Rapid environmental flow assessment for the Ruvu River

The Florida International Research iWASH Initiative Limited commissioned this study to conduct a rapid environmental flow assessment that will inform the ongoing environmental impact assessment work in the Ruvu sub-basin, as related to the proposed Kidunda Dam. The work utilizes the historical river flow data from Ruvu River at Morogoro Road Bridge in the sub-basin. The estimates from this work are initial estimates and will be followed by a detailed environmental flow assessment thereafter. A desktop reserve model developed in South Africa that is purely hydrologic-based and which is intended to quantify environmental flow requirements in situations when a rapid appraisal is required and where there is limited ecological data was used to provide initial estimates of ecological flow requirements. Also Flow Duration Curve analysis was conducted. The flow requirements were evaluated considering the various Ecological Management Classes (A-D; A/B, B/C and C/D) at the selected site - Ruvu River at Morogoro Road Bridge. The total maintenance flow requirement was estimated at 55.62% of the Mean Annual Runoff (1951-1977) for Ecological Management Class A and decreases to 15.23% for class D. These estimates are distributed appropriately across the year and take into consideration the high and low flow months provisioning. For the most dry months (September and October), the total maintenance flow requirement is estimated to be 15.03 and 14.86 m 3s-1 respectively for ecological management class A and 3.14 and 3.23 m 3s-1 for ecological management class D. Such a wide range provides the stakeholders, managers and decision makers a better idea of the possible range of required flows to maintain the river in different conditions. This study was purely hydrological with limited social and ecological considerations. It is therefore recommended that a follow-up detailed environmental flow study should look onto the functional elements of the river ecosystem and socio-economic issues. As such, the study should established a relationship between the ecological characteristics and the river flow regimes, the geomorphological aspects, the effects of climate change on flow recommendations and the socio-economic aspect and a detailed analysis of Kidunda Dam regulation on environmental recommendations