Quantitative methodology for downstream impact evaluation and dam operation optimisation: the case of the Tams hydropower project in Ethiopia

Abstract

While hydropower dam projects can surely benefit communities, they may also adversely affect people and environment. Studies of dam impact evaluation are often qualitative, allowing bias and manipulation when assessing feasibility. When they are quantitative, they generally do not comprehensively describe where, for how long, and at what magnitude downstream impact occurs. Moreover, little attention is given to quantitatively evaluating localised types of impact, such as social (e.g., on recession-based agriculture activities) or environmental (e.g., on wetlands). This makes it challenging to develop justifiable dam operation scenarios that balance energy production maximisation and downstream impact minimisation.

Focusing on the planned hydropower TAMS dam on the Baro River (southwest Ethiopia), this study develops a quantitative method for mapping a dam's downstream hydrological impact in space and time using the Deviation from Natural Flow (DvNF) metric. The DvNF is computed from the spatially distributed output of a two-dimensional hydrodynamic model, calibrated against historically observed maximum flooding from satellite imagery, using a newly developed method for main channel bathymetry reconstruction. The proposed method of hydrological impact mapping provides an accurate assessment of the dam’s downstream impact, specifically pinpointing impacted areas and periods of impact. It also allows for selecting an optimal protocol of dam operation (in the case of the TAMS dam, a three-year protocol rather than a one-year protocol).

Furthermore, a method based on DvNF-duration curves is developed to evaluate the localised impact of different protocols on wetlands, needing water recharge from overbank flooding, and villages, needing flooding for recession-based agriculture. The DvNF-duration curve approach complements the spatial DvNF mapping approach to support the selection of an optimal dam operation protocol, depending on the stakeholders’ specific interests.

The proposed methods can be applied elsewhere, including areas with limited river bathymetry data, to support decision-making in new dam construction or relicensing projects.

Date of Award	27 Jun 2024
Original language	English
Awarding Institution	Northumbria University
Supervisor	Davide Motta (Supervisor) & Martin Crapper (Supervisor)