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THE NEXT FLOOD  HOW SAFE IS YOUR HOUSE?

Are you living or planning to buy property in a potentially flood prone area? There are several ways of assessing that risk.

Setting levees back creates a bigger floodway and improves conveyance of flood flows (adapted from Mount, California Rivers and Streams).

The problems at Folsom Dam, however, are indicative of difficulties for other facilities around the state. In addition to conflicting demands between flood space and water storage, historic operating rules for flood events may be inadequate for currently known hydrologic and scientific information.

As an example, despite great advances in weather forecasting, there are only three dams in California that have flood operation rules taking into account the potential for a major storm event. in all other cases, dam operators simply have a rule book--based on the amount of water in their reservoir for a particular day of a particular month--which dictates the reservoir water levels they must maintain.

A more prudent operating criteria would be to incorporate flood forecasting into decision making, so that dam operators could make "pre-flood" water releases in order to have plenty of space available for anticipated flood flows. This, of course, would not happen ever single year; it would only occur when major storms were predicted.

New operating rules for dams, coupled with bigger and more

THE GALLOWAY REPORT

This frequently cited report stems from the 1993 Midwest floods, after which the Clinton Administration created a special panel of engineers and physical, social and biological experts to investigate the flooding and offer suggestions improving national flood policies. The Interagency Floodplain Management Review Committee was chaired by retired Brigadier General Gerald E. Galloway.

Formally known as “Sharing the Challenge: Floodplain Managment into the 21st Century,” the Galloway Report made a number of important and potentially far-reaching suggestion. It identified the increase of downstream flood peaks (higher, more dangerous flows) as being due to the elimination of floodplains.

The report also recommended expanded the “regulatory floodplain” so as to identify flood potential well beyond the 100-year zone. It suggested mapping  the biggest, reasonably foreseeable storm events possible (known as a “standard project flood”) in order to provide a more realistic land use planning gauge.

Another key finding was that “floods are natural repetitive phenomena” that will continue to occur. This subtle observation offered what some view as the first step toward living with flood rather than assuming that they always can be controlled.

The report suggested ways of becoming less reliant on dams and levees and more reliant on good land use planning, plus it called for trying to move people out of risky, low-lying areas (“hazard mitigation”) that are subject to dangerous floods.

  • Check with your community's flood protection administrator. Local officials who Usually hold this position can be found with the planning or public utilities departments, or with a city manager's office.
  • Assume you're at risk from flooding if you're within a government flood map.
  • Government flood maps don’t include the entire floodplain You should ask the floodplain administrator if your community has mapped larger floods (larger than 100-year), or floods resulting from predicted failures of the flood control system.
  • Floodplain administrators will give you fairly accurate answers for areas within the 100-year floodplain; they will give you wildly inaccurate answers for those areas outside the 100-year floodplain.

reliable floodway capacities downstream improves public safety. Having the desire to make flood releases from dams, having the ability to make those releases (by fixing existing structures and enhancing downstream floodways) would go a long way toward those flood protection improvements.

Downstream Design Problems: The need for bigger flood channels and better conveyance

Not only do some dam structures suffer from design flaws, but the assumptions that originally went into their construction (particularly along the San Joaquin River and its tributaries) have helped to create problems downstream. Since it was assumed that dams and reservoirs could absorb whatever flood nature tossed their way, some downstream river channels and floodways were drastically reduced in size.

Unfortunately, a key part of controlling floods is having better and more reliable "conveyance": the ability to safely convey flood flows through a river system. Once floodplains below dams are urbanized or narrowed by levees, big floods cannot be passed downstream without

the potentially disastrous consequences of levee failure or overtopping. Indeed, some watersheds currently have river and floodways downstream of dams with a conveyance capability little different from that of a good-sized creek. The Stanislaus, Tuolumne and San Joaquin rivers are all examples of this problem.

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