Stillaguamish Delta Dredged Sediment Re-use
- Salish Sea References
- Wiki Rules
- What Links To This Page?
The Stillaguamish Flood Control District has proposed to use dredged sediment to enhance tidal marshes in the estuary, to assist with Chinook recovery. This project is being considered within the context of Snohomish County’s Sustainable Lands Strategy, an effort among fish and farm interests to develop strategies that generate net gains for both. Re-use of dredged sediment to enhance tidal marshes has been done in other regions, but is uncommon in Puget Sound. A process has been initiated to evaluate whether dredged sediment re-use should be considered, and to determine the technical and scientific issues that need to be clarified in a feasibility study.
Proposal to Re-Use Dredged Sediment
The Stillaguamish Flood Control District has proposed to dredge the Old Stilly river channel in order to reduce drainage problems on farmland. As part of the proposal, they pose the idea of using the dredged sediment to enhance tidal marshes in the estuary, to assist with Chinook recovery. This project is being considered within the context of Snohomish County’s Sustainable Lands Strategy, an effort among fish and farm interests to develop strategies that generate net gains for both. With the possible exception of some small urban estuary projects, re-use of dredged sediment to enhance tidal marshes has not been done in Puget Sound. For this reason, Snohomish County requested that Western Washington University convene technical experts who could develop a conceptual framework for evaluating the potential to re-use dredged sediment in marshes. This effort, and the content of this page, was not intended to address the complex issues of whether a channel should be dredged, or what the potential ecological impacts of dredging might be. We focus here only on the question of whether sediment should be used to enhance tidal marshes.
Resources including some literature references and links to examples of sediment re-use projects around the U.S. can be found on the Beneficial use of dredge materials page.
Expert Workshop: Science and Technical Issues Identification - February 5, 2014
A small workshop with 12 people was held to frame the technical discussion of sediment re-use.
Workshop Objective: Identify a technical and scientific framework to guide an evaluation of sediment reuse opportunities. These issues include how to determine whether dredged sediment re-use should be considered, and if so, what information is needed in order to determine where and how to apply the sediment.
Meeting Summary: Appendix 1 of File:Fuller 2014 stillaguamish dredged sediment reuse.pdf
This summary includes discussion of:
- Current Conditions in the Stillaguamish Estuary
- Sediment re-use in the estuary: is there a potential positive role?
- What information is needed to determine where and how to apply sediment?
Stillaguamish Technical Advisory Group Discussion - February 12, 2014
Meeting Objective: Update the TAG on the development of the concept of re-using dredged sediment to enhance tidal wetlands. Discuss with the TAG the general issues and concerns that any further evaluation of sediment reuse opportunity will need to address.
The Discussion Summary can be found in Appendix 2 of File:Fuller 2014 stillaguamish dredged sediment reuse.pdf.
Conceptual Framework for Evaluating the Beneficial Re-Use of Dredged Sediment
Please feel free to edit this section
Based on the output from the expert workshop, the Stillaguamish TAG discussion, and other resources, the following conceptual framework was developed. A report summarizing the expert input and a first draft of this conceptual framework was first provided to Snohomish County in March 2014, and can be found in Fuller 2014.
Three Options for Sediment Re-use
The expert workshop determined that clean dredged sediment could be a beneficial tool to improve tidal wetlands in the Stillaguamish estuary under certain conditions. Re-using dredged sediment to enhance tidal wetlands does not address the underlying ecological processes that drive sediment or habitat dynamics. As such, these types of projects are not considered process-based restoration, but they may enhance or restore structural attributes of habitat.
Three broad categories of use were identified. See Appendix 1 in Fuller 2014 for a summary of expert comments and issues identified for these options. There are many uncertainties that would need to be clarified before a decision could be made about sediment re-use, and the following recommendations provide guidance for addressing those uncertainties.
The three potential methods of re-use include:
1. Add the sediment to a subsided restoration site so that it more closely matches adjacent marsh elevations and to improve marsh and channel habitat development.
Summary of discussion
Adding sediment to subsided restoration sites provides the greatest immediate potential benefits with the lowest levels of uncertainty. Applying it to restoration sites that have not yet been constructed eliminates fears of destabilizing existing on-site marshes. In addition, there are target elevations that will deliver the greatest marsh and channel productivity outcomes, and where site elevations are below the target, adding sediment could substantially improve project ecological performance. In addition, re-using sediment in the context of restoration projects would likely substantially decrease the permitting complexities compared to applying in existing habitat areas.
At present there are three potential restoration sites where the sediment could be used (see Figure 3 in Fuller 2014), though future sites may also become available. The Leque, Matterand and Nature Conservancy (TNC) sites are all substantially subsided compared to marsh elevations outside the sites.
The TNC site
Of the three, the TNC site is considerably lower in elevation than the others, and includes elevations that are likely too low to support tidal marsh. Unlike the two northern sites, the TNC site has already been restored (fall of 2012), so use of sediment here would be more complicated than at the other two sites. However, it could be incorporated as part of an adaptive management program for the project.
Early monitoring at the TNC site suggests that it is not performing ecologically at a level similar to reference sites and will likely take many decades to reach that level, due to the level of subsidence relative to adjacent natural marsh. Although the old dike was completely removed, the site behaves similarly, with respect to hydrodynamics, as a diked site that has had two breaches cut in the dike. The level of subsidence at the site means that there is over-marsh flow between the old marsh and the new marsh only at higher tide levels. To facilitate tidal connectivity with the adjacent distributary channel, two breaches were carved across the marsh during the restoration, so that the site would drain completely at low tide. However, because the site is so subsided, most of the hydraulic energy of moving tides is focused through the two breach sites. As a result, this will tend to cause all new blind tidal channel systems within the site to develop as drainages through those two sites, rather than forming more frequent, smaller channel connections through the adjacent marsh area.
The Leque and Matterand sites
The Leque and Matterand sites would likely be cheaper and simpler places to apply the sediment, given that they have not yet been restored and are closer to the site of proposed dredging. Both sites are subsided relative to adjacent natural marsh areas, so may benefit from sediment addition. Though subsided, both sites appear to be entirely within the expected elevation range of low marsh, which would be dominated by maritime and American bulrush. Both sites are at the north end of the bay, near the area that has been experiencing erosion over past decades. For this reason, sediment addition may be especially important to slowing or counteracting erosion. If the threat of erosion is greatest in those areas grazed by snow geese, as yet an unanswered question, it may be possible to adjust elevations in the restoration zones to emphasize vegetation more resistant to rhizome grazing, such as Carex, high marsh, or perhaps even some tidal transitional scrub-shrub. However, estuary topography will always adjust to the dominant hydrodynamic environment, so sustainable application of sediment must consider the processes that will ultimately determine surface elevations and slopes. Evaluation of these sites, as well as the TNC site, with respect to elevation, hydrodynamics, and projected development of vegetation and channels is necessary to determine the relative value of adding sediment.
2. Apply sediment to existing habitat areas
- Apply in areas of erosion risk, to slow, reverse or prevent marsh loss due to erosion. Examples include the northern marshes where significant marsh loss has already occurred, and the north end of the “new” prograding delta in front of Hatt Slough where there has been some marsh loss, but where proximity to the TNC restoration may improve connectivity with the river mouth.
- Increase the marsh area on the prograding delta near Hatt Slough by creating new marsh islands at the mouth, or building new marsh seaward of the existing marsh boundary. This area is not currently eroding and natural processes might therefore maintain the added sediment and continue to build elevation.
- Add sediment to existing marshes near Hatt Slough to increase their resilience to sea level rise, and/or to accelerate development of higher marsh habitat.
Summary of discussion
Using sediment to enhance existing marsh or create new marsh islands, has potential to slow the loss of marsh in eroding areas or to expand marsh in areas where natural processes are likely to maintain the new marsh, at least under current rates of accretion. However this approach faces significant risks and uncertainties. This approach has been used extensively on the saltmarshes of the Gulf and East coasts, and there is substantial literature emerging on the best practices and effectiveness of these applications. However, Puget Sound tidal marshes are mostly brackish rather than saltmarshes, and the plant species are different than found on the other coasts. As a result, the effects of sediment addition to existing marshes could be quite different than seen elsewhere. Each plant species has its own sensitivities and thresholds with regards to environmental changes, so Puget Sound species may respond differently to sediment addition.
Sediment addition at too great a rate could smother existing plants (and animals), or weaken them to the extent that a large marsh area is destabilized and eroded by winter storms. Erosion usually triggers an irreversible state change from marsh to tide flat, due to the loss of elevation. Sediment addition could also alter soil biogeochemical processes such as gas and ion exchange, which could destabilize marshes. An approach using Option Two could be developed under a rigorous, plot-based experimental regime to test the idea on a small scale without risking large marsh areas. Applying sediment more broadly to existing tidal habitats is not recommended without first taking a very careful experimental approach on a small scale.
It is possible that beneficial re-use of sediment to enhance marshes will become an important tool in the future for Puget Sound, particularly as the effects of sea level rise unfold. In areas where marshes can’t migrate landward, they may become threatened with drowning in the absence of sufficient sediment addition to keep up with rising waters. Most dredged sediment is currently disposed in deep water. At least some portion of that sediment could be used to enhance critical habitats at risk from sea level rise. Dredged sediment from the Swinomish Channel, for example, may be suitable for beneficial re-use.
However, re-use in this manner must be carefully considered, because applying sediment to existing marshes may unintentionally alter biogeochemical processes in a way that could destabilize large areas of marsh. In addition, replacing one habitat with another, such as replacing tide flat with marsh, may simply shift the impact from one group of species and ecological services to another group of equally valuable species and services. For these reasons, a controlled, small scale and experimental approach to Option 2 could be considered now, as a learning opportunity to test these ideas which may become important adaptive management tools in the future.
3. Add sediment as an offshore sub-tidal berm and allow tidal/wave action to move the sediment up into the marshes
Summary of discussion
This option has worked for feeding sand to beaches exposed to open ocean wave dynamics, but the wave dynamics in protected Port Susan Bay, combined with the energy of river flows and floods make the performance and ultimate destination of a sediment berm highly uncertain. The dredged sediment may contain high levels of fine grain material, which will perform much differently than coarser, sandier material. Fines may be carried further and in unintended directions.
Of these three broad categories, the first has the greatest potential for benefit, the least amount of uncertainty with regards to impacts and outcomes, and the lowest risk of damage to the ecosystem.
Recommendations for Evaluating Sites for Potential to Increase Ecological Outcomes by Adding Sediment
1. Review the literature
Review the literature on sediment enhancement of tidal marshes. There is a considerable history of sediment re-use in Coos Bay, San Francisco Bay, the Gulf coast, and the east coast, and this literature should be reviewed and summarized for lessons learned and best practices from the perspective of Pacific Northwest estuaries. This review will guide evaluation of potential sediment re-use sites, and any future project design that emerges.
2. Seek advisory input from scientists and practitioners
Seek advisory input from scientists and practitioners experienced in sediment re-use. While literature is a critical resource, the practical advice of experts doesn’t always make it into the literature. In this region, learn from experts involved in sediment re-use at Jetty Island, the eelgrass beds near Bainbridge Island, and Columbia River marsh islands. In addition, experts in other regions have long experience in re-using sediment specifically in tidal marshes. Expert input will be essential to guiding the evaluation of alternative sites, and the possible development of project designs.
3. Evaluate potential sediment addition sites by assessing the following elements
The workshop determined that, at a conceptual level, there may be significant potential to improve the ecological performance of the three subsided restoration projects by raising elevations using added sediment. The key guiding principle for evaluation should be that use of sediment at a site must substantially improve ecological performance with respect to the site’s restoration goals. To assess sites for their potential need for added sediment, the following information should be evaluated.
- Current conditions: site and adjacent marsh elevations, adjacent marsh species composition, site and adjacent marsh soil particle size distribution and soil salinity, presence of non-native species on site or nearby.
- Accretion rates near the sites. [a network of SETs and feldspar marker horizons already exists, though additional horizons need to be installed soon in marsh areas near the two northern sites]
- Relative climate change vulnerability. Evaluate relative vulnerability of different regions of estuary, considering rates of marsh accretion, vertical land movement, sea level rise, and potential summer low flow effects on salinity.
- A vegetation-elevation-substrate model for the Stillaguamish estuary. Evaluate the effect of estuary region on plant elevation ranges, as species appear to have different distributions in different parts of the estuary, possibly in response to soil salinity and sediment grain size. [A veg-elevation model has been developed for the TNC site and may be under development for the other sites, a full veg-elevation-substrate model is in development for the entire estuary]. This model will inform the next task.
- Projected vegetation and elevation at each site, with and without sediment addition. Evaluate occurrence probabilities for dominant species, bare ground, and for known invasives including reed canary grass and narrow leaved cattail. Also, expected vegetation development in context of climate change. Choose a timeframe to evaluate, such as 20 or 50 years, and use the relative climate change vulnerability assessment to project future vegetation.
- Projected channel development after restoration, with and without sediment addition. [A channel model has been developed for the TNC site, and may be in development for the other sites].
- Erosional dynamics at north end of bay, and at each alternative site. There has been a high rate of marsh erosion in the north, with the seaward marsh edge moving closer to the dike at rates up to 21m/year. Some work is underway to evaluate causes. It will be important to determine which factors are playing a role, and whether the northern restoration sites will be affected. Site-scale erosion potential should also be evaluated for each potential site.
- Projected site use by Chinook with and without sediment addition. This would be based on an analysis of areas of marsh and channel habitats.
4. Consider potential sediment sources.
If an evaluation of restoration sites suggests that sediment addition may be key to meeting the project’s desired ecological performance, consider the options for obtaining sediment. The project proposed by the Flood Control District to dredge the Old Stilly channel instigated this process and would clearly be the first source to consider. However, if the project doesn't advance due to permitting hurdles, or does not generate the right type of sediment, or will be insufficient in quantity, there may be other sources of clean dredge material, such as the Swinomish Slough Dredging Program. Whatever the source of sediment, an evaluation will need to happen to insure that the sediment is of the right quality for use in tidal marshes. It should approximate the soil qualities of existing marsh, including particle size distributions, organic/inorganic proportions, and other factors. It should be free of a seedbank of invasive weeds. When applying sediment to a site, the top layer is most important in terms of colonization by native plant and animal species, and so greatest care should be taken with the qualities of the surface layers.
5. Consider a small scale, experimental approach to Option 2
Options 2 (adding sediment to existing marsh areas) and 3 (creating an off-shore sediment berm) are not recommended at this time, at least in any large application, due to higher uncertainties and risks. However, in the context of climate change vulnerability, the use of sediment to improve the resilience of existing marshes may become an important tool in the region in the near future. For this reason, it may be worth considering a small, carefully controlled experimental project as part of this first effort, to inform future adaptive management approaches in the region. Such a project must not be at a scale or location that would risk destabilizing large habitat areas. An experimental project under this option should not happen at the expense of improving the performance of one of the current restoration projects.
6. Any sediment re-use project that is implemented should take an experimental approach
Re-use of sediment to enhance tidal marshes is new in Puget Sound, and the outcomes of this project would be very important to improving the design and performance of future projects. Any re-use project should therefore be implemented with an experimental approach, based on explicit hypotheses and associated monitoring, in order to maximize learning opportunity.