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| Title: | REGIONAL FLOOD FREQUENCY ANALYSIS FOR UPPER AWASH SUB- BASIN (UPSTREAM OF KOKA) |
| Authors: | Mengistu Demissie |
| Advisors: | Dr. Yonas Michael |
| Copyright: | 2008 |
| Date Added: | 18-Oct-2008 |
| Publisher: | Addis Ababa University |
| Abstract: | 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.In order to improve the estimation of the Q-T relationship, the need to use at-site and regional
information arises, so that stabilizing site specific estimates based on limited data can be
handled. The RFFA procedure involves the definition and identification of homogeneous
region based on the given criteria. In the present the application of index-flood for at-site and
regional method of FFA are considered as one of the tools in overcoming problems of
ungauged catchments and a stream having small size of observation (n). In this thesis
application of index flood is only for the derivation of standardized flood that help in
derivation of growth curve for station and regions in the sub- basin.
The use of regional and at-site information to estimate flood magnitude for a catchments with
little or no observed data has become increasingly important since many projects which
require design flood information are located in areas where observed flood data are either
missing or inadequate. Regional analysis consists of analyzing the record of all gauged sites
in a hydrologically homogeneous region and stations, in order to be able to use or transfer
information contained in the record of many sites to estimate quantiles at any individual
gauged or ungauged catchments in the region.
Hosking and Wallis (1993) have discussed various aspects of regional frequency analysis
such as identification of homogeneous region and describing the different steps of regional
analysis. In the present application, the discharge return period (Q-T) relationships for all sites
as obtained from extreme value and lognormal distribution analysis that were plotted together
with the discharge being expressed in dimensionless or standardized form. |
| Description: | A dissertation submitted in partial fulfillment of the requirements for the degree
of Master of Science (Engineering) of the Addis Ababa University |
| URI: | http://hdl.handle.net/123456789/1490 |
| Appears in: | Thesis - Civil Engineering
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