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Baseline specialist study on the freshwater requirements of the Mkurumudzi Estuary, Kenya
Bornman, T. G.; Jennings, M. (2006). Baseline specialist study on the freshwater requirements of the Mkurumudzi Estuary, Kenya. [S.n.]: [s.l.]. 22 pp.

Available in Authors 
    VLIZ: Non-open access 245675

Keywords
Author keywords
    freshwater requirements

Authors  Top 
  • Bornman, T. G.
  • Jennings, M.

Abstract
    Tiomin Kenya Limited intends to use a dry mining method for the excavation of heavy minerals from the Kwale deposit, located in the southern coastal region of Kenya. Large volumes of water will be required to separate the heavy minerals from the raw material and it is expected that the gross demand could be as high as 2800 m3.hr-1 (CES 2006). Numerous water sources have been identified and several of the alternatives involve freshwater abstraction (through existing and new dams) from the Mkurumudzi River. The Mkurumudzi River is the most important surface water source in the vicinity of the mining area. This perennial river arises in the Shimba Hills and flows from the north-west to the southeast and drains into the Indian Ocean in Gazi Bay. River flow is governed by the seasonal rainfall. The Gazi area has a similar climate to Mombasa that received an average annual rainfall of 1137.47 mm for the period 1890 to 2001 (Dahdouh-Guebas et al. 2004). The rainfall along the Kenyan coast shows a bimodal rainfall pattern that is termed the long rains (April - July) and the short rains (October – November). Droughts and floods are common features of the Kenyan climate. The risk of long dry spells is relatively high and prolonged droughts (with an average monthly rainfall of 70 mm extending for 15 to 24 months) have an expected recurrence interval of five years (CES 2006). Significant flood events occur on a similar frequency to droughts. The base flow of the Mukurumudzi River is primarily maintained by the discharge of numerous perennial springs. The construction of any dam with the purpose of freshwater abstraction will reduce the freshwater flow downstream and alter the sediment balance of an estuary (Taylor et al. 2003). A major threat of reduced river inflow is the risk of reducing natural variability in driving components (i.e. hydrodynamics, sediment dynamics and biogeochemistry) which, ultimately, play an important role in determining the biodiversity and processes operating in an estuary. However, the crucial decision that needs to be made in terms of Estuarine Freshwater Requirements (EFRs) is to predict at what river flows, or ranges of river inflows, these changes start to have significant effects on the biophysical environment of the estuary. The main purpose for estimating EFRs is to provide decision-makers with a means of quantifying the water quantity (and quality) requirements of the biophysical environment of an estuary. It is important to understand at the outset that any reduction in river inflow to an estuary will result in change, albeit very small. However, the underlying goal of any EFR is to prevent measurable adverse effects on the ecosystem and, where a system is already in a degraded state, to recommend measures to improve the future management condition of the estuary. The purpose of this study was to determine the freshwater requirements of the Mkurumudzi Estuary using available literature, flow data and limited fieldwork, i.e. a desktop assessment. Although Gazi Bay has been intensively studied, very little research has been done on the Mkurumudzi Estuary that drains into the Bay. Therefore it was necessary to study the spatial distribution of the mangroves and the water quality of the estuary in greater detail.

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