A Fresh Look at Flood Risk

This past winter’s Northern California floods appear to have put to rest the myth that communities are guaranteed safety if they are protected against the so-called 100-year flood. That is the statistical calculation--or perhaps more accurately, guess--that levees and other fixtures are adequate to safeguard a community against the worst flooding that can be expected within a century.

One weakness of this rule of thumb is that flooding history cannot provide an accurate forecast. The corollary is that occurrence of a 100-year flood does not ensure that a place is safe for a century against flooding of such magnitude. In fact, the Sacramento area has suffered two 100-year floods within the past 11 years, and levees presumed adequate in these circumstances failed in both instances.

So it is welcome news that the state now will take a fresh look at flood danger and recommend flood plain management programs. One obvious way to minimize the threat would be to prohibit development in some high-risk areas.

Making a flood plain study was recommended to Gov. Pete Wilson last week in a 289-page final report from the 1997 flood emergency action team led by state Resources Secretary Douglas P. Wheeler. Wilson now has signed an executive order creating a state flood plain management task force, which is due to report back by March 1.

The task force studies will include one of particular interest in the arid regions in Southern California--an examination of urban development on alluvial fans, the sloping deposits left by mountain rivers as they flow to valley floors. The panel will make a statewide inventory of the flooding risks in such areas, in part to help guide development.

The lessons of Northern California’s 1997 flood were tragic--at least nine deaths and $2 billion in property damage in the Sacramento and San Joaquin river basins. A tough new approach to planning and development in high-risk zones in California could prevent a repeat of those sodden, deadly days--or worse.