Flood Risk Management and the American River Basin An Evaluation (1995) / Chapter Skim
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2 IDENTIFICATION AND EVALUATION OF ALTERNATIVES
2 IDENTIFICATION AND EVALUATION OF ALTERNATIVES
Pages 32-84
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From page 32... ...
In a more recent document, the 1994 Alternatives Report (USAGE, Sacramento District, 1994a) the Sacramento District presented a revised set of alternative plans, including estimates of 32
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From page 33... ...
Flood Control Measures In developing project alternatives, USACE begins by identifying flood control measures that can be used alone or in combination. In the 1991 ARWI, the Sacramento District identified 23 flood hazard reduction measures, 13 pertaining to the main stem of the American River and 10 pertaining to Natomas.
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From page 34... ...
The latter included 4 measures for increasing the outlet efficiency of Folsom Dam, in addition to measures for increasing downstream channel capacity, increased flood control storage space in Folsom Reservoir, construction of a dam at Auburn, and raising of Folsom Dam and its spillway. The 1991 and 1994 flood control measures are summarized in Table 2.1.
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From page 35... ...
No No Yes/No Use of existing diversion tunnel No No Yes/No Improved flood forecasting and reservoir operation Yes/No No Yes/No Increased Flood Releases from Folsom Reservoir Levee/channel modifications Yes/Yes Yes Yes/Yes Setback levees Yes/No No Yes/No Flood control bypass south of Sacramento (Deer Creek) Yes/No No Yes/No Increased Flood Storage in the American River Basin Flood detention at Auburn Yes/Yes No Yes/Yes Existing upstream reservoirs Yes/No No Yes/No Multiple small-detention reservoirs Yes/No No Yes/No Offstream storage near Folsom Yes/No No No Out-of-basin storage on Deer Creek Yes/No No Yes/No Increased flood space in Folsom Yes/Yes Yes Yes/Yes Raised Folsom Dam and spillway Yes/No No Yes/Nod Other Measures Divert flood flows into Sacramento River deep water ship channel Yes/No No No Miscellaneous nonstructural Yes/No No No aMeasures listed for consideration in the 1991 American River Watershed Investigation, Sacramento District, U.S.
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From page 36... ...
Increase maximum Folsom flood control storage to 650,000 acre-feet Lower Folsom spillway Increase objective release to 130,000 cfs Folsom Modification and Reoperation (2) Increase maximum Folsom flood control storage to 470,000 acre-feet Lower Folsom spillway Increase objective release to 130,000 cfs Levee Modification Increase objective release to 145,000 cfs Increased Folsom Flood Storage Maximum flood control storage-590,000 acre-feet 1994 Alternatives Report Auburn Dam 894,000 acre-feet Auburn Dam 545,000 acre-feet Auburn Dam 380,000 acre-feet Folsom Modification and Reoperation (3)
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From page 37... ...
Most notable is a reoperation plan for Folsom Reservoir that will increase the winter flood control space based on the availability of storage space in the three largest reservoirs in the upper American River basin. This plan is expected to be implemented independently of the ongoing planning process and hence is considered an existing condition in the 1994 Alternatives Report.
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From page 38... ...
Folsom Reservoir is severely limited in this regard. For example, the primary floodrelease structures, the five main spillway bays, cannot discharge water at the objective release rate of 115,000 cfs until the flood control storage has been filled to about half of total capacity.
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From page 39... ...
The remaining constraints are administrative and legal and could be changed by appropriate agreements. As noted above, the 1991 ARWI considered a number of measures for improving the flood control effectiveness of Folsom Reservoir, including lowering the main spillway, using flood forecasting to draw down Folsom Reservoir in advance of a potentially severe storm, increasing the objective release, increasing the allocated flood space in Folsom, use of storage in upstream reservoirs, and raising Folsom Dam.
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From page 40... ...
Flood Forecasting and Flood Control Effectiveness In both the 1991 ARWI and the 1994 Alternatives Report, the Sacramento District considered and then rejected a measure involving the use of weather forecasts to draw down Folsom Reservoir in advance of a storm. This decision
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From page 41... ...
At a flood storage space of 500,000 acre-feet, the main bays cannot pass any water. The original operation of Folsom Reservoir depended on the concurrent use of the river outlets and the five main spillway gates.
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From page 42... ...
made a number of recommendations for improving the efficiency of Folsom Reservoir with the existing structures. These include concurrent operation of the river outlets and five main spillway gates and use of the three auxiliary spillway gates during normal flood operations.
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From page 43... ...
But, in spite of these changes, the committee was uncertain about the current and future operating efficiency of Folsom Reservoir. The reasons for this uncertainty include the following: · The Folsom Flood Management Plan, referred to in the 1994 Alternatives report, was not completed in time for committee inspection.
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From page 44... ...
It Is important to stress that while the committee was uncertain about the current and future operating efficiency of Folsom Reservoir, it did not believe that these uncertainties were sufficiently large to compromise the validity of the 1994 Alternatives Report. Hence the committee does not suggest that these uncertainties must be resolved before a flood control alternative is selected for Sacramento.
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From page 45... ...
=~#I-~ i3~ = all ~ ~= art ha= a! A -ad miki!
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From page 47... ...
..,..,.,... ..~ ~ · justification of constraints on release rates; and · operator training and other means of improving operator performance, including use of continuous interactive simulation of storm events.
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From page 48... ...
arises from a number of factors, including potential changes in flood regime due to changes in climate and watershed conditions and to changes in political and economic demands on reservoir storage space. Gated Auburn Dam In the 1991 ARWI, the Sacramento District was criticized because it included gated outlet structures in its preliminary designs of the proposed flood control dams at Auburn.
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From page 49... ...
Design of the Deer Creek project assumes that the seasonal flood control storage in Folsom Reservoir will remain at 400,000 acre-feet. Releases would be made from Folsom Reservoir to the Deer Creek detention basin only after it had been determined that the American River had achieved the objective release of 115,000 cfs from Folsom Reservoir.
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From page 50... ...
The Sacramento District's analysis apparently only investigated the potential for a 600,000-acre-foot storage reservoir that would not receive any water from Folsom Reservoir until the objective release of 115,000 cfs from Folsom Reservoir had been reached. The study did not report on the possibility of including a smaller Deer Creek reservoir together with a combination of other measures in order to produce an overall package of flood control measures to provide 200year protection to Sacramento.
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From page 51... ...
The starting point in the analysis is Folsom Reservoir. On the basis of a long-term streamflow record from the gaging station just downstream of the reservoir and the record of storage changes in Folsom Reservoir, the Sacramento District estimated the inflows to Folsom Reservoir and the probability distribution of rain-flood inflow volumes for various durations.
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From page 52... ...
Consider the design situation on the American River. The design events are a set of "balanced" inflow hydrographs to Folsom Reservoir, each with an assumed exceedance probability.
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From page 53... ...
Several critical issues that emerged during the committee's review of the USACE analysis are also discussed. Development of Inflow Design Hydrographs for Unregulated Conditions Development of the unregulated design hydrographs for use as inflow hydrographs for Folsom Reservoir is a key component of the design process, because it is here that probabilities are introduced into the process.
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From page 54... ...
Series of adjusted annual maximum flows, representing unregulated flows to Folsom Reservoir, were developed for durations of 1, 3, 5, 7, 10, 15, and 30 days, for both rainfall and spring snowmelt floods. Probability distributions for these series were estimated using the program REGFQ: Regional Frequency Computation (USAGE, 1982)
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From page 55... ...
The purpose for which the adjustment was developed is not relevant in the Sacramento situation, and, as explained in Chapter 4, the adjustment yields biased estimates of level of protection and expected damages. It appears that the Sacramento District did not use the correction in its analysis supporting the 1994 Alternatives Report; however, as discussed in Chapter 4, the committee disagrees with the procedure the District did use in the 1994 Alternatives Report to estimate level of protection.
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From page 56... ...
o- ~ 1 1 1 1 1 1900 1910 1920 1930 1940 1950 Year I . I 1960 1970 1980 1990 FIGURE 2.2 Annual maximum daily flow, American River at Fair Oaks, 1906-1986, adjusted for affects of regulation.
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From page 57... ...
The Sacramento District's response to these criticisms is that the upstream reservoirs are not operated for flood control and cannot be counted on for storage in large floods. Furthermore, the reservoirs are located in the upper part of the watershed, capturing runoff from only 14 percent of the total watershed area.
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From page 58... ...
This can be done by adjusting the Folsom flood control space in concert with the available storage in the upstream reservoirs, as specified in the Folsom reoperation plan discussed previously in this chapter. Development of Design Hydrographs Below Folsom Accounting for Effects of Folsom Dam For events with an exceedance probability of greater than about 2 percent, the effects of Folsom storage were accounted for by directly using 32 years of flows measured at Fair Oaks.
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From page 59... ...
Given the state of flux in the operations of Folsom, the committee did not find the 1991 operational assumptions to be unreasonable. Because the committee did not have documentation supporting the 1994 Alternatives Report, it was not able to examine in detail the methods used to evaluate the operational assumptions regarding Folsom Reservoir.
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From page 60... ...
The committee was not aware of any criticisms of the Sacramento District's approach to this portion of the analysis and thus did not investigate the issue in depth. Hydraulic Modeling The next step in the analysis was to estimate stage hydrographs for critical locations on the lower American River.
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From page 61... ...
HEC-2 is a USACE model for computing water-surface profiles of one-dimensional, steady-state, gradually varied flows. In recognition of the unsteady nature of flood flows, USACE subsequently has developed a one-dimensional unsteady flow network model of the lower American River, based on the USACE model UNET.
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From page 62... ...
The objective releases investigated by the Sacramento District in the 1991 American River Watershed Investigation are 115,000, 130,000, 145,00O, and 180,000 cfs. The feasibility of conveying these objective releases in the American River channel downstream from Folsom Dam is determined in part by the adequacy of the downstream levees and revetments to contain flows within the channel without failure.
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From page 63... ...
Today, with the existence of Folsom Reservoir, flood flows can be attenuated for longer duration, but the levees cannot safely pass a sustained flow of 180,000 cfs. After the February 1986 flood, extensive geotechnical evaluation of the levees was conducted (USAGE, Sacramento District, 1991, Appendix M)
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From page 64... ...
NEMDC (west levee) American River Levee System North (right)
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From page 65... ...
levee modifications included slurry walls, toe drains, new levees, levee raising, bank riprap, levee riprap, and various combinations of these projects. Subsequent Investigations The 1991 ARWI generally concluded that the levee system was stable for the original design flow (i.e., objective flow)
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From page 66... ...
Recent Work A new risk and uncertainty methodology is under development by USACE, and that methodology was extended by the Sacramento District for this study. The committee was provided with a series of working papers and calculations concerning the risk and uncertainty analysis being developed by the Sacramento District for evaluating the various flood control alternatives, including levees, under consideration for the American River.
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From page 67... ...
The 1994 Alternatives Report, however, does not provide additional information concerning the actual calculation procedures employed in the risk and uncertainty analysis. With respect to the analysis of channel capacity to convey the objective releases, USACE will no longer treat stage-discharge and stage-damage functions in a deterministic fashion, but will regard these as stochastic functions and will estimate probability distributions for these functions.
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From page 68... ...
Because the various reports available express uncertainty concerning levee stability, the committee has concerns about the Sacramento District's proposed alternatives for repairing and enlarging the levees to permit conveyance of "objective releases" from Folsom Reservoir larger than 1 15,000 cfs. Before alternatives involving raising and enlarging the levees to permit conveyance of 130,000, 145,000, or 180,000 cfs are used in the flood damage reduction project, sufficient data concerning levee stability must be available to provide assurance that the repaired or raised levees can contain these higher flows.
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From page 69... ...
for 1 905-1 949 go 1905-1 949 25 50 100 200 500 RECURRENCE INTERVAL (yrs) FIGURE 2.3 Split-record quartile estimates for 3-day deregulated rain floods, American River at Fair Oaks, estimated by Bulletin 17B.
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From page 70... ...
Storms that occur later in the winter are likely to have more precipitation in the form of snow, which does not generally contribute to storm-induced flooding. However, the timing of rainfall floods on the American River does not appear to have shifted during the period of record at the Fair Oaks gage.
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From page 71... ...
Systematic errors in flow monitoring and in the adjustment of the gaged flows for the effects of regulation by Folsom Reservoir are another potential cause of the apparent nonrandomness of the Fair Oaks unregulated series. The gage has been located at several sites and was non recording prior to 1930.
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From page 72... ...
Geological Survey stream-flow measurements at the Fair Oaks gage. Evaluations of potential temporal changes in flood conveyance in the lower American River must consider channel stability, which in turn is dependent on channel morphology and stratigraphy.
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From page 73... ...
, and mining sediment still dominates the active sediment. Field visits in 1994 located much historical sediment stored along the lower American River.
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From page 74... ...
At river mile 21, about six feet of historical sediment cap about 3 feet of older sediment. (Allen James, University of South Carolina.)
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From page 75... ...
On the basis of historical aerial photographs and field evidence, consultants for SAFCA (WRCSwanson, 1992) concluded that bank erosion potential is high, and that sustained bank erosion since 1955 can be attributed to Folsom Dam closure and levee construction.
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From page 76... ...
For example, closure of Oroville Dam in 1968 caused complex channel changes downstream on the Feather River at least through 1975 (Porterfield et al., 1978~. It has also been argued that the lower American River has been degrading in recent decades, encouraged by the closure of Folsom Dam and levee construction in the 1950s (WRC-Environmental and Swanson, 1992)
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From page 77... ...
In fact, due to surplus energy from decreased sediment loads and decreased channel capacities from levees and historical deposits, and due to observed channel erosion and lack of sediment replacement from above Folsom Dam, ongoing net channel erosion could be expected for the lower American River. In spite of these reasons to suspect channel enlargement and the ramifications to channel conveyance and environmental concerns along the parkway, evidence of channel change in the lower American River has not been adequately studied.
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From page 78... ...
.2. Fair Oaks Gage at Hazel Avenue cable (53 River Miles o M i I e s 1 0 Base Map: Sacramento District, 1991 FIGURE 2.4 Locations of gages and levees on the lower American River.
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From page 79... ...
. Fair Oaks Gage near Old Bridge Cross-section plots (1913 to 1950)
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From page 80... ...
Temporal patterns of flood stage changes are illustrated by a time series plot of regression residuals (Figure 2.7~. Flow stages at the old Fair Oaks gage rose slightly from 1905 to 1912, lowered about 2 feet by 1920, rose about 2.5 feet in the late 1930s, and dropped about 3.5 feet by 1950 to about 1.5 foot below the mean for the period.
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From page 81... ...
FIGURE 2.6 Stage-discharge relationship from the Fair Oaks gage at the bridge and early cable site. Several distinct periods of high and low stages can be identified.
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From page 82... ...
o o ._ co En a) CY -2 a a' ~- 1 Cn -4 1 900 1 920 FLOOD RISK MANAGEMENT AND THE AMERICAN RIVER BASIN l 1 Fair Oaks Gage at Hazel Avenue : day.
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From page 83... ...
Evidence from two Fair Oaks gage sites indicates substantial local channel bed scour. From 1913 to 1958, flow stages at the Fair Oaks bridge changed considerably, showing two periods of increasing stages and two of decreasing stages, interpreted as periods of aggradation and degradation, respectively.
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From page 84... ...
As pointed out above, implementation of risk and uncertainty analysis in the lower American River will require appraisals of channel and levee stability (USAGE, Sacramento District, 1994a)
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