Flooding is and will be our most significant urban adaptation challenge. Recent scientific estimates of future sea level rise predict 4-6 feet by 2100, rising rapidly after that. Some experts think a better estimate would be 6-9 feet by 2060. Groundwater levels will also rise, on top of sea levels, causing extensive freshwater flooding in coastal cities that may well double the area that floods by salt water. In addition, rainfall intensity has already increased and will continue to get worse. If cities wait, they’ll be in triage mode and will have to abandon districts that can’t afford to adapt using local funds. “Abandonment” is another word for economic and environmental disaster—when it happens in under-funded cities, it will leave behind underground infrastructure, building ruins, and soil contamination.
The most important new urban designs are those for floodable development, examples of which already exist in Europe. We could build a multi-functional version in North America, turning new housing investments into a hybrid infrastructure that protects existing neighborhoods and infrastructure systems. Low-rise or mid-rise housing can be built on pile foundations in areas that will be permanently flooded as a result of rising groundwater or salt/ brackish water. The housing can be built in artificially excavated ponds surrounded by open water on three sides, with access on the fourth side to an earthen levee with a road on top and infrastructure hook-ups. These ponds would form a honeycomb pattern in a floodable area, functioning as a “micro-polder” that is able to absorb several feet of additional water from temporary flood events that otherwise would damage surrounding areas. These “micro-polders” could be protected from waves and debris by a perimeter system of larger storm-protection levees. By building this new kind of urban district with “wet feet,” we can provide critical protection for existing urban districts.
Cities can implement this by creating a legal entity like a public development authority to prepare the ground. This authority would excavate the ground to build the levees, build roads on top, and supply the new floodable housing with electricity, water and other public services. Development fees from the new housing would be used to help pay for this infrastructure, and for other amenities such as artificial beaches and wetlands that add recreation and habitat value to the floodable district. Cities with flooding shores would be able to accommodate increased density while reinvesting in a resilient new system of stormwater infrastructure.
Hill’s Resiliency Top 3:
1. The Sand Engine (Zandmotor), near The Hague in The Netherlands: This is an example of an artificial landform, a sand spit made of dredge material, that was placed in a design-build operation to nourish the coast around it by utilizing the energy of waves to move the sand along the coastline and widen the surrounding beaches. It has produced recreational value, habitat value, and storm protection value, all in one project design that is less expensive than traditional beach nourishment.
2. LEAP for San Francisco Bay (pictured above): A set of unbuilt landscape-based proposals that expand on the Sand Engine concept with other forms, this was illustrated and developed by a young designer based on ideas from my work and many experts he has spoken with around the Bay.
3. European versions of floodable development: (A.) Nesselande housing, Rotterdam, The Netherlands; (B.) the site preparation for the HafenCity District, Hamburg, Germany; (C.) Backenhafen Water Houses (Studio Gang design), HafenCity District, Hamburg, Germany: All three of these projects provide examples of floodable development—in (A.) it’s housing on pile foundations in a high-water table environment; in (B.) it’s a whole urban district built on mounded earth, with hardened or waterproofed first stories for the buildings; and in (C.) it’s mid-rise buildings on pile foundations standing in the water of a canal.
Kristina Hill is an associate professor at the University of California, Berkeley, where she studies international strategies for adapting urban infrastructure and coastal districts to sea level rise. She has worked on urban water systems in the Pacific Northwest, New Orleans, the Mid-Atlantic and New England coasts. Her current focus is on the San Francisco Bay region, where unique opportunities exist to bring new housing strategies into a dynamic metropolitan landscape of wetlands, sand dunes, and beaches.
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