Limitations in the Process of Peak Discharge Estimation for Design of Major Road Crossing Drainage Structures (Based on Current Practices in Ethiopia)
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Date
2015-12
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AAU
Abstract
Drainage structures of a road are one of the major components which contribute for proper function
of the road system. Under designing might result in over flooding of the road surface which
undermines the safety of the road as well as cause damage to the road pavement structure.
Overdesigning will result in unnecessary expenditure of fund. There have been documented instances
of occurrences these issues based on the current practice. One of the reasons for this problem is
improper estimation of design discharges. This has been evident on many constructed roads
throughout the country. Therefore it is apparent that identification as well as proposition of remedial
measure of drawbacks during peak flood estimation for road drainage design is critical issue.
The main objective of the proposed research is to identify areas of limitations and drawbacks in the
process of peak discharge estimation for design of major road crossing drainage structures and
propose possible remedial solutions. The analysis is based on current practice in Ethiopia, i.e.
basically ERADDM and AACRADDM procedures will be followed.
The methodology adopted to achieve this objective was taking sample rivers with sufficient and
adequate gauge data, from different parts of the country, and performing detailed hydrological study
based on the current practice in Ethiopia. This was done using two methodologies, i.e., by estimation
of flood using rainfall-runoff relationships as dictated by the currently applied manuals in the country
initially and later conducting flood estimation for the same catchments based on the historical gauged
data using probabilistic methods, such as Log Normal Distribution or Gumbel EVI Distribution or
Log Pearson Type III Distribution, which ever fits best.
The gauged rivers selected for further analysis were Akaki, Geba near Suppi, Gheba near Mekele,
Weib near Agarfa and Weito. Based on the results obtained from the two methods (SCS and
Statistical) it was concluded that the variation considerable, except that of Gheba nr Mekele and
Weito. The variation of results obtained with the methods (SCS and Statistical) are considerably high
in three cases out of five which is in the range of 83% to 142% while in the case of Gheba nr Mekele
and Weito the variation is not significant (2% and 7% respectively). At this point it should be noted
that the comparison is made just to show the magnitude of variation in estimated flood and there
shouldn’t be any implication that one estimation method is more accurate than another.
Some of the setbacks observed using the SCS method are unavailability of topographic, land use and
soil maps of large scale, inaccuracy in determination of Time of Concentration and unavailability as
well as insufficiency of rainfall data. Setbacks using statistical method are insufficiency and reliability
of data and manual collection of data (not automatically) which results in human error and omission
of peak floods.
ERADDM 2013 section 5.5 emphasizes that, “Regression equations and derivations from stream
gauging (Gumbel, Log Pearson, General Extreme Value) are often preferred but rely on data not
available. For this reason, only the Rational Method and the SCS method are given in this chapter.”
The result of the research reinforces this statement. Similarly it is recommended that to get a lasting
solution the density and reliability of river gauge data should be sought after. This can be achieved
by installing automatic gauging stations in new as well as already existing river locations to get
sufficient and accurate data. This could be achieved for instance along existing or newly constructed
roads. After some time elapses regional formulas can be developed using flood frequency analysis to
get a reliable estimate of peak discharge.
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Keywords
Peak Discharge, Road Crossing, Drainage Structures, Limitations