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A Review of Information on Temporarily Open/Closed Estuaries

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A Review of Information on Temporarily Open/Closed Estuaries A Review of Information on Temporarily Open/Closed Estuaries in the Warm and Cool Temperate Biogeographic Regions of South Africa, with Particular Emphasis on the Influence of River Flow on These Systems Edited by Alan Whitfield1 & Guy Bate2 1South African Institute for Aquatic Biodiversity 2Nelson Mandela Metropolitan University and University of KwaZulu-Natal Interim report to the Water Research Commission on the Project “The freshwater requirements of intermittently open Cape estuaries” WRC Report No 1581/1/07 ISBN 978-1-77005-518-6 Set No 978-1-77005-517-9 February 2007 DISCLAIMER This report has been reviewed by the Water Research Commission (WRC) and approved for publication. Approval does not signify that the contents necessarily reflect the views and policies of the WRC, nor does mention of trade names or commercial products constitute endorsement or recommendation for use ii EXECUTIVE SUMMARY Along the Eastern Cape coast there are more than 80 temporarily open/closed estuaries (TOCEs) that fall within the warm temperate biogeographical region and 5 in the cool temperate region of the Western Cape. TOCEs are predominantly regulated by the amount of river inflow received, the magnitude of which is governed primarily by catchment size and the regional climate that dictates rainfall patterns. There is very little published information on the hydrodynamics of TOCEs, hence the approach followed has been to focus on the forces that govern hydrodynamic processes and mouth conditions in estuaries and develop a conceptual model of possible mouth states that can play a role in the control of mouth state. This overview focuses on the similarities and differences between the structure and functioning of TOCEs and permanently open estuaries (POEs) in these regions and the similarities and differences between Cape and KwaZulu-Natal systems. It also aims to inform the Resource Directed Measures (ecological water requirement) process of DWAF, emphasising the importance that needs to be placed on the consequences of reduced river flow/increased closed mouth phases on Cape TOCEs. Variations in climate, topography and catchment geology give rise to a wide variety of estuarine types, but from a hydrodynamic perspective, all estuaries in South Africa fall into one of two main categories, namely POEs or TOCEs. POEs normally have large catchments and a relatively high runoff throughout the year. Their estuary volume is normally large enough for tidal flow to play a significant role in maintaining open mouth conditions. An examination of the data for all South African estuaries indicates a good relationship between mean annual run-off (MAR) and whether an estuary is a POE or a TOCE, and that a model for indicating this difference is a function of estuary water area x MAR x effective tidal amplitude. TOCEs are isolated by the formation of a sand berm across the estuary mouth during periods of low or no river inflow. They stay closed until their basins fill up and the berm is breached. Mouth breaching results in the removal of significant amounts of sediment, but infilling by sediment commences once tidal conditions are established. Hence the major forces that maintain open mouth conditions are river and tidal flow and the major closing forces are marine wave energy and sediment availability. In small and medium size estuaries (<150 ha) tidal flow can maintain open mouth conditions during spring tides but often closure occurs during neap tides. Thus, during periods of low river flow in small to medium sized estuaries, the tidal prism is insufficient to maintain an open mouth. The semi-closed mouth state normally only occurs in small estuaries (<100 ha) with a very limited marine sand supply. Factors prolonging an open mouth include features that dissipate wave energy and turbulence, which reduce the ability of waves to carry sediment into the mouth. Factors that contribute to iii mouth closure include structures or conditions that inhibit river or tidal flow. Strong tidal outflow and the protection of the mouth from wave action by a rocky promontory, if present, play a major roll in maintaining open mouth conditions. Structures that inhibit tidal exchange (e.g. a rocky sill) reduce the scouring ability of the tides and can shorten the time that the mouth remains open. Each estuary is unique because of the various factors that influence its structure and sensitivity to flow, and two similar sized estuaries adjacent to one another can be quite different. At any one time, the mouth of an estuary is either open or closed. However, when closed, some marine ‘overwash’ may occur during high wave or storm events. There may also be a semi-closed state with only a shallow ‘trickle’ of water flowing out to sea. This is termed ‘overtopping’. Very little reliable information is available on the hydrodynamics of TOCEs. Even less is known about systems that stay closed for prolonged periods. Hence, to develop an in-depth understanding of the long-term cycles and hydrodynamic processes within TOCEs, data urgently need to be collected on the mouth status of South Africa’s smaller estuaries. Most of the studies evaluated in this report indicated that long-term information on estuary mouth status is very limited and that runoff data from small river catchments are of very low confidence. There is therefore an urgent need to improve the quality of the runoff data provided to estuarine scientists. Two different mechanisms have been proposed for the inlet closure (by sediment) of small estuaries on micro-tidal, wave-dominated coasts with strong seasonal variations in river discharge. The first is the interaction between inlet currents and longshore sediment transport, while the second is the interaction between inlet currents and onshore sediment transport. Both sediment transport mechanisms are likely to be applicable to South African TOCEs. Mouth closure often occurs during sea storms when high sediment loads are entrained by turbulent storm wave action and carried into the estuary and mouth area where it is deposited in the lower energy environment. When this deposition rate exceeds the erosion potential of tidal flow, a net sediment build-up occurs. If this situation continues for long enough, the mouth closes. Within the coastal zone, there are a number of processes that can transport varying amounts of marine sediments to estuary mouths. Marine sediments that have been transported close to the mouth by such processes are potentially available to be transported into the estuary itself, mainly by means of tidal flow through the mouth but sometimes in conjunction with wave action. There are also a few processes that can transport marine sediment directly into the estuary. Actual net (usually north-easterly) longshore sediment transport of about 400 000 m3 to 1 200 000 m3 per annum (on average) is estimated along most of the exposed, open South African coast, while the potential transport (due to wave energy) is sometimes even higher. Along rocky shorelines, or sheltered areas, the net average longshore transport rate is mostly between about 10 000 m3 and 400 000 m3 per annum. Extreme cross-shore transport rates are iv estimated to be as high as 150 000 m3 over short periods during very large sea storms and a shoreline length of 500 m. River floods are the most important natural means of eroding and transporting sediments out of estuaries. Large volumes of sediments can be removed in a very short time during major floods with a return period of 1 in 50 years and more. Smaller floods with return periods of 1-2 years can sometimes also have a significant influence, particularly in the estuary mouth region. Floods therefore play a major role in the equilibrium between sedimentation and erosion in estuaries Depending on factors such as estuary size, beach profile and mouth protection, three different estuary states can occur. These are (a) where the mouth is open to the sea allowing seawater intrusion during flood tides, with river inflow continuing to introduce freshwater into the system under all tidal regimes; (b) where the mouth is semi-closed and the berm prevents seawater intrusion except during high tides; and (c) where the mouth is closed and the height of the berm prevents seawater from entering the system, as well as estuary water draining into the sea. Low volumes of river water could still be entering the estuary and sporadic overwash of seawater can occur, depending on wave conditions at sea and the berm height. When the mouth of a TOCE is open, a longitudinal salinity gradient normally exists and the position of the haloclines depends on the tidal regime and extent of river inflow. In some systems vertical stratification can also develop, usually in systems where the middle and upper reaches are characterised by deep water (> 3 m). The semi-closed state is typical during periods of low rather than high fresh water input when the mouth will be fully open. If inflowing fresh water is restricted to the surface of the estuary, then more saline water will be ‘trapped’ in deep areas, thus leading to strong vertical salinity stratification. Entrainment of freshwater into deeper layers and wind mixing causes the water column to gradually change into a homogenous brackish water body. During the closed mouth state, salinity levels throughout the water column are usually similar, although some vertical and longitudinal stratification may be evident immediately after closure. A decrease in dissolved oxygen is mainly caused by decomposing organic material, which has a high biological oxygen demand. Low dissolved oxygen levels can cause organisms to become stressed or die, with oxygen concentrations less than 3 mg l-1 regarded as hypoxic. During the open state, TOCEs normally have > 6 mg l-1 oxygen but during the semi-closed state, stratified bottom water may have a concentration of < 3 mg l-1.
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