Can sediment sluice solve reservoir sediment deposition problem?

认识水库淤积的主因 Understanding the Main Cause of Sediment Deposition in Reservoirs 合理设置排沙底孔 Reasonably set up sediment discharge bottom sluices 梅志宏 mr.meizhihong@qq.com MEI Zhihong 1. 鱼与熊掌不可兼得，修建大坝有好处就有坏处 1. Every coin has two sides, building dams has advantages and disadvantages 河流，裹挟着泥沙从高山向海洋进发，水沙一体犹如人体的血液，流淌在高山 峡谷和丘陵平原，输送着大量的物质和能量，造就了河流沿岸发达的农业和渔 业；也像人体的神经系统，传播交流着信息文化。哪里有大河，特别是河流下 游平原地区，哪里就有发达的农渔业文明，并以此为基础发展出现代工业文明。 可以说，人类文明，离不开河流。物质、能量、信息的流动、交换，是自然界 和人类社会演化发展的基本要素。 River, carrying silt marches from mountain towards ocean, water-and- silt-inone, like the blood inside the human body, runs through mountains, valleys, hills and plains, transporting large quantities of substance and energy, created flourishing agriculture and fishery along the banks; it’s also like the nervous system of the human body, transmitting and communicating information and culture. Where there is a big river, there is advanced agricultural and fishing civilization, particularly in the downstream plain area, serving as a basis on which modern industrial civilization develops. It can be said that human civilization can hardly do without rivers. The flow and exchange of substance, energy and information is the fundamental element for the evolution and development of the nature and human society. 河水，有涨有落，泥沙，悬浮沉落。来水来沙虽然随着季节呈现出一定的周而 复始的规律，但，有时，洪水泛滥，淹没良田，有时，水枯石落，人畜饥渴！ 江河裹挟泥沙变迁无常，“三十年河东，四十年河西”，河流，在造福人类的 同时，水患沙患也无穷！ Water rises and falls; silt suspends and sinks. Although the incoming water and sediment exhibit certain repeated rules with the seasons, sometimes, however, there are floods inundating fertile farmlands while other times, droughts threatening human and livestock. Things change constantly over time. Rivers carrying silt keep changing unpredictably to bring benefits as well as endless floods and sediment problems to the mankind. 千百年来，为了解决人畜饮水，灌溉防洪，通航发电，世界各地的劳动人民在 河流上修筑堤坝，开发利用水资源，兴利避害。最常见的是在江河上修筑大坝， 抬高水位蓄水，形成水库，按照人们的需要供应水量等。这就像家中修建粮仓， 将秋天时丰收的稻谷存储起来供应其他季节，一切都随其自然，没有什么不妥。 然而，也不可避免的，大坝有负面影响。最常见的是：

For thousands of years, in order to provide water to men and livestock, irrigate crops, prevent floods, navigate vessel and generate electricity, people around the world built dams on rivers to exploit water resources, promote advantages while prevent disadvantages. Building big dams on rivers is most common to store water by elevating water level and form reservoirs to supply water for people’s need. Same as building barns at home to store the harvest in the fall for other seasons’ supply, all are natural, nothing inappropriate. Inevitably, however, dams have negative effect, including mostly the following: 首先，大坝可能会阻断船只的自由通行，阻隔鱼类自由洄游，阻断了泥沙输移 的连续性； Firstly, dams may block free passage of vessels, free migration of fishes, and continuity of sediment transport; 其次，水位抬高可能淹没大量的土地，特别是良田； Secondly, elevated water level may inundate vast land area, especially fertile farmland. 再者，水位的抬高，意味着大坝上游的水流流速的减缓。人们从生活常识中可 知，如果裹挟着大量泥沙的水流流速变缓到一定程度，河流将会淤积堵塞，水 质可能恶化！中国有句成语，叫“流水不腐，户枢不蠹”就是描述的这个现象。 对于大坝下游而言，泥沙在上游沉积后，水流中泥沙含量相对不足，形成了 “饥饿之水”，缺乏泥沙的“清水”将冲刷河底和河岸，洲滩消失或变迁，河 岸坍塌，河道移动，对沿岸的居民、鱼类栖息地等造成威胁。由于河道经常难 以捉摸地变动，加上有机质和营养物的来源减少，大坝将对下游航道、取水口、 工农业、渔业等产生负面影响。 Thirdly, elevated water level means slowed-down flow velocity in the upstream of the dam. People know it from common sense that if the flow velocity of water carrying massive silt slows down to a certain degree, sediment would deposit, river clog and water quality deteriorate. “Running water never stales and doorhinges never get worm-eaten” is the Chinese idiom describing this phenomenon.

For the downstream of the dam, sediment concentration of the current becomes comparatively deficient after sediment deposits in the upstream, resulting in “water of hunger”. Sediment- starved “plain water” will erode the riverbed and river banks, bottomland will diminish or change, riversides collapse, river course shifts, posing threats to the coastal inhabitants and habitat for fishes. Due to unpredictable river course changes, plus decreased source for organic matters and nutrients, dams will have negative effect on downstream waterway, intakes, industrial, agricultural and fishery industries etc. 可以说，人类利用河流，得到了好处，也不可避免的带来了负面作用。我们如 何平衡这种关系？ It is fair to say that river exploiting benefits mankind also brings inevitable negative effects. How can we balance the relationship?

一种回答是，在河流上不要做任何工程开发。有一个笑话可以回答这种看法： 人可能因为吃饭被噎死了，因此，人不要吃饭。显然，这不是智慧人类具备的 风险思维方式，否则，人类不可能发展到今天。 One answer is “No engineering development on the rivers”. There is a joke about this kind of viewpoint: a man should not eat because he may get choked to death.

Obviously, it is not the way of risk thinking that intelligent humans have, otherwise we would never have developed to be what we are today. 另一种回答是，有限度的开发利用自然资源，承担可接受的风险，走可持续的 发展之路。历史经验表明，后者的选择是明智的。为了实现可持续开发，人类 在实践中总结了许多工程和管理措施。例如，大坝造成的阻断负面作用，可以 通过修建船闸、升船机、鱼道等缓解；对于水库淹没丧失大量田地负面作用， 则通过合理选择坝址和水库水位，避免水库有大量淹没，采取土地补偿、替代 发展等措施加以缓解。在此暂时不做进一步的展开分析。 Another answer is “Sustainable development with limited exploiting of natural resources and acceptable risks taking”. Historical experience shows that the latter choice is wise. To achieve sustainable development, human beings have developed many engineering and managerial measures through practice. For example, by building ship locks, ship lifts, and fish ways etc. to mitigate the negative blocking effects caused by dams; the impact of vast farmland inundation can be eased through reasonable site selection and reservoir level to avoid dam inundation as well as land compensation and alternative development measures.

This article will not elaborate on this for the moment. 湄公河干流沙耶武里电站为了解决水库排沙问题，减少了 3 孔溢流表，增加了 4 孔高程较低的排沙底孔。有人认为，这是解决水库泥沙的必要措施，似乎所 有的电站都应该利用排沙底孔来解决整个水库的排沙问题。究竟是不是这样呢？ 湄公河干流的北本电站又是如何考虑的？ To solve issue of reservoir sediment discharge, the Xayaburi Hydropower Plant located on the mainstream of Mekong River reduced three crest overflowing orifices and increased four lower-elevation sediment discharge bottom sluices.

Some people think that these are necessary measures to cope with reservoir sediment deposit issue and it seems that all hydropower stations should use sediment discharge bottom sluice to solve sediment discharge problem. Is it really the case? How does the Pak Beng Hydropower Station on the mainstream of Mekong River think about that?

2、“敌退我进，敌进我退”，现有技术手段可以实现可持续开发，关键在降低 水位 2. “When the enemy retreats, we advance. When the enemy advances, we retreat”, existing technologies can achieve sustainable development, and the key is to minimize the water level 从上面的叙述中可以看出，水库中泥沙淤积产生的主要原因，是水库形成以后， 水流的流速变慢了，泥沙沉积到水库中。流速变慢的原因，是因为水库抬高了 水位。因此，大坝修建后和修建前相比，最明显的变化是水位抬高了。

From the above account, we know that the main cause of reservoir sediment is sediment deposition in the reservoir due to slowed down water velocity after the reservoir is built. The reason for slowed down velocity is because the reservoir elevates the water level. So, the most distinct change before and after a dam is constructed is the elevated water level. 假设在自然的河流情况下，大坝坝址处的来流量为 Q，相应的水位为 Z。修建 水库以后，在同样的来流情况下，水库的水位将被抬高到 Z+dz。dz 就表示抬 高的程度，它越大就意味着水库水位抬高的程度越大。如果 dz=0,就是水位没有 抬高，河流流速没有减缓，泥沙的运动和修建大坝前是一样的。大坝上游水流 的泥沙输沙能力与天然水流一致，河流不会有额外的淤积，下游也不会因为上 游淤积加剧改变水流泥沙含量，从而造成下游河道冲刷、变迁。 Assuming, under a natural river condition, the discharge at the dam site is Q and the corresponding water level is Z. After the dam is built, the reservoir level rises to Z+dz with the same discharge. “dz” represents the elevated degree, the bigger the value is, the higher the reservoir level is elevated. The water level is not elevated if dz equals to 0 and velocity is not slowed down, sediment transport remains the same as it is before constructing the dam. In upstream of the dam, the sediment transport capacity of the current is consistent with that of natural current, the river has no additional sediment deposition, so downstream sediment concentration does not change due to intensified sediment deposition in upstream, resulting in scouring and change of river way in downstream. 一方面，人类需要修筑大坝抬高水位，便于水资源的存储，通过蓄丰补枯，提 供水资源给居民生活和工农业利用。同时，河道水位抬高以后水深加深、河宽 展宽、水流平稳，便于船只航行。另一方面，为了避免大量泥沙淤积在水库里， 又要求不要抬高水位，显然两种要求是相互矛盾的！ On the one hand, mankind needs to build dams to elevate water stage to store water resource easily and by accumulating water in high flow years and discharging water in low flow years to provide water supply for people’s daily use and agricultural and industrial utilization. Meanwhile, water depth deepens, river width widens and flow smoothens after water level gets elevated, making navigation easier. One the other hand, water level should not be elevated to prevent huge sediment deposition in the reservoir. Obviously, these are contradictory. 如果我们观察河流可以发现，河流泥沙的含量和大小并不是每时每刻都是一样 的。一方面，不同的流量，泥沙含量有明显差异。对于大部分河流而言，尤其 在湄公河流域，旱季，水清，泥沙含量接近零。雨季，一般情况下，水浑，含 量也比较小，大部分是悬浮在水体中的细颗粒泥沙。某些时期由于暴雨产生洪 水，可以看到这样的景象：洪水滔滔、沙洲变迁，泥沙颗粒大到几十厘米以上， 坚硬的岩石河床抵抗住洪水的力量，但软弱的河床河岸可能发生剧烈变化。通 过分析澜沧江水文站的有关资料可以发现，在没有发生大洪水的时期，澜沧江 湄公河中的泥沙含量通常是非常小的。若发生大洪水则会有大量泥沙一起向下 游输移。并且，洪水越大，沙量越大。 If we observe the river, we can find that the sediment concentration and sizes are not always the same at all times. On one hand, sediment concentration varies

significantly in different discharge. For most rivers, particularly in the Mekong River Basin, sediment concentration is close to zero during dry season and when water is clear. In rainy season, the water is muddy in most cases but sediment concentration is also relatively low, mostly are small sediment grains suspending in the water. Some times, heavy rainstorm brings floods with scenes like surging floods and changing bottomland, sediment grains can be bigger than tens of centimeters; rock riverbed is strong to resist floods, but soft riverbed and riversides may experience dramatic changes. Analysis on the related data from hydrometrical stations along the Lancang River suggests that sediment concentration in the Lancang River/Mekong River is very small during the time when there is no major flood. When a big flood occurs, massive sediment moves towards downstream. The bigger the flood is, the greater the sediment concentration will be. 另一方面，泥沙颗粒大小差异很大。中小流量下水流中泥沙除了含量很小，颗 粒也极其细小，水流轻微的流动就可以将泥沙带到下游。少数粗颗粒的砾石卵 石、粗沙等，往往在较大洪水时期间歇性的在河床附近滚动、跳动、移动（可 以称为“推移质”），走走停停，对流量大小更加敏感。如果流速不足够大， 河床表面的的推移质是不会运动的。 On the on hand, the sizes of sediment grains vary widely. Besides low sediment concentration, sediment grains in medium to low discharge are extremely small, which can be carried to downstream by gentle current. A few bigger grains, gravels, cobbles, and coarse sand etc. often roll, jump or move (the so-called “bedload”) intermittently near the riverbed in bigger floods, taking breaks occasionally and are more sensitive to the size of the discharge. If the velocity is not high enough, the bedload on riverbed will not move. 了解了上述河流泥沙输移的规律，人们就可以根据它制定出一套规则，在流量 较大例如在发生大洪水时期，降低水库的水位，让大坝抬高的水位与自然状态 相比非常接近，洪水流量越大，要求大坝抬高水位作用要越小，越接近天然情 况下的水位，对水库的排沙作用越好。 Basing on the understanding of the above laws of sediment transport, people can develop a set of rules to minimize reservoir water level to be very close to the natural condition during higher discharge period like big floods, the bigger the floods are, the lower the elevated water level by the dam will be; and the closer it is to the natural water level in natural condition, the better for reservoir sediment discharge. 把上述措施简述为：“当上游大量泥沙来了，降低水位恢复流速，将泥沙从水 库中排到大坝下游，没有泥沙或者泥沙较少，抬高水位以利于供水、灌溉、航 运、发电”，类似于战争中采取“敌进我退，敌退我进”的战术。这种措施可 以实现水库的水资源开发利用与水库排沙之间的平衡，并且可以避免水库泥沙 大量淤积，避免“清水”（饥饿之水）对下游河岸河床的冲刷，及时平衡上下 游河流之间的物质、能量、信息的输移交流。 In a nutshell, in case a big quantity of sediment coming from upstream, to minimize the water level and restore velocity to discharge sediment to downstream of the dam; when there is no or little sediment, to elevate water level for water supply, irrigation, navigation and electricity generation. It’s just

like the tactics of “When the enemy retreats, we advance. When the enemy advances, we retreat” applied in warfare. This measure can achieve the balance between water resource development and reservoir sediment discharge, and avoid “plain water” (hungry water) to scour downstream riverbed and banks and promptly balance the transport and exchange of substance, energy and information between upstream and downstream.

3、如何有效的降低水库水位？ 3. How to minimize reservoir water level effectively? 由以上叙述可知，在上游泥沙集中进入水库前，及时、有效降低水库水位、将 水位尽量恢复到天然水位，是有效排除水库泥沙的关键！ We know from the above that, before upstream sediment collectively enters the reservoir, to promptly and effectively minimize the water level as much as possible to natural condition is the key to effective releasing of reservoir sediment. 那么，如何做到？ Then, how do we do it? 首先，水库的运行水位与天然水位相比，抬高程度不宜过高，即尽量修筑低坝 来开发利用河流。这样的好处在于，容易做到较快速、平稳的将水库水位恢复 到天然状态，也容易解决过鱼、通航的问题。同时，低坝方式不会造成大量的 土地被淹没。 Firstly, it’s inadvisable to have the reservoir’s operational water level elevated too high, comparing with the natural water level; in other words, try to construct low dams to exploit rivers. The benefits are, it is easy to restore reservoir water level to natural condition more quickly and smoothly, and to solve the problems of fish pass and vessel navigation. Meanwhile, low dams do not inundate vast land are. 第二，让水流进入水库以后，尽快流出水库，停留时间越短越好，即不要将来 水长时间地存储在水库中，这样对水流、泥沙的运动状态的改变最小，水库不 容易淤积，水质也不容易发生恶化，水温也基本不变，对水环境影响最小。即 “径流式”开发方式。（Run-off）。 Secondly, after the water enters the reservoir, let it run off as soon as possible, the shorter time it stays, the better; that is to say, do not store the incoming water in the reservoir for a long time, thus to change the current and sediment movement state to the minimum, sediment is difficult to deposit, water quality not easy to deteriorate, water temperature barely changes, and water environment gets minimum impact. This is the so-called “run-off” development mode. 第三，必须设置足够多的泄水建筑物，让洪水在接近天然水位的状态下通过大 坝。 Thirdly, must set up sufficient water releasing facilities to let the flood pass through the dam in close to natural water level condition.

为了发电、通航、过鱼等需要，必然要修建电站厂房、通航建筑物、过鱼建筑 物，这意味着原河道的泄洪断面可能被侵占了一部分，造成水位的抬高，流速 变缓，而此时水流携带大量泥沙，势必造成水库泥沙淤积。水位的抬高程度越 大，产生泥沙淤积的程度越大。因此，必须满足“泄洪时水位接近天然状态” 的原则。设置足够多、能力足够强的泄水建筑物就是必然选择。 Power house, navigation structure and fish pass facilities must be built for electricity generation, vessel navigation, and fish pass etc., meaning parts of the original river way’s discharge cross-section may be occupied, resulting in elevated water level and slower velocity, when the current carries huge quantities of sediment, reservoir sediment is bound to deposit. The higher the water level is elevated, the greater degree of sediment deposition will be.

Therefore, the principle of “water level to be close to natural condition during flood discharging” must be followed. To set up sufficient and strong enough water discharging structure is a must. 从生活中我们可以发现，河流的表面水流流速远远大于河流底部的水流，即河 面流速最大，河底流速接近零。原因在于越接近水面，水体受到的阻力主要是 空气和河岸，此时阻力小，流速大；越接近河床，受到的阻力主要是河床和河 岸，还有下游水体的阻碍，此时阻力大，流速小。 We observe from daily life that the velocity of river surface is much higher than that of the river bottom, in other words, river surface has the maximum velocity while the velocity of river bottom is close to zero. The reason is that when it is closer to the surface, the resistance is smaller, mainly from air and riversides, hence the velocity is high; When it is closer to the riverbed, more resistance comes from riverbed, riversides and the downstream water, hence the velocity is lower. 在泄水建筑物中，溢流表孔位于水面以下一定深度。而底孔则位于高程较低的 位置，受到下游河道水流的阻碍影响较大，加上河床阻力作用，在同样的过水 面积情况下，溢流表孔的泄水能力远远超过底孔的能力。为了满足“泄洪时水 位接近天然状态”的原则，使水库尽量不拦截泥沙，顺利地将上游来水来沙排 到下游，应该优先使用溢流表孔降低水位。从实际情况看，布置了电站厂房、 通航建筑物、过鱼建筑物后，河道的泄水面积被迫减少。为了便于建筑物布置 和施工方便，同时为了泄洪排沙恢复到“自然状态”，大坝的选址时候往往是 在较宽的河段，宽度不够时还要扩宽坝址河道宽度。因此，大坝设计泄洪建筑 物和水库排沙建筑物时，应优先选择溢流表孔。 Inside the water releasing structure, crest overflowing orifices are located in a certain depth under the water. Bottom sluices are located in area with lower elevation, which is affected more by the resistance from downstream current; plus the resistance from the riverbed, with the same discharging cross-section, the capacity of crest overflowing orifice is far beyond that of bottom sluice In order to follow the principle of “water level to be close to natural condition during flood discharging” to let the reservoir trap the least sediment and to discharge the water and sediment coming from upstream to downstream smoothly, crest overflowing orifices should be given priority to use to minimize the water level. From reality’s perspective, river way’s discharging area is forced to reduce after setting up of the power house, navigation structure and fish pass.

For easy structure layout and convenient construction, as well as for water and sediment discharge to restore to “natural condition”, dam site selection is usually in comparatively wider cross-section of the river, or if it is not wide enough the dam site river needs widening. Therefore, crest overflowing orifice should be given priority when designing the dam’s water discharging structure and sediment discharge structure. 北本电站的设计中，除了电站厂房、通航建筑物以外，其余的断面全部用于泄 洪排沙，并且扩宽了原河道断面，溢流表孔的泄水断面又宽又深，做到了“泄 洪时水位接近天然状态”。从下图看，对比修建大坝前后，流量较小时泥沙非 常小，水库可以抬高水位，便于发电、通航等。当流量较大，水库水位已经非 常接近天然的水位。 In the design of Pak Beng Hydropower Plant, except for the power house and navigation structure, the rest of the cross-section is used for water and sediment discharge. The original river way cross-section is also widened and the discharging cross-section of the crest overflowing orifice is wide and deep, meeting the “water level to be close to natural condition during flood discharging” principle. The following chart illustrates that, comparing before and after the dam construction, sediment is very small when discharge is low and the reservoir can elevate the water level for electricity generation and navigation; when discharge is high, reservoir water level is already pretty much close to natural condition.

4、如何理解冲沙底孔的作用？ 4. How to understand the roles of sediment discharge bottom sluice • 也许有人会问，北本电站设置表孔已经具备恢复到天然的泄洪和排沙能 力了，为什么仍然设置了冲沙底孔？它与沙耶武里的排沙底孔有什么区别？

• Some people may ask, why would Pak Beng Hydropower Plant set up bottom sluices when it’s crest overflowing orifices already have the capacity to restore to natural water and sediment discharge? How are they different from the sediment discharge bottom sluice of Xayaburi Hydropower Station? 其实，北本电站也设置了 8 孔冲沙底孔，并且还是尺寸较小、数量较多，分布 在电站进水口附近。 Actually, Pak Beng Hydropower Plant also has eight sediment discharge bottom sluices, smaller in size, more in quantity, located near the power plant’s intakes area. 原因在于，冲沙底孔的作用不是解决大范围的水库泄洪和排沙，而是为了小范 围局部的冲沙。 The reason is that the role of sediment discharge bottom sluice is not to solve wide-range reservoir water and sediment discharge but rather small-scope and partial sediment sluicing. 实际上，通过生活的观察，或者理论和实践知识可以知道，冲沙底孔在泄水时 候，作用的范围仅仅在上游不远处，一般就几十米到几百米范围。对于靠近冲 沙底孔几十米到几百米范围的淤积物有效。如下图： In fact, we know from daily life observation, or theoretical and practical knowledge, that during water discharge the sediment discharge bottom sluice only covers a scope of tens to hundreds of meters near the upstream. It is effective to deposition within a scope of tens of meters to hundreds of meters near the sediment discharge bottom sluice. See below figure:

Note: When the low level outlet opens, local velocity will increase significantly, sedimentation near the gate will discharge to downstream with the water, the affected scope of the low level outlet is very limited near the dam. The affection is not for discharging sedimentation in all reservoir.

对于北本水库而言，设立冲沙底孔，就是为了排除淤积在电站进水口附近的淤 积物，而不可能用于排除整个水库的泥沙淤积物。 To Pak Beng Hydropower Plant, setting up sediment discharge bottom sluices is for sluicing sediment deposition near the intakes of the power plant, not for sediment deposition discharge of the whole reservoir. 沙耶武里电站减少了 3 孔溢流表，增加了 4 孔排沙底孔，有利于大坝上游几十 米到几百米范围淤积物的排出，但作用范围非常局部。由于整个水库的泄洪排 沙能力降低了，因此，反而不利于整个水库的泥沙输移。 Xayaburi Hydropower Plant reduced three crest overflowing orifices and increased four sediment discharge bottom sluices, helping discharge deposition in a range of tens of meters to hundreds of meters in the upstream, but the coverage is very partial. Because the capacity of water discharge and sediment discharge of the whole reservoir reduced, it is bad for sediment transport of the whole reservoir.

• 还有人会问，中国黄河上著名的三门峡电站，由于泥沙淤积太严重，后 来改造时增加排沙底孔，这又如何解释？ • People may ask, the famous sedimentation-plagued Sanmenxia Hydropower Plant on China’s Huanghe River also had sediment discharge bottom sluices installed in renovation, how to explain that? 首先，黄河泥沙数量非常大，水流流速稍微变缓就会造成河道淤积。当时在开 发三门峡电站时，无论在理论上还是实际经验都缺乏对黄河泥沙的深入认识， 并且，当时的人们预测，上游植被将恢复、水土流失将减轻，黄河泥沙来量会 大幅度减少。 First, the sediment of Huanghe River is massive, and a slight slowdown in velocity would result in sedimentation. There was a lack of deep understanding about Huanghe River’s sediment in terms of both theory and practical experience at the time when the power plant was built. Furthermore, people at that time were forecasting Huanghe River upstream vegetation would restore, water and soil erosion would ease, and thus sediment quantity would dramatically decrease. 其次，没有遵循上文所述的“低坝”、“径流式”开发原则，大坝抬高水位明 显，水流流速减缓明显。大坝设置的泄水建筑物的泄洪能力远远不够，不能将 水位恢复到天然状态。水库建成投入运行，为了水资源开发的需要，必须大幅 度抬高水位蓄水，水流和泥沙在水库的滞留效应非常严重，严重的泥沙淤积必 然发生。 Second, project development did not follow the above said “low dam” and “runoff” principle. The dam elevated water level significantly and velocity slowed down evidently. The discharge capacity of the dam was far from sufficient and was unable to restore the water level to natural condition. After operation, the reservoir had to elevate water level dramatically to store water for the need of water resource development. Water and sediment retention effect in the reservoir was very serous, severe sediment deposition was inevitable.

后期对电站改造，主要是增加排沙底孔。其作用首先是增加大坝的泄洪能力。 原来大坝设计的泄洪能力太小，水位不能充分降低到接近天然水平，增加排沙 底孔以后，有利于降低水库水位，从而减少水库淤积。另外是有利于排出底孔 上游附近的淤积物。但由于底孔的泄洪能力十分有限，同时为了水资源的开发 利用，做不到“恢复到自然的泄洪排沙状态”。实际上，三门峡的底孔改造效 果，仍然存在着较大的争议。 Power plant renovation in later stage mainly had sediment discharge bottom sluices installed. The function was firstly to increase the dam’s discharging capacity. The original design had too small discharging capacity and the water level was unable to fully lower to close to natural condition. Adding sediment discharge bottom sluices helped minimize reservoir water level to reduce reservoir sedimentation. Moreover, it also helped discharge deposition in the reservoir bottom near the upstream. However, it failed to “restore to natural flood discharging and sediment discharge condition” because the sediment discharge bottom sluices’ capacity is very limited, as well as for the need of water resource development. In fact, the result of Sanmenxia Dam bottom sluices innovation still remains highly controversial. 今天，许多水利工程在开发和保护中取得某种平衡，就是吸取了类似“三门峡 电站”失败的经验教训。 Today, with the lessons learnt from the failures of the Samenxia Hydropower Plant, many hydro projects achieve some balance between development and protection.

• 设置排沙底孔有利于底孔上游短距离的冲沙。但要注意一下几个原则： • setting up sediment discharge bottom sluices is beneficial to small scope sediment discharge near its upstream. However, the following principles need attention: （1） 排沙底孔的目的是解决局部范围的泥沙淤积； （1） the purpose of sediment discharge bottom sluice is to cope with local sediment deposition; （2） 为了经常可以使用排沙底孔，孔口尺寸不宜太大，否则大坝前的水位可 能造成较大波动，影响水库正常功能发挥。同时，也可避免较大流量对下游河 岸冲刷。还要避免长时间不使用，造成泥沙淤积堵塞闸门。总结为一句话： “小孔口，小流量，勤使用”。 （2） to be able to use the sediment discharge bottom sluice often, the sluice size should not be too big, otherwise the water level before the dam may fluctuate greatly and affect the reservoir to work normally. Meanwhile it can also avoid comparatively bigger discharge to erode downstream watersides. Try to avoid not using them for long time, which may result in sediment blocking the gate. To sum up: small orifice, low discharge, use often. （3） 底孔高程不宜太低，特别要距离河床一定高度。一方面，孔口太靠近河 床，水流通过的效率低。另一方面，河床底部通过的大量卵石砾石可能磨损底 孔本身及闸门部件。留有一定的安全高度，可以利用河床附近的空间沉淀最大

颗粒的淤积物，它们对底孔的磨损作用最强。一句话，为了排出淤积物，首先 要保证自己长久的安全运行，也要控制好投资。 （3） the bottom sluice elevation should not be too low, especially the distance to the riverbed should be a certain height. On one hand, the flow efficiency is low if the sluice is too close to the riverbed; on the other hand, a large number of gravels and pebbles in the bottom of riverbed may abrade bottom sluices themselves and the components of the gate. Put aside a certain safety height and make full use of the space near the riverbed to deposit the biggest sediment grains which is most harmful to bottom sluices. In one sentence, in order to discharge deposition, first need to ensure safe operation for long time, and then control investment well. （4） 对于水深较深的蓄水型高坝大库，泥沙与周边水体形成一股密度较大的 整体，可能从水库尾部潜入到水库底部，长距离的运动到大坝前（称为“异重 流”），此时设置底孔可以有效的排沙。但对于“低坝”、“径流式”开发， 不属于此类。 （4） for deeper-water storage-type high dam and big reservoirs, sediment and surrounding water forms into one with higher density. It may sneak from the tail of the reservoir into the bottom and finally arrives in front of the dam from a long distance (the so called “density current”). In this case, setting up bottom sluice can effectively discharge sediment, however, “low dam” and “run-off” development does not fall into this category. （5） 对于湄公河干流“低坝-径流式”水库，水库长度往往有几十到 100 多 km。进入水库的粗颗粒泥沙（推移质，Bedload），首先会淤积在距离大坝几 十到 100 多 km 的地方，这部分泥沙走走停停，移动到大坝坝前，估计要几十 年以后。因此，利用低高程的冲沙底孔排出这部分泥沙，出现效果的时间将是 大坝运行几十年以后。 （5） for “low dam-runoff” reservoirs on the Mekong River mainstream, the length of the reservoir is often tens to over 100 kilometers long. Big-grain sediment (bedload) entering the reservoir will first settle down in place with about tens to over 100 kilometers distance from the dam, then keep on marching forward and take breaks once in a while before they finally arrive in front of the dam after maybe decades of years. Therefore, the effect of using low-elevation sediment discharge bottom sluice to discharge this part of sediment may emerge after decades of years after the dam kicks off operation.

5、结语 5. Conclusions 在河流开发的过程中，人们积累了大量的理论和实践经验，不断地加深对河流 的认识。“低坝”“径流式”开发方式，是实现开发与保护之间平衡的一种可 持续的开发方式。 During the process of river development, people have accumulated a large quantity of theories and practical experience as well as deepening understanding about rivers. “Low dam” and “run-off” are sustainable development ways to achieve the balance between development and protection.

每条河流有自己的特点，任何开发要根据开发目标，考虑水文和泥沙特点，结 合其他的工程经验加以选择。切不可盲目抄袭，也不可因为有一定的负面影响， 完全拒绝开发。须知，人类走路，也有摔死的时候，不可因为有任何的风险而 拒绝机遇。人类文明的历史已经告诉我们，只能基于风险的思维人类才可能生 存和发展，关键要有“可持续发展”的理念。 Each river has its own features. Basing on the goals, any development must take into account the hydrological and sediment characteristics of the river to select the mode with the lessons learnt from other engineering projects. Neither can one copy blindly nor should he refuse development completely because of certain negative effects. Please bear it in mind that one may fall and die even while walking, so mankind should not turn down opportunities due to risks. The history of human civilization told us that only human beings with risk-thinking can survive and develop, and it is key to have the “sustainable development” philosophy. 对于“低坝”“径流式”水库，水库排沙的关键是设置足够的泄洪建筑物，要 优先设置表孔泄洪排沙。在来沙量较大的时期，让水库水位恢复到天然状态， 让水流和泥沙都不要滞留在水库里面。 For “low dam”, “run-off” reservoirs, the key to sediment discharge is to set up sufficient discharging facilities and to give crest overflowing orifice priority for water and sediment discharge. During the period of big sediment, let the reservoir water level restore to natural condition and prevent water and sediment from being trapped in the reservoir. 排沙底孔的作用仅仅是局部范围冲刷淤积物。在电站取水口等泥沙淤积可能妨 碍其功能的地方，可以设置低高程的排沙底孔。其原则是“小孔口，小流量， 勤使用”，同时为了保证自身安全运行，底孔高程不宜太低。 Sediment discharge bottom sluice only scours sediment deposition partially.

Low-elevation sediment discharge bottom sluice can be set up in areas near the intakes of the power plant, where sediment deposition may obstruct its function.

The principle is “small orifice, low discharge, use often”; meanwhile bottom sluice elevation should not be too low in order to ensure its safe operation.