美国建坝与拆坝

美国有着漫长的筑坝历史，大坝为其经济发展做出了巨大的贡献，但同时也对生态环境造成了一定程度的影响。本文介绍了美国已建大坝、反坝、拆坝、修坝、建建坝一些情况，其建坝、拆坝等方面的经验对中国乃至世界其它国家的水电开发管理具有非常重要的借鉴作用。     

美国拆坝情况简析

美国1999～2003年底前共拆坝168座,所拆的坝绝大多数为小型坝,寿命超过使用年限、功能已丧失或本身就是病险坝,且维修费用高昂,拆除是最经济的选择.在拆除过程中非常注重对拆坝环境影响的观测与评价.美国拆坝的原则和经验可供我国借鉴.     

美国百年拆坝历史回顾

自1912年至今,美国拆坝历史已有百年之久,期间拆坝共计1108座。从拆坝数量、分布范围、坝高和坝龄等多个角度对这些闸坝的资料进行分析比较,得出不同年代美国的拆坝特点:1980年以前拆坝数量少,分布范围较小,但闸坝的负面影响及拆坝必要性开始被人们所认识;20世纪80年代以后拆坝数量和范围逐渐扩大,坝高仍小于20 m,拆坝原因以生态修复为主,且更加关注闸坝安全问题;20世纪90年代拆坝数量继续扩大,拆坝高度实现突破,编制完成《大坝及水电设施退役导则》;21世纪,拆坝数量、高度和长度都打破以往记录,被拆闸坝以混凝土坝为主且绝大多数超过了使用寿命。同时,分析表明,闸坝的拆除与当时的社会经济状况有着密不可分的联系。     

拆坝有缘由建坝须谨慎——从美国拆坝看水电开发政策的演变

关于水电开发和水利工程建设对环境与生态的影响的争论仍在继续。怒江、金沙江上诸多待开发的水电工程因争议在缓缓推进,接近尾声的三峡工程也由于逐渐暴露出来的一些问题而受到环保主义者及其他相关人士的密切关注甚至批评。水电开发和水坝建设的不当对生态环境造成破坏,重新评价和调整一些河流的水电开发方案乃至我国整体的水电开发政策,向那些严重破坏生态的水电工程说不,等等,这些声音仍然不绝于耳。在这些声音的背后或者说那些持反坝观点的人,他们的理论依据或许更多地来自国外的反坝运动和拆坝案例。那么,像美国这样的水电大国,他们究竟出于怎样的原因在拆坝,     

拆坝措施在河流修复中的运用

大坝对社会具有重大效益,但同时造成了河流功能的显著变化和生态系统的退化.另外,许多大坝经过长期的运行,已丧失了原设计的功能,大坝的老化产生安全问题也需要考虑拆坝.对拆坝的理论研究和管理还处于起步阶段,对拆坝的标准、措施研究都不够深入.介绍了国外有关拆除大坝的管理实践,讨论了建坝和拆坝对生态系统产生的主要影响.对拆坝在未来作为河流修复措施在我国的应用前景进行了分析.     

美国退役坝拆除背景与原因剖析

为全面、客观反映美国退役坝拆除的真实情况,通过文献检索和专题调研,在综述美国退役坝拆除背景及现状的基础上,分析了美国拆坝的主要原因,即环境、安全和经济因素。案例研究表明,美国拆坝旨在消除老化坝的安全风险,恢复溯河性鱼类洄游以及节省无成本效益的开支。水坝退役是水坝生命周期管理中的一个重要阶段,拆坝并不意味着不能建坝,目前水电仍然是美国最大的可再生能源电力来源。美国在水坝退役方面的理念和经验可为我国水库降等与报废管理提供借鉴。     

拆坝对河流生态环境影响利弊研究综述

为研究拆坝对生态环境影响的利弊,介绍了拆坝对水文情势、泥沙输运、河道地貌、河道水质、栖息地质量及生物多样性等生态的影响,综述了拆坝后河道物理化学及重要生态特性之间响应关系的研究进展。拆坝对生态环境的影响具有时空复杂性,且各生态因子相互影响;水文情势的变化是所有生态响应的触发条件,泥沙输运是生态响应的关键性因子。为了河道长期连续性的开发与利用,研究泥沙输运及泥沙污染物的释放规律,预测拆坝后河道水质时空变化过程,分析流域尺度上生态系统之间的响应关系等对拆坝的评估决策十分重要。     

美国哥伦比亚河流域下游鱼类保护工程、拆坝之争及思考

许多江河在水电开发过程中面临着对鱼类如何保护等环境课题,建立过鱼通道是鱼类保护的措施之一。自20世纪30年代起,美国陆军工程师团在哥伦比亚河及斯内克河下游修建了8座梯级水电站,这些水电站通过各种工程措施维持鱼类洄游通道。近二十年来,环保主义者不断要求拆除斯内克河下游的4座大坝,争论波及全美。美国政府最终拒绝拆坝,认为拆坝不是促进鲑鱼洄游所必须的,采取其它工程措施同样能改善鱼类洄游条件。文章最后提出了一些个人思考,有助于正确认识和处理水电开发与环境保护的关系。     

浅析美国建坝与拆坝

分析了美国101座建坝(坝高100 m或库容30亿m3以上)和248座拆坝(1999~2005年)的特征。得出如下结论:①20世纪60年代为美国建坝高峰期,建坝集中于中西部偏远地区的小河或大河支流上,密西西比河干流未建坝;建坝开发目标依次为防洪、供水灌溉、发电、旅游生态、航运,坝型依次为土石坝、重力坝、堆石坝、拱坝、混合坝及其他;最大坝高234 m,最大库容348.5亿m3。②拆坝的建坝时间为1708~1970年,拆坝的坝高大多小于5 m,最大坝高18 m,拆除的主要是建在小支流或溪沟上的老、弃、废和"无名"坝;拆坝的原因依次为"生态环境"、"水坝安全"、"经济效益"和"泥沙淤积"。③美国建坝与拆坝具有"中西部河流建大坝、东部小河拆小坝"的总体特征。     

Fracturing dams,fractured data: Empirical trends and characteristics of existing and removed dams in the United States

Dam removals in the United States continue to accelerate in pace and scope, but no national analyses have examined how removed dams compare with existing dam stock. Here, we review and analyse the best available national data on dams from the National Inventory of Dams (NID), dam removals from American Rivers, the U.S. Geological Survey, and the National River Restoration Science Synthesis databases to compare trends and characteristics of removed versus existing dams in the United States. If historical trends continue, by 2050 the United States can expect between 4,000 and 36,000 total removals, including 2,000–10,000 removals of NID dams. Best‐fit regression models estimate total costs between $50.5 million and $25.1 billion (mean $10.5 billion, median $416.5 million) for all removals and $29.6 million to $18.9 billion (mean $7.2 billion, median $285 million) for NID removals, a significant cost savings over present stated dam rehabilitation needs. Structural characteristics and ages of documented removals are not representative of existing dams, with privately owned hydroelectric dams subject to public oversight and water supply dams the most disproportionately removed. We conclude that dam removal science would benefit from the creation of an interdisciplinary framework for studying dams as environmental, social, and technological interventions, facilitated by transparent datasets around dams and removals and reflexive research approaches that combine statistical approaches with place‐based analyses.     

黄土高原淤地坝的减沙作用及其时效性

在黄河近年来沙锐减的背景下,需要深入探究黄土高原淤地坝的减沙作用。本文基于实测数据分析了淤地坝拦沙与入黄泥沙减少量的关系、拦沙作用的时效性以及坝地的减蚀作用。研究结果表明,淤地坝拦沙量一般大于或等于相应的入黄泥沙减少量;流域的林草梯田覆盖率越大、地表土壤粒径越粗,淤地坝因拦沙所致的减沙量占其拦沙量的比例越小。淤地坝拦沙作用的时效性非常突出,拦沙库容淤满即基本失去拦沙能力。失去拦沙能力后,淤地坝仍可依靠拦沙所形成的坝地发挥减沙作用;如果流域的林草梯田覆盖状况较差,坝地减沙作用的"空间影响范围"可达自身面积的4倍。不过,随着流域的林草梯田覆盖率增大,单位面积坝地的实际减沙量会逐渐降低。     

The impact of dams on flood flows in the United States

Natural flood regimes provide a wide array of important ecological functions. Our goal is to assess the hydrologic impact of dams on flood flows throughout the United States. Regional regression models of the median annual 1‐day maximum flow were developed as a function of natural watershed characteristics, dam storage, and population density. Most of the regressions have adjusted R2 values in excess of 0.80, and overall the models covered 78% of the area of the continental U.S. Alteration of flood flows is present in every region of the country, and is more severe west of the Mississippi and especially in the southern Great Plains, desert Southwest, and northern California. The percent of U.S. rivers with greater than a 25% reduction in the median annual flood is 55% for large rivers, 25% for medium rivers, and 10% for small rivers. The majority of freshwater ecoregions in the country have at least 10% of their rivers with 25% or greater alteration in all three river size classes. A simple model based on the ratio of dam storage to mean annual runoff was developed for assessing alteration in ungauged rivers, and was found to be generally useful for classifying rivers into categories of potential alteration. Overall, we document the alteration of natural flood flows across the U.S. in more detail than has been previously accomplished, and demonstrate the efficacy of multivariate regional regression models and other indicators for assessing hydrologic alteration. Copyright © 2010 John Wiley & Sons, Ltd.     

Cumulative effects of dams on migratory fishes across the conterminous United States: Regional patterns in fish responses to river network fragmentation

Globally, dams fragment river networks, threatening migratory fishes which require access to distinct habitats to complete their life cycles. Efforts to understand how cumulative effects of multiple dams affect migratory fishes across large regions, such as a country or continent, could help to identify locations for connectivity-enhancing actions to conserve migratory fishes. To address this, we evaluated cumulative effects of dams on migratory fishes in rivers across nine ecoregions of the conterminous USA. First, using fish data from thousands of sites (N = 45,989), we summarized ecoregional patterns in assemblages, quantifying the number of migratory species comprising assemblages, showing the prominence of potamodromous species across the large region as well as differences in migratory life history traits among ecoregions. Next, we compared the importance of a set of river network fragmentation metrics that captured influences of multiple dams in networks versus other anthropogenic landscape stressors and natural landscape factors that impact migratory fishes by ecoregion. We found that migratory fishes were more sensitive to cumulative dam effects than other stressors including urbanization and agriculture in the eastern USA. To further identify specific effects of environmental variables on potamodromous fishes, we conducted Boosted Regression Trees analysis in the eastern ecoregions. Our results suggested that the key natural influences on river fishes included catchment area as well as river baseflow and air temperature, suggesting that migratory fishes may be affected by changing climate. Additionally, we found that downstream dams were more influential than other human stressors to potamodromous fishes, underscoring the importance of enhancing connectivity within river networks to conserve migratory fishes. Collectively, our results provide new insights in identifying threats to migratory fish species across the USA, providing information that can aid in conserving this vulnerable but ecologically and socioeconomically important group of fishes.     

长江流域泥沙输移与概算

泥沙概算是一种通过研究泥沙在流域不同位置和一定时期内的时空分布，从而揭示河流水沙平衡状态的方法。本文通过分析提出了长江泥沙概算方法，探讨长江流域产沙、输沙和泥沙沉积三者的关系。在此基础上，通过流域侵蚀、沉积物和悬移质泥沙粒径级配的对比分析，计算得出每年从长江上游侵蚀下来的泥沙中，粒径大于0．5mm的近12亿t泥沙几乎都沉积在长江流域上游的沟谷和支流中，粒径在0．05-0．5mm范围内的4．92亿t泥沙有3．13亿t沉积在上游沟谷和支流中，而粒径小于0．05mm的5．36亿t泥沙也有2．02亿t沉积在这些区域。当长江上游（宜昌站）和汉江总输沙量超过2．86亿t时，长江中下游将发生淤积，总输沙量超过5亿t时的淤积量超过1亿t；当宜昌输沙量超过1亿t时，洞庭湖区将发生淤积。文章还根据长江干流、部分支流和通江湖泊的水沙关系，建立水沙动态图，研究分析了长江水沙在时空上的分布特点。     

长江上游流域地表侵蚀与河流泥沙输移

在丰富的野外考察、调查及实测资料基础上,将长江上游流域地表侵蚀与河流输沙作为一个完整系统,综合分析了地表物质的侵蚀～堆积～归槽～输移过程,探讨了流域产沙特性及泥沙运动规律。     

Sedimentology of New Fluvial Deposits on the Elwha River,Washington, USA,Formed During Large‐Scale Dam Removal

Removal of two dams 32 m and 64 m high on the Elwha River, Washington, USA, provided the first opportunity to examine river response to a dam removal and controlled sediment influx on such a large scale. Although many recent river‐restoration efforts have included dam removal, large dam removals have been rare enough that their physical and ecological effects remain poorly understood. New sedimentary deposits that formed during this multi‐stage dam removal result from a unique, artificially created imbalance between fluvial sediment supply and transport capacity. River flows during dam removal were essentially natural and included no large floods in the first two years, while draining of the two reservoirs greatly increased the sediment supply available for fluvial transport. The resulting sedimentary deposits exhibited substantial spatial heterogeneity in thickness, stratal‐formation patterns, grain size and organic content. Initial mud deposition in the first year of dam removal filled pore spaces in the pre‐dam‐removal cobble bed, potentially causing ecological disturbance but not aggrading the bed substantially at first. During the second winter of dam removal, thicker and in some cases coarser deposits replaced the early mud deposits. By 18 months into dam removal, channel‐margin and floodplain deposits were commonly >0.5 m thick and, contrary to pre‐dam‐removal predictions that silt and clay would bypass the river system, included average mud content around 20%. Large wood and lenses of smaller organic particles were common in the new deposits, presumably contributing additional carbon and nutrients to the ecosystem downstream of the dam sites. Understanding initial sedimentary response to the Elwha River dam removals will inform subsequent analyses of longer‐term sedimentary, geomorphic and ecosystem changes in this fluvial and coastal system, and will provide important lessons for other river‐restoration efforts where large dam removal is planned or proposed. Published 2013. This article is a U.S. Government work and is in the public domain in the USA.