Emily White, Visiting Research Associate, Center for the Study of Land, Air, and Water, Bard College
大坝所有者在决定维修时有许多因素需要评估, 删除, 或者安装微型水电系统. 在做这个选择的时候, it is important to consider the fate of the sediment (and associated contaminants, 营养物质, 和有机物)积累在大坝后面的水库中. 通过捕获沉积物, dams can deprive downstream areas of the sediments needed to maintain the channel structure that supports the ecosystem of the adjacent areas (i.e.，河岸带).
Dam 删除 can be beneficial by re-establishing natural water and sediment flows, 水的温度, 以及本地生物的栖息地. 然而, 当大坝被拆除时，大量的泥沙会暴露出来并被动员起来, resulting in increased water turbidity that can adversely impact downstream habitat. 这通常被认为是短期效应. 在长期的, dam 删除 is typically thought to be ecologically beneficial on a watershed scale because aquatic connectivity is improved. But there are many uncertainties associated with the risks and benefits of dam 删除.
The effects of sediment release are difficult to quantify because changes occur over long time periods and data characterizing pre-dam conditions are often lacking. 在美国拆除的1200多座水坝中.S. (2014年之前)，不到10%的人进行了科学评估(Bellmore等人., 2016). This lack of knowledge makes it challenging to predict how a specific stream will respond to dam 删除.
National guidelines have been developed for assessing sediment-related effects from dam 删除 (USDOI, 2017). This approach uses an informal evaluation of risk (likelihood of sediment impact and magnitude of consequences) to determine the amount of required sediment investigation (e.g.数据收集、分析、建模和管理). The potential for sediment contamination and the reservoir sediment volume are important factors. 对污染沉积物的关注首先通过筛选调查进行评估. 如果怀疑有相关污染物, sediment sampling and analysis is then required to determine if sediments can be released or if mitigation is needed. 如果污染物浓度低到可以释放, 然后必须考虑泥沙体积和撞击概率. 然而, 蓄水池中沉积物很少的低水头大坝, 例如, has very little risk of sediment impacts so extensive sediment investigations are not needed. Negligible sediment volumes are considered to be less than 10% of the average annual load entering the reservoir. 因为每年的负荷很难确定, an alternative approach can be used that compares sediment volume with the size of a typical alluvial channel (e.g.，沙坝或砾石坝).
Installing a micro hydroelectric project on an existing dam should have negligible impacts on the suspension and transport of sediments downstream. 水量、流量和水质(e.g.，泥沙水平)主要受上游土地利用的控制. 除非进行了影响流的更改, 泥沙运动和洪水模式不应受到影响. 然而, the construction process during installation of a hydropower system can disturb impounded sediment, 特别是需要疏浚的时候. 在纽约州, guidance on dredging activities (as well as in-water and riparian management of dredged materials) is provided by the Department of 365体育备用al Conservation (NYSDEC, 2004).
An important step in gaining approval for a project that involves dredging is the design of a sampling and analysis plan to characterize sediment quality. 本质上, this is the process by which it will be determined if there are contaminants of concerns associated with the sediments. The sediment sampling plan flowchart below can be used to assess whether a project is exempt from submitting a plan to NYSDEC. 关于抽样计划的准备细节，请参阅《365体育备用网》 & 操作指南系列(TOGS.1.9 (NYSDEC, 2004).
如果沉积物取样是必需的, samples must be analyzed for a suite of contaminants in order to classify the sediment (as Class A, B, 或C). 这些沉积物类别是基于这些污染物的浓度, 根据它们对水生生物和生态系统的已知和假定影响. Appropriate management options are then selected given the class of sediment to be dredged.
- 氯化烃(多氯联苯，2,3,7,8- tcdd)
These analyses must be conducted by a laboratory that is accredited by the New York State Department of Health (NYSDOH) 365体育备用al Laboratory Approval Program (ELAP). Based on NYSDOH’s Analytical Services Protocol, sediment testing (to satisfy TOGS 5.1.9; NYSDEC, 2004) requires a full data deliverables package (ASP Category B). A list of certified commercial laboratories is maintained by DOH at the following website: http://www.wadsworth.org/regulatory/elap/certified-labs. 虽然价格可能有所不同，TOGS 5的成本报价.19 sediment testing (including the specific compounds listed above) from one accredited lab in Fall 2018 was approximately $1630 per sample. The number of samples that should be collected in order to adequately characterize the dredged material will depend on the project area and the expected sediment heterogeneity.
- Minimize resuspension by careful equipment operation and/or use of floating booms, 淤泥窗帘, 屏幕, 等.
- Avoid exposing benthic organisms (that live on or in the bottom of the stream) to more highly contaminated material
During dredging, best management practices should be employed that meet these objectives. 例如，应采取措施减少固体的再悬浮(e.g., 使用封闭桶), 应采用淤泥帷幕来保护敏感生境, 在鱼类迁徙和产卵期间，工作应该受到限制. 365体育备用al objectives and best management practices should also be considered when developing a management plan to place the dredged material in the water or riparian zone, 如果根据沉积物类别认为这种放置是适当的(i.e.,污染物存在).
To learn more about sediment transport and deposition, see the following website: http://www.fondriest.com/environmental-measurements/parameters/hydrology/sediment-transport-deposition/
Bellmore J.R.杜达,J.J.克雷格L.S.格林,年代.L.Torgersen C.E.柯林斯,米.J. 和Vittum K. 2016. 美国大坝拆除研究的现状与趋势. 电线的水. doi.org/10.1002 / wat2.1164
纽约州365体育备用保护部. 2004. 技术 & 操作指南系列(TOGS.1.9:泥沙和疏浚物在水中和河岸的管理. 水评估管理局水务局.
USDOI, United States Department of the Interior, Bureau of Reclamation, 技术 Service Center. 2017. 大坝拆除泥沙分析指南.