The Urban Ocean and its Connection to Boston

Coastal cities are home to more than 50 percent of the global population and more than 80 percent of the American population, and they continue to grow. Of the world’s megacities (defined as having a population of at least 10 million) more than 75 percent are situated on the coast. Over 400 million people inhabit the 136 port cities around the world. In the United States, the number of homes at risk from hurricane damage is almost 4.6 million. In Boston, it is estimated that almost 13,000 homes could be affected with more than $38 billion at risk (CoreLogic, 2016). As the population of Boston grows, it will be mostly concentrated in the coastal zones; therefore, societal risks are increasingly defined by urban, coastal issues.

Extreme weather events along the urban coasts of the world have dramatically increased over the last decade. Hurricane Sandy was a painful reminder that coastal storms are among the world’s costliest and deadliest disasters, capable of causing tens-to-hundreds of billions of dollars in damages and destroying entire neighborhoods and critical infrastructure. Sea level rise has given a boost to high tides, which are now regularly overtopping streets, floorboards, and other low-lying areas that had long existed in relatively dehydrated harmony with nearby waterfronts. The trend is projected to worsen sharply in the coming years.

Nowhere has our changing climate appeared more relevant than in the South Boston waterfront, which has a growing working and residential population, but is also a becoming a destination for others to visit and enjoy. Boston Harbor has always played an important role in the history of New England. Over 350 years ago, early settlers were drawn to the region largely because of its fine natural port. Today, harbor commerce generates $8 billion in annual revenue for the region and is at once an avenue of trade and transportation, a haven for sport and recreation, and potentially a rich fishing ground.

The uniqueness of this urban ocean is felt by its temporal and spatial variations, ranging from seasonal to millennial, often with catastrophic events mixed in. The urban ocean is the place where the ocean, land, and people all come together. Each of these component “systems” has a profound effect on the others. The ever-present changes in water level create complex currents, often in the context of an estuary environment, that influence the physical, chemical, and biological characteristics of the region, and thereby define its capability and capacity to support human life.¹

Boston Harbor, the urban ocean just off the shore of Boston, is a shallow, tidally dominated embayment with a surface area of 125 square kilometers and an average depth of five meters. The harbor is filled with a considerable number of islands, 34 of which dot the harbor landscape. The tides are primarily semi-diurnal (repeating every 12.42 hours) with a range of 2.45 meters almost everywhere in the harbor. The velocities, however, exhibit strong spatial variability due to complex bathymetry and coastline. The water that enters and leaves the harbor is exchanged through two 15-meter-deep passages that connect Boston Harbor to Massachusetts Bay. This nearshore environment has freshwater from land runoff flowing into the saltier offshore waters leading to freshwater plumes and recirculation. This urban ocean embodies the physics, chemistry, and biology of the waters and includes how humans have significantly influenced the behavior of each of these systems through land degradation, river management, a major sewage outfall, and other anthropogenic activities.

The coastal ocean and weather patterns drive processes and events that range from highly supportive of human populations (e.g., fishing, marine transportation, and tourism) to highly threatening (e.g., storm surges and flooding). The dredging of shipping channels intended to support safe navigation and economic prosperity has often led to the alteration of tidal- and wind-driven dynamics and transport processes, with significant consequences to ecosystems both inland and along the coast. The construction of seawalls to protect life and property often causes beach erosion and long-term shoreline retreat. These are only a few of the myriad ways in which the symbiotic relationship between humans and the coastal ocean has produced significant impacts, many of which were not expected and some of which are still poorly understood.

The balancing act that exists between ocean-as-sustainer and ocean-as-threat has produced a very wide range of coastal ocean “management” strategies that have themselves often resulted in significant short- and longterm changes to the balance. A new concept to be considered has to do with the all the islands that dot the harbor. Two keys question were posed in the “Frontier City” studio: using the existing islands, could one construct a set of offshore barrier dunes in the Boston Harbor that would lower storm surges by blocking and reflecting an incoming surge and therefore save lives, reduce damage, and safeguard the environment? If so, could the new shoreline be designed with natural processes in mind?

To test the hypothesis that a set of such dunes can exist, a series of hydrodynamic simulations based on the laws of fluid physics and the existing bathymetric configuration of Boston’s offshore waters were conducted to look at new landscapes. 2 The work coalesced around sketches exploring various configurations of a series of offshore dunes. The sketches were based on potential human benefits that would ultimately be measured against the downside risk of damaging the marine ecosystem. Initial hydrodynamic modeling demonstrated that there was a significant possibility for a set of offshore dunes that would save lives and protect property in times of climate change and sea level rise.

The system of dunes that achieves its objectives must minimize negative overlap with living marine resources and existing conflicts like the Massachusetts Bay outfall and planned uses of the coastal waters. The dunes will firstly change the water currents by redistributing the water level pathways. If water currents are changed, the distribution of fine and coarse sediments are changed, which in turn changes the locations where bottom-dwelling species live. The construction of the dunes will require unprecedented billions of cubic meters of sand, clay, and rock. The new dunes will have the attributes of a beach, a shore face, and a core structure. The intent will be for the dunes to not just resemble their natural equivalent but also operate as nearly equal analogs. Some of the sand used in the dunes’ formation will be transported away under the natural processes of the coastal ocean possibly forming stable beaches in the surrounding area. We could also imagine these dunes while decreasing the height of storm surge, enabling lower, softer, and less disruptive landside storm protections, a tremendous benefit to those who live on the present shoreline. Walls and levees being proposed for Boston could potentially be reduced by as much as half.

1 Water level is dictated by two aspects: the tide, which is due to the moon and the sun, is highly repetitive; while the other, meteorological effects of the wind and pressure, is more difficult to predict because atmosphere is hard to forecast.

2 This is tested using sECOM as the successor model to the Princeton Ocean Model (POM). The model is a finite-difference free-surface estuarine and coastal ocean circulation model coupled to a surface wave model. The modules solve the three-dimensional primitive fluid equations (representing conservation of mass and momentum, and heat and salt transfer) subject to the hydrostatic and Boussinesq approximations, and the surface wave momentum conservation equation. sECOM simulations have been extensively validated in dozens of applications.