To breach or not to breach

By Sue Matthews, Overstrand Estuary Management Coordinator 

The Bot River Estuary Forum (BREF) convened at Middlevlei Estate for its quarterly meeting on the 6 October. Among the issues under discussion was the mouth management policy, governing artificial breaching of the estuary. 

Over the years, water runoff into the Bot system has been reduced by at least 25% from the norm due to alien infestation in the catchment and water abstraction for agricultural and domestic use. This means that there is insufficient freshwater flow to prevent the mouth from being closed by marine sediments. Artificial breaching aims to restore the estuary’s connection to the sea, which helps maintain its ecological functioning and its value as a nursery area for fish. 

However, breaching at low levels results in inadequate scouring of the estuary, causing sediment build-up and mouth closure shortly after breaching. Organic material in the sediments is also not flushed out, negatively impacting water quality and benthic habitat. 

The mouth management policy was debated at the Bot Indaba held in April last year under the auspices of the old Bot River Estuary Advisory Committee (BREAC). The Indaba was attended by a number of current BREF members, as well as invited specialists, and the revised criteria for breaching, approved by consensus, were as follows:

These criteria have been adopted by BREF and will form part of a mouth management document that must be submitted to the provincial environmental authorities. In terms of the Environmental Impact Assessment (EIA) Regulations, breaching is not permitted   without an environmental authorisation - unless it is being done “for maintenance purposes undertaken in accordance with a management plan agreed to by the relevant environmental authority”. Application for special dispensation to implement the mouth management policy for a period of five years will therefore be made on the basis of ecosystem maintenance. 

Since salinity and water level are key components of the breaching criteria, Cape Nature conducts monthly monitoring of a number of sites along the Bot estuary and through the Lamloch swamps to Kleinmond. Terrence Coller presented the 2010 results at the meeting, which clearly show that the main body of the Bot estuary is a lot more saline than the Kleinmond arm of the system.

Given the criteria, the mouth of the Bot estuary has not qualified for artificial breaching this year, and is unlikely to open naturally without a major unseasonal rainfall event.

Sue is contracted to the Overstrand Municipality in a position funded by The Table Mountain Fund, an associated trust of WWF-SA.