Regional Flood Frequency Analysis For Upper Awash Sub- Basin (Upstream of Koka)
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Date
2008-05
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Addis Ababa University
Abstract
The conventional and L-moment method of flood frequency analysis are applied in
determining flood magnitude of defined return periods by selecting the best-fit theoretical
probability distribution. The most important part of the distribution is the tail as far as extreme
flooding phenomena. These phenomena of floods are of utmost concern in water resources
development and management. In most cases the central part of the theoretical distribution fits
satisfactorily with the empirical points. In FFA the objective is to estimate flood magnitude
(Q) corresponding to any specified recurrence interval of (T) years. The estimation is
complicated due to lack of a physical basis for determining the form of the underlying flood
frequency distribution and the necessity of evaluating flood event for return periods that
exceed the observation period (Leulseged, 2002).
Regionalization is generally accepted term to explain the transfer of information about flood
peak at one catchment derived from other catchments with similar characteristics. The
advantage of such procedure is particularly great in the estimation of frequencies for higher
flood magnitudes with limited at site data do exist and inference in the tail of probability
distributions makes the stabilization of the estimators difficult. It is quite clear that
regionalization is most viable way of improving flood quantile estimation. Although there
remain researchable topics in development and application of regionalization method. The
performance of regional Extreme value and LN distributions are found to be highly
satisfactory and can be widely applied in this paper. More attention is given to at-site
homogeneity test to group stations in the upper Awash sub-basin (u/s koka) in to two regions
after checking them for the consistency and independency testes and estimation of standard
error. The upper Awash sub-basin (u/s of koka) has 10-selected gauged stations consisting of
stream flow record varying from 12 to 37 years, out of which 6 stations are found in the upper
region and 4 of the stations are found in the lower region. An Extreme value EV1, GEV and
Lognormal LN2, LN3 distributions are selected as the best fit distribution for the stations in
the sub-basin. The sub-basin has been divided into two regions the upper and lower region,
the upper region covers 18.46% of the sub-basin and the lower region covers 81.54% of the
sub-basin.
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Keywords
Upstream ; koka