Sediment Management in Savin Hill Cove 1

Sediment Management in Savin Hill Cove 1

Sediment Management in Savin Hill Cove

A pilot management plan for the Boston Harbor

Gwen Richards and Caitlyn Mello

UMASS Boston

March 7, 2011

Sediment Management in Savin Hill Cove 2

Introduction

Savin Hill Cove, located in Dorchester, MA, has been experiencing excessive sediment deposition causing navigation channels, used by the University of Massachusetts Boston’s research vessel and the Savin Hill Yacht Club, to fill with sediment. The same sediment deposition problem is affecting the function of the University of Massachusetts Boston’s heating and cooling system. Due to the accretion of sediment, cooler water becomes less available for the in-take pump resulting in an inefficient heating and cooling system. Marine growth, sedimentary and contaminating materials also cause damage to the pumps themselves, and previously had to be replaced every two years (Sweeney 2005). A dredging project was implemented in 2006 to address both of the issues at hand, but unfortunately is not a final solution. Sediment deposition must be monitored and evaluated to maintain the height of sediment desired. Our goal is to research a variety of sediment management solutions to keep sediment out, keep sediment moving and remove sediment already deposited, to improve deposition conditions at Savin Hill Cove. We would also like to determine more solid suggestions for the frequency and volume of sediments needed to be dredged in the future as well as the alteration in these figures that could be caused by using alternative solutions.

Dredging

Most harbors around the world have to address dredging at some point in their history. The human alterations that have occurred to make marinas and harbors more accessible are only temporary. Sediments transported by water and wind continue to affect these sites and can cause their navigation channels to fill up with sediment. Most tackle the issue by dredging the sediments when they determine it is necessary. After research in flora, fauna, sediment cores and sediment contamination, a 2006 dredging project was implemented at Savin Hill Cove to increase water depth, clean the harbor, enhance navigation channels, and to remove unwanted sediments that were building up around UMASS Boston’s heating and cooling in-take pump (Sweeny 2005). This took place in the spring of 2006 and it was the first time since the 1960s that this site was dredged. Two sites were chosen for dredging in Savin Hill Cove, one being dredged at ten feet below low tide and the other at eight feet below low tide, totaling in 22,000

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cubic yards of sediments removed. After examination of the quality of the sediment the Army Corps of Engineers permitted dredging and offshore disposal of the sediments. Although this was the overall cheapest option for disposal, the dredging project still cost $3 million (Sweeney 2005). Dredging in general also puts a toll on the marine environment where it is taking place. Dredging sediments causes a decrease in water quality and visibility, by introducing a large amount of sediments into the pelagic zone. Dredging also can disrupt benthic marine life, as well as pelagic. Although none of the fauna studies conducted for the 2006 project show signs of significant marine life declines due to dredging, it still could have had some kind of impact still unknown. Another issue we have with conventional dredging is the fact that it is not a permanent solution. Sediment deposition must be monitored and assessed continually so future dredging projects can be planned and implemented. However, we are also aware that most solutions in nature are not permanent, but we feel by using or combining more ecologically beneficial and less damaging solution to this sediment deposition issue we could also reduce the frequency and volume at which the site needs to be dredged, or even disregard future dredging altogether.

Information to Be Gathered

Identifying problems due to sediment accumulation in Savin Hill cove is simple. Most are quite obvious, like the clogging on the UMass intake pump, large debris that are evident at low tide, and the constant need for dredging, among others. Addressing these issues though, requires a vast amount of information- scientific, social, and political.

Scientific

A sound management plan begins with an environmental assessment. This assessment will provide necessary information on the living and non-living components of the cove and surrounding areas; how they are affected, how they interact, and any trends that emerge. It is important to understand how the cove has changed over time as well. Several other analyses, physical, chemical, geological, hydrologic, and topographical, must be completed. This must be completed in order to determine exactly where sediments are coming from, what toxins they may contain, and how they react when mixed.

Social

A sound management plan is one that includes the local community in its development, passage, and implementation. Savin Hill Cove exists within several overlapping communities:

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the residents of Dorchester, The members of the yacht club, UMass Boston, and the Boston Harbor as a whole. Offering information sessions and open community forums can involve the community so that we may better understand their desires when developing a management plan. This will also bring in a variety of opinions and viewpoints, which will be quite useful.

Political

Finally, a sound management plan is one that anticipates and plans for the necessary policy measures. The “red tape” of politics, especially zoning codes, often impedes necessary actions, especially when they are non-conventional, like the alternatives discussed in this proposal. Frequently consulting and working with local authorities and officials can save time and money in development, passage, and implementation.

Investigating how other areas in similar situations have been managed can also save time and resources. Fortunately, most alternative methods are currently being used in harbors around the world. By looking to these examples, and learning from their successes and failures, the best possible option can be decided upon.

Sediment Source

Where is this sediment coming from anyway? Most do not address where the sediment is coming from and why it is collecting there in the first place. The lack of addressing sedimentation issues further up the watershed only diverts people from getting to the core issues. That is why we will be examining the Neponset River Watershed and other nearby water features to better understand how to stabilize the sediment source. The Neponset River Watershed covers about 130 square miles of land southwest of Boston and is part of the larger Boston Harbor Watershed which includes two other rivers, the Charles River and the Mystic River (The Neponset River Watershed Association 2011). The Neponset River ultimately leads to the Boston Harbor, which includes our site Savin Hill Cove. Not only water is passed through here, garbage, sediments, salt from winter roadways and marine life all travel through this waterway. Dam construction has negatively impacted the watershed as well. Sedimentation builds up behind the dams causing a fluctuation in sedimentation loads along the river. This can introduce sediment into the water column disrupting marine life and slowing down the water’s movement. The Neponset River Watershed Association is proposing the removal of the T&H dam and the Baker dam to improve water quality and allow migration for marine species (The Neponset River Watershed 2011). These removal projects could improve or worsen the sedimentation issues at

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Savin Hill Cove. However, information found at the Neoponset River Watershed Association website leads me to believe that water mobility will increase with the removal of the dams, which may allow for more movement downstream at Savin Hill Cove. Some areas throughout the watershed, and specifically in the harbor, collect this variety of debris. Overall the removal of the dams should improve natural conditions of the watershed and improve sediment mobility throughout.

From personal speculation there seems to be a lack of water movement at Savin Hill Cove. Anthropological changes, such as the construction of Morrissey Boulevard and marinas, most likely have also negatively impacted the hydrology of Savin Hill Cove. For instance, there is no longer a solid connection from Patten Cove, which is on the opposite side of Morrissey Boulevard, to Savin Hill Cove. This could also be a contributing factor to the decrease in sediment mobility. During the winter months a large amount of sediment is introduced into the watershed from salts and sands used to deice roadways. This only adds to increase in sediment, and with Savin Hill Cove almost enclosed by roadways this source of sediment is hard to escape from. A lack of vegetation along the waterways is also a major issue to consider in sediment accumulation (The Neponset River Watershed Association 2011). Using a Google map image of Savin Hill Cove and its surrounding coastlines, you can instantly detect a lack of vegetation along the water. One alternative sediment management solution we are looking into is restoring native salt marshes to coastlines that are now lined with hard structures like seawalls or riprap. We hope to collaborate with Nicole and Alyssa, who are working on restoring salt marshes to the area, to determine the benefits of implementing this natural solution. If we can promote sediment deposition to occur in salt marshes, maybe we can decrease the amount of sediment that is currently deposited in Savin Hill Cove. Many factors may contribute to poor sediment mobility at Savin Hill Cove, both anthropogenic and natural. Investigating further in sediment sources of Savin Hill Cove is vital to our current research.

Alternatives

Alternatives in sediment management have become more common as the dangers of dredging and other conventional methods have been proven quite destructive and costly. As discussed earlier, Savin Hill Cove is in need of alternative methods of sediment management in

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order to save money and the small, sensitive environment. When looking at alternatives, there are three key factors that must be kept in mind: keeping sediments out, keeping sediments moving, and safe removal of accumulated sediments and debris (Davis 2010).

Keeping sediment out:

While methods other than dredging can be used to remove sediment and debris from Savin Hill Cove, it would be ideal to act in a preventative manner and focus also on keeping sediments out of the cove. Stabilizing sediment sources is a critical component of sediment management, and can be done in this case by monitoring and stabilizing the Neponset River and the Morrissey Boulevard storm water drainpipes. Diversion of flow, in some cases, can also be successful.

If sediments cannot be properly stabilized, methods to either trap incoming sediment, or simply block them from entering the cove should be examined. Silt screens are successful at trapping sediment and debris, but are more commonly used as temporary barriers during activities such as construction and other activities that contribute and unnatural amount of debris. Also, the yacht club located in the cove required uninhibited access to the cove, and these screens may become a problem. However, a compromise involving scheduled retractions of the silt screen to accommodate boaters can be negotiated, as the future of the yacht club relies on this sediment management.

Another method of blocking incoming sediment is a pneumatic barrier, or “bubble wall” (Davis 2010). These can be installed in long, straight lines, or in small clusters, and will disturb the flow of water and the sediments that it contains. Along similar lines, sediment “traps” may also be installed, so that only small portions will need to be dredged.

Keep sediment moving:

Much of Boston’s land mass is man-made, including the land around Savin Hill Cove. Thus the original currents that flowed in and out of the cove no longer exist. The water and currents in the cove now are rather stagnant and allow for massive amounts of sediments and debris to settle on its bottom. Keeping currents moving and sediments suspended is another critical component of sediment management in Savin Hill Cove. Training dams, cleverly placed rip raps, sea walls, and other structural elements can keep water flowing and sediments suspended (Davis 2010).

Sediment removal:

If sediments cannot be substantially controlled via traps, screens, and increased suspensions dredging will again be considered. However the cost, both economic and environmental, is too great to not come up with alternatives. The problem though, is that

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acquiring permits for sediment removal, other than dredging, is a difficult and lengthy process, often requiring permits from the U.S. Corps of Engineers (Davis 2010).

Ideally, dredging will never be considered again because alternative methods of management will be sufficient enough to never warrant such destructive activities.

Sources

Davis, Trey, & McAnally, William. (2010).Sediment Management Aalternatives for the Port of Gulfport, Mississippi. Informally published report, Department of Civil and Environmental Engineering, Mississippi State University, MS. Retrieved fromhttp://www.northerngulfinstitute.org/publications/docs/2010/02/7189Gulfport2010.02.pdf

Sweeney, Chris. (2005). Savin Hill Cove Dredging Proposal. PowerPoint Presentation.

The Neponset River Watershed Association. (2011). River Issues. Retrieved from http://www.neponset.org/River-Issues,RestorOpps.htm