The three preceding columns in this series were partly a cautionary tale of sea-level rise, and partly a short course on how water and sediment behave in the Bay. They may have left you with a sense of foreboding. Stay with me. This column will present a three-act play with a happier ending.
Act I. Two characters with critical needs search for solutions. Meet the Authorities from the Port of Baltimore and its fellow agencies, and the Ghosts of Chesapeake Bay, Past, Present, and Future.
The Ghosts begin: Where could the vast volumes of sediment be found to replace our islands that are disappearing, and have already disappeared?
The Authorities proclaim: Periodic dredging is the only way to keep Baltimore’s shipping lanes open for business, but what can be done with these enormous volumes of dredged sediment?
The idea to use dredged material from Baltimore’s shipping channels to restore Bay islands was first proposed in the early 1900s. Baltimore has one of only two deep-water ports in Maryland (Cambridge is the other one). As described in the second column, fine silt, clay, fine plant and other organic debris is constantly accumulating in deeper areas of the Bay, like Baltimore’s port.
Oxygen dissolved in the water is used up as the organic matter decays. The bottom water and the water in the top layers of mud (‘pore water’) are close to becoming, and periodically become, an anaerobic ‘dead zone.’ Hazardous industrial heavy metals (e.g., lead, mercury, chromium) and organic chemicals accumulate and persist under these conditions.
Act II. The characters confront and resolve their differences. The process of reconstructing Poplar Island can begin.
The Ghosts Express Concern: Using your dredged mud to replace our missing islands sounds good, but what about the hazardous chemicals in the harbor?
The Authorities Respond: You’re right to be concerned. All the dredging will be done by the Army Corps of Engineers, and they follow Federal EPA guidelines for testing sediment before it’s dredged.
The opportunity to join these two needs together for mutual benefit became a topic of discussion in the 1970s. Hundreds of sites wanted to use the dredged material. At the same time, environmental and citizen advocacy groups raise concerns that dredging and redepositing sediments would stir up harmful chemicals and mix them into the Bay’s water, endangering people and wildlife.
The MD Dredged Material Management Act (DMMA) was passed into law in 2001, mandating protocols for pre-dredging chemical testing to determine if, and where, sediments can be dredged and then deposited without environmental risk. These guidelines are updated periodically, most recently in 2019. Hart-Miller Island became the first site to receive dredged material in 1984 and Poplar Island followed in 2001.
For the Poplar Island, the Army Corps of Engineers proposed to use only dredged material from the approach channels to the harbor, south of the Patapsco River and north of the Bay Bridge. Testing showed that sediment dredged this far from Baltimore would be safe on Poplar Island.
The process of transporting and spreading bottom sediments is slow and methodical. Dredging barges dig large volumes of sediment using a huge bucket loader that empties into a flat-bottom barge called a scow. Once filled, the scow is towed to Poplar Island and bay water is pumped in.
The resulting slurry is then pumped into one of twenty defined areas (‘cells’) that will constitute the new island. Man-made dikes separate and contain the cells. A perimeter sand dike standing three meters above sea-level protects the island’s perimeter from incoming waves. Once the sand settles and compacts, it will be two meters above sea-level.
The slurry dries out over months, the water that seeps out is returned to the bay, and the process is repeated until each cell reaches the proper height (low and high marshes, uplands). Each cell’s surface is then graded and canals are excavated for drainage and tidal currents. It can take several years to fill a cell.
Act III. Once each cell is filled, activity moves from engineering to biology.
The Authorities Ask: We’ll keep delivering dredged material until the island is complete, but the biologists and wildlife people have to bring it to life.
The Ghosts reply: The sediments you’ve delivered are rich in nutrients, but it will take time, monitoring, and perhaps adjustments to create a healthy and biodiverse ecosystem.
Researchers from Horn Point Labs in Cambridge began the work of introducing plants and monitoring soil microbial activity and plant growth. Inlets to tidal marshes were excavated through the perimeter dike and tides started flowing. Over a 15-year period, one species of Spartina (a marsh grass known as Cordgrass) was introduced into low marshes and a different species was planted in high marshes. Different ways of connecting marshes were tried to find ways to attract fish from the bay into the marshes. Wildlife began to arrive and the island’s biodiversity (especially fish, reptiles, and birds) and population sizes grew.
As plants took hold, they began to trap sediment carried by tidal currents. Rates of vertical accretion are relevant because ideally, sediment entrapment should keep pace with sea-level rise. If it doesn’t, tidal currents and wind-generated waves will erode and carry off sediment.
If all goes as planned, Poplar Island will soon be a self-regulating system, growing to keep pace with rising sea-level. If it doesn’t, the island could begin to disappear again.
