Ten Principles to Determine Environmental Flow Requirements for Temporarily Closed Estuaries

Abstract

<jats:p> Temporarily closed estuaries require seasonal opening to tidal flows to maintain normal ecological processes. Each estuary has specific environmental flow (EFlow) requirements based on the relationship between freshwater inflow, coastal dynamics, rate of sandbar formation, and the open/closed state of the mouth. Key abiotic processes and ecosystem services linked to mouth state were highlighted. We reviewed completed EFlow requirement studies for temporarily closed estuaries in South Africa and found that the formulation of these requirements should consider the timing and magnitude of flows in relation to the morphology of an estuary, its mouth structure, catchment size, and climate. We identified ten key principles that could be adapted to similar systems in equivalent climatic settings. Principle 1 recognizes that each estuary is unique in terms of its EFlow requirements because size, scale, and sensitivity of core elements to freshwater inflow are specific for each system; EFlows cannot be extrapolated from one estuary to another. Principle 2 highlights the importance of baseflows in keeping an estuary mouth open because a small reduction in flow can cause the mouth to close and alter essential ecological processes. Principle 3 outlines the role of floods in resetting natural processes by flushing out large volumes of sediment and establishing the equilibrium between erosion and sedimentation. Principle 4 emphasizes the need for open mouth conditions to allow regular tidal flushing that maintains water quality through reducing retention times and preventing the onset of eutrophic conditions. Principle 5 advises artificial breaching to be practiced with caution because execution at low water levels encourages sedimentation that reduces the scouring effect of flushing. Principle 6 holds that elevated inflow volumes from wastewater treatment works or agricultural return flows can increase the frequency of mouth opening and cause ecological instability. Principle 7 states that water released from dams to supply the environmental flow cannot mimic the natural flow regime. Principle 8 specifies the need for short- and long-term data to increase the confidence levels of EFlow assessments, with data to be collected during the open and closed mouth states. Principle 9 advocates the implementation of a monitoring program to track the achievement of EFlow objectives as part of a strategic adaptive management cycle. Finally, Principle 10 recommends the adoption of a holistic catchment-to-coast management approach underpinned by collaboration with regulatory authorities and stakeholders across a range of sectors. These principles can be used to guide the formulation and management of EFlows, an essential strategy that links the maintenance of estuarine ecological integrity with social well-being.</jats:p>

Environmental Flows in the Lower Ebro River and Delta: Current Status and Guidelines for a Holistic Approach

“…Deltas are a particular type of estuarine system in which the dependence on river flow (water, sediments, nutrients) is very strong, especially in river-dominated deltas such as the Mediterranean ones [16,17], but e-flow proposals for deltaic systems are scarce [18][19][20][21][22]. E-flow requirements of estuaries have been ignored in the past, mostly because of the lack of long-term monitoring data or understanding of the responses to changes in freshwater inflow [23]. Three main countries have developed methods for estuaries (i.e., Australia, South Africa and USA), from practical applications and a learning-by-doing approach [23][24][25][26][27][28], but some efforts have also been made in other countries like China [22,29].…”

“…E-flow requirements of estuaries have been ignored in the past, mostly because of the lack of long-term monitoring data or understanding of the responses to changes in freshwater inflow [23]. Three main countries have developed methods for estuaries (i.e., Australia, South Africa and USA), from practical applications and a learning-by-doing approach [23][24][25][26][27][28], but some efforts have also been made in other countries like China [22,29]. Recent methods take a holistic and adaptive standpoint and are presented as frameworks that include a number of steps and have elements of risk assessment and adaptive management [23].…”

Advancing the Science of Environmental Flow Management for Protection of Temporarily Closed Estuaries and Coastal Lagoons

“…River inflow is a major factor that maintains open mouth conditions in wave-dominated TCEs [15,18]. It is often the only driving force that maintains the mouth in the open state as tidal flows are limited as a result of constricted mouths and small surface area.…”

“…Shallow, wave dominated estuaries are the least studied and arguably the most vulnerable to global change because of their relatively small volumes, high water retention times, perceived low social value and lack of scientific understanding. For this type of estuary, the flow regime needed to maintain an open mouth is strongly dependent on sediment availability and wave conditions [15]. For example, baseflow requirements would be significantly higher for estuaries situated along high-energy coastlines, such as those found along the South Africa, Australia, California (USA) coast.…”

“…The health of estuaries is fundamentally associated with freshwater inflow, which includes the magnitude of flow, as well as the timing, frequency and duration of flow events. These dynamic aspects of flow regime play an important role in mouth closure [31], with low flows being associated with closed mouth state and high flows with open mouth state [15]. Flow alteration in the surrounding watershed from, e.g., dams, channelization, urbanization and abstraction, can have a substantial impact on estuarine mouth dynamics, which in turn can influence biotic processes and ecosystem functions and services (Table 1).…”