In April 1782, just six years after the Declaration of Independence was signed, John Adams arrived in Amsterdam as the first U.S. Ambassador to The Netherlands. Three months later, a consortium of Dutch bankers provided a 5 million guilder loan (equivalent to $150 billion today) to the new republic, a clear sign of my country’s confidence in the U.S. While I can’t provide a loan, as a Dutch water engineer I can offer something else to Americans: my country’s five centuries of experience living, working, and thriving below sea level. This is surely knowledge and knowhow that the U.S. will desperately need as water levels continue to rise and countless coastal communities are threatened.
Sea-level rise will, of course, affect coastal communities all over the world. However, due to its vast amount of coastline—the National Oceanic and Atmospheric Administration estimates it to be about 95,000 miles—the U.S. will be particularly vulnerable. It is difficult to put a precise number on exactly how high sea levels will rise—it’s what scientists call a fluid model—but according to the government agency, “a worst-case scenario of as much as 8.2 feet (2.5 meters) above 2000 levels by 2100 cannot be ruled out.” And water levels won’t rise independent of other ecological factors; increasingly strong hurricanes and storms due to climate change, rising air and water temperatures, coastal erosion, and flooding will further exacerbate stress on coastal communities.
A 2018 report from the Global Change Research Project estimated that there are nearly 50 million U.S. housing units in danger of eventual flooding. Some communities—in limited, halting numbers—have already begun their begrudging retreat. Some cities are considering storm-surge barriers and seawalls. However, such structures are not only expensive CO2 bombs because of the heavy reliance on concrete as a building material, but they are of limited effectiveness, good for a few decades at most. And, truth be told, not all of the coastline will be defensible. Some land will have to be surrendered back to the sea. But the decision to abandon heavily populated cities would be economically catastrophic and politically fraught. I (and other engineers) strongly believe that some form of the Dutch Polder Solution, an integration of hard and soft, nature-based infrastructure, would be capable of protecting threatened coastal communities in the U.S.
Before outlining our pilot projects for three American cities, some history is in order. Most people in The Netherlands live, as their predecessors have for 485 years, within the boundaries of the great European Rhine/Meus/Scheldt River Delta, in low-lying polders. This includes the inhabitants of our largest cities: Amsterdam (the capital), Rotterdam (the world’s third-largest port), The Hague (the government seat), Utrecht, and Haarlem.
Polders are large land-and-water areas, fully surrounded by dikes, where the ground elevation is situated below mean sea level (MSL) and the water table within the polder is controlled by engineers. About 50% of the overall land and water mass of The Netherlands is situated in polders, and always below MSL.
To be able to live safely in such an environment, extensive infrastructure is required: sea and river dikes, drainage canals, temporary water-storage basins, and pumps, along with sufficient financing for the management and maintenance of all of these systems. The financing for all necessary infrastructure in The Netherlands is primarily provided by the central government, but residents benefitting from these protective structures and services pay for it with a monthly fee. A fully integrated, public, statewide water-management agency, the so-called Water Board, is responsible for designing, constructing, managing, and maintaining all polder infrastructure.
Is some version of the Dutch Polder solution a feasible one for the U.S.? Culturally and politically, it’s an open question. We know the polder-and-dike system works for us. We also understand the low levels of trust that Americans now have for their government, and how state and local governments have struggled to complete large infrastructure projects on time and within budget. If we take the imminent dangers of sea level rise seriously, this must change. Time is already perilously short. Considering the increasing rate of sea-level rise and the slow pace of global efforts to reduce carbon emissions, the U.S. faces a stark choice: design, build, and maintain the appropriate water infrastructure, or move to higher ground.
To successfully execute and complete the design and construction phases of these projects, we recommend the establishment of a federal government agency, working in collaboration with the U.S. Army Corps of Engineers; it could be called the Coastal Security Agency. To guarantee adequate maintenance and management, it would be staffed with engineers and scientists and sufficiently financed for decades to come. Now, let’s be honest here: the costs will be high (but the cost of doing nothing will be multiples higher), the timelines long, and the politics no doubt very complicated. But there aren’t many other options at this point. We’ve outlined plans for three cities, but it goes without saying that many other U.S. cities, towns, and regions are already at severe risk.
We propose completely separating the greater Charles, Mystic, and Neponset River estuaries and the waters of Boston Harbor from the rising waters of the Atlantic Ocean by means of a robust, nature-based sea dike. This would comprise three sections, starting from the mainland at the Pemberton-Hull Peninsula, toward Georges Island and Lovell Island, until it reached Deer Island in the north. These three sections will significantly shorten the current shoreline along Boston Harbor Bay and safeguard the metropolitan area against sea-level rise and coastal flooding.
Within the now-enclosed Boston Harbor Bay, auxiliary dikes would need to be built, along with a water level monitoring system, with pumping stations and spillway/water inlet structures, enabling full control of the harbor water table, including control of algae growth. In addition, depending on specific site and shoreline conditions, other structures and services will be necessary to protect buildings, infrastructures, transportation systems, sewage systems, and water-treatment plants, as well the inner bay wetlands and their crucial biodiversity.
Two locks will also be included within the sea dike: a large one to facilitate ocean vessels serving the port, and a small one for recreational boating. Some of these ideas were already presented in a report, Designing With WATER: Creative Solutions From Across the Globe, prepared in 2014 on behalf of the Boston Harbor Association. The study proposed moving the port facilities to the bayside of their proposed Harbor Island Barrier, a move that can easily be accommodated in our Dutch polder solution.
The coastal situation in Miami-Dade County is very different and certainly much more challenging than the other two proposals presented in this essay. Last year the county commission rejected a $4.6 billion proposal to install floodgates and 10-foot-high seawalls to protect downtown Miami. Instead, the county wants natural solutions, such as barrier islands and mangrove trees. Other cities are exploring similar approaches: installing living shorelines, recreating salt marshes and wetlands, creating manmade oyster beds, constructing local flood protections with parks and open spaces. These measures are certainly useful and should continue, but they will probably not be sufficient enough to stay ahead of the current rate of sea-level rise and the intensifying forces of storms and hurricanes.
Our proposal for Miami-Dade: Coastal.Retrofit 2.0, is a nature-based solution that involves building a barrier reef (1) approximately 500 meters from the shoreline. A hidden sea-dike (6), situated directly underneath the raised beach (5), will be the main structure protecting the city. In between these two structures, a quiet lagoon (8) is created. The wetlands (2) created by the new barrier reef facing the lagoon will be extensively planted with mangrove trees (2a) and below the water table with eelgrass meadows (2b). The sand required for raising the beaches (5) and establishing the barrier reef wetland (2) will be excavated from the Atlantic Ocean bottom with hopper dredges, at least 10 miles away from the present coastline.
Additional structures, including inland cross-dikes, will be required to compartmentalize and control the inland water table. Using the existing canals and waterways for these cross-dikes will minimize landscape and waterway disturbances. Large water-pumping stations and spillway and water-inlet structures will be required, enabling full control of the water table, including the ability to control algae growth and protect biodiversity. A large lock will have to be built to facilitate the passage of cruise ships and other commercial ocean vessels, as well an additional smaller lock for recreational boating.
A related side note: Our joint-venture firm, Van den Herik Sliedrecht and Nautilus Coastal-Solutions, both family-owned businesses from The Netherlands, is also exploring a possible solution for an even graver challenge: mitigating the gradual (and frightening) disappearance of the subsoil limestone-rocks underlying large parts of Florida due to the acidification of ocean waters.
SAN FRANCISCO BAY
We propose completely separating the tributaries and the waters of San Francisco Bay from the progressively rising waters of the Pacific Ocean by means of a nature-based sea dike, approximately 3.5 miles long, built immediately east of the Golden Gate Bridge. This dike will significantly shorten the current, approximately 400-mile-long shoreline along San Francisco Bay and will safeguard towns, communities, and businesses against sea-level rise and coastal flooding.
Within the boundary of the sea dike, inside the now-enclosed San Francisco Bay estuary, auxiliary dikes would need to be built, along with a water-level monitoring system, pumping stations and spillway and water inlet-structures, enabling full control of the Bay Area water table, including the control of algae growth. In addition, depending on specific site and shoreline conditions, other structures and services are necessary to protect buildings, infrastructures, transportation systems, sewage systems, and water-treatment plants, as well as protection of the inner bay wetlands and their crucial biodiversity. Two locks will also be included within the sea dike, a large one to facilitate ocean vessels serving the port, and a small lock for recreational boating. The sea dike and all other structures must be designed and built to be fully earthquake proof, up to at least 7.8 on the Richter scale.
Featured image: Jisp, The Netherlands. Photo by Frans Lemmens.