Evaluating the Potential Risk from Impounded Sediment

Emily White, Research Associate, Bard College/Hudsonia

Dam owners have many factors to evaluate when deciding between maintenance, removal, or the installation of a micro hydropower system. When making this choice, it is important to consider the fate of the sediment (and associated contaminants, nutrients, and organic material) that has accumulated in the reservoir behind the dam. By trapping sediments, dams can deprive downstream areas of the sediments needed to maintain the channel structure that supports the ecosystem of the adjacent areas (i.e., the riparian zone).

Dam removal can be beneficial by re-establishing natural water and sediment flows, water temperature, and habitat for native organisms. However, large volumes of sediment can be exposed and mobilized when a dam is removed, resulting in increased water turbidity that can adversely impact downstream habitat. This is generally considered to be a short-term effect. In the long-term, dam removal 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 removal.

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. Of the more than 1200 dams that were removed in the U.S. (before 2014), less than 10% were scientifically evaluated (Bellmore et al., 2016). This lack of knowledge makes it challenging to predict how a specific stream will respond to dam removal.

National guidelines have been developed for assessing sediment-related effects from dam removal (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., data collection, analysis, modeling, and management). The potential for sediment contamination and the reservoir sediment volume are important factors. Concerns of contaminated sediments are first evaluated with a screening survey. If contaminants of concern are suspected, sediment sampling and analysis is then required to determine if sediments can be released or if mitigation is needed. If contaminant concentrations are low enough to warrant release, sediment volume and probability of impact must then be considered. However, a low-head dam with very little sediment in the impoundment, for example, 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. As the annual load can be difficult to determine, an alternative approach can be used that compares sediment volume with the size of a typical alluvial channel (e.g., sand or gravel bars).

Installing a micro hydroelectric project on an existing dam should have negligible impacts on the suspension and transport of sediments downstream. Water quantity, flow rate, and water quality (e.g., sediment levels) are primarily controlled by upstream land use. Unless changes are made that impact streamflow, sediment movement and flooding patterns should not be affected. However, the construction process during installation of a hydropower system can disturb impounded sediment, particularly when dredging is required. In New York State, guidance on dredging activities (as well as in-water and riparian management of dredged materials) is provided by the Department of Environmental 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. Essentially, 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. Details regarding the preparation of a sampling plan are given in Technical & Operational Guidance Series (TOGS) 5.1.9 (NYSDEC, 2004).

If sediment sampling is required, samples must be analyzed for a suite of contaminants in order to classify the sediment (as Class A, B, or C). These sediment classes are based on the concentration of these contaminants, based on their known and presumed impacts on aquatic organisms and ecosystems. Appropriate management options are then selected given the class of sediment to be dredged.

Compounds to be analyzed for sediment classification:

• Metals (arsenic, cadmium, copper, lead, and mercury)
• PAHs and Petroleum-Related Compounds (benzene, total BTEX, total PAHs)
• Pesticides (DDT+DDD+DDE, Mirex, Chlordane, Dieldrin)
• Chlorinated hydrocarbons (PCBs, 2,3,7,8-TCDD)
• Other potential water quality limiting substances (site specific)

These analyses must be conducted by a laboratory that is accredited by the New York State Department of Health (NYSDOH) Environmental 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: https://www.wadsworth.org/regulatory/elap/certified-labs. While prices may vary, the cost quoted for TOGS 5.19 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.

In order to protect aquatic and wetland habitats as well as riparian areas, the NYSDEC guidelines include the following environmental objectives:

• Minimize resuspension by careful equipment operation and/or use of floating booms, silt curtains, screens, etc.
• Minimize amount of material disturbed or returned to water
• Avoid damage to nearby wetlands and habitats
• Avoid known historical or archaeological sites
• Avoid dredging during fish migration and spawning periods
• Avoid littoral zones (shore between high and low water marks)
• 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. For example, measures should be taken to reduce resuspension of solids (e.g., use of closed bucket), silt curtains should used be used to protect sensitive habitats, and work should be restricted during periods of fish migration and spawning. Environmental 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, if such placement is deemed appropriate based on the sediment class (i.e., contaminants present).

To learn more about sediment transport and deposition, see the following website: https://www.fondriest.com/environmental-measurements/parameters/hydrology/sediment-transport-deposition/

References

Bellmore, J.R., Duda, J.J., Craig, L.S., Greene, S.L., Torgersen, C.E.,  Collins, M.J. and Vittum, K. 2016. Status and trends of dam removal research in the United States. WIREs Water. doi.org/10.1002/wat2.1164

NYSDEC, New York State Department of Environmental Conservation. Designing a Dredging Sediment Sampling and Analysis Plan.

NYSDEC, New York State Department of Environmental Conservation. 2004. Technical & Operational Guidance Series (TOGS) 5.1.9: In-Water and Riparian Management of Sediment and Dredged Material. Division of Water, Bureau of Water Assessment and Management. 

USDOI, United States Department of the Interior, Bureau of Reclamation, Technical Service Center. 2017. Dam Removal Analysis Guidelines for Sediment.