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Please use this identifier to cite or link to this item: http://hdl.handle.net/123456789/1490

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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