Societal Responses to Floods
People generally don't like it when their homes and buisnesses are inundated with floodwaters. Thus, a great deal of effort has gone into trying to stop this from occuring, with mixed results. There are basically two approaches to the problem:- Hard Responses
- Dams
- Levees
- Channelization
- Soft Responses
- Watershed Management
- Zoning and Land Use
- Insurance
- Erosion Control
- Forecasting
Dams and Reservoirs
Approximately 30% of US reservoir capacity is devoted to flood control. Small dams are effective in upstream areas while large dams protect the downstream areas. Some dams may also produce hydroelectric power. Dams have enormous negative environmental impacts - they destroy ecosystems in the areas flooded by the reservoir waters, prevent sediment from reaching the ocean, and reduce water to downstream regions.
Dikes and levees
These structures are very common world-wide and are used to protect fertile flood plains. Unfortunately, they increase downstream flood peaks by eliminating flood plain water storage. And, their failure often results in disaster (remember Hurricane Katrina?)
Flood Control Channels
The Los Angeles River is a fully lined reinforced concrete. The river was channelized after a devistating flood inundated LA and Orange counties in the early 1930s. It is designed to control high velocity flow. However, the concrete lining prevents scour and prevents sediments from reaching the coast.
The LA River in all it's channelized glory.
Map of the greater Los Angeles area with the LA and San Gabriel Rivers highlighted in red.
Recently, efforts have been made to return the Los Angeles River back to it's natural state. This idea was first met with skeptisism. However, progress is being made and a large stretch of the river has been restored. The City of Los Angeles has a great website on the Los Angeles River Revitalization. The Friends of the Los Angeles River is another great site to learn more.
Photo of a revitalized section of the LA River near Marsh Park.
Watershed Management
Flood Plain Zoning - There are many ways state and local governments can create flood management zones. One way are flood inundation maps. These maps are easy to prepare for zoning purposes. However, zoning is hard to do because of extensive flood plain development - people are reluctant to relocate their homes or businesses. Urban development is the worst use of flood plains. Unfortunately, large cities have grown up alongside major rivers. Farming, recreation, and natural areas are best. Many communities are rezoning areas alongside rivers as parks or other recreational facilities.
Federal Flood Disaster Protection Act of 1973
This Act provides flood insurance at affordable rates, but, it also requires:- Stringent control on new construction in flood plains
- Comprehensive flood management plans
FEMA Flood Insurance Rate Maps (FIRMS) are a result of this act. Cartographers map Special Flood Hazard Areas (SFHAs) on maps; SFHAs are areas that are subject to inundation by a 100-year flood. Homes and businesses in these SFHA zones are required by law to have flood insurance. Thanks to the channizing of the Los Angeles River, the Santa Clarita Valley is the only area in L.A. County to be affected by FIRMS.
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