Abstract:
Diversion weir built across the river or canal to divert water from the original course.
Assessment of design practice and performance evaluation of case study carried out in
western Oromia, Ethiopia on six selected diversion weir and analysis the design method,
parameters and assumptions that make change the weir dimension and cause low
performance.
Secondary and primary data are collected and analyzed for determination of peak flood
design discharge and design of diversion weir. Four meteorological stations were used for
design runoff estimation by g umbles‟ distribution formula of 50 years return period. Sibu
Sire rainfall station 88.7 mm for Indris and Jalale, Baco 107 mm for Gibe Lamu and
Gambela Tare, Boneya Boshe 118.54 mm for Chokorsa and Nekemt 133.07 mm designed
rainfall used for Fite diversion weir design. Land use/cover and sub watershed analysis
done by GIS 10.1 and peak flood design determined by unit hydrograph for Indris 158
m
3
/s, Jalale 79 m
3
/s, Chokorsa 106 m
3
/s, Fite 50 m
3
/s, Gibe Lamu 105 m
3
/s and Gembela
Tare 72 m
3
/s.
The weir dimensions top and bottom width, cutoffs, creep length, seepage head and apron
thickness were determined for all selected weirs. The seepage head and apron thickness
determined by Bligh‟s, Lane‟s and Khosla‟s . After hydraulic dimension of weir design
the estimated exit gradient is safe under Bligh‟s, Lane‟s and Khosla‟s with compared to
natural soil exit gradient. Structural stability analyzed indicates Indris, Fite and Chokorsa
are safe at the hydraulic design and in the case of G/Lamu, G/Tare and Jalale the stable
value obtain ed after further stability analyzed done by changing the weir bottom width.
As performance evaluation revealed that the designed weir cross section of In dris,
G/Tare, Fite and Chokorsa were accommodating peak flood while Jalale cannot and flow
shift occurred out of G/Lamu weir cross section. Piping, uplift pressure, scouring, sliding,
overturning and over stress did not occurred on the five selected weir however, in the case
of Chokorsa weir downstream floor totally scoured and burst due to uplift pressure
occurred. In general the constructed weirs have no more engineering design problems
which cause low performance but lack of well schemes administration the major
problems occurred as field observation result indicates.
