GEORGE NEWS - In anticipation of heavy rains expected this week, preparatory channels were made by SANParks in three estuaries in their management area.
The Swartvlei lake in Sedgefield, Touw River in Wilderness and the Groot River at Nature’s Valley are expected to rise during the rains expected to fall from Tuesday afternoon (today) to Wednesday morning.
Machinery to help fast-track the opening of the mouth (called breaching) is ready to open the estuary mouths when the water reaches the correct levels as stipulated in the Garden Route National Park’s management plan.
According to Jonathan Britton, a ranger in Wilderness, "The necessary preparatory channels are ready both for the Touw River and in Sedgefield, so should it rain and water levels rise to 2m, we can open the river mouth."
Rangers are monitoring the water levels very closely.
Periodic breaching
The method used for this is called 'periodic artificial breaching' of the estuary mouth.
"The set standards for breaching are when water levels in the estuary reach 2m above mean sea level (amsl) for Swartvlei and between 2,1 and 2,4m for the Touw River.
Artificial breaching is associated with a reduced head of water behind the sand berm at the mouth. The estuary can be highly productive for fish, birds, water plants and biological diversity when it is closed."
Circumspection
Dr Ian Russel explained SANParks’ cautious approach. “When an estuary closes, benthic microalgal populations begin to recover and build up. (These play a key role in regulating carbon and nutrient turnover and in supporting food webs in shallow water.)
"This forms an important base of the food web, providing nutrition for invertebrates that are consumed by fish and birds.
"The stable conditions of the closed phase also provide suitable conditions for the growth of submerged plants. The plants also provide foraging and refuge habitat for juvenile fish.”
He adds the estuary mouth will generally close naturally during the low rainfall periods as there is a reduction of the inflow of fresh water.
Repeated low-level breaching will almost certainly lead to gradual shallowing of the estuary from sediment accumulation.
Definitions
Estuary: An estuary is defined as a partially enclosed coastal body of water, which is either permanently or periodically open to the sea, and within which there is mixture of seawater and freshwater derived from land drainage.
Lagoon: A coastal lagoon is a shallow, coastal body of water, separated from the ocean by a barrier. This barrier can be formed by a coral reef, barrier islands, a sandbar or less frequently, rocks. The depth of a lagoon seldom exceeds a few meters.
Read a related article: Severe weather warning
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