Housing Solutions for Rising Sea Levels

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Housing Solutions for Rising Sea Levels

Climate change requires developers to get creative to protect buildings from flooding.
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Wednesday, October 12, 2016 - 11:00am

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Climate change requires developers to get creative to protect buildings from flooding.

By 2050, the Union of Concerned Scientists expects sea levels to rise 6 to 16 inches above current averages. Many are already preparing for the rise. Following are four ways housing developers and communities can reduce the risk of flooding, despite rising levels.

Elevated Development

What it is: Raising the height of land or existing development and protecting it with coastal armoring.

New fill is a strategy being used by the Netherlands to expand and elevate land for settlement. In other locations, elevated development would more likely be confined to elevating existing fill land and protecting it with armoring. The elevation could also be used to protect infrastructure such as airports, roads or railways.


Treasure Island Master Plan, which would elevate the building pad for the island’s proposed developed area, and concentrate development there. The plan also calls for protecting the buildings with a levee and a wide setback.

Elevated houses in post-Katrina New Orleans. New Orleans actually requires new or rehabilitated housing in levee-protected areas to be elevated either 3 feet above grade, or to the base flood elevation established by the FEMA, whichever is higher.

Boardwalks and pole houses. Some boardwalk-type communities are built on poles over tidal salt marshes with a predictable tidal range.

The Good, the Bad and the Unknown

Advantages: This strategy allows structures to be built on an encroaching shoreline or in a vulnerable area with a low risk of flooding. Although it is probably only cost effective for new buildings, it may be a good tool for retrofitting certain low-lying infrastructure, such as airports.

Disadvantages: It’s a short-term strategy. Elevated development, unless it only consists of stilts directly over water, also alters the characteristics of shorelines. It also will need protection, just like low-lying development. Its advantage is merely that it is not threatened by sea level rise for a longer time.

Unknowns: No one knows if higher land or structures will support high-density, transit-oriented new development. Elevated new development in areas close to shorelines could be a good strategy to accommodate growth without worsening sprawl. This is particularly true if constructed near existing transit or built densely enough to fund the creation of new transit—similar to the plan for Treasure Island. However, if low-density development is allowed to proceed along the shoreline, it could increase global warming emissions and may not warrant expensive protection measures in the future.

Floating Development

What it is: Structures that float on the surface of the water, or may be floated occasionally during a flood—making them largely invulnerable to changing tides.

Floating homes can be moored to the shore or anchored to the sea floor. Floating development differs from boats in that it lacks propulsion systems. The challenge of sea level rise has spurred designs for floating forms of development that are much larger than single-family homes, such as an offshore airport in the Netherlands’ North Sea and floating hotels and restaurants off of Dubai’s coast. Electricity, water and wastewater infrastructure must be supplied to floating development through flexible pipes.


Floating homes in Sausalito, where the first houseboat communities were established in the 1960s. Floating homes also are popular in other waterfront cities with relatively protected waterfronts, such as Redwood City, Calif., Seattle and Amsterdam.

Floating infrastructure. A floating greenhouse was built as a prototype in the Netherlands, and floating or pontoon bridges have been built in several places in Washington.

A floatable home prototype for New Orleans. A new home designed to be used in New Orleans would rest on its engineered foundation most of the time. However, during a catastrophic flood it could float as high as 12 feet, anchored to poles.

The Good, the Bad and the Unknown

Advantages: Floating development manages the uncertainty of high tides and earthquakes. It could work in spite of the uncertainty surrounding the timing and nature of sea level rise. Floating development is very resilient to seismic activity.

Disadvantages: Floating development works only in protected areas. Floating structures do not work well in places subject to wind and wave action from storms, such as the ocean coastline.

Unknowns: This type of development has not yet been demonstrated in high-density cities. In fact, floating development is just beginning to be thought of as a sea level rise strategy for larger cities or islands. From an engineering perspective, many structures can be built to float, though they cannot be retrofitted to do so. As they have not been demonstrated anywhere as a model for high-density cities, floating developments could end up facilitating growth in parts of the country that are not close to jobs transit or services.

Floodable Development

What it is: Structures that are designed to withstand flooding or to retain stormwater.

The idea of floodable development is two-pronged. One idea is to design buildings and infrastructure to resist damage by occasional, or even periodic, flooding. This could also be a back-up strategy in case shoreline armoring fails. A second idea is to create retention areas for ocean surges or heavy rainfall. In those areas, water would be captured and then later released to receiving waters—or to the wastewater system when floodwaters recede.

Floodable development is an extreme form of low-impact development (LID). LID is a set of tools for beneficially reusing stormwater by infiltrating it into the ground and creating green space and habitat while reducing the demands on urban wastewater treatment systems. New floodable development built to handle sea level rise may be designed to manage stormwater, both salt and fresh, at orders of magnitude above most LID tools.


LID tools are widely used in Seattle and Portland, and are starting to come online in San Francisco. They include swales or contoured ground, rain gardens, trees, constructed wetlands, green roofs and permeable pavement. They also include large cisterns to store water for future use. Floodable development, at a larger scale, is being pioneered with ideas such as landscape-scale flood parks, floodable streets (possibly channeled by temporary flood walls) and water plazas where floodwater rushes in to create an interesting feature in otherwise dry public space.

A large underground parking garage is under construction in Rotterdam, Netherlands, and will hold water instead of cars during peak floods.
A “village blue” has been proposed for several towns in the United Kingdom experiencing increasing river flood problems and growing populations. This concept is a central recreation area, complete with swimming and boating facilities, that becomes an expandable lake in flood conditions.

The Good, the Bad and the Unknown

Advantages: Floodable development could take an effective small-scale toolbox to the next level. It may best be thought of as a suite of tools (including LID) that can be selected based on which work best at a given site. But unlike LID, the floodable development suite of tools is still experimental. Some tools may be better for fresh water, salt water or a mix, while some may be better in urban areas versus agricultural or rural areas.

Disadvantages: Floodable development could be hazardous. Stormwater, particularly at the seaward end of a watershed, is usually polluted with heavy metals and organic chemicals, in addition to sediment and bacteria. Large quantities of stormwater sitting on the surface or in underground storage facilities could pose a public health hazard during a flood or leave contamination behind. This could be a particular problem in areas with combined sewer systems, such as San Francisco, where wastewater and street runoff go to the same treatment system.

Also, wastewater treatment systems that commonly treat the hazards of combined sewer effluent before releasing it into natural waters do not work well with salt water mixed in. If floodable development strategies are designed to hold and release brackish water, new treatment methods will be needed for the released water to meet water quality standards. Finally, emergency communication tools and extensive public outreach and management would be required to prevent people from misusing or getting trapped in flooding zones.

Unknowns: Floodable development is untested. We don’t know if buildings and infrastructure can be designed or retrofitted to accommodate occasional flooding in a cost-effective way. Also, it is not clear exactly how much volume new floodable development tools will hold. Some of the more heavily engineered solutions, such as a water-holding parking garage, may not turn out to be more beneficial than armoring or investments in upsizing an existing wastewater system.

Managed Retreat

What it is: The planned abandonment of threatened areas near the shoreline.

Managed retreat is a strategy that safely removes settlement from encroaching shorelines, allowing the water to advance unimpeded. It involves abandoning, demolishing or moving existing buildings and infrastructure to higher ground. It also includes banning new development in areas likely to be inundated. It is used when coastal armoring and other shoreline protection efforts become very expensive, or are judged to be a losing battle.

The “managed” part of retreating from the shoreline involves establishing thresholds to trigger activities such as demolishing buildings or abandoning efforts to control shoreline erosion. These thresholds can be coupled with buy-back programs to compensate property owners for loss, plus strict building codes that allow only certain types of relocatable or floodable structures.


Construction setback rules in most coastal areas and Great Lakes states allow development only within a certain distance of the water’s edge. California has delegated this authority to local coastal programs in partnership with the California Coastal Commission. Historically, California set the line using calculations for a 100-year storm. Of course, many older structures don’t comply with current setback rules.

Funding programs in North Carolina and New Jersey allow states to purchase storm-threatened or coastal buffer properties.

Relocation is being tried with larger and larger structures.

In 1999, the historic Cape Hatteras Lighthouse was moved 3,000 feet inland from an eroding beach in the Outer Banks of North Carolina.

Voluntary erosion-control “rolling” easements in Texas and South Carolina vary landward from a rising high tide line. These rolling easements allow for continued private property ownership and development, with restrictions only on armoring and public access.

The Good, the Bad and the Unknown

Advantages: Managed retreat minimizes human suffering by relocating buildings and infrastructure to safer ground before a catastrophic flood. Managed retreat is usually less expensive than armoring strategies, especially when structural measures can only be temporary. It also can be designed to allow for the restoration of flood-buffering wetlands and natural shoreline habitat, which affords protection.

Disadvantages: This strategy is a political quagmire. It involves tremendous legal and equity issues, because not all property owners are willing sellers. In many places, shoreline communities are already disadvantaged and lack the adaptive capacity to relocate.

In addition, retreat may require costs beyond relocation or property costs if site cleanup is needed following demolition. Managed retreat can potentially be very expensive for areas that are significantly developed, and it will cause a loss of coastal property values if setback lines are moved. It is a measure of last resort for many highly developed coastal areas that have no places for buildings to be relocated.

Laura Tam is SPUR’s sustainable development policy director. To view the original and extended article, “Strategies for Managing Sea Level Rise,” visit http://bit.ly/2ax3sfw.

Keywords: Environment & Climate Change | Environment & Climate Change | Green Builder Media | Green Building

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