Environmental impacts of a reduced flow stretch on hydropower plants

Abstract In Brazil, given its privileged hydrology, the unexplored economic use of water resources has many dimensions, such as hydroelectric power. This energy will face increasingly rigorous social and environmental impact assessments (40% of potential is located in the Amazon region). Hydropower inventory studies conducted over decades, with solutions such as ecological river flows, that flood smaller areas and reduce natural river flows modifications, are being reviewed. The river extension from dam to the point where the waters are returned after the powerhouse is known as the Reduced Flow Stretch (RFS). Even mega-projects, such as the 11.3 GW Belo Monte dam, are designed with deviating flows reaching an astounding 13,000 m 3/s (excavated material higher than Panama Canal). RFS requires to be carefully studied to achieve appropriate ecological flows, since RFS flows increased reduces the plant's electricity production to the same installed capacity. Balancing RFS requirements and hydroelectric power remains a challenge and, clearly, there is no consensus. Here, we performed an analysis of the main environmental impacts caused by RFS requirements, considering the multiple water use specific for each dam site. The natural variability of river flows provides diversity of habitats and maintains the richness and complexity of biological communities. Therefore, the present study has great ecological, social and economic relevance, since proper evaluation of the RFS requirements avoids potential destabilization of biological communities and even loss of biodiversity. This type of arrangement was more common in dams located in headwaters of rivers, as in the slopes of the Andes mountain range, and in regions like the Alps. There are many hydroelectric plants in South America and Europe that have this type of arrangement of engineering works. But the times are different and the environmental impacts have to be better evaluated. A final aspect also involves the maintenance of ecological flows downstream of dams. Regularization reservoirs need to keep downstream, even if they do not have a TVR, adequate flows that represent minimally the seasonality of the river, with floods and droughts, that propitiate the maintenance of the ecosystems downstream. There are cases such as the Sobradinho Plant in the São Francisco River that has been much questioned in this regard, especially when the climate is changing in the basin, with long periods of drought, and with increasing water use. So this is a very important and increasingly current issue.

摘要 鉴于巴西得天独厚的水文条件，其尚未开发的水资源经济用途涉及多个维度，水力发电便是其中之一。该能源领域正面临愈发严格的社会与环境影响评估——其40%的潜在开发潜力位于亚马逊区域。数十年来开展的水电清查研究，诸如采用生态基流（ecological river flows）以减少淹没范围、降低天然径流改造程度的解决方案，正被重新审视。大坝至电厂尾水点的河段被称为减水河段（Reduced Flow Stretch, RFS）。即便像11.3吉瓦的贝卢蒙蒂大坝（Belo Monte dam）这类巨型工程，其设计分流流量也高达惊人的13000立方米每秒，开挖土石方量超过巴拿马运河。减水河段需经细致研究以确定适宜的生态基流，因为该河段流量的提升会降低电厂的额定发电量。平衡减水河段的相关要求与水电开发仍是一项挑战，且目前尚无共识。本研究针对减水河段要求带来的主要环境影响展开分析，同时结合各坝址的多元用水需求。天然径流的自然变异性为河流提供了多样栖息地，维系了生物群落的丰富性与复杂性。因此，本研究具备重要的生态、社会与经济价值：合理评估减水河段的相关要求，可避免生物群落出现潜在失衡，乃至生物多样性丧失。这类工程布局最初多见于河流源头区域，例如安第斯山脉坡地以及阿尔卑斯山区。南美与欧洲有多座水电站采用此类工程设计，但时代已然不同，需对其环境影响开展更为严谨的评估。最后一个方面涉及大坝下游生态基流的维持问题。调节型水库需维持下游流量，即便未配备TVR，也需确保下游流量至少能反映河流的季节性特征，包括洪水与干旱过程，以此维系下游生态系统。圣弗朗西斯科河（São Francisco River）上的索布拉迪纽水电站（Sobradinho Plant）便是因此饱受争议的案例之一，尤其是在流域气候变化、出现长期干旱且用水需求不断增长的背景下。因此，这是一个至关重要且愈发凸显的当下议题。