River Murray Bank Erosion: Investigation of Control Strategies - MDBC project M210 / MDFRC project M/03/5119 (SB/1/25)
MDFRC Techincal Report
Associated publications: (Conference - Frankenberg, J. (1990) Protection of river banks from erosion. Boating on Inland Waterways, Issues in Water Management, IV. Wodonga, Vic, 3 to 5 August) & (journal article - Frankenberg, J. and Tilleard, J.W. (1991) Protecting river banks from erosion: Results of a survey of sites on the Murray River. Australian Planner 29: 107-110)
The River Murray between Lake Hume and Yarrawonga carries very high volumes of water within the channel, to supply the irrigation needs of the Murray Valley irrigation industry. The seasonality of the flow regime has changed markedly with regulation and this is widely believed to have had a impact on riparian vegetation and bank stability. Bank erosion is a concern to landholders and to river management authorities. It causes sediment input to the river, loss of valuable land, damages the remnant riparian vegetation, and creates the risk of changes in river course. In an earlier project, between 1987 and 1990, permanent survey sites were established on the river in the vicinity of Albury-Wodonga, where representative profiles were surveyed annually. The aim of this project was to continue those surveys, to document the actual erosion rate at several sites, to investigate the role of vegetation in bank stability, and to identify and trial potential control strategies, including vegetation and physical structures. Over 50 permanent bank profiles were surveyed annually, for three years, providing an accurate measurement of erosion rates of the river bank at eight sites. The Common Reed, Phragmites australis, was identified as the most useful plant for bank stabilisation, and large numbers of plants were propagated and planted on the river bank, to explore a range of establishment techniques. Timber needle groynes were constructed at two sites to stabilise rapidly eroding banks, and provide better establishment conditions for vegetation. The surveys have shown that the banks of the River Murray are eroding significantly in some reaches, The erosion is not confined to meander movement processes, and is resulting in a widening of some sections of the river channel. Significant erosion is occurring on inside bends, as well as on outside bends and straight reaches. Erosion rates are greater on bare banks than on Phragmites covered banks. Erosion is not confined to flood years, but also occurs as a result of regulated irrigation flows . Phragmites can effectively stabilise the bank in some situations, and is not adversely affected by the changed flow regime. Timber needle groyne construction can be effective in slowing erosion rates and improving establishment of Phragmites under normal flow conditions. However construction design must be adequate for the position on the reach, and the light construction, and possibly the spacing, of the groynes at one site was not adequate to prevent damage during two large floods. The more heavily constructed groynes at the second site have successfully protected the bank during floods and Phragmites is well established. The stability and diversity of the riparian zone has increased markedly. This has proved to be a useful method of reducing bank erosion sufficiently to enable restoration of riparian vegetation. Experience in growing and planting of Phragmites has shown that it can be propagated in large numbers and successfully established on river banks which are not actively eroding. On more difficult sites the use of larger more mature plants increases the success rate. A manual describing the propagation and use of Phragmites, Guidelines for Growing Phragmites for Erosion Control has been prepared and widely circulated. Cattle grazing has a significant impact on the distribution of Phragmites and its effectiveness as a bank stabiliser. Where Phragmites occurs, either naturally or planted, fencing of the bank to prevent grazing is essential to obtain maximum stabilising benefit from the Phragmites. Fencing should be encouraged to protect the remnant stands of Phragmites, which are a valuable resource in the riparian environment.