Intensity and frequency assessment of natural disasters

The intensity of natural disasters refers to the ability of disaster-causing factors to act on the disaster-bearing body, and usually reflects the intensity of natural disasters through the damage degree of the disaster-bearing body. For example, Wang Jinfeng et al. (1993) used the annual precipitation anomaly percentage (≤ 12.5) to determine the flood intensity, and used indicators such as ① affected area, ② duration, ③ death toll and ④ economic loss to indicate the degree of disaster loss, ① ≥ 65438+100000 hm2, ② ≥/kloc-.

The frequency of natural disasters refers to the number of disasters with a certain intensity in a certain period of time, and its reciprocal is its recurrence period, that is, how many years it usually occurs, which is generally obtained through historical data statistics.

For example, the calculation method of flood return period is as follows:

① N= year of investigation-year of occurrence = 1975 (investigation)-1849.

(referring to the biggest flood so far)

② N= year of investigation-year of occurrence/ranking position

(refers to the ranking number of known floods)

(Rank refers to the ranking of the catastrophic floods that occurred during the investigation period, which is used when the historical flood year is closer to the present. )

③ Nˊ=2N/ 1+a

(Used when compared with other historical floods of a- 1)

In the risk assessment of natural disasters, the "exceeding probability" exceeding a certain intensity is often used as the scope of risk assessment, such as the inundation range exceeding "once every 20 years".

Risk assessment not only evaluates the time distribution of natural disasters, but also pays attention to the possible range of disasters, that is, the risk area or disaster-prone area as a risk grading unit. Generally, the disaster risk area can be determined according to the dynamic changes of local disaster-prone environment, historical disaster intensity and disaster-causing conditions. For example, flood risk areas are usually delineated according to the possible inundation range of each return period, and flood risk assessment pays more attention to flood level and possible inundation time of each return period, and less attention to flood flow and other factors. For historical floods, hydrological departments have generally done a lot of investigation work, and most counties and cities have compiled flood diversion manuals. On this basis, the flood risk assessment work is to compile the flood inundation range map of each return period for risk assessment. The dangerous areas of other disasters such as collapse, landslide and debris flow can be determined according to the quantity and distribution of loose deposits on the slope and the dynamic changes of inducing factors (weather and earthquake). Storm surge, geological drought and other disasters can be accurately judged on the basis of a large number of investigations and studies on meteorology, regional geology, soil and vegetation, with reference to the local disaster history. However, it is difficult to determine the disaster risk areas such as earthquake, typhoon, meteorological drought, hail, insect pests and rodents, and further research is needed.