Program

Discussion Topics

Major Drivers Affecting Flood Hazards: Land Use Change

Land use is a major driver of changing flood hazards; it is also a reflection of societal needs and values. Over time, people have modified land to derive benefits: to grow food, to decrease how often valuable or populated areas are flooded, to enable navigation and commerce, to support growing urban areas. These changes have affected the nature of flood hazards. More urbanized watersheds typically have more impermeable surfaces, increasing total runoff from a rain event. Agricultural practices may eliminate water micro-storage areas and accelerate surface drainage. Channels and levees speed waters downstream, sometimes increasing peak flood flows. Levees reduce flooded acreage for less frequent flood events while raising river levels. Diminished wetlands can reduce natural storage areas; diminished vegetation can reduce natural barriers to coastal storms. Seawalls and breakwaters can alter sediment movement, changing the landscape of coastal areas.

Land use is also a major driver of changing flood consequences. Increasing population density in flood-prone areas will put more people at risk. A growing population even in areas subject to infrequent flooding poses increased challenges during major flood events. At some point, sheer numbers may strain the ability of a government to respond effectively. Areas perceived as being protected by structures from flooding may be subject to development pressures. It may be difficult to ensure that areas intended for needed water storage during infrequent flood events remain free from encroachment. Already-stressed populations may be particularly vulnerable during and after flood events and may need increased assistance.

Land use changes will continue to occur and be driven by societal needs and values. Anticipating these changes, and taking a more holistic approach, will enable us to anticipate and prepare for changing flood risks. The presentations in this session focused on the flood risk challenges posed by land use changes and offered specific approaches to anticipate and reduce those risks while meeting other societal goals.

revised 18 Feb 2011