FLOOD MONITORING IN GREATER COLOMBO RIVER BASIN USING HEC-RAS FLOODPLAIN ANALYSIS

摘要

Study aims to monitor flood by analyzing floodplains for flood discharge of different return periods, using one-dimensional hydraulic model. This will discuss the capability of GIS approach, incorporate with HEC-RAS for analyzing flood extents. Extracting dynamic flooding information and mapping flooded area accurately, made the main objective of monitoring floods and assess the damages accomplished. Methodology is applied to the canal network in Greater Colombo river basin. A digital terrain model is synthesized from measured bathymetric and Photogrammetric data. Watersheds are delineated using hydrological function in ArcGIS & flood dischargers for different return periods are derived by the rational equation. Then water surface profiles are calculated for considered return periods (2, 5, 10, 25 &50). Finally, Results of the hydrological analysis are applied to monitor flood by developing raster and vector floodplains. Inundated areas are monitored using selected factors such as, Flood Depths, Affected Buildings, Inundated Roads, Crop Lands submerged and number of Schools affected. Flood depths of each floodplain are found by subtracting flood levels from area heights average. When consider the affected buildings, floodplain of Kotte Swamp is the most vulnerable. 497 buildings are affected in 2-year probable flood while 948 in 50-year probable flood. The highest Road inundation is in Parliament Lake area. Local roads, length in 2258.52m are flooded in 2-year probable flood while 9215.4m in 50-year flood. Meanwhile, the largest extent of Paddy lands drowned, also in Parliament Lake floodplain (6.59ha in 2-year flood while 34.42ha in 10 year flood) compare only to the Kolonnawa Canal (0.04ha in 2year flood while 0.09ha in 10 year flood) floodplain, as no other floodplains with flooded Paddy. Three affected schools identified only in St. Sebestian Canal, Kirillapone Canal and Mahawatta Canal floodplains, when water level rises in 25 and 50 year probable floods.