Cities at the Frontline
Coastal cities concentrate population, economic activity, and critical infrastructure in areas increasingly exposed to sea level rise and coastal flooding. With over 800 million people living in coastal cities worldwide and coastal urban populations projected to continue growing, adapting urban areas to rising seas is among the most pressing challenges of our time. This chapter examines the range of strategies available to urban planners, engineers, and policymakers for building resilient coastal cities.
Urban adaptation encompasses far more than building higher seawalls. It requires fundamental reconsideration of how cities are planned, designed, built, and governed in an era of rising seas. The WIA-SLR-001 standard provides a framework for urban adaptation that integrates physical interventions with land use planning, building regulations, emergency management, and community engagement.
The Three A's of Urban Adaptation
Urban sea level rise adaptation is commonly organized around three complementary strategies: Accommodate (modify buildings and infrastructure to coexist with periodic flooding), Protect (build defenses to keep floodwaters out), and Retreat (relocate development away from high-risk areas). Most cities will employ a combination of all three approaches, varying spatially based on local conditions and values.
Accommodation Strategies
Accommodation strategies accept that flooding will occur and design buildings, infrastructure, and systems to function despite periodic inundation. This approach is often most appropriate for areas with moderate flood risk, lower density development, or where protection and retreat are impractical.
Flood-Resilient Building Design
Designing structures that can withstand and recover from floodingFlood-resilient buildings are designed to minimize damage from floodwaters and enable rapid recovery after flood events. Key strategies include elevating critical systems and living spaces above anticipated flood levels, using water-resistant materials at ground level, designing for rapid drainage and drying, and locating utilities above flood elevation.
- Elevated construction on piles, columns, or filled foundations
- Breakaway walls at ground level that allow floodwaters to pass through
- Flood-resistant materials (concrete, marine-grade metals, closed-cell foam)
- Elevated mechanical, electrical, and plumbing systems
- Backflow prevention on sewer connections
- Emergency power and water supplies above flood level
Freeboard Requirements
Requiring building finished floor elevations to exceed the base flood elevation (BFE) by a specified margin, typically 1-3 feet, to account for uncertainty and future sea level rise.
Wet Floodproofing
Designing lower levels to flood with minimal damage through flood openings, water-resistant materials, and relocating valuables above flood level.
Dry Floodproofing
Sealing buildings below flood level to prevent water entry, using waterproof coatings, flood shields, and structural reinforcement against hydrostatic pressure.
Critical Facility Standards
Enhanced requirements for hospitals, emergency services, and utilities, typically requiring protection to the 500-year flood level plus freeboard.
Flood-Resilient Infrastructure
Beyond individual buildings, urban infrastructure systems must be adapted to maintain function during and after flood events. This includes transportation networks, water and wastewater systems, power and communications, and stormwater management.
- Transportation: Elevating roads and rail lines, installing flood gates at tunnels, designating flood-resilient emergency routes, and developing real-time flood routing systems.
- Water Systems: Protecting water treatment plants and pumping stations, installing backflow preventers, elevating wells and intakes, and maintaining emergency interconnections.
- Power and Communications: Relocating substations and switching stations, burying power lines where feasible, elevating transformers, and ensuring redundant communications pathways.
- Stormwater: Increasing system capacity, installing tide gates on outfalls, implementing distributed green infrastructure, and planning for compound flooding from rain and surge.
Managed Retreat
Managed retreat involves the planned relocation of people and development away from high-risk coastal areas. While often viewed as a last resort, managed retreat is increasingly recognized as a necessary component of adaptation strategies, particularly in areas where protection is not feasible or sustainable over the long term.
When Retreat Makes Sense
Managed retreat may be the most appropriate strategy when: flood risk is severe and increasing; protection costs exceed the value of assets protected; natural systems would benefit from restoration; repetitive flooding is already causing hardship; community consensus supports relocation; or receiving areas can accommodate displaced residents. Retreat is not surrender but rather strategic redeployment of resources to sustainable locations.
Retreat Implementation Approaches
Tools and mechanisms for relocating development from high-risk areas- Voluntary Buyouts: Government purchase of at-risk properties at fair market value, with demolished structures and land converted to open space.
- Land Swaps: Exchange of at-risk properties for safer parcels, maintaining property ownership while enabling relocation.
- Transfer of Development Rights: Allowing development rights from high-risk parcels to be used elsewhere, compensating owners while restricting coastal development.
- Rolling Easements: Legal mechanisms that allow development but require removal as sea level reaches specified thresholds.
- Building Relocation: Physical moving of historic or high-value structures to safer locations.
- Planned Community Relocation: Coordinated relocation of entire communities to maintain social cohesion.
The relocation of the Isle de Jean Charles community represents one of the first federally-funded climate relocations in the United States. The Biloxi-Chitimacha-Choctaw tribal community has lost 98% of their island to erosion and sea level rise since 1955. In 2016, the community received $48 million through the HUD National Disaster Resilience Competition to plan and implement relocation to a new site 40 miles inland.
The process has highlighted both the possibilities and challenges of managed retreat. Key issues include maintaining community cohesion, preserving cultural identity, ensuring equitable treatment, and navigating the complex legal and jurisdictional landscape of community relocation. The experience offers important lessons for future climate relocations worldwide.
Social Dimensions of Retreat
Managed retreat raises profound questions of justice, identity, and community. Displacement can sever connections to place, disrupt social networks, and threaten cultural traditions tied to coastal locations. Equitable retreat requires meaningful community participation, fair compensation, attention to vulnerable populations, and support for maintaining community cohesion in new locations. The WIA-SLR-001 standard emphasizes that retreat must be truly voluntary where possible and designed with affected communities rather than imposed upon them.
Adaptation Pathways
Adaptation pathways provide a framework for sequencing adaptation actions over time, recognizing that different strategies may be appropriate as sea level rise progresses and conditions change. Rather than selecting a single strategy for all time, pathways approaches identify decision points where current strategies may become inadequate and alternative pathways should be initiated.
Key Concepts in Pathways Planning
- Adaptation Tipping Points: The conditions under which current strategies become inadequate and new approaches are needed.
- Decision Points: Times when decisions must be made about whether to continue current pathways or switch to alternatives.
- Lead Times: The time required to plan and implement new adaptation measures, which must be factored into decision timing.
- Lock-in: Situations where current decisions constrain future options, either enabling or foreclosing adaptation pathways.
- Flexibility: Maintaining options and avoiding irreversible decisions that may prove maladaptive under future conditions.
Land Use Planning
Land use planning is among the most powerful tools for long-term coastal adaptation, shaping development patterns that will persist for decades to centuries. By directing development away from high-risk areas and ensuring that new development is flood-resilient, planning can significantly reduce future exposure and vulnerability.
Planning Strategies
- Hazard Zoning: Designating flood-prone areas for uses compatible with periodic inundation, such as parks, agriculture, or water-dependent development.
- Setback Requirements: Requiring new development to be located a specified distance from the shoreline or flood zone boundary.
- Density Restrictions: Limiting development intensity in high-risk areas to reduce exposure while not necessarily prohibiting all development.
- Overlay Districts: Applying additional development standards to properties within flood hazard zones.
- Transfer of Development Rights: Enabling development potential from high-risk areas to be used in lower-risk receiving areas.
- Comprehensive Plan Policies: Integrating sea level rise considerations into overall land use policy and infrastructure planning.
Post-Disaster Planning Windows
Major flood events, while devastating, can create opportunities for transformative adaptation that would be difficult to achieve under normal circumstances. Pre-disaster recovery planning enables communities to think through how they would rebuild differently after a major event, identifying opportunities to relocate or redesign flood-damaged areas rather than simply rebuilding in place. Having plans ready before disasters strike is essential for seizing these windows of opportunity.
Integrated Water Management
Coastal cities face compound water challenges from sea level rise, storm surge, rainfall flooding, and groundwater rise. Integrated water management approaches address these interconnected challenges through coordinated strategies that manage water at the urban system scale.
Green Infrastructure
Green infrastructure uses natural and semi-natural systems to manage stormwater, reduce flooding, and provide multiple co-benefits including urban cooling, air quality improvement, and habitat creation. Key elements include:
- Bioswales and rain gardens that infiltrate and filter stormwater
- Green roofs that retain rainfall and reduce runoff
- Permeable pavements that allow infiltration
- Urban forests and tree canopy that intercept rainfall
- Constructed wetlands for stormwater treatment and storage
- Floodable parks and plazas that provide storage during extreme events
Groundwater Management
Sea level rise causes groundwater tables to rise in coastal areas, which can lead to flooding from below even in areas protected from surface flooding, damage building foundations, and compromise septic systems and underground infrastructure. Adaptation strategies include:
- Mapping groundwater vulnerability and projecting future conditions
- Upgrading septic systems to mounded or advanced treatment systems
- Designing foundations and basements for higher water tables
- Managing saltwater intrusion in aquifers used for water supply
- Installing groundwater drainage systems where appropriate
Community Engagement and Equity
Successful urban adaptation requires meaningful engagement with affected communities, particularly those most vulnerable to sea level rise impacts. Low-income communities, communities of color, and marginalized groups often face disproportionate exposure to flood risk while possessing fewer resources for adaptation. Equitable adaptation must address these disparities.
Principles of Equitable Adaptation
- Inclusive Process: Ensuring all community members, including traditionally marginalized groups, have meaningful opportunities to participate in planning.
- Targeted Investment: Prioritizing adaptation investments in underserved communities with high vulnerability.
- Anti-Displacement: Designing adaptation measures to avoid displacing existing residents through increased costs or gentrification.
- Cultural Preservation: Recognizing and protecting the cultural heritage and identity of affected communities.
- Procedural Justice: Ensuring fair processes for decision-making, not just equitable outcomes.
- Capacity Building: Supporting community capacity to participate in and lead adaptation efforts.
Summary
Urban adaptation to sea level rise requires a comprehensive approach that integrates accommodation, protection, and retreat strategies within a long-term planning framework. Adaptation pathways provide a structure for sequencing actions over time, maintaining flexibility as conditions evolve and scientific understanding improves.
The most effective urban adaptation combines physical interventions with land use planning, building codes, infrastructure upgrades, and integrated water management. Throughout all aspects of adaptation, equity and community engagement must be central concerns, ensuring that the burdens and benefits of adaptation are fairly distributed and that affected communities have meaningful voice in shaping their futures.
The WIA-SLR-001 standard provides guidance for cities navigating these complex adaptation decisions, emphasizing the importance of science-based planning, stakeholder engagement, adaptive management, and attention to equity. As sea levels continue to rise, urban adaptation will remain an evolving challenge requiring ongoing learning, adjustment, and commitment.