Determination of design discharge and environmental flow in micro hydropower plants based on flow duration curve in small rivers: A Case Study of Hhaynu River - Mbulu,Tanzania

In designing micro hydropower plants capacity, the amount of water flow discharge determination is very essential in estimating the power output for the micro hydropower scheme due to the fact that in recent years there has been an increase in water demand due irrigation activities because of an increase in agriculture activities [1]. From the literature, there are several methods that are used to determine hydro turbine water flow discharge but the most widely used method is the hydrological method which is based on the formulation of flow duration curve or hydrograph for the respective river flow based on site historical and measured hydrological data. The hydrograph represents the amount of water flow in m3/s that is available for a particular river or stream in percentage from the historical hydrological data which has been computed over a period of more than one year. From the study of Hhaynu River which is a small river in Tanzania, the computed flow results show that, 8% of the time the flow is at 2.3 m3/s while at 50% of the time the river flow was at 1.86 m3/s and at 100% of the time the water flow was at 0.60 m3/s. When determining design discharge for run-of-river schemes, provision have to be considered for environmental flow on which for the Hhaynu River this has been analysed to be 0.2 m3/s (33.3% of river base flow) which resulted to the hydro-turbine development design water flow discharge of 0.4 m3/s from the computed flow duration curve with provision for environmental flow. The usefulness of developing flow duration curve for rivers is to determine the available water flow in a particular small river and its suitability for micro hydropower development from estimates of the amount of water flow discharge.