Laserfiche WebLink
<br />, <br /> <br />DISCHARGE FREQUENCY ANALYSIS <br /> <br />1. BACKGROUND INFORMATION. <br /> <br />Frequency analysis of recorded streamflow data is an important. flood- <br />runoff analysis tool. This chapter describes the role of frequency analysis and <br />summarizes the technical procedures. EM 1110-2-1415, "Hydrologic Frequency <br />Analysis", describes the procedures in greater detail. <br /> <br />1.1. ROLE OF FREQUENCY ANALYSIS. The traditional solution to <br />water-resource planning, designing, or operating problems is a deterministic solution. <br />With such a solution, a critical hydrometeorological event is selected. This event is <br />designated the design event. Plans, designs, or operating policies are selected to <br />accommodate that design event. For example, the maximum discharge observed in <br />the last forty years may be designated the design event. A channel modification may <br />be designed to pass, without damage, this design event. If this design event is not. <br />exceeded in the next 1000 years, the design may not be justified. On the other hand, <br />if the discharge exceeds the design event 20 times in the next 30 years, the channel <br />modification may be underdesigned. <br /> <br />A probabilistic solution employs principles of statistics to quantify the risk <br />that various hydrometeorological events will be exceeded. Risk is quantified in terms <br />of probability. The greater the risk, the greater the probability. If an event is certain <br />to occur, its probability is 1.00. If an event is impossible, its probability is 0.00. For <br />flood-runoff analyses, the probability of exceedance is usually the primary interest. <br />This is a measure of the risk that discharge will exceed a specified value. Decisions <br />are taken so that the risk of exceedance is acceptable. For example, the channel <br />modification described above could be designed for a discharge magnitude with an <br />annual exceedance probability of 0.01. In that case, the risk is known and is <br />accounted for explicitly in the decision making. <br /> <br />1.2. DEFINITION OF FREQUENCY ANALYSIS. The objective of <br />streamflow frequency analysis is to infer the probability of exceedance of all possible <br />discharge values (the parent population) from observed discharge values (a sample of <br />the parent population). This process is accomplished by selecting a statistical model <br />that represents the relationship of discharge magnitude and exceedance probability for <br />the parent population. The parameters of the models are estimated from the sample. <br />With the calibrated model, the hydrologic engineer can predict the probability of <br />exceedance for a specified magnitude or the magnitude with specified exceedance <br />probability. This magnitude is referred to as a quantile. <br /> <br />7-90 <br />