Methodology - Response of Flood Events to Land Use and Climate Change - page 84

3.11 Flood Frequency Analysis
Analyzing the extreme event is mostly impacted in hydrologic systems. For any
design, river engineering and hydraulic planning is not enough to say that the
maximum observed flood was, for example, 480 m 3 /s, it is also necessary to
consider the frequency of occurrence of the mentioned flood. Parodi [ 94 ] realized
that the magnitude of an extreme event is inversely related to its frequency of
occurrence e.g., very severe events have less frequency compared to more mod-
erate events. Therefore, according to Parodi [ 94 ], ''the frequency analysis is
related to the magnitude of extreme events to their frequency of occurrence
through the use of probability distributions''. The hydrologic data in frequency
analysis are assumed to be independent and identically distributed. For this pur-
pose, the annual maximum of the variable, e.g., the annual maximum discharge,
should be considered.
These variables are the largest instantaneous peak flow occurring at any time
during the year with the expectation that successive observations of this variable
from year to year will be independent [ 84 , 95 - 100 ]. According to Parodi [ 102 ],
the results of flood frequency analysis can be used for many engineering purposes:
for example, the design of bridges, dams, dykes and flood control structures;
economical purpose of flood control projects; and for delineation of floodplains
and to determine the effect of encroachments on the floodplain [ 94 ].
3.11.1 Return Period
Flood hazard assessments are usually based on a number of flood scenarios. Each
scenario is associated with a probability P E or return period T. In many countries
around the world, such as Canada, UK, USA, Italy, France, Spain, Germany, and
New Zealand, the floodplain area is affected by a 100 years return period flood that
plays an essential role for flood mitigation strategies. In Europe, according to the
proposed directive of the European Union on the assessment and management of
floods, two flood scenarios with return periods of 10 and 100 years and an extreme
scenario with a higher return period were defined [ 84 ].
Stedinger et al. [ 101 ] suggested that in flood hazard assessments two steps
should be considered: the first is to estimate the T-year discharge along the river or
water course, and the second is to transfer the discharge values into floodplain. The
most common approach to estimate the T-year discharge along rivers is flood
frequency analysis. Hosking and Wallis [ 102 ] have a discussion related to the
scale. They concluded that in many cases, local frequency analysis is comple-
mented by regional flood frequency analysis by applying the data from those
gauging stations that have similar flood behavior or characteristics.
Apel [ 84 ] had referred to some limitations faced by the frequency analysis. For
example, he mentioned that in any flood frequency analysis, the results are affected