VARIATION OF ROUGHNESS COEFFICIENTS FOR UNSUBMERGED AND SUBMERGED VEGETATIONa

be from pipe and connection breaks rather than structural failure of distribution reservoirs (as long as the reservoirs have been designed for lateral loading). Most of the examples of storage tanks and distribution reservoirs impacted by the 1994 Northridge and 1995 Hyogo-ken Nanbu earthquakes appear to have been of steel construction rather than of reinforced concrete or prestressed concrete construction, which are predominant in New Zealand for water supply reservoirs. There appears to be a lack of evidence regarding the behavior of reinforced concrete and prestressed concrete reservoirs in large earthquake events. However, it is difficult to conceive that reservoirs of such construction could collapse catastrophically in an earthquake, particularly if they are designed for lateral loading (as they are in New Zealand). Concrete tanks are more likely to experience severe cracking (with the steel reinforcing or prestressing continuing to provide some residual structural integrity) or severed pipe connections. In the event of either scenario, the contents of the reservoir would tend to be released gradually over time rather than catastrophically. For a large earthquake event, catastrophic failure of a water supply reservoir is, however, an appropriate failure scenario for ferro-cement tanks or wood stave tanks with wire rope wound round externally to provide circumferential strength. Similarly catastrophic failure is a realistic failure scenario for reservoirs of any type of construction where an earthquakeinduced slope failure affects the platform on which the reservoir is sited. In summary, before embarking on a detailed analysis of the dam break type flood event resulting from catastrophic collapse of a water supply reservoir, hazard assessments for such reservoirs need to consider the nature of reservoir construction, the potential extreme loading conditions on the reservoir, the behavior of the reservoir under those loading conditions, and potential failure scenarios.