Holistic behavior of urban pond systems for flood risk mitigation-a case study in Jaffna municipal council area

Abstract

In many drought-affected areas, it is very common that the peak rainfall intensities are very high even though the total annual rainfall is considerably low and it results in severe flood hazards. During the rainy season, about 25% of the Jaffna Municipal area gets flooded due to the lack of a proper stormwater management system and it has significant social consequences for the communities and individuals living in the flat, low lying terrain area. The main objective of this case study is to find the effect of pond cascade system against flood hazards in the Jaffna peninsula. The study was carried out considering seven ponds (Paalkulam cascade) in Jaffna Municipal Council (JMC) area which used to behave as a typical cascade. HEC-ResSim model based on water balance approach was used to study its holistic behaviour in flood mitigation. Daily rainfall data of the years 2002~2017 was collected from Department of Meteorology, Colombo and pond survey maps, land use pattern maps, existing canal network and natural stream network were collected from Strategic Cities Development Project (SCDP), Jaffna. It was found that when the pond cascade connectivity is considered instead of treating them as individual units, the water retention capacity increases reducing the flooding in the region. The surveyed 2017 flood depths and flood extents were used for model calibration and when Paalkulam cascade model results are compared with a predicted 10-year return period flood map where a 27.5% of the total land area of the cascade catchment is flooded, it was found that the flood affected area could be reduced to 13.2%, if the cascade connectivity is restored. Total catchment area of Paalkulam cascade is 152 ha and the results show that 21.73 ha area (14.3%) could have been saved from a similar 10-year flood hazard. Therefore, it can be concluded that when the pond cascade system is implemented, the effect of flood hazards can be reduced