Floodplain Management

Traditional floodplain management
Floods the most common natural disaster in California
Misleading 100 year flood concept
Concept of a “Reasonably Foreseeable” flood event
Multi-objective management
Floodplain management for the 21st Century
Articles in our periodical Linkages
Links to web sites and on-line reports
Key books and articles not available on-line

Traditional Floodplain Management Destroys River Functions and Ecological Wealth

The natural floodplains of rivers and streams are rich ecological areas and part of a functioning river or stream system. In valley floors, a dynamic river will move its course to and fro over a long time frame within its “meander belt.” Riparian vegetation and wetlands associated with streams and rivers are essential habitats for a great number of animal and plant species.

For over a century our society has worked to “tame” California’s waterways. Most of California’s rivers are now flanked by levees, divorcing them from their historic floodplains and increasing the elevation of floods. This destroys both the ecological benefits of floodwaters periodically spreading over floodplains and also a variety of natural river ecological and physical functions. The native vegetation of these floodplains has been largely replaced by human uses. For example, California has lost over 95 percent of its Central Valley riparian woodlands and over 95% of its wetlands, first to agriculture and then urban-suburban development. Vital natural functions of rivers and floodplains, essential for long-term ecological health and provision of ecosystem services to society, are ignored.

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Floods are the Most Common Natural Disaster in California

But while systems of levees and dams give a sense of protection from floods, in reality many of these areas are threatened by catastrophic flood events. Flooding remains a major problem in California, and there have been several disastrous events since 1850. In January 1862, four weeks of rain produced vast inland seas across most of the Central Valley and in Orange County. Major floods occurred in 1907, 1909, 1937, 1955, 1962, 1964, 1986, 1995 and 1997.

About 90 percent of the disasters in California have been floods. Since 1950 each of California’s 58 counties have been flood disaster areas at least three times. Most of the flooding is in the floodplains along rivers, but there are also problems with coastal flooding and the potential for catastrophic flooding of southern California alluvial fans, where fast moving water is mixed with rocks and large boulders.

The engineering approach to flood events and the use of floodplains fails time after time. The 1993 floods along the Mississippi river, which would have had far worse consequences but for an excess of 1,000 levee breaks in the upper river basin, woke society to these past errors and suggested a different approach to flood management. The federal government’s Galloway Report called for major reform.

Past flood events give us clear lessons. In Congressional testimony, UC Davis’s Dr. Jeffrey Mount identified four key lessons from the 1997 California floods (See Linkages #4)

* Levees and dams cannot eliminate flooding
* California’s multipurpose dams are for water supply
* Levees fail to prevent large floods and exacerbate damage
* A cycle of cyclical engineering increases potential for flood damage

A further complexity is that the state of California is now liable for damages if a levee it is responsible for fails. This was the November 2003 decision of the state's Third District Court of Appeals, which ruled that the state was liable for 1986 flood damages in community of Linda,Yuba County (Paterno et al v State of California. CO40553.) An old levee, built by local interests on hydraulic mining debris, collapsed. The state had assumed responsibility for this levee in 1953, when it took over responsibility for the Sacramento River Flood Control Project. The Appeals Court ruled that the local Reclamation District, to whom the state had given levee operation and maintenance responsibility, was not liable. In March 2004 the California Supreme Court rejected the state's petition and so the Appeals Court ruling stands. This has huge implications for floodplain management.

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The Misleading 100 Year Flood Concept

Local governments and residents rely on an artificial system of determining which areas may be flooded. The system gives a false sense of security, belying the real danger of catastrophic flooding. The 1997 California floods showed the consequence of this, when most of the flooded areas were lands mapped as being outside the floodplains. Continuing the current approach to floodplain use over the next few decades of California’s seemingly remorseless growth will exacerbate the current problems greatly.

Currently, local communities think in terms of the “100 year flood” as a result of the flood insurance requirements of the Federal Emergency Management Agency (FEMA) and its flood insurance rate maps. Flood insurance is required for properties within the 100 year flood areas of FEMA maps, but not for other areas. If land is declared protected from the 100 year flood, for instance by construction of a levee, then its properties do not require flood insurance. A big mistake is thinking land outside areas labeled as subject to a 100 year flood is not in the floodplain.

The 100 year flood is a statistical phenomenon based on the levels of past recorded floods. If there is a big flood next year in an area, the level will be re-calculated. Flood protection from the 100 year event may suddenly become protection from only a 60 year event.
Secondly, this concept does not mean that such a flood will only happen once every 100 years. It may happen two years in a row as the probability is always 1% a year.

Thirdly, lands outside the 100 year flood areas can easily be inundated by bigger events. This is what occurred on the Mississippi in 1993 and in Central Europe in 2002, when there were floods of a magnitude that occurs only once in 500 years.

The threat includes risks to lands behind levees. Levees can fail, even during a smaller flood event than they should handle. Larger floods will over-top levees. The areas behind levees have a residual risk of a catastrophic flood which in some cases would be 20 feet deep. If that behind-levee land is in a basin, then much of the water will not drain away after the floods recede.

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The Concept of a “Reasonably Foreseeable” Flood Event

A better approach is to determine the “reasonably foreseeable” flood event, and use this for local land use planning and flood protection. This may be significantly greater than the 100 flood, or may be less, depending on the location. In 2002, the California Floodplain Management Task Force developed this idea of a “reasonably foreseeable flood” and recommended its adoption. However, its use in any area, will require funding for the technical work to determine lands would be impacted by a reasonably foreseeable flood.

A variety of data is needed to make this determination, including historic and paleo-flood data and hydrologic modeling. On many streams, it is also important to consider the impacts of future development which will increase impermeable surfaces on a watershed and so increase the amount and speed of stormwater run off unless best management practices are used to contain extra runoff.

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Multi-objective Management

In 2002, the California Floodplain Management Task Force recognized the variety of positive values of undeveloped floodplains, including conservation of agricultural lands, wildlife habitat and groundwater recharge areas. It made important recommendations for promoting multi-objective management.

One recommendation is “flood management programs and projects, while providing for public safety, should maximize opportunities for agricultural conservation and ecosystem protection and restoration, where feasible.”

Another recommendation gets away from the mind-set of channelized streams narrowly bordered by levees. “In planning new or upgraded floodwater management programs and projects, including structural projects, local and state agencies should encourage as part of the design, where appropriate, nonstructural approaches and the conservation of beneficial uses and functions of floodplains”. The use of setbacks levees to provide more room for flood flows, coupled with allowing a natural, often meandering, stream channel is an example of this approach.

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Floodplain Management for the 21st Century

  • Flood-proof existing communities against reasonably foreseeable floods rather than the 100 year event. This approach was supported by the California Floodplain Management Task Force through a number of recommendations.
  • Utilize innovate flood control projects, such as the Napa River Flood Management Plan, a community led project to protect the City of Napa. This is based on “living river” principles that re-connected the river to its floodplain and restored tidal wetlands
  • Halt the continual encroachment of metropolitan development into rural floodplains and alluvial fans, the vast majority of which is unnecessary if society switches to true Smart Growth. Not only is it expensive to keep expanding the geographic area requiring extensive flood protection but also this consumes high quality agricultural lands and wildlife habitat, while foreclosing opportunities for restoration of riparian areas and river functions.
  • Utilize multi-objective management to maximize opportunities to conserve and restore more natural rivers and streams in rural areas, including use of levee set-backs and riparian restoration, while providing agricultural lands with added protection from flooding.

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Additional Floodplain management information in Linkages

Linkages #4, Spring 1997

Changing Flood Management to Prevent Future Disasters


Excerpted from testimony by Dr. Jeffrey Mount to the House Subcommittee on Water Resources and Environment, March 1997.
Examines the lessons learned from past flood events and proposes a new flooding paradigm

Flood Management and Ecological Enhancement Goals on the Cosumnes River


Explores the potential to couple more effective flood management with ecological restoration

San Joaquin River Possibilities & Southern California Constraints


Outlines the special problems of the San Joaquin River and the potential for re-engineering, with levee setbacks and a bypass. Visits the problems of Southern California’s alluvial fans

Linkages # 1, Fall 1997

What Future for the Santa Clara River?


Overview of Santa Clara River corridor’s biological values, urban growth issues and the beginnings of a management plan

Linkages # 15, Fall 2003

Planning for a Floodplain: Southern California’s Santa Clara River


Examination of the Santa Clara River Enhancement and Management Plan

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Links to Useful Web Sites and on-line Reports

Beyond Flood Control: Flood Management and River Restoration. Friends of the River (1997)

California Floodplain Management Task Force, Final Recommendations Report (2002)

Comprehensive Study, December 2002 Interim Report to Congress.
U.S. Army Corps of Engineers study on flood damage reduction and ecosystem restoration along the main stems of the Sacramento and San Joaquin Rivers.

Ecological Issues in Floodplains and Riparian Corridors (2001) Bolton SM, and J Shellberg
[Click on floodplain.pdf to download]
From the Center for Streamside Studies, University of Washington. Details impacts of stream alteration and techniques to protect habitat. Extensive references

Final Report of the Flood Emergency Action Team.
California Department of Water Resources report after the 1997 floods. Chapter 6 addresses floodplain management issues

The Gathering Storm A Five Part Sacramento Bee Special Report on food dangers and issues in the western United States. By Tom Knudson and Nancy Vogel. November 1997

Floods Warn Valley of Worse to Come. Storms, Growth Expose Region's Rising Risk
San Francisco Chronicle, March 3, 1997. Interesting article on Central Valley Flood issues, plus maps of Central Valley floodplain and levee system

Alluvial Amnesia: How Officials Imperil Communities by Downplaying Flood Risks (2002)
The Center for Government Studies. Explores alluvial fan issues, using Deer Creek above Rancho Cucamonga, San Bernardino County as the example.

Cosumnes Research Group
Extensive research and monitoring of the Cosumnes River by UC Davis and other partners. Addresses hydrology, geomorphology, water quality, aquatic resources and data.

Federal Emergency Management Agency (FEMA) Floodplain Management web page

Floodplain Management Association

Friends of the Santa Clara River

Inventory of California Watershed Groups

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Books and Articles not Available On-line

California Rivers and Streams: The Conflict Between Fluvial Process and Land Use (1995). By Mount, JF. University of California Press.

Sharing the Challenge: Floodplain Management into the 21st Century. (1994) Galloway GE. Federal Interagency Floodplain Management Task Force. Report on the 1993 floods in the Mississippi River Basin, with lessons and far-reaching recommendations.
Available from the Government Printing Office [ISBN 0-16-045078-0]

Need for Ecosystem Management of Large Rivers and Their Floodplains. Sparks RE (March 1995) Bioscience 45:168-182

Understanding Large River-Floodplain Ecosystems. Bayley PB (March 1995) Bioscience 45: 153-158.

Naturalization of the Flood Regime in Regulated Rivers Nelson JC et.al. (September 1998). Bioscience 48:706-721.

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