The Dredgeful Story of Poplar Island EarthCache

WELCOME TO POPLAR ISLAND!!!

The Chesapeake Bay was once the valley of the Susquehanna River which used to empty directly into the Atlantic Ocean. Over thousands of years of glacial retreat and sea level change, the River receded and the Bay filled in its place. In a generally north-south orientation, the Chesapeake Bay Islands – which include Poplar Island – dot the Bay’s Eastern Shore. Once connected to the mainland when sea level was lower, they are remnants of the Susquehanna River Valley, and their shape and size continue to transform in response to erosion and sea level rise.

Within the Chesapeake Bay, a relative sea level increase of 1 foot has been experienced over the past century and is the result of two phenomena: sea level rise and local subsidence or sinking of landmass. According to the Maryland Commission on Climate Change Act in 2015, The University of Maryland Center for Environmental Science is responsible for establishing sea-level rise projections for Maryland's coastal areas and update them every 5 years. The most recent report in 2018 shows that Maryland is likely to see sea level rise at rates of 2 to 4 feet over the next century. Islands like Poplar Island are barrier islands that protect the mainland from impacts of severe weather by acting as a buffer. As a result, these islands are more susceptible to erosion from sea level change because they disappear faster than they can naturally rebuild. Barrier islands also provide a relatively predator-free habitat for birds and other animals. When the islands disappear, the ecosystems they support disappear as well.

Researchers have estimated Poplar Island was 2,000 acres in size during Colonial times. The first recorded land transaction in the 1670s reported an area of 1,400 acres. When it was officially surveyed in 1847, it had been reduced to approximately 1,140 acres. By the late 1800s, the island had split into three distinct landmasses with the main island supporting a town called Valliant. Almost 100 people lived on the island at this time with several farms, a school, a church, a post office, and a sawmill. Removal of trees for the sawmill is believed to be a major contributor to the island’s erosion because trees help protect against land loss. By the 1920s, people began to move away as erosion accelerated to 15 feet lost per year and harsh winters away from the mainland began to take their toll. By 1930 all the residents had left Valliant. Over time, Poplar Island was owned by many different groups including hunting groups, Campbell’s Soup, and even the Smithsonian Institution to name a few. But by 1993, Poplar was a mere five acres spread out across four small remnant islands.

DREDGING AND POPLAR ISLAND

Dredging is the removal of accumulated sediments that have settled to the bottom of a waterway to keep the waterway navigable. In 1998, the United States Army Corps of Engineers (USACE) partnered with the Maryland Department of Transportation Maryland Port Administration (MDOT MPA) to begin rebuilding Poplar Island using dredged material from the Chesapeake Bay approach channels to the Port of Baltimore. This partnership resulted in two major wins, one ensuring the shipping channels are maintained to keep the Port of Baltimore open for business and two, beneficially reusing the dredged material to restore critical habitat that is currently dwindling in the Chesapeake Bay.

The first step for expanding the remnants of the island is to create a dike to contain the dredged material. Dikes are elongated structures that regulate water levels by separating, containing, channeling, or draining water from a specific area. Many are naturally occurring, but for Poplar Island, all 35,000 feet of containment dike that you see in the aerial photographs is man-made. At Poplar, the dikes contain the dredged material while separating it from the surrounding bay water. The dikes are constructed by first placing stone to form a protective breakwater. This structure, referred to as the toe dike, just breaks the surface of the water and forms the toe of the final dike. Behind this stone structure, sand is placed and compacted to the desired geometry and elevations. Sand to construct the dikes is harvested from Chesapeake Bay sources immediately adjacent to Poplar Island. Finally, more layers of armor stone, with sizes varying depending on expected wave conditions, are placed on the slope to protect the sand dike. The impacts of sea level change are incorporated into the design of all of these components.

Once the dikes are formed, dredged material is pumped into individual cells through a process referred to as inflow. The dredged material is already very wet, but the total volume is nearly doubled by adding water to make it into a pumpable slurry for inflow. As dredged material is placed into cells, geotechnical engineers carefully calculate the final elevation of the dredged solids once the water is removed. For the upland cells, the engineers work very diligently to get just enough material to add a three foot lift per application.

At this point, Poplar Island's cells are pools of muddy water. How do they create the solid land? The answer is through the process of consolidation, which is the settlement of soil through the removal of water. First, the excess water that was added to make the slurry must be decanted. This is done by discharging water through each cell's spillway when it meets state water quality standards, leaving only the mud behind in the cell. Then follows the process of crust management where trenches are cut to drain water trapped in the settling soil. Evaporation also occurs. This drying and dewatering combination is how the soil settles, or consolidates, under its own self-weight without the use of heavy equipment. The loss of water and air voids allows the soils to decrease in volume and the soil particles pack together more tightly. In some of the cells you can see where enough water has been removed from the upper crust to cause desiccation cracks. Desiccation cracking is a technical term for the muddy soil drying and cracking as it loses water and contracts. Periodically, soil samples are bored from cells to test the moisture content, to determine how much the soil has consolidated, and to determine if more slurry should be added. In this way, engineers are expediting a specific geologic process that usually take hundreds of thousands of years.

This process is repeated many times depending on the final elevation of the area. About half of Poplar Island will be tidal wetlands at elevations between two and three feet while the other half will be forested uplands at an elevation of 25 feet.

When Poplar Island is finished, currently estimated to be in the mid-2040’s, the project will be approximately 1,715 acres in size and provide placement capacity for about 68 mcy of dredged material. The island will have restored approximately 776 acres of wetland, 829 acres of upland, and 110 acres of open-water embayment habitat in the middle of the Chesapeake Bay. To date, over 400 different species of wildlife have been documented and over 30 different birds have been confirmed as nesting onsite.

In order to log this cache you must submit the following:

1. Which cell(s) contain remnants of the original island?
2. Armor rocks form the dikes and therefore the edge of the island. Large rocks are used on the open water side while smaller ones are placed inland and on the bay side. Why do you think it is built this way, how does this pertain to Poplar being a barrier island?
3. In 1992 barges were placed in cell 1A, are they still there? What was the purpose of their placement? Do you think it was successful?
4. If the shipping channels need to be periodically dredged to remove excess soil due to sedimentation, why do the soil particles settle there? How deep are the channels?
5. Where can you see crust management? What does it look like and why is it an important stage of habitat restoration?
6. How is soil consolidation different from compaction?
7. What is the total estimated dredge material to be received (in million cubic yards) for the project?
8. What species are returning to or using the habitat created at Poplar Island (identifying two or three is fine)?
9. Please submit a photo of you and your group on the island (completely optional but a great way to remember your joyful visit)

In order to find this cache, you must sign up for a free tour. You MAY NOT utilize your own transportation to get to the island. Reserve your spot early as the tours fill up quickly. More specific information on parking and meeting the tour will be provided by the tour coordinator when you make your booking. The boat ride is approximately 25-30 minutes each way and is included in the tour for free. Tour reservations can be made by calling 410-770-6503 or emailing poplartours@menv.com and are only held Monday through Friday from March through the end of October. Please talk to the tour coordinator about tour logistics. For more information, visit www.poplarislandrestoration.com!

Poplar Island Tour Rules

Poplar Island is open to the public through prior arrangement ONLY

Please realize that Poplar Island is an active construction site. It is expected and appreciated that you obey the following safety regulations while on the tour:

1. All posted signs and verbal notices must be obeyed.
2. Alcohol is prohibited.
3. Pets are prohibited.
4. The group must stay together at all times (it’s a bus tour).
5. Poplar Island is open to the public through prior arrangement only.
6. Closed-toed shoes with appropriate soles such as sneakers or hiking shoes/boots are required.
7. High visibility vests (provided) must be worn at all times while onsite and outside of the vehicle.
8. Participants will not be allowed off the bus in areas designated as active construction zones.
9. All participants are required to wear a life jacket while on the boat (provided). Visitors under 50 lbs. must provide their own life jackets.
10. The tour boat has a strict 24-passenger limit. If your group exceeds this number, we reserve the right to deny any unscheduled visitors.

To make the tour more enjoyable for all participants, please take the following into consideration:

1. Participants must be prepared to board a boat and climb a few steps; however the amount of walking is minimal.
2. Tours run rain or shine. However, in the event of severe weather we may need to cancel the tour, in which case we will contact you as soon as a decision has been made.
3. The boat will depart promptly at the above scheduled time. Please make sure you arrive on time so that you are not left behind. We cannot allow alternative boat transportation to dock on Poplar Island.
4. The tour does not include a designated lunch break but feel free to bring snacks.
5. Tours can be enjoyed by people of all ages, but please keep in mind that some of the topics covered are slightly advanced for our younger visitors. We also encourage you to bring binoculars and cameras.

In order to gain and hold our community support and understanding, Poplar Island tours and programs are offered free of charge, we simply ask for your full attention and cooperation. Please bring your confirmation letter (distributed upon reservation) with you as your ticket onto the boat.

Permission to place this cache has been granted by the US Army Corps of Engineers and Maryland Department of Transportation Maryland Port Administration project team.