Mount Roland

CONSTRUCTING A RIFFLE

"The Telstra riffle" was our first major river construction. Built to solve a specific problem, this riffle in fact solved some other issues as well. The situation was that a major Telstra cable, originally submerged under the Dasher River, was hanging several feet in the air due to prolonged erosion of the river banks and bed. This left the cable very vulnerable to damage from debris carried by floodwaters.

After discussing the problem with DPIW's river engineers, our co-ordinators suggested to Telstra that a solidly constructed riffle would both protect their cable and prevent further erosion at the site. Telstra agreed, and also supplied funding for the construction. DPIW provided plans, and construction began.

This is a section of the Dasher river before the construction of the riffle.

This is where we learned that a substantial riffle requires a lot of rock - more than the engineers had estimated (roughly twice as much). The picture below shows rock being delivered to site.

Large boulders were strategically placed to form the basis of the riffle across the whole width of the riverbed. Some of the boulders were very big indeed!

Below is the completed riffle, viewed from upstream during summer at low water levels. The calm area above the riffle is a deep pool and a favourite spot for fish and the riffle contains passages allowing fish to move freely up and down the rivers.

The picture below was taken from the vantage point of the bridge on the downstream side of the riffle and shows the riffle flooded after heavy rain in winter. The rocks downstream to either side of the riffle prevent erosion of the banks.

We also found, after the first year's floods had passed, just how far riffles need to be embedded into the banks. Generally speaking, the riffle should be cut into the bank on each side of the river far enough to reach solid ground - preferably clay. In some cases, this means we have gone up to three metres into the banks, though one metre is more common.

Below is a picture of the same riffle taken in November 2007.