Author(s): T. A. J. G. Sirisena; C. N. Rajapaksha; D. E. N. Senarathne; I. Abeygoonasekara; K. Ariyarathne; K. Maiyourathaan; S. M. Hewavidana; T. M. N. Wijayaratna
Keywords: Flood depths; Flood paths; Inundation maps; Mitigation measures; Retention areas; Return periods
Abstract: This study explores existing and proposed structural measures as long-term flood mitigation strategy for Moragoda Ela watershed in Galle municipal council area. Study area is low lying, densely populated and most of the drains are polluted and has very low conveyance capacity. Available rainfall retention areas are vanishing rapidly due to land filling. Since Moragoda Ela has a great impact on flood mitigation in the Galle city, a proper and detailed Engineering study of this drainage system is important and essential for identifying an effective and efficient flood control mechanism. MIKE 11hydrological and hydrodynamic modeling has been carried out to assess the existing conditions and simulate the proposed options under rainfall return periods of 5 and 10 years. Proposed flood mitigation interventions consist of rehabilitation of drain network, preservation of retention areas, alternative flood paths and flood bunds. The results show that the proposed interventions can achieve considerable water depth reduction and 50%reduction of inundation time at important locations like hospital roads and junctions with respect to 10 year return rainfall event under existing condition. Inundation flood maps show approximately 55%reduction in flooding area compared to the present flood area with 10 years return period.