The Physical Aspects of the Mpenjati Estuary System

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A study on the physical aspects of the Mpenjati estuary- beach system.

Roxanne Munsamy
University Of KwaZulu-Natal

Abstract
A study was conducted at the Mpenjati estuary on the 4th of August 2012 to determine the physical aspects within the estuary beach system. This paper gives an overview of the factors affecting the slope of the estuary, sediment sizes and the tidal prisms. The beach slope was measured using the Emery board method and it was found to be characteristic of the KwaZulu-Natal coastline. Wave energy and sediment transport rates along the beach morphology are interlinked, high energy storms tend to erode and flatten beaches whilst lower energy waves tend to be responsible for the reverse process- rebuilding breaches. A flow duration curve was produced and it was established that the Mpenjati estuary mouth is opened with the flow duration of 0.26 m3/s. Sewage works found to play a greater influence on flow rate as opposed to abstractions for irrigation. The estuary was open for sixty five percent of the time between the periods 2000 to 2006.

Introduction
A general understanding of an estuary is a region through which a river discharges into a sea and in a Southern African context the following is a widely accepted and concise definition of an estuary “It is a partially enclosed coastal body of water in which is either permanently or periodically open to the sea and within which there is a reasonable variation of salinity due to the mixture of sea water and fresh water derived from land drainage” (Day, 1980). Hence estuaries are the transition zones from land to sea, and fresh water to salt water. Although estuaries are influenced by the tides, they are protected from the full force of ocean waves, wind, and storms by landforms such as barrier islands (water.epa.gov). Interactions between salt water and freshwater forms the basis of estuary hydrodynamics, together with the movement of sediment, the effects of wind, waves, anthropogenic inputs and biotic processes. Estuaries are divided into four different types depending on how they are formed. The four types are the coastal plain estuaries that is formed by rising sea levels that fill a already existing river valley, tectonic estuaries that is formed by folding or faulting of the land, bar built estuaries are formed when a shallow lagoon or bay is protected or covered by a sand bar or barrier land and fjords that are formed in a u shaped valley formed by glacial action (Garrison, 1995, Essentials of Oceanography). The sensitivity of an estuary mouth to closure can roughly be correlated to the river inflow, particularly during low flow periods, required to keep the mouth open. For many estuaries, especially the smaller ones such as the Mpenjati estuary, the most important factor in keeping the mouth open is river flow, and in particularly base flows (Department of Water Affairs and Forestry, 2008). Catchment sediment yields are said to have important affects on estuaries and since some estuaries are closed for significant amounts of time, during the closed periods they hence act as repositions for the net accumulation of sediment carried down by inflowing rivers (Perissinotto et al, 2010). The mean grain size on a beach influences the generalized beach slope, coarser sand is associated with steeper beach slopes whilst flatter beaches have slopes that has mainly finer grains of sand ( Bascom, 1959).The largest sand particles are found at the plunge point which is just seaward of the back rush and which ultimately is the maximum point of turbulence, the finest sand grains are found predominantly along the sand dunes ( Bascom, 1959).Tidal amplitude varies from ~1.0 m to at a spring tide to about 0.3m at neap tide conditions and in most smaller estuaries the tides are normally described as truncated due to the constriction of the outflow from channel depth. Residence time is normally only a few days to weeks under this particular condition. However...
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