Abstract:
Many irrigation scames have been design and built in ethiopia in recent year; about 90% of
small scale irrigation projects are under performance. Offiya small scale irrigation project is
one of non-functional after it constructed due to destruction of different head work structure
components. This study aimed to assess the design practice and performance of Offiya
diversion weir through technical performance indicator of hydraulic and structural analysis.
The main input data used were collected from Ethiopian metrological services agency,
minister of water irrigation and energy office department of GIS, Kaffa zone water mine and
energy department, Chena woreda water mine and energy department and direct field survey.
The tools that used in this study were Tape, Leveling, GPS Garmin 72, GIS 10.4.1, and Excel
spread sheet, DEM, Bentley flow master, HEC-HMS and Geostudio2018. Peak rainfall from
Chena metrological station analyzed for feature 50 years return by using Gumbel extreme
value distribution method was 140.5846mm. The analysis resulted the river peak design flood
of 133.08m3/s and the discharge capacity of the weir over the crest was 80.77988m3/s where
about 60.7% of design flood computed by HEC HMS.The weir dimensions top and bottom wi
dth, cutoffs, creep length, seepage head and apron thickness were determined or Offiya weirs
. The stability analysis of weir body and its appurtenance structures was analyzed and compa
red with standard safety factor. The overturning and sliding stability factor for weir body
were 2.4 and 1.43 this shows the weir was safe against overturning but not sliding. The
upstream retaining wall and wing wall was safe against overturning and sliding but not safe
for overstress and bearing capacity of foundation as analysis result. The downstream
retaining wall was safe for stability against overturning, but not for sliding, overstress and
bearing capacity. The quantity of seepage flux for water at pond level and high flood
condition were 1.3479 x 10^-5 m3/s/m and 1.2131 x 10^-5 m3/s/m respectively. The safe exit
gradient of silt soil weir foundation was 0.17 whereas the computed was 0.225 which
indicate the foundation soil not safe. The thickness of impervious floor for static
and dynamic case were than the provided thickness of impervious floor these show the under
performance the weir structure to the design period. The other underperformance indicator w
as the absence of river training, energy dissipater and up and downstream protection work
