Drawdown and river bank stability

Deposited with permission of the author. © Samuel John GreenDrawdown is the lowering of the water level, and can refer to the groundwater, or the level of a river. In this thesis it will generally refer to a river going from a high flow condition to a lower flow condition. The rate of drawdown is expressed as either the change in flow per unit time, or the change in stage per unit time. The later is of most importance in terms of bank stability. It is commonly thought that 'rapid' drawdown of river levels adversely affects river bank stability. This concern over river banks stems largely from work done following the failures of some earth dams once the storage had been drawn down. Little further work has been done in looking at whether the same approach can be used for assessing the likely impact of drawdown on river banks. Given this apparent lack of information and knowledge on drawdown failures, this work investigated the effect of drawdown rates on river bank stability, and in so doing a better understanding of the key parameters is gained. The task was approached from three angles. The first was an analysis of the historical records to see what the rates of fall prior to regulation were, and whether they had changed since regulation. The results indicated that it can not be shown conclusively that the rate of draw down has increased since regulation. They also show that there has been a marked change in the frequency at which drawdown events of a given size occur, particularly the high incidence of lower drawdown rates. The second approach to the investigation was the field work component. The focus of the field work was the response of the near bank watertable to changes in river level. Other data collected were soil particle size distribution, bank profile monitoring (through the use of photogrammetry) and permeability testing. The results of this work show that the near bank watertable was easily able to match the rate of drawdown seen in the river channel. The particle size analysis and permeability data confirm this observation. The third prong was the smallest and consisted of some very simple modelling which further showed that drawdown rate is not so critical for river banks. The findings of this thesis are that draw down at the current rates is not a factor in the failures observed on northern Victorian rivers. The rate of drawdown has not significantly changed since regulation, although the frequency at which specific rates of drawdown occur, has changed. Observational data shows that elevated and sustained nearbank watertable levels cause seepage failures that look like drawdown failures. Permeability is the key factor in the response of the near bank watertable and should be used as a guide to the maximum drawdown rate