Abstract:
A unique characteristic of dry zone rainfall is its higher peak rainfall, even though the annual rainfall is relatively low. Jaffna peninsula is located in the northernmost part of Sri Lanka and faces these critical storm events during the second inter-monsoon. The pond system in the area was acting as the major water retention body, however due to the lack of awareness and proper rehabilitation, their retention efficiencies have severely been reduced.
Research methodology was developed to check the effect of reservoir connectivity for flood mitigation. HEC-ResSim computer simulation application was used to model Paalkulam and Nayanmarkaddu kulam pond cascades in the Jaffna Municipal Council region. Model results were used to quantify the flood affected area and the results were validated based on a water balance model. Daily rainfall data of year 2017, pond survey (contour) maps, canal network and natural stream network were used. Two scenario analyses were followed to identify the reduction in inundation area after the inclusion of reservoir cascade behaviour and the two rehabilitation approaches for sustainable pond restoration.
HEC-ResSim modeling was continued for scenario analyses, considering the insignificant deviations (6~8%) with the water balance model results. During the 2017 flood hazard, 27.5% of Paalkulam cascade catchment area out of total 156.7 ha was flooded and it was found that the affected area could have been reduced to 13.2% saving 21.73 ha area (14.3% of the Cascade land area), had the cascade connectivity been restored. The bund raising and bed dredging approaches showed a flood area reduction of 4.5 ha (20.2%) and 7.2 ha (33.3%) for downstream reservoirs and 5.8 ha (26.4%) and 3.9 ha (17.2%) for upstream reservoirs.
Furthermore, the second scenario analysis for rehabilitation approaches confirmed that the most suited rehabilitation approach for upstream and downstream reservoirs are reservoir bund raising and reservoir bed dredging, respectively.
Therefore, for a sustainable pond rehabilitation approach, proper accounting of the cascade connectivity is vital. Moreover, the best pond rehabilitation approach highly depends on the corresponding pond location in the cascade.
