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Abstract:Hydrological modeling is a commonly used tool by water resource planners to simulate the hydrological response in a basin due to precipitation for the purpose of management of basin water. With the increasing demand for limited water resources in every basin, careful management of water resources becomes more important. The Deduru Oya river in Sri Lanka supplies water to number of new and ancient irrigation systems and the management of water resources in the Deduru Oya river basin, which has an area of 2620 km 2 , is important for optimum utilization of water for these irrigation systems. This paper describes a case study of continuous rainfall-runoff modeling in part of the Deduru Oya basin with intra-basin diversions and storage irrigation systems using the Hydrologic Engineering Center -Hydrologic Modeling System (HEC-HMS) version 3.0.1 to estimate runoff in the Deduru Oya river.Long term daily rainfall data at several rain gauging stations, evaporation, land use and soil data in the river basin, daily river runoff at a stream gauging station, intra-basin diversions from the river into a storage reservoir, irrigation releases from the reservoir and drainage flow returned to the river from irrigation systems were used to set up the HEC-HMS model. Five-layer soil moisture accounting loss method, Clark unit hydrograph transformation method, and recession base flow method of the HEC-HMS model were used. Temporally varying irrigation water uses, storages and losses in the basin were taken into account in the analysis. The results depict the capability of HEC-HMS to reproduce stream flows in the basin to a high accuracy with averaged computed Nash Sutcliffe efficiencies of 0.80. The study demonstrates potential HEC-HMS application in flow estimation from tropical catchments with intra-basin diversions and irrigation storages. The model developed is a tool for water management in the Deduru Oya river basin.

The downstream low lying region of the Kelani River including the Colombo suburbs, experience severe inundation due to heavy rainfalls in the upper basin of the Kelani River. Occurrence of heavy rainfalls is expected to be more frequent in the tropics with the impact of climatic change (IPCC, 2007). Therefore, understanding future rainfall intensity in the river basin and inundation in the low lying region along the lower reach of the Kelani River is extremely important as this is a region with a high population density and economic activities in the suburbs of the capital. The present study analyses the potential extreme rainfalls and resulting flood inundation along the lower Kelani River. Coarse grid atmospheric parameters provided by Global Climate Model (GCM) models for A2 (high emission scenario) and B2 (low emission scenario) scenarios of Intergovernmental Panel on Climate Change (IPCC, 2007) were downscaled to local scale by applying Statistical Downscaling Model (SDSM). Flood discharge and inundation along the Kelani River reach below Hanwella were analyzed by applying two-dimensional flood simulation model (FLO-2D). Inflow to the model at Hanwella, is estimated by the Hydrologic Engineering Center-Hydrologic Modeling System (HEC-HMS) model under future extreme rainfall events. Areas vulnerable to inundation under the above climatic change scenarios are presented.

A depth-averaged elliptic computational model in curvilinear coordinates is presented for velocity and depth computations in shallow water river confluences of any geometry. The pressure correction equation has been used with SIMPLE or SIMPLE-like procedures in depth-averaged models to date; here this is replaced by a depth correction equation to improve the convergence. This is a significant enhancement to the SIMPLE procedure when applied to depth-averaged flow computations of river elements with irregular bed topography. First, the model is validated using laboratory measurements from a flat-bed confluence with flow recirculation. Second, it is validated using laboratory measurements from a confluence with irregular bed topography derived at the dynamic equilibrium of a movable bed experiment. Next the model is employed to simulate the complex confluence flow at a bend of a river with a floodplain. The model predictions give satisfactory agreement with field observations. Though the flow in the scour hole region of a river confluence is three-dimensional, if proper parameters are used to represent bottom friction and mixing, a depth-averaged model can be used to obtain sufficiently accurate velocity and water surface elevation information for the design of flood control measures in shallow water river confluences.