Home
Forgot your password?
Theft of metals hampers monitoring at Jonkershoek
|
Excitement over finally sorting out the last technical gremlins in the new data transmission system for Jonkershoek’s water level data has been short lived. Data transmission is after all only useful if the data is accurate.
High prices of copper and brass seem to have led to the Jonkershoek monitoring site joining an international trend of increased theft of items made of these metals. Monitoring equipment at Jonkershoek has been targeted in the same way as copper pipes, electric cables, letter box covers and fire hydrants have been.
Compromising the structural integrity of the weirs
Where in the past copper rain gauges in Jonkershoek had been the main target, the last couple of weeks saw the focus shift to weirs. Here people have been removing brass, presumably for re-sale to scrap metal buyers. While theft and vandalism is not new at Jonkershoek sites, what makes it particularly problematic this time around is the fact that the structural integrity of the weirs has been compromised.
V-notch gauging weirs such as the ones at Jonkershoek are constructed from concrete, steel and brass. The most important section of the weir, where the water flows over, consists of a steel plate lined by a brass edge. The brass edge is machined at a 45 degree angle that forms the top edge of the V-notch. This ensures the smooth passage of the water over the lip of the structure and hence a very accurate reading of water level.
Accuracy issues
Removing this part of the weir structure changes the weir profile and leaves bolt holes for water to flow through. This means that data collected afterwards cannot be compared with previous data. It also means that discharge tables created recently during a precise survey of the weir profiles by the Department of Water Affairs do not apply anymore and will need to be re-done after the weirs have been repaired. Typically these discharge tables allow water levels measured in the V-notch to be translated accurately to flow rate.
Why build outdoor structures out of valuable metals that are impossible to secure, one may ask? The answer to this is that these structures, and the rain gauges that measure input in the surrounding catchments, are from an era when these metals were both cheap and considered the appropriate corrosion resistant materials to use in construction. They have lasted for decades, despite being located in one of the highest rainfall areas of South Africa, and in contact with acidic stream water. The onus is now on the Fynbos Node to replace these parts with something equally long lasting that has virtually no re-sale value.
Putting a time to the crime
The latest round of thefts was spotted almost immediately because our new water level recorders send data to the office twice a day, allowing us to respond more quickly to anomalies in water level data. It has been very useful in this case because data downloaded on a Tuesday morning documents changes in water level that occurred the previous afternoon. This data showed that the water level dropped after 13h00 on the previous afternoon at two weirs, shortly after I had last been to the weirs and left them in an intact state.
We were immediately able to confirm that the drop in water level was due to the loss of the brass on the weir structure. Putting a time to the crime may narrow down the list of likely suspects and allow for additional evidence to be collected while it is fresh. This has allowed us to submit a detailed statement to the police in Stellenbosch.
|
