Hydrological Modelling Of The Black Volta Basin In Ghana For Flood Forecasting

Abstract

The occurrence and devastating impacts of floods have been on the increase in recent times at the global scale. This is probably one of the manifestations of climate change/variability. The Black Volta basin is one of the flood-prone areas in Ghana, and hosts one of the most vulnerable communities in the country. The frequency and severity of these disasters require concerted efforts towards mitigation. There is therefore the urgent need for a decision support system for effective management of floods and related disasters in the basin. The objective of this study is to develop a hydrological model based on existing data, duly calibrated with field observations, for the prediction of flood events and potential high flows in the Black Volta Basin. The outcome and output of this research will prove useful to basin managers, policymakers and disaster management organizations in the basin. In this study, the Hydrologic Engineering Center – Hydrologic Modeling System (HEC-HMS) version 4.5 hydrologic model is used for event based modelling of the Black Volta catchment within Ghana. Firstly, the Spatio-temporal trend of monthly and annual rainfall amount and intensity was analyzed for the basin for the period of 1980 to 2018 using the Mann-Kendal trend test and Sen’s slope estimator. On an annual scale, the basin recorded decreasing trend of rainfall amount, while the rainfall intensity showed an increasing trend. With this increasing trend of rainfall intensity, frequency of flooding in the area is expected to increase. In the event-based modeling, the Soil Conservation service (SCS) Curve Number loss approach was used to account for infiltration loss. Direct runoff and base flow were simulated using the SCS unit hydrograph method and the recession method, respectively, and the Muskingum-Cunge model used as channel routing. Calibration of the model was carried out using events in September 1996 and 2007, whiles August–September 1999 and September 2010 event were used for validation. The results yielded an overall NSE value of 0.919 for the calibration and 0.851 for the validation, indicating a good fit between the simulated and observed discharge. The calibrated model was then applied to forecast the runoffs for 2, 5, 10, 15, 20, 25, 50, 75 and 100 years return periods of rainfall using the 2016 IDF curves developed for the area. The simulated peak flows for 2 to 100 years return periods ranges from 1,568.8 m3/s to 2375.4 m3/s at Chache, 3251.7 m3/s to 5655.0 m3/s at Bui, and 8677.7 m3/s to 14941.4 m3/s at Bamboi station. Comparing these values to historical peak flow values within the basin, it is inferred that the range of predicted peak flows for the return periods have the potential of causing flood. This result forms the basis for characterization of floods in terms of return periods, and subsequent application in flood risk assessment within the basin.