Hydrological Modelling Approach for Flood and Water Pollution Control in an Ungauged Catchment - A Case Study in Erewwala Catchment in Bolgoda River Basin, Sri Lanka

Abstract

The data scarcity is a widespread, global issue and unavailability of reliable hydro meteorological data is one of the major issues the hydrologists and researchers are facing in Sri Lanka when it comes to water resources planning and management. A significant increase in occurrence of flash flood incidents and water quality degradation in surface water bodies have been noted in the recent past. Hence, developing an approach to identify underlying causes and recommend mitigation or preventive measures for floods and water pollution is a timely requirement that planners, designers and researchers should attempt. The objective of this project is to formulate a hydrological modeling approach to recommend preventive or migratory measures for floods and water pollution in ungauged catchments based on the findings of a case study in Erewwala catchment in Bolgoda River Basin, Sri Lanka. The parameters of hydrological models for ungauged catchments can be estimated using regional information. For this case study, a rainfall-runoff model was developed in spreadsheet and graphical format where the monthly runoff coefficient and base flow were the model calibration parameters. The model was calibrated based on observed data for three years and validated for two years. Observed discharge data at Millakanda gauging station, basin rainfall obtained from Rakwana, Horagoda and Usk Valley rainfall stations and evaporation data from Colombo were used for the model developed for Kaluganga. The calibrated parameters of Kaluganga basin were used for the water resources assessment in Erewwala catchment (2.9 km2). The catchment was divided into three sub-catchments for water pollution control purpose and the incremental runoff at the each sub-catchment outlet node was estimated using the rainfall- runoff model. The types of water uses were identified for each sub-catchment unit and the discharges due to each water use in individual catchments were calculated, subsequently deriving the contribution of each catchment to its overall water pollution. These values were compared with the stipulated permissible pollutant level in surface water bodies. For flood control measures, the HEC-HMS (US-ACE) software was used to estimate the peak discharge with 10 year return period storm event and the peak discharge in each catchment node was obtained. The required flood controlling measures were identified and recommended for the critical catchments which contribute to the highest discharge leading to flashflood conditions in the downstream areas. Based on the peak flow and pollutant source analyses, the sub-catchment No. 1 was identified as the most polluted catchment as well as the one which contributes the most to the flash floods in Erewwala catchment.