Flood Risk Management and the American River Basin An Evaluation (1995) / Chapter Skim
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7 FINDINGS AND RECOMMENDATIONS
7 FINDINGS AND RECOMMENDATIONS
Pages 203-218
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From page 203... ...
The key issue in the planning process, and in this report, is how to reduce flood risk in the lower American River basin given a decisionmaking arena that includes significant scientific uncertainty and organized opposition to some of the possible risk reduction alternatives. This report discusses the uncertainties that confront floodplain managers and offers suggestions in many areas, including the need for additional research.
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From page 204... ...
It is important to understand that even if Sacramento achieves its stated goal of a "200-year level of protection," the city will still face a significant residual flood risk. The risk would be equal to a probability of flooding of 1 in 200 per year or about 22 percent over a 50-year period, essentially a 1 in 5 chance over 50 years.
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From page 205... ...
The issue decisionmakers face is how best to determine and then implement an acceptable flood risk management program for the American River basin. Beyond all the complexities and subtleties, the ultimate question is whether the flood damage reduction offered through a combination of measures not including a new dam is acceptable, or whether a new upstream dam is necessary to reduce risk to a more acceptable level.
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From page 206... ...
Some model and data uncertainty is inevitable, and if that uncertainty about levee adequacy is unacceptable either to the public or decisionmakers, then flood risk reduction alternatives beyond levees such as building a dam at the Auburn site- may prove unavoidable. Recommendations · Before the option of raising and enlarging the levees to permit conveyance of 130,000, 145,000, or 180,000 cfs is included in the flood damage reduction project, the Sacramento District, in concert with SAFCA and other local interests, should ensure that sufficient data and professional consensus concern
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From page 207... ...
Recommendation · USACE should assess the magnitude of uncertainty in the American River flood risk and damage estimates by performing a sensitivity analysis involving re-computation of the estimates using just the second half of the American River flood record, from 1950 to the present. Hydraulic Modeling of the Sacramento-American River System A better understanding of the complex flow behavior in the neighborhood of Fremont weir and Sacramento weir, Yolo Bypass, and the river junctions between the Feather and Sacramento rivers and the Sacramento and American rivers is needed over the long-term to support water management decision-mak 1ng in the system.
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From page 208... ...
Hydrologic Monitoring in the Watershed Soil moisture and snowpack water content affect flood risk in the basin, as does the storage level in reservoirs in the upper American River basin. Measurements of snowpack and streamflow levels are routinely made and used as input to a hydrologic forecasting system operated by the National Weather Service and the California Department of Water Resources that describes the hydrologic status of the basin and provides streamflow forecasts using current hydrologic con .
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From page 209... ...
Maintaining Efficiency of Flood Control System Operation Population growth, increasing development, and other changes in the American River watershed create a dynamic flood risk, and it is critical that system operating plans be revisited and revised periodically. For example, as discussed in Chapter 2, it was evident that during the 1986 flood existing operating rules did not adequately consider the existence of 100,000 acre-feet of storage behind the cofferdam upstream of Folsom, even though the dam was designed to breach in a 30-year flood event.
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From page 210... ...
Improving Resource Management Traditional environmental impact assessments fail to evaluate flood risk management alternatives in an ecosystem context because they use a speciesoriented framework. This approach has limited usefulness.
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From page 211... ...
Recommendation · Should any alternative that includes a dam at the Auburn site be considered or pursued as a flood risk reduction measure, then the gate design and operating policies should provide options to control the depth and frequency of inundation, allowing operators to reduce plant mortality while keeping drawdown rates low to reduce environmental impacts from landslides. RISK METHODOLOGY Risk and Uncertainty Planning Methodologies The new USACE flood risk and uncertainty analysis procedures are an innovative and timely development that should improve national flood protection planning.
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From page 212... ...
(The committee did not have the resources to determine or evaluate the actual distortion for the American River study.) Recommendations · USACE needs to develop a consistent scientific methodology and an effective vocabulary for description of residual flood risks and uncertainties to technical and public audiences.
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From page 213... ...
In addition, flood risk will continue for the already developed parts of the city. The committee does not sanction the development of Natomas, but in acknowledgment of the development pressures and in recognition of extensive existing development, the committee recommends that future federal participation in flood damage reduction projects for the American River be conditioned upon the following: Recommendations · Congress should explicitly determine whether flood control projects on the American River warrant federal involvement based on the presence of widespread national benefits from flood protection or on a limited ability of the community to provide its own flood protection.
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From page 214... ...
To obtain maximum benefits, a comprehensive flood risk management strategy should consider nonstructural flood damage reduction measures together with structural measures, especially as applicable in currently undeveloped areas such as the Natomas Basin. These measures should include appropriate floodplain zoning, floodproofing, education, and, when feasible, relocation.
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From page 215... ...
Thus the probability of at least one catastrophic flood within the lifetime of most residents is roughly equal to the probability of flipping a coin and getting heads. Similarly, the Sacramento District and SAFCA should interpret the flood risk on the American River with a 200-year "level of protection" as a 22 percent chance over the next 50 years that flood waters will overtop the levees and inundate the Sacramento area.
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From page 216... ...
The traditional term "level of protection" may mislead the public and is not consistent with the analytical outcomes expected from the new USACE procedures. Recommendations · USACE should select a technically sound risk communication vocabulary and approach to communicating flood risk likelihood and consequences and use it consistently in all reports and presentations.
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From page 217... ...
State leadership is critical to build a consensus on technical and institutional strategies to manage competing water demands. · If no agreement is reached on an acceptable approach to flood risk management in the near future, the Sacramento District and SAFCA should expand the consensus-building efforts of the Lower American River Task Force.
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From page 218... ...
These are offered in a spirit of constructive criticism and to encourage the continued progress in reducing the American River basin's flood risk and in the evolution of the nation's understanding of flood risk management. The committee reiterates its concern that nothing stated in this report should be used as an excuse for delaying action in the American River basin.
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