Abstract:
Irrigation water scarcity in Sri Lanka is
intensified by conveyance losses in unlined tank-cascade
canals. This study quantifies flow inefficiencies in a 650 m
reach of the Kaddumurivu Irrigation System (KIS) by
integrating field measurements with a one-dimensional
steady-flow HEC-RAS model. Eight canal stations were
surveyed using a Total Station, while discharge and
water-surface elevations were measured under steady
operating conditions using a current meter and staff
gauges. The model was calibrated by adjusting Manning's
roughness coefficient until simulated water-surface
elevations agreed with observations within ±0.05 m and
was subsequently validated using an independent flow
dataset. The measured discharge decreased from 1.0052
m3/s upstream to 0.7681 m3/s downstream, giving a
volumetric loss of 0.2371 m3/s equivalent to 23.6% of
inflow, and a conveyance efficiency of 76.4%. Energy-
based analysis gave a total head loss of 0.30 m, of which
0.12 m was attributed to friction and 0.18 m (60%) to non-
friction inefficiency. The calibrated Manning's n values
were 0.039 for the upper unlined reach, 0.012 for the
concrete-lined reach, and 0.020 for the lower unlined
reach. Spatial analysis identified the lower unlined reach
and short structural-transition segments as critical loss
locations. The results suggest that non-friction
mechanisms, potentially including seepage, are
significant contributors to the observed inefficiency,
supporting targeted lining, maintenance, structural
inspection, and continued monitoring rather than
uniform rehabilitation of the entire canal.
