Ten Principles to Determine Environmental Flow Requirements for Temporarily Closed Estuaries

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 eect 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.