Uncertainty in evaluating peak discharges of extreme floods

摘要

In 1997 and 2002 there were floods in the Czech Republic with peak discharges that in manycases exceeded the currently stated values of Q100.During the flood in 2002, at 15 dam reservoirs the peak dischargesreached values that significantly exceeded the design discharge values.This meant that water passedthrough the bottom outlets and mainly over the safety spillways, after reaching a level in the dam reservoir thathad not been allowed for in the project parameters.The dams and their outlets and safety spillways, however, have represented and will probably continue to representthe main monitoring profiles where the value of peak discharges can be most reliably estimated, both inreal time during a flood event and during subsequent evaluation of the flood.During the extreme flood in 2002,the gauging stations on many of the tributaries flowing into the dam reservoirs were destroyed or were put out ofoperation (instruments malfunctioned or were destroyed, power supply failed, stations were inaccessible).The totalinflow therefore formed a major basis for determining the discharges into the dam reservoirs during the courseof the flood.This inflow is evaluated from the increase or decrease in the volume of water in the reservoir in agiven time unit – most frequently one hour – and thus expresses the total average inflow into the reservoir withinthis time unit.The values of the runoffs from the dam reservoirs and the total inflows into the reservoirs are thencompared with the results from other methods, in particular with discharges estimated by the "slope-area"method.These results mostly came from determining the course of the peak discharge water level obtained fromthe after-flood traces.Using three real cases, this paper shows the uncertainties in evaluating the results of peakdischarge using different evaluating methods.There are uncertainties at the overflow on the spillway of theHusinec dam on the Blanice river, uncertainties in evaluating the total peak inflow of the Orlik dam reservoirs,and further uncertainties in determining the peak discharges on the Blanice river upstream and downstreamfrom Husinec dam.With the help of these examples, the paper concludes with a presentation of differences inthe presentation of peak discharges from the way in which they are stated in the final report on the flood event(Ministry of Environment of the Czech Republic, 2003).