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Flood Risk Management and the American River Basin: An Evaluation (1995)
Commission on Geosciences, Environment and Resources (CGER)

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Identification and Evaluation of Alternatives In the 1991 American River Watershed Investigation (ARWI), the Sacra- mento District presented various alternative plans to provide flood control to Sacramento, including supporting analysis (USAGE, Sacramento District, 1991~. For each alternative plan, the 1991 ARWI provided estimates of the cost, ex- pected benefits, and net benefits; the level of protection; and the environmental impacts and proposed environmental mitigation. Formal decisionmaking on the alternative plans was then based on these estimates. In the USAGE's planning process, the benefit-cost ratio is calculated to screen out inefficient alternative plans, as plans with negative net benefits are not eligible for federal funding. The alternative plan with the highest expected net benefits, consistent with applicable environmental laws and regulations, is desig- nated the National Economic Development plan (NED) and is generally the plan recommended by the federal government. In the American River case, the NED plan included construction of a dam and 894,000-acre-foot reservoir at a site near Auburn. However local interests, as represented by the Sacramento Area Flood Control Agency (SAFCA), preferred a plan featuring a smaller dam and after consultation a plan including a smaller structure, offering, a 200-year rather than 400-year level of protection, became the selected plan. During review of the 1991 ARWI by federal and state agencies and by public interest groups, concern about a number of technical issues emerged. These issues played some role in the rejection of the selected plan by Congress in 1992 and ultimately led to the creation of this committee. In a more recent document, the 1994 Alternatives Report (USAGE, Sacramento District, 1994a) the Sacra- mento District presented a revised set of alternative plans, including estimates of 32

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IDENTIFICATION AND EVALUATION OF ALTERNATIVES 33 costs and benefits. Unfortunately, the analysis supporting those new estimates is not scheduled for release until July 1995. In preparing the 1994 Alternatives Report, the Sacramento District had the opportunity to benefit from the technical debate that was generated by the 1991 ARWI and from interactions with this committee and many other parties. In addition, the 1994 Alternatives Report previewed the first application of USAGE's new approach to evaluating flood control projects, an approach based on risk and uncertainty analysis. This chapter discusses the development of alternative plans and the technical analysis used to estimate costs, benefits, and levels of protection. Subsequent chapters consider the analysis of environmental impacts and the new USACE approach to risk and uncertainty analysis. The committee's consideration of these issues was based largely on written and oral information provided by USACE, SAFCA and its consultants, and various critics of the 1991 ARWI. The committee was able to make firm recommendations on a number of technical issues, but many issues remain unresolved owing to lack of data and to the fact that the supporting technical analysis is not yet available. This latter fact has proven particularly problematic. Information related to that future document, received informally during briefings, indicates that the analysis supporting the 1994 Alternative Report is significantly different in many crucial respects from that which supported the 1991 ARWI. But the committee did not have formal written documentation of the analysis, and in most cases was uncomfortable about commenting on oral presentations and the few supporting documents that were available. SELECTION OF PROJECT ALTERNATIVES Perhaps the most critical step in the development of a flood control project is the selection of alternatives that will receive detailed analysis. Regardless of the potential effectiveness of a particular alternative, if it is not identified, it will not be selected. Furthermore, if popular alternatives are not selected for detailed analysis, it may be difficult to win support for the selected alternative, regardless of the potential effectiveness of the popular choices. Thus, this section looks specifically at the selection of alternatives in the American River planning pro- cess. (Additional discussion of the selection of alternatives and project planning in general is found in Chapter 6.) Flood Control Measures In developing project alternatives, USACE begins by identifying flood con- trol 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. Of the 13 main stem measures, 4 were retained for further consideration and incorporated

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34 FLOOD RISK MANAGEMENT AND THE AMERICAN RIVER BASIN into flood protection alternative plans: (1) structural modifications to Folsom Dam to increase outlet efficiency; (2) increased downstream channel capacity to allow greater flood releases (so-called "objective releases") from Folsom Reser- voir; (3) increased allocation of storage space in Folsom Reservoir to flood control; and (4) construction of a dam upstream of Folsom Reservoir (at Auburn). In the 1994 Alternatives Report, which excluded consideration of the Natomas Basin, the Sacramento District presented 17 measures, 8 of which were retained for further consideration. 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. Flood Control Alternative Plans In the 1991 ARWI, the 4 surviving flood control measures were bundled into 6 alternative plans. Two alternatives were based on construction of a flood control dam at Auburn. Two other alternatives combined increasing flood con- trol storage and outlet efficiency at Folsom with increasing downstream flow capacity. The fifth alternative was based solely on increasing the downstream channel capacity. The final alternative was based solely on increasing the pro- portion of flood control storage in Folsom Reservoir. Seven alternative plans were presented in the 1994 Alternatives Report. Three of these were based on construction of a flood control dam at Auburn. Three other alternatives combined increasing flood control storage and outlet efficiency at Folsom with increasing downstream flow capacity. The final alter- native combined increasing flood control storage and outlet efficiency at Folsom, without increasing the downstream flow capacity. The alternative plans presented in the 1991 ARWI and 1994 Alternatives Report are summarized in Table 2.2, along with the estimated levels of protection and ratios of the net benefits to the net benefits of the NED plan. Note that the methods that the Sacramento District used to estimate the levels of protection in 1991 differed from those used in 1994; hence the estimates are not strictly com- parable. Criticisms of the 1991 Measures and Alternatives The measures and alternatives presented in the 1991 ARWI were criticized on a number of grounds. Many of these criticisms focused on the evaluations of the alternatives; these are addressed in subsequent sections. However, some of the criticisms had to do with the perceived failure of the Sacramento District to consider and evaluate potentially effective alternatives. The most serious criti- cisms focused on Folsom Reservoir. In particular, critics argued that the district

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IDENTIFICATION AND EVALUATION OF ALTERNATIVES TABLE 2.1 American River Flood Control Measures (Excluding Natomas) 35 Measure 1991 Reporta 1994 Preprojectb 1994 ReportC Listed/Retained Condition Listed/Retained Increased Outlet Efficiency of Folsom Dam and Reservoir Normalized use of auxiliary spillway No No Yes/No Structural modifications Lower main spillway Yes/Yes No Yes/Yes Enlarged river outlets No No Yes/Yes New river outlets No No Yes/Yes New tunnel outlets No No Yes/No Conjunctive use of river outlets and main spillway (without modifying outlets) 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, Sacra- mento District, U.S. Army Corps of Engineers. bMeasures from the 1991 ARWI that were treated as part of the pre-project condition (i.e., mea- sures already or planned to be implemented) in the 1994 Alternatives Report, Sacramento District, U.S. Army Corps of Engineers. CMeasures listed for consideration in the 1994 Alternatives Report, Sacramento District, U.S. Army Corps of Engineers. dMeasures that may be reconsidered before final recommendations are made.

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36 FLOOD RISK MANAGEMENT AND THE AMERICAN RIVER BASIN TABLE 2.2 American River Flood Control Alternative Plans Alternative Level of Protectiona (years) Net Benefits/ NED Net Benefitsb 1991 ARWI Auburn Dam 894,000 acre-feet Auburn Dam 545,000 acre-feet Folsom Modification and Reoperation (1) 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) Modify Folsom outlet works Increase objective release to 180,000 cfs Folsom Modification and Reoperation (4) Variable Folsom flood control storage 450/670,000 acre-feet Modify Folsom outlet works Increase objective release to 145,000 cfs Folsom Modification and Reoperation (5) Variable Folsom flood control storage 475/670,000 acre-feet Modify Folsom outlet works Increase objective release to 130,000 cfs Folsom Modification and Reoperation (6) Variable Folsom flood control storage 495/670,000 acre-feet Modify Folsom outlet works Maintain objective release at 115,000 cfs 400 1.0 200 0.80 0.56 100 0~30 00 100 455 270 200 244 217 0.30 0.34 1.0 0.70 0.30 0.24 0.21 185 0.19 152 0.32 aLevel of protection was computed differently in 1991 and 1994. bFor the 1991 ARWI, the divisor is the net expected benefit for the 1991 NED plan; for the 1994 Alternatives Report, the divisor is the net expected benefit for the 1994 NED plan.

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IDENTIFICATION AND EVALUATION OF ALTERNATIVES 37 failed to adequately consider modification of the operation of Folsom Dam, which, coupled with improvements in the dam's outlet capacity, might signifi- cantly increase the effectiveness of the existing storage. Some of these criticism were addressed in the 1994 Alternatives Report. 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. Issues of Importance in the 1991 and 1994 Alternative Plans In considering the alternative flood control plans in both the 1991 and 1994 reports, the committee elected to focus on four specific elements: use of Folsom Reservoir, the question of gates in the Auburn Dam alternatives, the Deer Creek alternative, and nonstructural measures. Folsom Reservoir As noted above, the 1991 ARWI was criticized for failing to give sufficient consideration to ways to maximize the flood mitigation potential of Folsom Reservoir, including the use of flood forecasts. How valid is that criticism? Before addressing this question, consider how the operation of Folsom Reservoir determines its effectiveness at reducing flood risk in Sacramento. Folsom Reservoir provides the primary means of reducing flood flow in the lower American River. The flood reduction potential of the reservoir depends on the amount of water that can be stored as compared to the difference between the amount that enters the reservoir during major flood events and the amount that can be safely released. At full pool, Folsom Reservoir has a storage capacity of about one million acre-feet. But Folsom is a multipurpose reservoir; in addition to flood control, its purposes are water supply, hydropower, and recreation. Un- fortunately, there are conflicts among these objectives. If the reservoir were to be operated for an assured water supply alone, the optimal strategy would be to keep the reservoir as full as possible. If the reservoir were to be operated for flood control alone, the optimal strategy would be to keep the reservoir as empty as possible. Clearly, the reservoir cannot be operated to maximize both of these objectives simultaneously. One solution to this dilemma is to allocate storage amounts separately to flood control and water supply. Nominally, the top 400,000 acre-feet of storage space in Folsom Reservoir is allocated to flood control; the remainder is allocated for water supply. This allocation is not rigid, however, owing to the timing of flood events in the watershed. Potentially damaging floods occur only during the winter storm season, which lasts from the beginning of November through the

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38 FLOOD RISK MANAGEMENT AND THE AMERICAN RIVER BASIN end of March. Hence the full flood storage pool need be available only during this period. The manner in which the flood storage space is managed is specified by a flood control diagram that was originally formulated in 1956 and modified in 1977 and 1987. Under the 1987 diagram (USAGE, 1987), the flood control storage space must be increased from zero on October 1 to a maximum of 400,000 acre-feet on November 17, at which level it must be maintained until February 8. Between February 8 and May 31 the flood control space is to be varied according to the accumulated seasonal precipitation, which is closely related to the depth of snowpack in the upper American River watershed. This currently used approach to managing the flood control space in Folsom Reservoir could be modified to improve flood control effectiveness (as is being considered with the Folsom reoperation, discussed below). Such improvements may or may not come at the expense of water supply or other water resources purposes (see Chapter 6 for additional discussion). The seasonal allocation of flood storage determines the amount of storage available for flood control prior to a flood. The effectiveness of the available storage depends on how it is used during a flood event. Obviously, it is desirable to release water as rapidly as possible without causing downstream damage dur- ing a flood, since that frees up storage space in the reservoir. But there are constraints on how rapidly water can and should be released. First, there are physical limitations on the maximum discharge rate from the reservoir. Folsom Reservoir is severely limited in this regard. For example, the primary flood- release 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. (The objective release rate is the design discharge capacity of the channel and levee system downstream of the reservoir; sustained flows in excess of this rate could cause levee failure.) Second, there are admin- istrative and legal limitations on releases. The 1987 Water Control Manualfor Folsom Reservoir (USAGE, 1987) provides that as an operating guide, "releases from Folsom Dam shall not be increased more than 15,000 cfs or decreased more than 10,000 cfs during any 2 hour period. . ." This limit on the rate of increase of discharge rates (the so-called "ramping rate") is intended to minimize bank sloughing and caving downstream and to allow time to prevent downstream loss of life and damage to property. The 1987 Water Control Manual also limits the maximum controlled release to 115,000 cfs, up until the time at which the storage level of the reservoir reaches full pool. At full pool the release policy is governed by an emergency spillway release diagram that is designed to protect the reser- voir from failure due to overtopping. There is one additional constraint that is applied to the operation of the reservoir during floods: while inflows are rising, the controlled discharge from the reservoir cannot exceed the inflow rate. This requirement ensures that in no flood event will the peak discharge below the reservoir exceed the peak discharge into the reservoir. Note that this is a de facto policy that is not explicitly specified in the 1987 Water Control Manual.

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IDENTIFICATION AND EVALUATION OF ALTERNATIVES 39 All of the above constraints on the operation of Folsom Reservoir can be modified to some extent. Changing the physical constraints, of course, requires structural modifications to the reservoir and levees. 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 im- proving 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. Of these, the use of flood forecasting, use of storage in upstream reservoirs, and raising Folsom Dam were not incorporated into any of the proposed alternatives. In the 1994 Alternatives Report, the original 1991 measures were reconsidered, although increasing the Folsom flood space in accordance with the amount of water stored in upstream reservoirs (Folsom reoperation) was considered to be a without- project condition. New measures in 1994 included construction of new outlet works, as well as altered use of the existing outlet works. As in 1991, measures involving flood forecasting and the raising of Folsom Dam were not incorporated into alternatives, although appar- ently the latter measure is still being considered. It is clear that the Sacramento District considered a number of strategies for increasing the flood control effectiveness of Folsom Reservoir. The most notable of these is the Folsom reoperation, which is considered a without-project condi- tion in the 1994 Alternatives report. Also relevant is the decision by the Sacra- mento District to reject use of flood forecasts, as well as some other approaches to Folsom operation. Folsom Reoperation One measure considered in the 1991 ARWI was increasing the Folsom flood control storage allocation to 650,000 acre-feet. This measure was included with lowering the Folsom spillway and increasing the objective releases in an alterna- tive that provided an estimated 150-year level of protection. The lost water supply resulting from the increased flood control allocation was computed to cost about $10 million per year, or about 20 percent of the total annual cost of the alternative. Subsequently it was realized that if the Folsom pool were lowered in accordance with the water stored in the largest upstream reservoirs, the expansion of the flood pool would not necessarily represent a loss to water supply. On the basis of this realization, several potential operating rules were considered; of these, the so-called "670 plan" became a without-project condition in the 1994 Alternatives Report. Under this plan, the flood control space in Folsom Reser- voir would vary between 400,000 and 670,000 acre-feet, based on the day of the year and the reservoir storage space available in the French Meadows, Hell Hole, and Union Valley reservoirs. Between December 1 and March 1, the Folsom

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40 FLOOD RISK MANAGEMENT AND THE AMERICAN RIVER BASIN TABLE 2.3 Estimated Volume of Water That Must Be Stored in Order to Control the Flood of the Given Recurrence Interval to the Given Objective Release Required Volume (1,000 acre-feet) Recurrence Interval (years) Objective Release of 1 15,000 cfs Objective Release of 180,000 cfs 100 498 232 200 770 452 400 1,115 748 NOTE: The volume estimates are based on the USACE flood quartile estimates for the 3- and 5-day floods, without the expected probability correction, and on the design hydrograph used in the 1991 ARWI, with- out any adjustments for upstream storage. flood control space would be maintained at 400,000 acre-feet if the empty space in the three upstream reservoirs totaled at least 200,000 acre-feet. Any incremen- tal reduction in the upstream space would require a corresponding incremental increase in Folsom's flood space. When all of the empty space in the upstream reservoirs was filled, the flood-storage space at Folsom would be maintained at 670,000 acre-feet (SAFCA, 1994a). Although Folsom reoperation was consid- ered a without-project condition in the 1994 Alternatives Report, it still must be approved prior to its adoption. This proposed modification of the operation of Folsom Reservoir represents a significant increase in the flood control effectiveness of the reservoir. An idea of the relative magnitude of this increase can be obtained from Table 2.3, which gives for different levels of protection the volume of water that must be con- trolled if the corresponding flood peak is to be kept from exceeding an objective release of either 115,000 or 180,000 cfs. The table was developed by computing the area enclosed above the objective release and below the design hydrograph for the given recurrence interval. It is based on the design hydrographs used in the 1991 ARWI, without the expected probability correction. From Table 2.3 it can be seen that the maximum additional storage of 270,000 acre-feet provided by the proposed modification represents about 35 percent of the volume required to control the 200-year event to 115,000 cfs. For the 400-year events, the amount is 24 percent. 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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IDENTIFICATION AND EVALUATION OF ALTERNATIVES 41 was based on the conclusion that weather forecasting was not sufficiently accu- rate. The committee also doubts the efficacy of early releases, given the current limitations of precipitation and runoff forecasting, physical and administrative limits on pre-flood-peak release rates from Folsom, and the fact that Folsom reoperation will enable use of about 70 percent of the available storage space in the reservoir. The committee thinks, however, that forecasting may be of value in devising strategies for regulating floods that exceed the Folsom flood pool capac- ity so as to minimize the amount by which the actual Folsom outflows exceed the objective release. In addition, dam operation decisions that clearly take available forecast information into account are more likely to be acceptable to both the dam operators and the public than decisions that do not make use of all available information. The committee recommends, therefore, that the Sacramento Dis- trict, the Bureau of Reclamation, and the state of California keep abreast of developments in precipitation forecasting and develop the capability to exploit major improvements in forecasting accuracy. Folsom Operation During Flood Events As previously discussed, maximum flood-reduction effectiveness requires rapid discharge of water during a flood event. In this regard, Folsom Reservoir presents three issues: limitations in the outlet structures at Folsom; appropriate- ness of the rules governing the release of water from Folsom during floods; and actual operation of the reservoirs during past floods. During a flood event, Folsom releases water over the main spillway, through river outlets in the spillway, and through the power penstocks. The main spill- way has eight gated bays. Five of these bays discharge down the spillway into a stilling basin at the base of the dam; they constitute the main release mechanism. The river outlets were designed to operate concurrently with the five main spill- way bays. The remaining three spillway bays, called the auxiliary spillway bays, discharge to a flip-bucket energy dissipator. These bays were designed to help pass water during extreme floods to protect the dam against overtopping. Unfortunately, the existing outlet facilities are inadequate and limit the flood control effectiveness of Folsom Reservoir. When the pool is at the bottom of the current flood space (40O,OOO acre-feet of storage), the five main spillway bays can pass only 6,500 cfs. At a flood storage space of 500,000 acre-feet, the main bays cannot pass any water. The original operation of Folsom Reservoir de- pended on the concurrent use of the river outlets and the five main spillway gates. Shortly after the dam became operational, however, it was discovered that con- current use caused cavitation damage to the spillway. Even with subsequent modifications to the river gates, concurrent operation of the river and spillway gates has been avoided. These limitations on flow releases severely constrain the current operation of Folsom and would be especially constraining under the proposed reoperation.

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42 FLOOD RISK MANAGEMENT AND THE AMERICAN RIVER BASIN For this reason, several of the proposed measures involve construction of new outlet structures. In addition, Countryman (1993) made a number of recommen- dations for improving the efficiency of Folsom Reservoir with the existing struc- tures. 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. Countryman calculated that use of his "maximum outlet plan" would increase the releases during the FEMA 100-year flood by over 60,000 acre-feet. This represents about 8 percent of the volume required to control the 200-year flood to 115,000 cfs (Table 2.3~. Although this is not a large percentage, given the low level of protection currently provided Sacramento, the recommendations of Countryman (1993) should be considered seriously. The committee was told that the main spillway gates and the river outlets are assumed to operate concur- rently in the analysis supporting the 1994 Alternatives Report. The committee did not attempt to evaluate in detail the appropriateness of the ramping rates or of the de facto requirement that outflows be less than inflows during the period of increasing inflow. The committee was told that in the analysis supporting the 1994 Alternatives Report the ramping rates were in- creased by 33 percent for flow up to 25,000 cfs and increased by 100 percent for flows above 25,000 cfs. Operating with these new rates would improve the flood-reduction effectiveness of the reservoir. The committee conducted its own analysis of the increases in water levels and velocities associated with the ramp- ing rates. The results of this analysis show no reason why ramping rates must be held at 15,000 cfs per 2 hours. The committee recommends that the Bureau of Reclamation and the Sacramento District consider the impacts of operating Folsom with higher ramping rates. The more critical issue is the way the reservoir is actually operated in prac- tice. Up to the present, the operator has had to compute reservoir inflows on the basis of observed increases in water levels. This problem alone results in a 4- hour delay in releases. It is the committee's understanding that the flow measure- ment issue is being remedied by the installation of telemetering equipment at flow monitoring stations in the three main upstream tributaries. The committee strongly supports the development of real-time capacity for monitoring inflows to Folsom Reservoir and of a means for accurately gaging outflows from Folsom and Nimbus reservoirs. Another important operational problem is the failure of operators to follow the rules. In its discussion of the 1986 operation of Folsom Dam, the Bureau of Reclamation stated that prescribed rule curve operation should be viewed as "hypothetical." The agency goes on to say (Bureau of Reclamation, 1986) operators are reluctant to rapidly increase the volume of outflow and conse quently affect the floodplain unless such increases are clearly warranted. It is estimated that actual operating efficiencies, when compared to hypothetical op eration, are about 80 percent.

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74 FLOOD RISK MANAGEMENT AND THE AMERICAN RIVER BASIN .. , ~ I - ~ . ~.: N:~: . ~ . ~. _ . . PHOTO 2.1 Hydraulic mining was common in northern California in the late 1800s and delivered significant amounts of sediment into Sierra rivers. These historical deposits are visible, such as this terrace near river mile 21. A bike trail runs on top of the terrace. (Allen James, University of South Carolina.) PHOTO 2.2 Hydraulic mining sediment deposits often consist of erodible, unconsolidat- ed sand and gravel. 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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IDENTIFICATION AND EVALUATION OF ALTERNATIVES 75 PHOTO 2.3 The potential for channel instability is increased in areas with hydraulic mining deposits, such as this site along the left bank of the American River near Watt Avenue. The terrace surface extends upstream and down-stream, as well as beneath the levee. (Allen James, University of South Carolina.) Bank and Lateral Stability As pointed out in the "Geotechnical Analysis" section above, the 1991 ARWI states that banks and levees were structurally stable at flows up to 115,000 cfs, but would fail due to seepage or overtopping at higher flows. The 1991 ARWI was based largely on a geotechnical perspective, neglecting geomorphic pro- cesses. Three recent reports have introduced the geomorphic perspective (WET, 1991; WRC-Environmental and Swanson, 1992; RCE, 1993~. On the basis of historical aerial photographs and field evidence, consultants for SAFCA (WRC- Swanson, 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. Consultants for the District identified lateral instability and seepage failures as serious concerns, although the District does not believe that the bank erosion problem goes beyond what can be treated by standard maintenance practices (Sadoff, 1992~. Bank stability was evaluated based on stream power, which was highest in steep upper reaches below Folsom, where channels were presumed stable because of resistant strata in the bed and right banks (RCE, 19931. How- ever, extensive deposits of historical sediment on the left bank of these reaches

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76 FLOOD RISK MANAGEMENT AND THE AMERICAN RIVER BASIN could be prone to erosion. In the lower reaches, stream powers were high be- tween river mile (RM) 5 and RM 6, corroborating other findings that the bends below Howe Avenue are vulnerable to bank erosion. Comparisons of aerial photographs from 1968 and 1986 indicated that channel migration rates at five critical sites (RM 12.5 to 20.1) averaged 4.8 ft/yr and ranged between 1.1 and 8.0 ft/yr (WET, 19914. Migration rates as high as 13.9 ft/yr at other sites were not deemed critical because of the channel distance from a 50-foot buffer around the toes of levee slopes. These migration rates do not include substantial channel changes from the 1965 flood, which caused an avulsion near river mile 15. Agreement on the potential for lateral channel migration is important not only to bank stability, but also to channel enlargement. Lateral planation in meandering alluvial channels can maintain a natural equilibrium system, but with the dowr~-valley sediment supply cut off by dams, eroded bank material may not be entirely replaced and erosion could result in net channel enlargement over time. Channel Lowering and Enlargement Questions relevant to channel stability and potential changes in conveyance in the lower American River include the degree and timing of aggradation and degradation, whether channels have returned to presettlement base levels, and whether channel enlargement continues. Dam closures are often associated with channel erosion downstream (Williams and Wolman, 1984), although responses to dams may be complex and may include periods of local aggradation. 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), although little evidence has been cited. Vertical Incision Vertical changes on the lower American River have been the subject of several investigations. Gilbert's (1917) time series of Sacramento River bed elevations just below the American River confluence showed 10 feet of bed aggradation from 1855 to 1890, and about 8 feet of degradation by 1914. These responses to hydraulic mining sediment indicate that the lower American River also must have experienced substantial channel bed aggradation and degradation. Recent studies of historical incision, based primarily on California Debris Com- mission (CDC, 1907) and subsequent topographic maps (1955 and 1987), iden- tify 10 to 20 feet of degradation in the lower river from 1906 to 1986 and conclude that thalweg incision is ongoing at some locations (WET, 1991; WRC

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IDENTIFICATION AND EVALUATION OF ALTERNATIVES 77 Environmental and Swanson, 1992; RCE, 1993~. Ten channel cross-sections, resurveyed between 1987 and 1993, showed no systematic change (RCE, 1993), but these surveys were not separated by any major flood events. At some sites the channel bed rests on resistant premining strata, and removal of historical sedi- ment from the bed is complete at these sites (RCE, 19931. Incision of resistant Pleistocene strata can result in sustained channel degradation, however, as on the nearby Bear River in response to a 1955 flood (James, 1991a). Thalweg profiles indicate that most channel degradation between RM 6 and RM 11 was complete by 1955, but that considerable incision occurred between 1955 and 1987 from RM 6 to the mouth and between RM 1 1 and RM 14 (RCE, 19934. Channel incision of about 20 feet and considerable channel enlargement had occurred in the lower American River by 1960 (Olmsted and Davis, 19614. Changes in thalweg profiles on 1957 and 1987 maps indicate an average of about 18 feet of incision between RM 2 and 3 (WET, 19914. Bed stability was modeled using USACE design 100-year hydrographs and the Parker bedload transport equation based on the median bed material size (Dso) and Shields entrainment criteria (RCE, 1993~. Most simulated channel beds experienced no scour, and maximum bed elevation change under the worst sce- nario was less than 2 feet (at RM 7~. On the basis of the model, channel beds throughout the lower American River should be stable under relatively large and infrequent events. Channel Enlargement Vertical incision is only one form of channel erosion, and vertical stability would not preclude channel enlargement by erosion of sediment stored along channel margins. It is common in aggraded systems for channels to respond initially to decreased sediment loads by incising vertically, and later to widen out; particularly when channel top widths are confined by levees or terraces. For example, it has been shown experimentally that knickpoint retreat is often fol- lowed by lateral migration and bank erosion (Schumm, 1973; Schumm et al., 1987~. Following vertical regrading of the lower American River channel profile, a period of channel enlargement by bank and berm erosion and lateral migration cannot be ruled out. 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 ramifica- tions 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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78 FLOOD RISK MANAGEMENT AND THE AMERICAN RIVER BASIN Channel Changes at Streamflow Gages The nature of channel erosion since the closure of Folsom Dam has been examined using topographic maps and aerial photos with limited temporal and spatial resolutions (WRC-Environmental and Swanson, 1992; RCE, 1993~. To enhance the channel-change data base, the committee examined high-resolution U.S. Geological Survey cross-section measurements at the Fair Oaks gage. These analyses are based on only a few sites associated with various locations of Fair Oaks gages and soundings, so caution should be exercised before extrapolating results up- or down-stream. Channel changes are demonstrated by channel cross-sections and stage-dis- charge regression analysis. Data were derived from stream-flow measurement records (USGS archives). Cross-section plots were derived from depth sound- ings at three locations (Figure 2.4~: the old Fair Oaks Bridge (1913 to 1950), a cable about 300 feet below the bridge (1930 to 1957), and a cable 2.2 miles upstream below Hazel Street (1958 to 19941. All sections are from bridges or cables to control the longitudinal position. Numerous plots reproduced sections during stable periods indicating high accuracy of the procedure. For the sake of brevity, only five cross-sections at one site are presented here. Channel morphological changes are rarely related to changes in flood stages ~ k~ ~ ~ \ t0,,,~, i ' '. ~ ... iC'~~_b`\ 9) \,,,, ~ \ ~ ~ ,,\~ Gt / 3dc / W"-.."""222- ~\ G ~ ''' '' ".~.2...',2,.2NlV,tIr,~,,\ Ni2''..2.~.'.'2''2''''22.''. W'..,,,~,,.',,,~.,.,'..r 4'.''.2.'.~..".2.2. ~.'~ ' ' 2 '.,.2 _ e If. . ~,`4 f D a m W ~ E S fin _ Sacramento River Flood Coritrot Project Levees Em_ American River Project Levees .___ Private Levees ~_ Rae ~ .1 Fair Oaks Gage at old bridge & cable . .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. SOURCE: A. James, University of South Carolina (adapted from USACE, 19911.

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IDENTIFICATION AND EVALUATION OF ALTERNATIVES TABLE 2.7 Stage-Discharge Data 79 Location Total N Model N Model Years Bridge 528 497 Q Range R2 1905 to 1958 500 < Q < 20,000 0.85 Stage = 67.5 + 7.18 · 10 - Q - 2.00 - 10-8 Q2 + 2.30 - 10-13 Q3 Hazel Street 454 413 1958 to 1994 Q < 15,000 0.74 Stage = 76.1 + 1.52 · 103 Q- 1.66 107 Q2 7.05 10-12 Q3 in a simple manner. For example, main channel deepening may not result in lower stages of overbank floods if meander-belt flows develop greater turbulence at channel crossings (Ervine et al., 1993~. Thus, an independent analysis of stage-discharge relationships was conducted to evaluate temporal changes in stage at the two gage sites: the old Fair Oaks Bridge and Hazel Street sites. Stage integrates morphologic and hydraulic factors, providing an indicator of flow conveyance. Stage data represent gage readings at the time of discharge mea- surements (not rating curves), corrected for gage datum changes. Flow stage is strongly related to discharge, so stage was statistically re- gressed on discharge to control for these effects. A third-order polynomial pro- vided the best-fit model at both sites. Extreme discharge events were eliminated from regressions (Q-Range, Table 2.7J to emphasize changes within the inner channel rather than overbank characteristics that can be dominated by roughness elements. The regressions provide an objective estimate of the stage of a given discharge. Plots of residuals (errors in the predicted stage) against time reveal temporal changes in stages of flows up to moderate magnitude floods. These methods and some limitations to their morphologic interpretation (e.g., changes in roughness and energy gradient) are explained elsewhere (Knighton, 1974; James, 1991a). Fair Oaks Gage near Old Bridge Cross-section plots (1913 to 1950) at the Fair Oaks bridge indicate channel bed scour and fill with net thalweg erosion of about 8 feet (Figure 2.5~. Channel morphology is controlled by bridge piers and the right-bank bluff. A deep left- bank fill narrowed the channel by about 20 feet toward the end of the period suggesting that constriction by the bridge is not the dominant reason for erosion. A cable was installed about 300 feet below the bridge in 1930, where cross- section plots indicate about 5 feet of thalweg erosion from 1944 to 1952 followed by about 2 feet of fill by 1957 when the cable was moved. Deepening and narrowing of cross-sections at this site suggest that erosion at the bridge extended through the reach. Channel deepening and narrowing at this bridge site follow

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80 85 80 a) 7C ID 1 A __ o is 70 > LL 65 60 55 FLOOD RISK MANAGEME~AND THE AMERICk RIVER BASIN 1 -I 1913 ~ 1917 a 1928 ~1945 ~1950 Fair Oaks Bricige r. 500 600 700 800 Station (feet) 900 1 000 FIGURE 2.5 Representative channel cross-section plots at the Fair Oaks Bridge showing about 8 feet of thalweg degradation between 1913 and 1950. Data gaps indicate bridge piers. The view is downstream. the general response observed elsewhere where channels are incised through hydraulic mining sediment (James, 1991a). Stage-discharge relationships at the bridge site indicate a systematic group- ing by period with occasional changes in flow stages (Figure 2.6~. 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. The rapid incision during the 1940s may represent a response to de- creased sediment yields following the closure of North Fork Dam in 1939. Fair Oaks Gage at Hazel Avenue In 1957 the gage and cable were moved 2.2 miles upstream to the present Hazel Street site below Nimbus Dam. From 1958 to 1994 the channel at this

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IDENTIFICATION AND EVALUATION OF ALTERNATIVES 75 of_ - o ._ Be_ a) ~70 Be_ oh 65 81 Fair Oaks at O/d Bridge 1905-1958 x ax sx ss st a~t Aim; as 2e ~ ~a,. ~ ~ ~ ~ set, a_ ~ ~ mix ~ ~ -~' .~. o "~;,~ ~ ~- . '- ~<1912 ~<1918 ~<1928<1931 s <1945 ~<1956 ° <1959Water Yr 0 1 0,000 20,000 Discharge Offs) 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. location experienced episodes of thalweg deepening and bar deposition followed by stable periods lasting several years, and about 9 feet of net thalweg degrada- tion. The 1965 flood scoured the thalweg about 10 feet, but the channel partly refilled from 1965 to 1973 and was colonized by willows. From 1973 to 1986, the channel bed was stable, but the 1986 flood lowered the thalweg about 3 feet and widened the channel considerably. Analysis of flood stages at the Hazel Street site indicates two periods of relative stability from 1958 to 1967 (Figure 2.8~. Stage-discharge regression residuals reveal lowering of flow stages at this site, between the two stable periods (Figure 2.7~. The 1965 scour event had no effect on flow stages, presum- ably due to rapid refilling and increased vegetational roughness of the channel. From 1967 to 1970, however, flow stages rapidly lowered about 2 feet. Sus- tained incision over the period from 1958 to 1994, during which time flow stages dropped about 2 feet, suggests a long-term tendency for channel degradation and a mobile bed at this site. The close proximity of Nimbus Dam upstream severely limits replacement of eroded bed sediment, resulting in net degradation. Thalweg incision at the two gage sites was about 8 feet (1913-1950) and 9 feet (1958 to 1994), respectively. Although net stage lowering for the two periods was only about 1.75 feet and 2.5 feet, respectively, large rapid fluctua- tions characterize these changes. This evidence of rapid erosion at gages lends

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82 in ~ 2 ._ En . . a) 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. . .. -. 2, .. ~ ~;s,s8,,,'. ;`a!.,.,,.~.;, T -ma- -I.- ~ it; 500 ~ Q < 20~000 Fair Oaks Gage at Old Bridge Q < 15,000 , . ~ in; s. . . . I: :~~ :~ Gage Change 1 1 ! ~ 1 940 1 960 Water Year 1 980 2000 FIGURE 2.7 Stage-discharge regression residuals for the Fair Oaks gage. Left side is from Fair Oaks bridge site (see Figure 2.6) and shows two periods of low stages and two of high stages interpreted as degradation and aggravation, respectively. Right side is from Hazel Avenue site (see Figure 2.8) and shows a short period of rapid stage lowering interpreted as in response to channel degradation. Joining of the two series is approxi mate. credence to a hypothesis of continued channel deepening and enlargement in the upper reaches. If the gage sites are representative of other sections, the conclu- sion that extreme floods would cause little incision on the lower American River (RCE, 1993) could underestimate the potential for channel down-cutting. Geomorphic Conclusion Bank stability is a serious consideration when considering conveyance of high flows in the lower American River. Although the degree of hazard that bank erosion, lateral migration, or bed incision pose to lessee stability is contested, all parties appear to agree that a program of channel monitoring and maintenance is necessary. The belief that historical sediment in channels of the Sacramento Valley is now stable is based largely on evidence of elevations derived from topographic maps and numerical simulations of channel bed erosion. Thalweg

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IDENTIFICATION AND EVALUATION OF ALTERNATIVES 85 o ._ - _ ~ . a' 80 LLI 1 O) 1 can Fair Oaks at Hazel A venue 1958- 1994 i. ~ ,. ~ -Or ' - . ... it, ~ G '' X A__ a ~g ~ <- WaterYear · < 1967 ~< 1969 ~ . ~1 · < 1982 ~ < 1986 ~< 1995 7 5 1 ! ~! 1 1 I ~! 1 1 1 0 5,000 10,000 Discharge (cfs) 83 FIGURE 2.8 Stage-discharge relationship from the Fair Oaks gage at the Hazel Avenue cable site. Several distinct periods of high and low stages can be identified. elevations indicate that base-level adjustments have decelerated, but ongoing vertical adjustments should not be ruled out. Nor would stabilization of long profiles necessarily indicate an end to channel bank erosion, lateral migration, enlargement, or instability. 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. There was a net lowering of flow stages by almost 2 feet for this period, presumably due to erosion of historical sediment. From 1958 to 1994, flow stages at Hazel Street also lowered about 2 feet. If these sites are representative of the lower river as a whole, further channel incision may be anticipated. Given historical aggradation, cessation of sediment deliveries since dam construction, and evidence of erosion, the potential for net erosional tendencies in the lower river cannot be rejected. A sediment budget deficit exists in the lower river as dams arrest sediment deliveries from upstream while erosion removes sediment, and this deficit results in net erosion. The hypothesis that channel erosion and enlargement have resulted in increased channel conveyance over the last two decades should be tested further using hydraulic models. Analysis of

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84 FLOOD RISK MANAGEMENT AND THE AMERICAN RIVER BASIN stage-discharge time series provides empirical support for the hypothesis that channel stages of moderate magnitude floods have lowered by a modest amount at two locations over two different periods, but more information is needed to substantiate these results and extend them to other locations downstream. Given the critical nature of flood hazards in Sacramento and extensive nine- teenth century channel changes, the committee suggests three areas of study regarding the geomorphology of the lower American River: (1) ongoing moni- toring of channel changes, (2) historical reconstruction of channel changes, and (3) geomorphic mapping. Recent and ongoing channel changes should be docu- mented and monitored following large flood events by repeating channel cross- section surveys, and by registering aerial photos. Study of long-term historical changes should include consultation of early historical records to establish presettlement channel conditions that could estab- lish a baseline for changes to the fluvial regime that was presumably in equilib- rium with long-term flow conditions. In addition, historical changes should be documented through historical and field methods. For example, CalTrans bridge surveys could be collected and repeated, and California Debris Commission records of twentieth century hydraulic mining sediment production could be tabulated. Vast tracts of erodible historical sediment stored in the lower river should be studied and mapped. In the upper reaches they are relevant to channel enlarge- ment and sediment production, while in the lower reaches they are relevant to bank and levee stability and seepage. Mapping will reveal spatial patterns and allow more accurate interpolation between geotechnical sample points. 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). Assignment of PNP and PEP elevation for levees should be based in part on knowledge of lower American River stratigraphy with an emphasis on the spatial pattern of historical sediment and former channels.

Representative terms from entire chapter:

folsom reservoir