Hydroelectric schemes use channels for water transfer purposes, irrigation schemes use them almost exclusively for water distribution, and many water supply systems use them for water diversion or transfer activities.
The decision criteria for their maintenance and renewal are very often dependent on the relative economic benefit that can be derived.
Where the supply of water produces income and the resource is restricted, then passing through the greatest volume would result in substantial benefit. This benefit will vary from season to season and we need to be careful that we do not use the high demand scenario inappropriately in the economic evaluation, for the restoration of canal efficiency or the elimination of water loss. Evaluations of these benefits should consider the probability distribution of this occurring, based on long-term average hydrology calculations.
In most cases this will effect the justifiable long term maintenance on the channels (be they lined or unlined), maintenance such as passing automatic cleaning equipment down concrete lined channels to clean the walls of growth and surface roughness to improve the overall hydraulic efficiency.
However, the key issue for many channels is the possibility of total failure and the overall consequences of this to the organization.
This aspect is best dealt with in risk management terms, however it too needs to be based on the actual probability of failure and the other criteria that will impact on the degree of damage.
Dartmouth Dam, Victoria, Australia
During the final commission testing of the Dartmouth Power Station, an emergency shutdown procedure resulted in water rising in the surge chamber tower and discharging to atmosphere. In doing so the resulting water flow lifted two large beams on vertical axle (pivots) that were used for the installation and removal of the isolation valve drive shafts.
These large steel members then fell to the base of the surcharge chamber where they were carried by the water flow down to the main turbine, where one member passed through the inlet screens and vanes to wedge and stop the main turbine that was operating at full power. The resulting hydraulic pressures and dynamic forces virtually destroyed this brand new installation.
Failure to assess the consequences of one of these beams being accidentally displaced had enormous financial consequences ($40 million). Fortunately, there was no loss in life or severe injury.
In other similar facilities screen bars have been dislodged or have fallen off with decay. It is important that FMECAi analysis is done to ensure all possible failure modes are assessed and dealt with appropriately.
Dungowan Dam, Tamworth, New South Wales, Australia
As part of a regular dam safety audit, the department of water resources determined that a dam had some structure deficiencies and, using the latest rainfall and runoff calculations, the dam would be overtopped in a possible maximum flood (PMF) situation.
Taking into account the consequences of such a failure, the Authority assessed the cost benefit to carry out the works necessary to reduce this probability to acceptable levels.
The cost was considered too high to be borne by the number of users that benefited from this facility. The consequences of failure were not all that great providing the few people living in the flood plain were protected.
The decision was made to install an automatic telemetry system that would give adequate warning of the likelihood of the dam being overtopped and this would activate alarms in all dwellings located in the flood path. This was deemed to be a satisfactory response to the problem and it dealt with the issue of "user pays" and the cost effectiveness quite appropriately.
What the authority did not fully understand was the mental anguish and stress that this alarm system placed on the people concerned whenever there was a reasonable rainfall. The knowledge of this level of stress and the problems it was causing became known to the community as a whole. Although only a small number of ratepayers were affected by the failure, the entire community eventually agreed to meet the higher costs by rectifying the problems associated with the dam and thereby eliminating the need for the telemetry warning system.
Similar examples can be given for drainage and sewer overflows and even for water main bursts. In each case only a small percentage of the population is affected by the overall failures and it is important that we appreciate the fact that although it may not affect all customers, the organization as a whole has an obligation to lessen the effect on those who are most likely to suffer from probable failures.
