On first sighting this long, ground-hugging aqueduct (and a few others like it) the need for such a structure is the first question that comes to mind. Since taking this picture and assuming the fluming was required to overcome ground conditions, I have confirmed this.
The land is owned by Paul and Linda Borrie and a search of Papers Past, a brilliant and very useful scanned and OCR processed newspaper image database tool available to researchers in New Zealand, has revealed that this was indeed the situation.
The Borough Race runs for a significant part of its 49km length, along the north-facing face of the hills which form the upland separation of the Waitaki River and Waiareka/Kakanui River catchments. These hills are mainly formed of weather rounded and wind distributed glacial Loess resting on and completely covering a spine of discontinuous basalt cills. The basalt is roughly 10 metres below the surface and was encountered in all the tunnels further downstream from this point.
The Loess is loosely consolidated and full of under runners - the local parlance for erosion formed caves and tunnels, often metres below the pasture surface, where rain accumulated as run-off in small streams and rills has carved down into the Loess and then escaped along roughly horizontal layers where the material is a bit denser. They are often filled with loosely packed vegetative rubbish and wind blown dust and are most often invisible at the surface. They are a serious hazard for pastoral farmer tilling the hillsides as they can collapse without warning engulfing tractors and stock and the farmers themselves. At this time and after extensive cultivation and stocking of the hillsides along with a shift in weather patterns to a dryer climate than might have existed a few thousand years ago when they were formed, most have filled and are stable and they rarely cause trouble, but in 1880 when the hillside was virgin and had not had machinery or animals on it, they would have been frequently unearthed by the navigators and labourers digging the race.
Even if they were not visibly intersected by the race channel, some may have been present just centimetres below the formed bottom surface of the race.
Such was the case at this location.
In December 1884 just four years after the race was commissioned, the Engineer had to report to the Borough Council that a serous break in service (5 days without water) had been occasioned by two "blow outs" in the race (this is a modern term coined in the 1900s) which had taken some time to repair and which were threatening to leak subside and flood the downstream pastures again. Silt and huge volumes of water pouring across adjacent land were a recurrent nightmare for the Council as the race leaked catastrophically in a number of places over its whole lifespan. A large and destructive blow-out on the land of Collie Hurst in the early 1980s, was actually the death knell for the Borough Race, prompting the (then) District Council to seriously consider abandoning the race and establishing a new alternative water supply for Oamaru, which they did.
The Engineer reported to The Council that the condition of the ground "on the Borrie property" over a 50 yard length of the race was so problematical, even after the two time consuming repairs, that he had set in motion the construction and placement of ground level box fluming (patterned no doubt on the overhead fluming of the aqueducts but less substantially built) "the structure to be set up resting on the bottom of the race" and thus isolating the water from further contact with the soil.
The extant substantial flume of concrete piers and half-round steel ducting will be the 1940s replacement for that "repair" It is a bit longer than 50 metres, no doubt to totally eliminate all risk of the race being in dodgy ground.
Without examining council archives (which I have not yet started to do as of April 2012) it cannot be asserted that this relatively modern structure directly replaced McLeod's 1884 repair - as there may have been other fixes set up in between those dates.
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Photo taken in Peebles, New Zealand
Misplaced? Suggest new location