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Jonkershoek weirs restored


Fynbos Node Technical Intern, Siyasanga Mpehle, hard at work preparing a weir for painting. (Picture: Abri de Buys)


Lambrechtbos B weir freshly repaired and painted. (Picture: Abri de Buys)

By Abri de Buys, Field Technician and Siyasanga Mpehle, Technical Intern, SAEON Fynbos Node

In June 2012, SAEON eNews conveyed the bad news that Jonkershoek’s weirs had been vandalised by brass thieves looking to take advantage of the rising prices valuable metals fetch at scrap metal buyers.

The changed weir profile resulting from the damage caused concerns over data quality and comparability in the post-damage period.

The timing of the theft was unfortunately shortly before the start of the rainy season in one of South Africa’s wettest catchments. This meant that the time taken to manufacture and purchase replacement parts would result in us being ready to do the repairs when the rains had already started and stream flow was at its highest.

The new sets of edges were manufactured out of galvanized steel instead of brass.

For this reason, we decided to postpone the repairs until drier conditions could guarantee repair work of a high standard.

Data integrity

The rainy season was thus spent keeping a close watch on incoming post-damage data, to try and determine exactly what effect the changed weir profiles had and how data should be corrected.

Two types of data are routinely collected to ensure that our electronic loggers are correctly calibrated to the actual water level in the weir. One is a reference reading against which our electronic data loggers are calibrated. The other is a reading from the electronic loggers. Deviations can occur as a result of any disturbance to the loggers and it is therefore important to check them against a reference at regular intervals. Loggers can be reset to the reference if they are found to deviate beyond acceptable limits.

Damage to the weir structure where reference readings are taken makes accurate calibration impossible. Because of this, a gap opens up in the data between the water level read by the electronic loggers and the manually measured reference. This gap was monitored over time to see if any deviations occurred that could indicate disturbance to the loggers. The data revealed that after the initial change in water level precipitated by the damage, the gap remained sufficiently consistent in extent to allow us to assign a correction factor with confidence.

Raw data will be archived with the caveat that these conditions existed for specified periods of time. The routine reference data for this period could then be used by future users of the water level dataset to correct stream flow during the period that the weirs were damaged.


Figure 1. Comparison of reference (staff gauge) and logger (Orpheus Mini OM Instant) data during routine checks since September 2011 at a Jonkershoek weir. (Click to enlarge)

Weir repairs

While keeping an eye on data quality, a set of weir edge templates was prepared for each damaged weir. The new sets of edges were manufactured out of galvanized steel (instead of brass) this time. These were ready for installation in early November when the first extended period of drier weather presented itself.

Weir ponds were drained, V-notch plate surfaces prepared, new edges aligned and bolted into place and all given a fresh coat of rust-preventing paint. Paint was left to dry properly for a few days before outlets were closed and weirs allowed to resume functioning.

Method continuity

The newly installed edges are quite close to being exact replicas of their predecessors. It will, however, only be after the weir profiles have been re-surveyed and compared to pre-damage profiles that we can be sure we are still measuring water level to the same specifications as before.

Theft is unfortunately one of the uncontrolled variables that affect long-term monitoring and one that will have to be factored into the development of observation platforms. The SAEON Fynbos Node now has access to a set of computer-aided design (CAD) drawings of the weir edges that can be used to repeat this process if needed in future.

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