Abstract:
Dams, reservoirs, and downstream communities are identified as major components
of the irrigated agriculture system in Sri Lanka. However, the aging and
deterioration of ancient earthen embankment dams and their vulnerability to extreme
rainfall events have raised serious concerns regarding dam safety. Despite
the low likelihood, a dam breach could result in catastrophic consequences including
loss of life and severe damage to infrastructure and the environment. Thus,
hypothetical dam breach modeling has become an effective tool in dam safety
management to forecast breach flood exposure downstream. The objective of the
present study is to model a hypothetical dam breach scenario at Giritale Dam
in Polonnaruwa, triggered by seepage-induced piping. Breach analysis is carried
out using HEC-RAS. The initial condition is set as the reservoir at a 10 m headwater
stage being subjected to a Probable Maximum Flood (PMF) inflow, and a
two-dimensional unsteady flow analysis is simulated to assess dam breach flood
behavior downstream. Simulation results estimate a maximum breach discharge
of 893.45 m3/s occurring 3.5 hours after breach initiation. An inundation map for
the dam breach outflow flood was developed, revealing a total inundation area of
over 17.25 km2 spanning 12 Grama Niladari (GN) divisions downstream. According
to Land Use/Land Cover (LULC) overlay on the inundation map, a 15.08 km2
area of paddy cultivation is completely inundated (87% of the total inundation
area). Given that irrigated agriculture is the key source of livelihood in the Giritale
downstream area, a significant socio-economic impact would result from dam
failure. With a great number of aging earthen embankment dams in Sri Lanka
currently suffering from alarming seepage issues, insights from this hypothetical
dam breach modeling effort are timely for proactive dam safety management in
the country.
