River structures

Mount Roland Rivercare Catchment Inc has instigated or assisted with many bank stabilisation projects along the Dasher, Minnow and Upper Don rivers, in conjunction with landowners and with financial support from a number of organisations and government programmes.

In-stream structures of riffles, groynes, rip-rap or combinations of all three have been constructed on 30+ properties. These all act to slow the flow rate of water, thereby reducing the eroding effect of water on riverbanks.

The placement of large logs (woody debris) in the rivers has been used to encourage the return of fish and other native aquatic species, and to prevent erosion of banks. The logs trap smaller fragments which promote the growth of plants. Vegetation covers the area surprisingly quickly, leaving an attractive and stable bank. In most cases, the logs were sourced locally, and donated as in-kind support.

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.

Read more: Constructing a riffle

Groynes

Where a current needs to be diverted away from a river bank to prevent erosion, one or more groynes (a protective structure of stone or concrete, also spelled groin) may provide a viable option.

Positioning a groyne in a river forces the water to travel further, which slows the water, reducing the eroding effect. As with all river structures, groynes need to be substantial enough to withstand the pressure of the highest floods that occur.

Read more: Groynes

Woody Debris

The placement of large logs ('woody debris') in the rivers has been one of the major techniques we have used to encourage the safe return of fish and other native aquatic species and to prevent erosion. The logs trap smaller fragments which promote the growth of plants, again improving stabilisation of the river banks.

Read more: Woody Debris

Rip-Rap

Rip-rap is a method of reinforcing river banks to prevent erosion, particularly where this is caused by water hitting the bank with some force, or swirling round beside the bank. Rock is generally used, though sometimes logs will provide a good solution. Even when the gaps between the rocks are filled with gravel, and seeds or seedlings have been planted there, it can take up to two years before a bank stabilised with rip-rap looks natural and attractive.

This is an example of a degraded riverbank, which had been severely eroded to the point that the fence that was once on solid ground was left hanging in the air.

Read more: Rip-Rap

Additional information