06 June, 2019

Using Wetlands to Improve Nantucket’s Coastal Resiliency

As an island in the Atlantic Ocean, Nantucket intimately understands the impacts of increased flooding frequency and storm events. For our community, adopting ways to increase our Coastal Resiliency is essential to maintaining quality of life, community function and ecological integrity on our little island.
Maximum Annual Storm Tides and Storm Surges recorded at the Nantucket Harbor Tide Station 1965-2015. (Storm Surge and Inundation Pathways Report, 2016)
So what is Coastal Resiliency? Essentially, it is the ability of a coastal community to resist, absorb, adapt to, and recover from the impacts of sea level rise, the increasing magnitude and frequency of storm events and shoreline change. The faster and more effectively we can respond to these hazards or mitigate their impacts all together, the better our Resiliency as a community.  On Nantucket our Town government, municipal departments, historical preservation groups, community civic leagues and conservation groups are all talking about Coastal Resiliency and how to make our community more resilient to the environmental change already impacting us. On the Town website, you can learn more about the Town’s current efforts to develop a community level Coastal Resiliency Plan to direct actions we can take as a community as well as position Nantucket to receive grant money. On Tuesday, January 11, 2019, staff from NCF, Select Board members, DPW staff, Town Natural Resources staff, other conservation groups and many more all participated in a day long Community Resiliency Workshop with a number of objectives including:
  • Develop a mutual understand of the natural hazard risks, vulnerabilities and resilience options for Nantucket
  • Identify vulnerabilities and strengths of Nantucket in response to Climate Change
  • Identify ACTION ITEMS that can reduce Nantucket’s vulnerability
The workshop defined the risk Nantucket faces to the hazards of climate change as: Risk = Vulnerability x frequency The frequency (and intensity) of hazardous events is both increasing and largely out of our control to impact. The thing that our community CAN do is mitigate or reduce our VULNERABILITY to these hazards. And one of the top Action Items identified at the workshop was to preserve, restore and potential create coastal wetlands to function as buffers against storm surges and increased flooding hazards. Why Spend our Efforts on Coastal Wetlands? Coastal wetlands are naturally designed to protect and buffer uplands from the impacts of storm surge, coastal flooding and sea level rise – shielding our communities from the worst storm impacts and helping us recover more quickly after major storm events. One of the best steps we can take to provide longer term resiliency to our coastal communities is the restoration and protection of our natural coastal defenses. Research following the devastation of Hurricane Sandy showed that functioning coastal wetlands prevented an estimated $625 in potential property damages across the path of the hurricane. In part of New Jersey it was estimated that the presence of coastal wetlands reduced the expected losses to hurricane damage by 20% on average and up to 50% for area just above sea level.
Reductions in annual flood losses to properties that have a marsh in front (blue) versus properties that have lost the marshes in front (orange). Narayan et al., Nature Scientific Reports 7, 9463 (2017).CC BY
Communities along the eastern coast of the United States are actively working towards converting their coastlines back to natural communities that can buffer our infrastructure and our towns from the effects of increased floods and storm surges. Resilient Boston Harbor is maintaining and increasing open vegetated parks to buffer from storms but also provide areas for flood waters to sit, preventing inundation of adjacent roadways, homes and business. The state of North Carolina has implemented the Coastal Resilience Initiative to restore or protect more than 5,000 acres of wetlands over the next three years through the creation of living shorelines and the protection of existing wetlands.Nantucket already has significant wetland resources along both of Madaket and Nantucket Harbors. Protecting those wetlands and looking for ways to enhance them will help protect our island. Downtown is particularly susceptible to flooding during high tides and storm surges events, likely because it was all once a wetland itself! Softening how the town connects to the harbor by increasing vegetated parks (as the Land Bank has been doing) and restoring low-lying areas to wetlands and/or retention basins will go a long way to mitigating those flood waters.
Imagine vegetated waterfront parks to absorb and mitigate flood waters and storm surges (Parks being created at the base of Brooklyn Bridge in New York City following Hurricane Sandy).
Restoring and creating wetlands along Easton st and in the Brant Point area will help mitigate flooding. These areas are often underwater during normal high tide events, likely because they were once all one large connect wetland that has been filled for development. Check behind the Nantucket Hotel or in the few open lots along Easton st the next time you walk that part of Town – you’ll find the hints of wetlands that once were.
Extent of water flows over the Town of Nantucket under 5ft (green) and 7ft (blue) storm surges. This is without taking into account future sea level rise. (Storm Surge and Inundation Pathways Report, 2016)
Finding ways to soften the bulkheads and hard shorelines using many of the tested proven methods of Living Shoreline research could alleviate much of the storm and flood impacts.
A variety of shoreline protection options exist.
Pie in the sky action? Convert the bulkheads downtown to a living shoreline wetland backed by grassy parks. Protective harder structures can be incorporated to these living shorelines to provide stability as well. Living shoreline marshes are significantly better than strict bulkheads at protecting shorelines and mitigating storm damage and flooding. The significant flooding behind the Dreamland and near Straight Wharf could potentially be helped by these ideas.There are a lot of options to providing Coastal Resiliency while promoting and maintaining natural resources and hopefully retaining the natural beauty that makes Nantucket so special. Even mainstream media is recognizing that walls won’t protect our coastal areas from these hazards – thinking proactively may help save our island in the future. And as one of the largest land owners on the island, The Nantucket Conservation Foundation is excited to be par tof the conversation and hopefully part of the solution. Futher Resources: http://resilientma.org/https://www.nantucket-ma.gov/DocumentCenter/View/19053/Storm-Surge-and-Inundation-Pathways—2016http://www.sarasota.wateratlas.usf.edu/upload/documents/Guidance-for-Living-Shorelines-in-the-Sarasota-Bay-Watershed.pdfhttps://www.conservationgateway.org/ConservationPractices/Marine/crr/library/Documents/A%20Community%20Resource%20Guide%20for%20Planning%20Living%20Shorelines%20Projects.pdf  

Update on the Hither Creek Washover, Erosion and Salt Marshes

*This post summarizes a report prepared by the Nantucket Conservation Foundation. Read the full report here.
The landscape at the southern end of Hither Creek, in a place once called Millie’s Pond, has become dramatically different in the last year and half. The Halloween storm of 2017 permanently altered this iconic piece of Nantucket. Take a drive on down this weekend and marvel at what change a single storm can bring to our coastlines on Nantucket.
Top image depicts Millie’s Pond, surrounding vegetation and the Stilt House prior to the Oct 2017 Storm (painting by Karen Allen-Kelley). The bottom image was taken during the Storm and shows the dune erosion, deposited sand and overwash from the Atlantic Ocean (photo by Fabrizia Lu Macciavelli)
The Halloween storm came with high tides and large surf on the south shore of Nantucket dramatically eroding the dunes between the Atlantic Ocean and the southern end of Hither Creek. As the ocean rushed over the dunes and poured into Millie’s Pond, it brought the sand from that dune system with it. Prior to the storm, 
the southern end of Hither Creek, past Millie’s Bridge, consisted of a deep almost circular salt pond (Millie’s Pond) surrounded on three sides by well-established salt marsh, shrub wetland, and coastal shrubs (Image above). Following the storm, sand from the eroded dune was displaced onto the salt marsh and filled in a significant portion of Millie’s Pond. Once the storm subsided, the overwash ceased and normal tidal water flows connecting Hither Creek to Madaket Harbor resumed. While Millie’s Bridge itself remains stable, the vegetated coastal wetland, salt marshes, dune and coastal pond were all completely changed, buried under up to 2 meters of sand in some spots. 
Science staff from the Nantucket Conservation Foundation wanted to understand how the massive amount of sand would impact salt marshes and shrubby wetlands surrounding the Pond – or what was left of the Pond! For the past year and a half we have been monitoring sand extent and vegetation growth within the new sand dune.
Immediately following the Halloween 2017 Storm, we measured the area newly covered by sand from the top of the previous dune to the edge of the Pond – 2.2 acres of new, exposed sand dune had been created (image below).
Tan color represents the new sand covering previous shrubby wetland, salt marsh and open water at Hither Creek in Nov 2017.
Through settling, erosion and further washover events in the winter of 2018, the sand advanced further into the pond and on top of surrounding salt marshes. One year after the first storm, the new sand flat was 3.5 acres in size with the majority of salt marsh buried and the iconic Stilt House removed due to erosion (image below).
Once year after the initial storm, the sand (green) cover 3.5 acres at Hither Creek.
The initial Storm buried a significant amount of vegetation and continued to advance into the pond and across the salt marshes. By the summer of 2018, none of the shrubby wetlands around the pond were able to grow up through the sand. Additionally, only a few small patches of salt marsh survived and grew up through the sand (image below – June 2018). 
Photos taken from approximately the same spot on the eastern end of Millie’s Bridge showing changes in sand extent, loss of the Stilt House and salt marsh vegetation over time (photo credit to Susan Landmann for the Oct 2017 photo).
For the most part, the plants and wetlands that existed prior to the storm have been erased. Now Nature needs to begin building new plant communities onto the bare sand. In summer 2018, we did see many new plants moving in – obviously brought in on high tides. American beachgrass, groundsel, high tide bush and seaside sandmat were abundant on the exposed sand. And, excitingly, we found an abundant population of a state-listed plant of special concern that loves exposed sand – the seaside knotweed established and even set seed, hopefully to keep growing and expanding next year!
What Will the Future Bring for Millie’s Pond?
The fate of this area will depend, in large part, on the cycle and intensity of winter season storms over the next five years. The buried wetlands that once ringed the pond did not survive but new plants moved in and began growing. Hopefully, given time, the exposed sand will become vegetated and stabilize the sand dune and buffer surrounding areas from future high tide events. 
Will future storm events breach Hither Creek and threaten Millie’s Bridge? We can’t say no for certain but right now it doesn’t look likely. Sand fencing installed on the sand dune has allowed that to accumulate and build in elevation. The more vegetation that grows on the sand, the more stabilized it becomes. Reducing impacts to the sand by limiting foot traffic and vehicle access will go a long way to protecting this area.
Although the exposed sand is a dramatically different habitat than previously observed at the end of Hither Creek, observations over a year showed that a lot of ecological activity took place within the impacted area. The sand provided habitat for many shorebirds, even a pair of piping plovers, as well as fiddler crabs and a rare plant species. Horseshoe crabs very actively used the sand for feeding, mating and nesting at high tides. Given time, this habitat may prove as valuable and hopefully as aesthetically pleasing as what was present before.

Salt Marshes Buried Under - Head of Hither Creek

The photos and videos on social media October 30th, 2017 were dramatic. Ocean waves in Madaket washing over the dune separating Hither Creek from the Atlantic Ocean, swirling under and around the stilt house and pushing sand into the southern end of Hither Creek. For a while during this story, it looked like Hither Creek would open to the Atlantic Ocean and the water rushing in threatened Millie’s Bridge, sending Nantucket DPW workers out to shore up the bridge. When the storm cleared, luckily the bridge was stable, the stilt house was still standing and the Atlantic Ocean was back on the other side of dune, now much shorter than before. But there were still dramatic changes to the landscape. All that rushing ocean water had significantly eroded the sand dune separating the ocean and Hither Creek and that sand had been push into Hither Creek, filling in most of the small circular salt pond closest to the silt house and blanketing the surround salt marsh with a thick layer of sand.
Salt Marshes Buried Under Sand – Head of Hither Creek
After the storm: sand covering shrubs and salt marsh and filling most of Millie’s Pond
Once the storm subsided, waves have occasionally washed over from the Atlantic Ocean only during extreme high tide/wave events, but under normal conditions, the southern end of Hither Creek is reconnected to tidal flow from Madaket Harbor. Significant sand remained deposited on top of the salt marsh and within the salt pond. NCF’s Research Program Director, Jen Karberg became very interested in what that thick layer of sand would do to the salt marsh underneath it. Salt marshes typically respond well to sand additions of an inch or two, small scale wash over events are vital for building up marsh elevation, encouraging growth of marshes grasses and helping salt marshes respond to climate change. Very little research has documented the response of salt marshes to this much sand deposition. Events like this are likely going to increase and salt marshes play a vital role in stabilizing our eroding shorelines and buffering our uplands from intense storm events. Understanding how large sand washover events can impact salt marshes will help NCF and other island organizations prioritize management and protection on Nantucket.
Sand over salt marsh JMK
Sand over existing Salt marsh
So, with permission of the Massachusetts Audubon and the Town of Nantucket (owners of the majority of the salt marsh at the southern end of Hither Creek), NCF set out to study how the salt marsh would respond to these deep sand deposits as well as how long those sand deposits stay in place. Over the next year, NCF staff will be documenting sand extent and depth over the salt marshes, tracking the movement/retention of sand and examining salt marsh vegetation growth in response to the sand deposits.In November, Jen established randomly located plots at the site, measured sand depth over the marsh and documented any vegetation that was still present. The response of salt marsh vegetation will depend on how much sand remains in place come spring when the plants start growing again. This is just the beginning of this monitoring work, which we will continue for at least a year but a few observations can already be made. In November, sand depth over the salt marsh ranged from 1/3 of a meter to up to 2 meters in some places. Interestingly, the sand deposit within the creek rises so much higher than the remaining salt marsh that incoming tides preferentially cover the remaining salt marsh which has already lead to the death of much of the grasses in some places. This also means the tide is flowing right around the base of Millie’s Bridge so the concrete blocks put in place to protect the bridge will likely remain for a while!   Observing the area at low tide, significant amounts of sand appear to be washing off of the big sand deposit and out into Hither Creek. Where these new deposits of sand fall out in the creek may end up may changing the navigational bottom of the creek. Between November and January, the sand deposits have been advancing over the salt marsh and closer to the bridge with each major storm and the loss of salt marsh is already being felt. Salt marshes both buffer large water flows and can hold on to water, keeping it from flooding uplands during large storm events. A neighbor on the pond had their basement flood with water during the most recent winter storm. This had never happened before to them before, with the only difference being that the salt marsh in front of their house was swamped with sand. These are just some small changes already being seen as a result of this major landscape change. Continued monitoring over the next year will help us understand how this area is changing and how future storm changes will impact Nantucket.

The Winter Life of the Salt Marsh

As the shortest day of the year has passed and we officially enter the winter months, the natural world around us seems to be hibernating. Leaves have fallen off deciduous trees and grasses and flowers are mostly dormant, waiting for spring like the insects and many other creatures of the natural world – hunkered down until warm weather returns.

Snow and ice along a salt marsh creek bank
Looking out over the expanses of salt marsh at the Creeks or in Polpis Harbor, you might think that the salt marsh was completely dormant also – resting and waiting for spring but looks can be deceiving. The winter salt marsh is actually quite ecologically active, even if most of that is hidden from view. Salt marshes, the boundary between salty oceans and freshwater inlands and uplands comprise one of the most biologically productive ecosystems in the world. These oft overlooked transitional zones provide nursery grounds for many of the commercially and recreationally important shellfish and fish in New England, as well as being home to a variety of wildlife and birds. The unique placement of salt marshes, getting salt water from the oceans and freshwater from the uplands, makes these areas rich in a variety of minerals and nutrients. One of the biggest values of a salt marsh for human settlement is the ability to buffer the surge and waves created by large storms and to trap and hold excess water, slowing its release while filtering out excess nutrients like nitrogen and phosphorus from reaching our harbor areas.
Measure tide water levels during winter storms in a salt marsh
Measure tide water levels during winter storms in a salt marsh
In recent years, the biggest storms to hit Nantucket have come in the winter months – Nemo (Feb 2013), Sandy (Oct 2012), Juno (Jan 2015) etc. – and with each of these storms have come highest tides and storm surges, inundating our downtown streets where the salt marsh had long ago been filled in. In harbors where salt marshes still thrive, storm surge and high tide water was taken in, the rush of water slowed down and halted before reaching upland infrastructure. Salt marshes provided the barrier to protect areas of the island from more severe impacts.     And in winter, those higher tide storm events arrive at just the right time to help facilitate the next biggest function of the salt marsh. As winter comes, the dominant plant in the salt marsh, Smooth Cordgrass (Spartina alterniflora) goes dormant, waiting for spring to grow again. Cordgrass leaves brown and dieback to the salt marsh surface, slowly decaying and breaking off with the help of big storms and shifting ice sheets. Insects and microbes (detritivores) early work on consuming and breaking down this abundance of dead vegetation. An acre of salt marsh can produce up to 10 tons of cordgrass each year! Some of it stays on the salt marsh surface, helping build new organic soil layers. Some of the nutrients and minerals stored in the dying vegetation moves out of the salt marsh on retreating high tides, transporting much needed nutrients to nearby ocean areas. The nursery of the sea, this rich detritus provides food for young fish and shellfish in the spring. A variety of amphibians, insects, snails and crustaceans find their way to the salt marsh in the spring to feast on the rich detritus released each winter.
The mummichog, a diminutive salt marsh resident, lasting out the cold winter in small tidal pools.
Salt marsh tide pools, pockets of salt water held up on the surface of the marsh, away from the daily tidal pull, act as refugia for small fish and hunting grounds for birds in the winter. The diminutive mummichog or mud minnow (Fundulus heteroclitus) hunkers down in these sheltered, warmer pools in the winter until migrating to the salt marsh creeks in the warmer spring weather. Waterfowl and wading birds particularly haunt the winter salt marsh, foraging and grazing, searching out their invertebrate prey. You may not see fiddler crabs in the winter but they are still in the marsh, burrowed down deep in the soil, waiting for warm spring.
Covered in ice, the salt marsh is busy preparing for spring.
Covered in ice, the salt marsh is busy preparing for spring.
Through the long, cold, dark winter months the salt marsh is still functioning, protecting our coastlines and providing shelter and food for a diversity of species. And waiting, prepping for the warm weather of spring for the grass to grow again and brighten our coastlines with a swath of yellow/green!