The Klein River Estuary mouth opened in the early hours of the morning of Tuesday 14th August near the Grotto Beach car park. The water level was at 2.78m above mean sea level at 18:30 the previous night, the same level at which the artificial breach was conducted last year. The week before it was at 1.8m above mean sea level, so it rose rapidly with the previous weekend’s heavy rains.The breach was reported at 08:00 in the morning. By midday the mouth had opened considerably.
The nature of the opening is quite different from the artificial breach that was performed in 2011 and provides an opportunity to review the merits and demerits of the two. Last year’s artificial breach was well monitored and documented, and similar work has been planned following the opening that has just occurred.
Sue Matthews, Overstrand Estuaries
Management Coordinator, will conduct
a full physical-chemical monitoring
run along the length of the estuary,
replicating the monitoring conducted
last year. This is made possible thanks to an instrument borrowed from Department of Agriculture, Forests and Fisheries (DAFF) in Cape Town and the assistance of volunteer boat-owners. Another run is planned for a month later.
Nutrient samples will be collected at the same time, according to the monitoring plan drawn up with specialist input last year. However, since the Dept. Water Affairs (DWA) has not yet completed their procurement process to appoint a laboratory to analyse the nutrient samples, these will be frozen, as for the previous two sampling runs.
Chlorophyll sampling and analysis has been done by Sue thanks to funding of the purchase of filters by the Hermanus Lagoon Property Owners’ Association and to DAFF permitting Sue to use their laboratory facilities; this too is planned to continue.Data from the DWA water level gauge will be reviewed to determine the rate at which the estuary drains, and comparison made with that of the 2011 breach. A photographic record of the mouth area is being kept.
It will take several months of close observation and data collection before a clear picture emerges on the full consequences of this year’s mouth opening. Early tests show that the salinity levels in the main body of the lagoon are still low and the turbidity remains very high, indicating poor mixing of fresh and sea water at this stage. A review meeting with specialist input is planned for March 2013, by which time the mouth will have closed and we will have seen how the estuary responds to the summer season.
Sue has also investigated options for a bathymetric survey of the Klein River Estuary. It is proposed this should be done only after the mouth closes and the water level has risen, so that both the channels and shallows can be charted, and so that there is no need to compensate for tidal influence, which requires the hiring of expensive equipment.