![]() For larger return periods there are no observed data anymore and therefore the regression line is extrapolated. The maximum discharge measured per year is plotted after it was ordered from high to low, and the regression line shows the magnitude-frequency relation for these observed data. This is illustrated in Figure 3, where data is available for a 25 year period. However, you can say that for an entire watershed.Ī frequency-magnitude relationship is normally based on a historical record of hazardous events (See previous section 2.2). Thus you cannot say that small landslides occur often in the same location and large landslide less frequently. For instance the occurrence of landslides cannot be represented for a particular location as a magnitude-frequency relationship (except for debris flows and rock fall) as the occurrence of a landslide will modify the terrain completely. In other cases the frequency magnitude relationship cannot be established for an individual point, but is done for a larger area (e.g. ![]() The flood itself will also have its own discharge-frequency relationship for the entire catchment, but this can be used as input to calculate the height-frequency relationship for a particular point. This is the case for events like flooding, where each location will have its own height-frequency relationship depending on the local situation. a particular slope, x-y location, building site). The frequency magnitude relationship can be valid for the same location (e.g. This relationship might differ substantially depending on the hazard type. Most hazard types display a relationship between the likelihood of occurrence (probability) and the magnitude of the event, as shown in figure 2. Hazard A shows a relationship where the same magnitude may occur much more frequent than for hazard B. Another method is the calculation of the return period: it indicates the period in years in which the hazards is likely to occur based on historic records an example can be a flood with a return period of 100 years (100 years return period flood = 1 event in 100 years = 0.01 probability).įigure 2: Relation between magnitude and frequency. The exceedance probability can be shown as a percentage: a hazard, that statistically occurred once every 25 years, has an exceedance probability equal to 0.25 (or 25%). Frequency is generally expressed in terms of exceedance probability which is defined as the chance that during the year an event with a certain magnitude is likely to occur. For rainfall phenomena both small magnitudes as well as large magnitudes may be catastrophic as illustrated in figure 1.įigure 1: Graphs showing the magnitude - frequency relation for rainfall related events.įew hazards don't follow this rule an example of events with random relation between magnitude and frequency is lightning. Magnitude-frequency relationship is a relationship where events with a smaller magnitude happen more often than events with large magnitudes. small flood events occur every year while enormous and devastating inundations are likely to happen once every one or more centuries. The frequency of events with a low magnitude is high, while the frequency of events with great magnitude is low: i.e. In most of the cases there is a fixed relation between magnitude and frequency for natural events (see figure 1). The analysis of historical records and their frequency allows scientists to understand when a certain hazard with a certain magnitude is likely to occur in a given area. ![]() In hazard assessment, frequency is a key point to study the occurrence probability of hazardous events in the future. the (temporal) probability that a hazardous event with a given magnitude occurs in a certain area in a given period of time (years, decades, centuries etc.).the quotient of the number of times n a periodic phenomenon occurs over the time t in which it occurs: f = n / t.the number of occurrences within a certain period of time.the relationship between incidence and time period.One of the most important temporal characteristic of a hazardous event is the frequency of occurrence. Use case 8.2 on magnitude-frequency analysis.Īs described in section 2.1, the most important aspects of hazards are the spatial and temporal characteristics of the events.Keywords: magnitude, frequency, flooding, landslides. This section introduces the concept of magnitude frequency analysis and gives examples of the generation of magnitude-frequency relations for flooding and landslides.
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