Assessing the Impacts of Recreational Boating Activities on River Bank stability - M/BUS/226
MDFRC Technical Report
There is no doubt that high speed recreational boating contributes to bank erosion on the Murray River. However, the results from this and other studies would suggest that other factors - most notably high flows coupled to relatively constant water-levels for extended periods of time associated with river regulation - have a much more substantial impact. Recreational boat wakes have clearly been implicated in bank erosion in a wide variety of river types and sizes, both in Australia and internationally. Detailed studies of the rates of bank erosion and boat activity in river systems as different as the Gordon River (Tasmania), the Mississippi River (USA), the Waikato River (NZ) and the Kenai River (USA) have all shown measurable impacts of recreational boats on the river bank stability. However even these studies have not always established the exact quantitative relationship between boat speeds, intensity or frequency of boat use, types of boats and the observable bank erosion. Studies have been conducted to develop predictive formulas for the size of wakes generated by a given boat size or speed; however these are not easily applicable in settings where there may be multiple boats operating at different speeds. Despite these limitations, in many places management responses have ensued and policies have been implemented, or are proposed, to limit boat speeds and otherwise minimize the impact of boat wakes. On the Murray River there have been few studies examining the role of boat wakes but all studies to date suggest that other processes, specifically the seasonal cycle in flow rates in this highly regulated river, may in fact be equally or more significant than recreational boat use. When we measured the wave energy created by various types of recreational vessels we found that wake boats can produce waves with greater erosion potential than other types of powered vessels that use the Murray River near Echuca. This is not surprising given that wake boats are both designed and operated to produce maximum wash. That is not to say that other boat types do not have the capacity to contribute to bank erosion. Personal watercraft and runabouts also produced wakes with both high peak and total energy density; personal watercraft, runabouts and ski boats all produced waves with similar levels of total power. The amount of sediment suspension caused by boat traffic varied with location and was estimated at between 0.45 and 3.6 kg/ metre of bank/hour in the Echuca region. In comparison, based on background turbidity levels, the total suspended solid load passing a given point in the river was estimated to be 25.4 tonnes/hr. When the potential for erosion from boat induced waves is compared to the potential from other sources, including flow and wind induced waves, it was estimated that overall, boat wakes only contribute to a small percentage of the total energy expended on the river bank at Echuca. Most of the energy expended on the river bank in this region is associated with river flow, with a smaller additional contribution from wind-induced waves. Finally the results of a survey of bank profiles and edges in reaches of the river where boating was restricted (speed restrictions, no skiing or no wash zones) was equivocal. While substantial undercutting of the bank was observed between 2007 and 2009 at the most downstream sites, where high speed boating activity was common, similar erosion was observed in regulated zones.