Abstract:
The frequency of flood occurrence
in the Jaffna Peninsula mainly depends on
rainfall intensity, conditions of natural
drainage, associated terrain, and hydraulic
structures. Jaffna Municipal area has also been
facing recurrent severe flood events in the past
years. Since it is situated in the dry zone, the
seasonal rainfall occurs in the second inter monsoon and North-East monsoon periods with
daily rainfall often exceeding monthly average
rainfall under extreme conditions and there is
no proper management of drainage network or
associated pond cascades. However, in the past,
these cascade pond systems were used to retain
the upstream rainwater and enrich
groundwater storage through recharging before
it reaches the outlet. In the present study, a pond
cascade model was developed by using the HEC ResSim computer simulation application to
compare the efficiency of flood mitigation of
cascade systems and individual ponds. The
HEC-HMS computer application was used to
find runoff from the catchment. The flood
inundation area was identified by the HEC-RAS
5.0.7 modeling. According to the results
obtained from the model calibration based on
the 2017-year rainfall event and 10-year return
period flood event, the flood inundation area of
the individual pond approach was 0.366. When
the cascade pond behavior was introduced, it
was reduced to 0.2352 km2. The obtained results
obviously say that the flood inundation area was
reduced by the pond cascade system because a
considerable amount of flood was retained in
upstream ponds. The inundation area was
decreased by 0.1308 km2. It is possible to reduce
the flood inundation area further based on the
results by increasing the pond capacities. In
conclusion, the flood inundation can effectively
be reduced by ensuring the holistic behavior of
the pond cascade system meanwhile increasing
the groundwater recharge.
