The water is transported by the waterways from the reservoir, past the dam and further downstream in a hydropower station. Generally, the waterways can be categorized to be either of channel-type or tunnel-type. The waterways of tunneltype include a number of different structures such as intakes, headrace tunnels, penstocks, intake sumps, spirals, draft tubes, surge shafts, surge galleries and tailrace tunnels. These structures are usually either fully or partially filled with water, which means that they in many cases are hard to inspect. Moreover, it is seldom financially justifiable to drain the tunnels solely to perform an inspection due to the loss of energy production that this measure entails. However, in connection to stoppage of the energy production to repair, inspect or replace some of the energy-producing parts, an opportunity to inspect the waterways of tunneltype is usually given.
In this study, a review of documentation from inspections of waterways of tunneltypes has been performed on behalf of Energiforsk to improve the state of knowledge regarding common damage types and deterioration mechanisms in this type of waterways. The collection of data has been performed by inquiring data from representatives of the utility companies in the steering committee of the research program for concrete in hydropower at Energiforsk. The obtained material includes both reports from inspections as well as compilations of observed damage in the waterways for a total of 53 different hydropower stations in Sweden.
From the analysis of the obtained material in this study, it could be concluded that erosion was the most common type of damage observed in waterways of tunneltype if all types of such structures are considered. However, the results become a bit different if the different types of structures are studied individually, even though erosion often is observed to be one of the most commonly observed damage types. Previous research has, however, shown that an observed damage often is a consequence of a series of deterioration mechanisms that together cause a degradation of the material. Therefore, it is hard to isolate one single deterioration mechanism that is the only reason why erosion is the most commonly observed damage type in waterways of tunnel-type. Furthermore, it could be concluded that the documentation of performed inspections generally is inadequate at the utility companies and consequently also the documentation of observed damage and defects in the waterways. It should, however, be noticed that all companies stated that they have performed inspections but that the documentation from these is either missing or hard to find.
With results from the study in mind a need for improvement in the management of the inner waterways is obvious. Especially regarding the knowledge on the status of long headrace and tailrace tunnels in rock with different degree of support. For facilities with common head-/tailrace tunnels for several units, the consequences of a larger failure could be substantial in terms of financial losses from no production. It is recommended to develop a strategy for management of inner waterways.