Flood Control

Page Updated: May 10, 2010

 

A TNC paper on Floodplain Reconnection strategies

Integrated Floodplain-Reservoir Management as an Ecosystem-based Adaptation Strategy to Climate Change

 

Discussion of Freshwater Ecosystem Based Adaption—Floodplains

from Jeff Opperman (December 3, 2009)

Reduced flood risk. Floodplain conservation—including protection of existing floodplains or reconnection of currently disconnected floodplains—provides a strong example of ecosystem-based adaptation. Most climate forecasts suggest that the frequency and magnitude of flooding will increase and even without climate change flood risks are rising in much of the world due to changing land-use patterns (e.g., draining of wetlands, increase in impervious surfaces), increases in population and development within flood-prone areas, and aging infrastructure and insufficient maintenance (e.g., eroding levees). Rather than trying to stay ahead of this flood risk through expanded hard infrastructure (dams, levees, floodwalls), large-scale reconnection of floodplains can act as green infrastructure that reduces flood risk. Levees can be set back from the river in strategic locations to allow floodplains to store and convey floodwaters and reduce risk for nearby areas.

Biodiversity and ecosystem services. In addition to the flood-risk reduction benefits, floodplains are among the most important habitat types in the world for biodiversity and ecosystem services; one seminal study ranked floodplains as second only to estuaries in their per-hectare provision of ecosystem services. Other important services include groundwater recharge, water filtration, sediment and nutrient sequestration, carbon sequestration, recreation, habitat and open space. Importantly, floodplains are engines of productivity that drive river food webs and support the largest freshwater fisheries in the world. This floodplain ecosystem service is incredibly important to the rural poor in much of the developing world. For example, between 60 and 70 million people depend on the Mekong River fishery for their protein and this fishery is largely driven by floodplain productivity.   

Resiliency and flexibility of water management infrastructure. Climate models suggest an increase of hydrological extremes—the increase in flood frequency as described above and an increase in the frequency of droughts. Thus some parts of the world may experience a change in how precipitation is distributed through the year, with more falling in short intense events with longer dry periods in between. This pattern will exacerbate management challenges for multipurpose dams that strive to provide both flood control—which requires empty or partially empty reservoirs to capture floodwaters—and a range of purposes that benefit from full reservoirs (water supply, hydropower, environmental flows to support downstream rivers). Large-scale reconnection of floodplains can shift some of the floodwater storage from the reservoir to the floodplain, liberating an additional increment of storage in the reservoir that can be used for water supply, irrigation, hydropower or environmental flows. In summary, restoring the important ecosystem of a connected floodplain can increase the provision of benefits from multipurpose reservoirs, serve as a hedge against hydrological uncertainty due to climate change, and increase the resiliency of water management systems. In the developing-world context, this same approach can be applied to the protection of existing connected floodplains.    

 

Example TNC projects where restoration or management of floodplain ecosystems are providing benefits such as flood-risk reduction or increased resiliency for water management systems:

• Hamilton City (Sacramento River in California’s Central Valley). TNC was part of a coalition that crafted a multipurpose flood-risk reduction project that will move a levee away from a river, restoring 600 hectares of floodplain and reducing flood risk for a nearby town (Hamilton City).
• Mollicy Farms (Ouachita River, LA). TNC and partners are reconnecting 6,400 ha of floodplain. The large-scale reconnection will provide an opportunity to monitor and study the provision of ecosystem services (e.g., carbon and nutrient sequestration). The flood-risk benefits have not yet been rigorously quantified but residents in a town downstream reported a sudden drop in flood stage against their levee when the Mollicy Farm levee was breached and floodwaters entered that floodplain (the river breached the levee on its own a few months before it was scheduled to be intentionally breached as part of the restoration project).  
• Upper Delaware River. TNC is collaborating with the Army Corps of Engineers to study options for reconnecting floodplains along the upper Delaware River that will maximize both ecosystem benefits and public safety (i.e., flood-risk reduction).
• Yangtze River. TNC is currently coordinating a series of feasibility studies to examine how floodplain management can lead to enhanced hydropower revenue from multipurpose reservoirs. A cascade of four dams in the Upper Yangtze are now under construction and the dams are currently designed to provide both hydropower and flood control. The series of studies are examining the feasibility of eliminating flood storage in the reservoirs and instead managing flood risk in the downstream floodplain (e.g., floodplain reconnection and storage, floodproofing or moving structures). Initial results suggest that removing flood control storage, and thus increasing the elevation and hydraulic head above the turbines, will result in hundreds of millions of dollars in additional annual hydropower revenue. A portion of this additional revenue can be dedicated to freshwater conservation and the necessary floodplain management actions. It is likely that the floodplain management actions will reduce flood risk for the downstream floodplain across a larger range of flood events than the reservoirs could have provided (i.e., the floodplain management actions will be more effective at reducing flood risk than would be the reservoirs).