotechnik Gmbh, Essen; Watermeyer, Legge, Piesold & Uhlmann, London Consulting Engineers, UK.
Hetland R D, 2004. Water mass structure of wind forced river plumes. Submitted to the Physical Oceanography, 31p.
Hetland R D, 2005. Relating river plume to vertical mixing. Journal of Physical Oceanography, 35: 1667-1688.
Johnson D R, Nguli M M, Kimani E J, 1982. Response to annually reversing monsoon winds at the southern boundary of the Somali Current. Deep Sea Research, 29, 104: 217-227.
Isobe A, 2005. Ballooning river-plume bulge and its stabilization by tidal currents. Journal of Physical Oceanography, 35: 2337-2351.
Kairu K K, 1997. Vulnerability of the Kenya shoreline to coastal instability. UNESCO-Kenya National Seminar on Sustainable Development, Mombasa, Kenya, 13-25.
Kitheka J U, Obiero M, Nthenge P, 2003a. Dynamics of sediment transport and exchange in the Tana Estuary in Kenya. Technical Report, KMFRI, Mombasa, Kenya, 48p.
Kitheka J U, Ongwenyi G S, Mavuti K M, 2003b. Fluxes and exchange of suspended sediments in tidal inlets draining a degraded mangrove forest in Kenya. Estuarine, Coastal Shelf Science, 56: 655-667.
KMFRI, 2002. The current status of Ungwana Bay fishery. Report of the Kenya Marine and Fisheries Research Institute, Mombasa, Kenya.
Linsley R K, Kohler M A, Paulhus J L H, 1988. Hydrology for Engineers. New York: McGraw-Hill Book Company, p492.
Lund-Hansen L C, Pejrup M, Valeur J et al., 1994. Eddy diffusion coefficients of suspended particulate matter: effects of wind energy transfer and stratification. Estuarine, Coastal and Shelf Science, 38: 559-568.
Mahongo S B, Julius F, 2010. Monthly variations in sea level at the Island of Zanzibar. Western Indian Ocean J. Mar. Science, 9: 1-16.
Maren van D S, Hoekstra P, 2005. Dispersal of suspended sediments in the turbid and highly stratified Red River plume. Continental Shelf Science, 25: 503-519.
McClanahan T R, Obura D, 1997. Sedimentation effects on shallow coral communities in Kenya. J. Exp. Mar. Biol. Ecol., 209: 103-122.
McClanaham T R, Young T P, 1996. East African Ecosystems and their Conservation. New York: Oxford University Press, 100p.
Munyao T M, 2001. Dispersion of the Sabaki Sediments in the Indian Ocean
[D]. Kenya: School of Environmental Studies, Moi University.
Mwebesa M, 1978. Effects of the ITCZ on East Africa. In: Basic Meteorology. East African Literature Bureau, Nairobi, 144-155.
Ndung’u J, 2009. Mapping of total suspended sediments using Meteosat second generation
[D]. International Institute for Geo-Information Science and Earth Observation, the Netherlands, 75p.
Partners in Science, 1991. A Kenyan-The Netherlands Co-operative Research Programme in Marine Science off the Kenya Coast-1990-1995. Hague, the Netherlands, 74p.
Obura D O, 1995. Environmental stress and life history strategies: A case study of corals and river sediment from Malindi, Kenya
[D]. University of Miami, Florida, USA, 1-60
Ojany F F, Ogendo R B, 1986. Kenya: A Study in Physical and Human Geography. Nairobi, Kenya: Longman, 228.
Ongwenyi G S, 1983. Development of water resources. In: Ojany F F, Olson S (eds). The Kenyan Geographer--Special Issue on the Proceedings of the Nairobi Workshop (4-8th August 1981)--Strategies for Developing the Resources of the Semi-arid Areas of Kenya. 36-47.
Ruwa R K, Muthiga N, Esposito M et al., 2001. Report of the Scientific Information and Conservation Sub-Committee-Prawn Fishery in Kenya. Marine Waters. Prawn Trawling Task Force, 8p.
Schoellhamer D H, 1995. Sediment resuspension mechanisms in Old Tampa Bay, Florida. Estuarine, Coastal and Shelf Science, 40: 603-620.
Soi C N, 2010. Spatial-temporal analysis of shoreline movement: Causes and effects of development at River Sabaki Estuary from 1973 to 2000 in Kenya
[D]. Moi University, Kenya, 75p.
Thompson A O, 1956. Geology of Malindi area. Geological Survey. Report No.36.
Tole M, 2001. Budgets for Malindi Bay. LOICZ Biogeo-Aug. 24. LOICZ Report.
Trenberth K E, Jones P D, Ambeje P, 2007. Observations: Surface and atmospheric climate change. In: Solomon S, Quin D, Manning M et al. (eds). Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the IPCC AR4. Cambridge: Cambridge University Press, 235-336.
UNEP, 1998. East Africa Atlas of Coastal Resources-Kenya. UNEP, Nairobi, Kenya, 119.
van Katwijk M M, Meier N F, van Loon R et al., 1993. Sabaki River sediment load and coral stress: Correlation between sediments and condition of the Malindi-Watamu reefs in Kenya (Indian-Ocean). Mar. Biol., 117: 675-683.
Whitney M M, Garvine R W, 2005. Wind influences on a coastal buoyant outflow. Journal of Geophysical Research, 110(C0314), doi: 10.1029/2003JC002261.
Wolanski E, Gibbs R J, Spagnol S et al., 1998a. Inorganic sediment budget in the mangrove-fringed Fly River Delta, Papua New Guinea. Mangroves and Salt Marshes, 2: 85-98.
Wolanski E, Nhan N H, Spagnol S, 1998b. Sediment dynamics during low flow conditions in the Mekong River estuary, Vietnam. Journal of Coastal Research, 14(2): 472-482.
Wolanski E, Mazda Y, Furukawa K et al., 2001. Water-circulation in mangroves and its implications for biodiversity. In: Wolanski E. Oceanographic Processes of Coral Reefs: Physical and Biological Links in the Great Barrier Reefs. New York: CRC Press, 356p.