Sponge cities: a framework for reducing flood damages

Daniel Medina | 20 May 2016 | Comments

The concept of Green Infrastructure (GI) for stormwater management has existed for many years and is now a well-established approach and essential part of numerous land development guidelines and regulations worldwide. GI goes by several other names: Low Impact Development (LID) in North America, Sustainable Urban Drainage Systems (SUDS) in Europe and Latin America, and Water Sensitive Urban Development (WSUD) in Australia and New Zealand. Most recently the term “sponge city” (my favorite) has emerged in China, providing an accurate metaphor for the desired result.

Where conventional stormwater management aims to reduce peak stormwater flow by diverting runoff into detention basins, GI aims to prevent the runoff in the first place, by mitigating an urban development’s negative stormwater effects at the source. New developments typically replace natural areas of soil and plants that soak up water “like a sponge,” with impervious surfaces, such as concrete and asphalt, that create fast-moving water that gets directed to storm drains and discharged into streams, lakes, and the ocean.

permeable pavement
Permeable pavement used at Dolley Madison Library in Fairfax County, Virginia.

Sponge cities” focus on mimicking the hydrology that existed pre-development through the use of micro-controls distributed throughout a developed site. These micro-controls are located near the source where runoff is generated and help deliver it back to its natural pathway (through permeable materials into the ground, or through evaporation into the air). Micro-controls can include bioretention filters, green roofs, wetlands and other devices that reduce both runoff volume and speed. Rain can also be harvested in cisterns for landscape irrigation and other beneficial uses.

The U.S. Environmental Protection Agency (EPA) has developed documentation to promote the many benefits attributed to GI: improved water quality, groundwater replenishment, stream channel protection, and carbon sequestration, to name a few. But one powerful side benefit, flood loss avoidance, was going nearly unmentioned, even though it held the possibility of saving hundreds of millions of dollars each year.

Bioretention filter
Bioretention filter (Dolley Madison Library).

While most research had been aimed at demonstrating GI benefits for water quality and channel protection, limited efforts had been made to understand the benefits of flood loss avoidance. By using GI to reduce the volume of runoff that reaches a stream, peak stream flows are smaller and water surface elevations are lower. This means smaller floodplains (areas expected to experience floodwaters during a storm) and fewer homes and businesses exposed to flood hazards and damages.

Recognizing this gap, the EPA tasked Atkins with conducting a study to estimate the value of flood losses that could be avoided if GI were implemented consistently as part of future development and redevelopment projects in the U.S. We examined the impact of flooding with and without GI at 20 large watersheds (ranging from 1,300 to 7,700 square kilometers) and specifically targeted areas where significant growth is expected to occur between 2020 and 2040. The approach—which included hydrologic and hydraulic modeling, floodplain delineation, and flood damage assessment using Hazus software to calculate monetary losses—was vetted by a panel of experts from government, academia, and industry.

The final report, now available on EPA’s website, revealed that GI, in fact, could have a significant role in reducing flood losses (when applied on an entire watershed basis). Using growth projections for 2040, the study found that national savings in flood losses avoided would range from $100 to $300 million annually, and could possibly be as high as $700 million. The total value of losses avoided between 2020 and 2040 would range from $1 to $2 billion dollars, and could potentially reach $5 billion. And since the scope of the study only included residential and commercial properties, damage totals would have been much greater if roads, rail, and critical facilities such as hospitals and power plants would have been included.

Green Infrastructure is already a central concept in current land planning practices, however this is not the case for flood hazard mitigation planning. This shortcoming is rooted in a disconnect between comprehensive urban planning processes and disaster mitigation planning processes—primarily because of the different priorities and missions of the entities involved. As the study suggests, GI may hold the key to bridging the gap between urban planning and hazard mitigation actions to create a comprehensive framework for making safer, resilient, and more livable communities.

Incorporating Green Infrastructure, or building “sponge cities,” should be a core principle that helps us leverage land use planning while enhancing the effectiveness of flood hazard mitigation. There is a critical need for a framework that brings together these planning activities to bolster the resilience of communities everywhere. Collaboration among agencies in charge of comprehensive planning and hazard mitigation planning can greatly streamline the path to flood resilience. We were proud to be part of the EPA’s study, which takes us one step closer to this goal.