SAN FRANCISCO DISTRICT

Transcription

1 LETTER REPORT SAN FRANCISCO BAY SHORELINE STUDY U.S. ARMY CORPS OF ENGINEERS SAN FRANCISCO DISTRICT,APRIL 1992

2 LETTER REPORT SAN FRANCISCO BAY SHORELINE STUDY U.S. ARMY CORPS OF ENGINEERS SAN FRANCISCO DISTRICT 1. This letter report has been prepared in order to summarize results of the entire San Francisco Bay Shoreline Study, using excerpts from the office reports for interims 1, 2, and 3 (Southern Alameda and Santa Clara Counties Interim, dated October 1988; San Mateo and Northern Alameda Counties Interim, dated September 1989; and Napa, Solano and Sonoma Counties Interim, dated April 1992, respectively). As discussed below, further project actions could not be justified for the areas studied under the Interims 1 and 2 authorities. Two potential project sites within the area of study for Interim 3 appear promising and further studies will be pursued under Section 205 of the Continuing Authorities Program. 2. Studv Authority. The San Francisco Bay Shoreline Study was authorized by the Water Resources Development Act of 1976 (Public Law ). Section 142 of the Act reads: '#The Secretary of the Army, acting through the Chief of Engineers, is authorized and directed to investigate the flooding and related problems to those lands lying below the plane of mean higher high water along the San Francisco Bay shoreline of San Mateo, Santa Clara, Alameda, Napa, Sonoma, and Solano Counties to the confluence of the Sacramento and San Joaquin Rivers with a view toward determining the feasibility of and the Federal interest in providing protection against tidal and fluvial flooding. The investigation shall evaluate the effects of any proposed improvements on wildlife preservation, agriculture, municipal and urban interests in coordination with Federal, State, regional, and local agencies with particular reference to preservation of existing marshland in the San Francisco Bay region PurDose and Scope of Studv. The primary objective of the study was to determine the feasibility of and the Federal interest in providing protection against tidal and tidal-related fluvial flooding, f,or, deyeloped areas within the 500-year tidal floodplain. After' initial study, the San Francisco Bay Shoreline Study was divided into three geographic areas, referred to as Itinterimtt studies, in order to give priority to the portions of the study area which appeared to have the greatest potential for flood damages. Interim 1 consists of southern Alameda and Santa Clara counties, Interim 2 consists of San Mateo and Northern Alameda counties, and Interim 3 consists of Napa, Sonoma and Solano counties. This letter report summarizes the study findings for the three interims.

3 The study investigated all lands within the tidal floodplain, rather than just those lands below mean higher high water, in order to consider all of the effects of tidal flooding. The San Francisco Bay Shoreline Study is not a study of tidal flooding which may occur due to global sea level rise caused by the I1greenhouse effect". Rather, the study only considers the local regional history of sea level changes, as presently established by Corps of Engineers national policy, to determine the economic feasibilty of specific projects. 4. Methods. a. Tide Stage vs. Frequency Study. The elevations of the extreme high tide stages used in the Interims 1, Interim 2 and Suisun City of Interim 3 were derived from the San Francisco Bay Tide Stage vs. Frequency Study completed by the San Francisco District, Corps of Engineers in October That study used the annual maximum tides measured at the Golden Gate for the period of 1855 through 1983, a 129-year period of record, to compute a tide stage vs. frequency curve. The curve was adjusted in accordance with the observed increase in the elevation of the annual maximum tides over the period of record. The adopted water surface elevations for the white Slough and Edgerley Island reaches were based on high river stages taken from the Napa County Unincorporated Areas Flood Insurance Study of b. Potential Tidal Floodplains. The interim study areas were divided into hydrologic reaches. For Interim studies 1 and 2, the maximum extent of tidal flooding which could occur in each reach was determined by mapping the contour of the 500-year tide elevation on 1:6,000 scale phototopographic maps. This contour represented the inland boundary of the Ifworst-caself or maximum extent of t8potentialn tidal flaodplains which would result if the levees in each study reach failed during a 500-year tide, and all low-lying areas behind the levees flooded to the elevation of the tide. Reaches and separable portions of the reaches which did not have significant development within the delineated worst-case tidal floodplains were eliminated from further study because of the apparent lack of economic justification for any tidal flood damage reduction plan. For the Interim 3 study area, reaches were identified as those existing develop,ed- atreas' that were potentially subject to tidal and tidal-relatea afloodhg. c. Flood History. Information regarding historic flooding in the study areas was compiled from local newspaper accounts, flood insurance study reports, other government reports, and interviews with local public works officials. Information regarding historic levee failures and levee damage due to overtopping was obtained from Corps of Engineers files, local newspaper accounts, and interviews with local public works officials. -2-

4 d. Tidal Flooding Analysis. The Economic and Environmental Principles and Guidelines for Water and Related Land Resources Implementation studies require that the formulation and evaluation of alternative plans be based on the most likely conditions expected to exist in the future with and without a project. A worst-case analysis using potential tidal floodplains would, therefore, be inappropriate in determining whether a Federal project is economically justified. Instead, the extent of flooding in the study areas which is most likely to occur was estimated as accurately as possible. The effects of actual tide conditions, wind waves, levee overtopping, topography and storage capacity of each segment of the floodplain area were considered in estimating the floodplains. The differential elevation between relatively frequent and relatively infrequent tide events in the San Francisco Bay is very narrow; for example, the 100-year tide elevations are only 0.5 foot higher than the 10-year tide elevations. Therefore, in most cases, it is unlikely that the levee damage caused by extreme tide-and-wind events will greatly exceed the damage which has occurred during historic high tide events. Because the flood history in most of the study areas indicates that levee failure is not a likely mode of tidal flooding, tidal floodplains were estimated based on levee overtopping rather than levee failure. For each reach studied in detail, levee overtopping volumes for various tide-and-wind combinations were calculated using site-specific, wind generated wave runup elevations, levee degradation estimates, and appropriate hydraulic equations. The probability of winds of various speeds, durations, and compass directions occurring simultaneously with various extreme tide stages was calculated using wind data collected at local airport weather stations. A coincident frequency analysis of tides and winds was then performed to determine the recurrence intervals for various volumes of levee overtopping for a three-day tide-and-wind event, subtracting the volume of the tick: floodwaters which would be retained in undeveloped ponding areas (such as salt ponds), and comparing the remaining flood volume to an area-capacity curve. In estimating levee degradation, it was also necessary to consider the degree of protection provided by the existing levees within thb ktbdy area. Because Federal participation in water resources development is not intended to be a substitute for local maintenance of existing facilities, the tidal flooding analysis assumed that local interests would continue to maintain the existing levees in their present condition. e. Economic Analysis. Feasibility-level construction cost estimates were prepared for each of the alternatives studied in detail, which included real estate costs but not habitat mitigation costs. An inventory of structures within the tidal floodplain was used to determine future without-project flood -3-

5 damages. The categories of flood protection benefits that were taken into account included flood damage reduction, reduction of business losses, and savings in flood insurance administration costs. The ratio of average annual benefits to average annual costs was then computed for each reach using a project life of 100 years and then discounted at current Federal rates. The discount rate utilized for Interim 1 and 2 was 8-5/8 percent. A discount rate of 8-3/4 percent was used for Interim 3. f. Environmental Considerations. Many of the reaches within the three interim study areas are located on historic wetlands that have been modified or reclaimed. As there is considerable public interest in the health and preservation of these environments, any modifications to these areas will be subjected to heavy scrutiny. The environmental factors for each of the three interim areas have been reviewed and at this time, no further study is needed. If, in the future, any of the projects are undertaken and additional studies of proposed structural alternatives are pursued within the study area, further investigation and documentation will be necessary to satisfy the requirements of legislation and promulgated regulations relative to wetlands protection. 5. Major Conclusions and Sisnificant Findincrs. a. Potential Tidal Floodplains, Extensive areas along the San Francisco Bay shoreline are lower in elevation than the extreme high tides in the Bay and are therefore potentially subject ta tidal flooding. These low-lying areas are mostly diked former tidal areas. The potential tidal floodplain contains urban development, including roads, parks, airports, and sewage treatment facilities. Much of this development is protected from tidal flooding by substandard levees, including privately maintained salt pond levees. These levees are subject to overtopping during extreme high tides, particularly if wind-generated waves and high river stages occur in conjunction with the high tides. b. Tidal Flooding History. Although wave and tide overtopping of shoreline levees has occurred numerous times, damage to urban.~~,ve;lopment within the study area due to ti2.al flooding has historically been very limited. Along most of the study area shoreline, salt ponds and other undeveloped diked areas provide large buffer zones between the bayfront levees and areas of urban development. Isolated incidents of levee failures which have occurred in the past are believed to have been caused by either high fluvial flows or by a lack of adequate levee maintenance, rather than by high tides alone. Most of the flooding which has occurred in the study areas has been due to high fluvial runoff. In some cases, high tides have resulted in induced fluvial flooding due to reduction of channel capacity. However, the problem of coincident tidal and -4-

6 fluvial flooding is considered to primarily be a fluvial flooding problem which is occasionally exacerbated by high tides. Detailed investigation of every fluvial system draining into, but located outside of the study area, is beyond the scope of this study. Therefore, fluvial flooding was only considered in a general manner sufficient to ensure that any project proposed by this study would not induce additional fluvial flooding. c. Estimated Tidal Floodplains. The estimated tidal floodplains which were based on computed levee overtopping volumes are much less extensive than the "potentialst tidal floodplains. The estimated tidal floodplains indicate that significant tidal flooding of developed areas is likely to occur in Alviso, the Palo Alto Airport, northern Sunnyvale and the Palo Alto Flood Basin in the Interim 1 area; East Palo Alto, Menlo Park, Redwood City, San Mateo, and Burlingame in the Interim 2 area; and White Slough, Edgerley Island and Suisun City in the Interim 3 study area. Alternative structural alignments were considered in detail for each of the interim study areas in which significant flooding is expected and an interested local sponsor has been identified. The structural plans considered consisted mainly of improvements of existing levees or the construction of new levees or floodwalls along various alignments. Nonstructural alternatives were considered and found either to have already been implemented or to be economically unjustified or socially unacceptable. 6. Recommended Plan. a. For all of the reaches within the Interim 1 and Interim 2 study areas, the cost of the alternative action plans considered exceed the computed National Economic Development (NED) benefits. Therefore, the recommended plan for those areas is the No Action Alternative. b. The Interim 3 study area consisted of three reaches located at Edgerley Island, White Slough in the City of Vallejo, and Suisun City. The benefit-to-cost ratio for the Edgerley Island alternative action plan considered were determined to be marginal at 0.95,,.,baj;ed,on proposed improvements to the existing Napa River levees; 'and the Leslie salt pond levees. The alternative action plan considered does not provide full protection due to flooding from Mud Slough with sea level rise. If improvement to the Mud Slough levee system is considered as part of an alternative action plan, the increased cost of a project may result in further decrease of the NED benefits. Therefore, the recommended plan for the area under the authority of this study is the No Action Alternative. However, since Edgerley Island is also located within the study area of the Napa River Project, further analysis of the reach may be pursued under that authority. -5-

7 Two possible structural alternatives were found to provide economically justified flood protection for White Slough. The alternative that would provide the highest net benefit consists of constructing a new levee along Highway 37. The benefit-to-cost ratio of this project is 2.8 to 1. The other alternative consists of improving existing levees and floodwalls along the periphery of White Slough. The benefit-to-cost ratio of this second alternative is 2.2 to 1. Two possible alternatives were determined to provide economically justified flood protection for Suisun City. The alternative that would provide the highest net benefit consists of improving existing levees and floodwalls along the southside of the city and the periphery of Suisun Slough. The benefit-to-cost ratio of this project is 2.0 to 1. The other alternative would also improve levees in the Marina area. The benefit-to-cost ratio of this second alternative is 1.7 to 1. Due to the scope and cost of alternatives considered, it is recommended that additional studies for White Slough and Suisun City be pursued under Section 205 of the Continuing Authorities Program. 7. Description of Potentiallv-feasible Projects. a. White Slough, Vallejo. White Slough is a tidal tributary of the Napa River which can be described as a modified salt marsh. It is traversed by State Highway 37. The fill supporting the highway is, in effect, a dike dividing the slough drainage roughly in half. Tide gates under the highway permit the passage of some water. Substantial portions of the marsh have been filled and a variety of urban development surrounds the slough. Flooding of some of the developed areas has occurred due to the failure of levees on the Napa River which have not been repaired. Two plans have been devised to reduce" future flood damages. The first is a levee along the outboard side of Highway 37. The second is a combination of floodwalls and levees with the floodwalls along the north side of the lagoon where development encroaches closest to the water. An enhancement plan which incorporates recreation/interpretive uses and the creation of new wetland habi;a,t,with the floodwall/levee option has been developed by the'.city of Vallejo..Highway 37 is a major transportation artery in the area and Caltrans has plans to raise the highway and widen it consistent with existing roadway alignments at either edge of the slough. b. Suisun City. Suisun City is a community that is roughly half municipal/commercial/industrial and residential development. Suisun City is built on reclaimed land, and is protected to some extent by dikes and levees. The city's southern boundary is the Suisun Marsh, the largest remaining wetland in California. Suisun Slough is a navigable waterway that bisects the community. The project under consideration is -6-

8 enhanced flood protection with floodwalls along the banked sides of Suisun Slough and raised levees on the south side where more open space is available. A second alternative would provide additional floodwalls along the marina area on the east side of the city. c. Tables of Economic Data. Cost and benefit data for the potentially-feasible alternatives in the White Slough and Suisun City locations are provided on the attached Tables L-1 through L-4. The tables do not include habitat mitigation costs. Consideration of mitigations plans and associated costs is presented in the following paragraph. d. Mitigation Considerations. The consideration given to mitigation plans for the potentially-feasible projects in White Slough and Suisun City is only preliminary. The mitigation plans considered have not been formally coordinated with any other agency or authority. Proposed sheet pile floodwalls would affect a strip of land about three feet wide. The extent of wetland habitat affected by levee construction or improvements would be a function of the increase in levee footprint which is proportional to the increase in crest elevation. Plants of special concern within the construction area will need to be identified, marked, and avoided where possible. Avoidance measures will require that an engineer and a biologist site the precise alignment locations and, where possible, modify alignments accordingly. Additionally, installation of sheet piles from a barge should be considered in Suisun City, where possible, to minimize the disturbance of wetland habitat. The affected plants could be dug up, potted, and put under the care of a qualified horticulturist or transplanted to an enhancement site in the project area or suitable spot in the same drainage area, if engineering constraints will not permit the plants to be avoided. The slopes of improved levees and the disturbed land along the floodwalls can be stabilized with appropriate plantings, including those of special concern. Since this study was not intended as a decision document to proceed directly to PED, mitigation plans were not developed in sufficient detai>.,to, prepare detailed mitigation cost estimates associated with the potgntially-feasible projects. Assessment of affected habitats are based on the Corps' observation of existing land use and the proposed locations of the levee and floodwall alignments. In order to address whether the projects are still potentially-feasible when mitigation plans are considered, acquisition costs for site mitigation were based on the lands and easements costs for construction of the proposed flood control projects. Habitat renovation costs related to mitigation were based on recent findings of a Corps' Feasibility Study in San Rafael, Marin County, California. Even though the magnitudes of the mitigation plans described above are not included in the cost estimates shown in Tables L-1 through L-4, -7-

9 the benefits provided by the proposed flood control alternatives in White Slough and Suisun City still exceed the project costs significantly. e. Development of Formal Mitigation Plans During Future Studies. If future study of the potentially-feasible projects within the White Slough and Suisun City reaches are pursued under Section 205 of the Continuing Authorities Program, then it will be necessary to prepare an Environmental Impact Statement during a more detailed Feasibility Study phase level. The Feasibility Study phase will include a request to the FWS to provide a Fish and Wildlife Coordination Act Report which will be utilized by the Corps to precisely define environmental impacts. Detailed estimates of mitigation costs will require further coordination with FWS in order to develop appropriate mitigation alternatives, quantification of habitat values, losses or gains of such habitat, and an incremental analysis of the specific mitigation alternatives to identify the most cost effective plans. As planning continues, coordination with FWS and other agencies to ensure the appropriate survey work will be undertaken to develop detailed conservation measures for listed threatened, endangered and candidate species. 7. This letter report concludes the investigation of the three interim study areas of the San Francisco Bay Shoreline. Based on the findings and recommendations of the study, no further action will be taken under the San Francisco Bay Shoreline Study Authority. This report will be distributed to all interested parties to inform them of the findings and conclusions of the study. Attachments Colonel, Corps of Engineers District Engineer -8-

10 TABLE L-1 PRELIMINARY COST ESTIMATES AND BENEFIT/COST RATIO WHITE SLOUGH - ALTERNATIVE 1 OPTIMAL LEVEL OF PROTECTION (100-YEAR DESIGN) 1989 PRICE LEVELS TOTAL CONSTRUCTION FIRST COST TOTAL LANDS AND EASEMENTS. INTEREST DURING CONSTRUCTION (1 YEAR) TOTAL PROJECT INVESTMENT COST ANNUALIZED FIRST COSTS (CRF 100 YEAR, 8-3/4%) OPERATION AND MAINTENANCE (8%) AVERAGE ANNUAL COSTS AVERAGE ANNUAL BENEFITS NET ANNUALIZED BENEFITS BENEFIT-TO-COST RATIO $1,288,000 $1,020,000 $ 101, $2,409,000 $ 211,000 $ 17, $ 228,000 $ 644,000 $ 416, : I : Construction first costs includes contingency (25%), engineering and design (8%), and supervision and administration (7%). * The construction first costs were adjusted from 1990 price levels using a factor of (Civil Works Construction Index System). The above costs do not include habitat mitigation costs. See paragraph 8(d) and (e) of the letter report for discussion of mitigation considerations. -9-

11 TABLE L-2 PRELIMINARY COST ESTIMATES AND BENEFIT/COST RATIO WHITE SLOUGH - ALTERNATIVE 2 OPTIMAL LEVEL OF PROTECTION (100-YEAR DESIGN) 1989 PRICE LEVELS TOTAL CONSTRUCTION FIRST COST TOTAL LANDS AND EASEMENTS INTEREST DURING CONSTRUCTION (1 YEAR) TOTAL PROJECT INVESTMENT COST $1,977,000 $1,020,000 $ 131, $3,128,000 ANNNUALIZED FIRST COSTS (CRF 100 YEAR, 8-3/4%) $ 274,000 OPERATION AND MAINTENANCE (8%) $ 22,000 AVERAGE ANNUAL COSTS AVERAGE ANNUAL BENEFITS NET ANNUALIZED BENEFITS BENEFIT-TO-COST RATIO $ 644,000 $ 348, Construction first costs includes contingency (25%), engineering and design (8%), and supervision and administration (7%). The construction first costs were adjusted from 1990 price levels using a factor of (Civil Works Construction Index System). The above costs do not include habitat mitigation costs. See paragraph 8(d) and (e) of the letter report for discussion of mitigation considerations. -10-

12 TABLE L-3 PRELIMINARY COST ESTIMATES AND BENEFIT/COST RATIO SUISUN CITY - ALTERNATIVE 1 OPTIMAL LEVEL OF PROTECTION (100-YEAR DESIGN) 1989 PRICE LEVELS TOTAL CONSTRUCTION FIRST COST TOTAL LANDS AND EASEMENTS INTEREST DURING CONSTRUCTION (1 YEAR) TOTAL PROJECT INVESTMENT COST ANNUALIZED FIRST COSTS (CRF 100 YEAR, 8-3/4%) OPERATION AND MAINTENANCE (8%) AVERAGE ANNUAL COSTS AVERAGE ANNUAL BENEFITS NET ANNUALIZED BENEFITS BENEFIT-TO-COST RATIO $2,410,000 $1,134,000 $ 155, $3,699,000 $ 324,000 $ 26, $ 350,000 $ 682,000 $ 332, Construction first costs includes contingency (25%), engineering and design (8%), and supervision and administration (7%) The construction first costs were adjusted from 1990 price levels using a factor of (Civil Works Construction Index System). The above costs do not include habitat mitigation costs. See paragraph 8(d) and (e) of the letter report for discussion of mitigation considerations. -11-

13 TABLE L-4 PRELIMINARY COST ESTIMATES AND BENEFIT/COST RATIO SUISUN CITY - ALTERNATIVE 2 OPTIMAL LEVEL OF PROTECTION (100-YEAR DESIGN) 1989 PRICE LEVELS TOTAL CONSTRUCTION FIRST COST TOTAL LANDS AND EASEMENTS INTEREST DURING CONSTRUCTION (1 YEAR) TOTAL PROJECT INVESTMENT COST ANNUALIZED FIRST COSTS (CRF 100 YEAR, 8-3/4%) OPERATION AND MAINTENANCE (8%) AVERAGE ANNUAL COSTS $2,886,000 $1,134,000 $ 176, $4,196,000 $ 367,000 $ 30, $ 397,000 AVERAGE ANNUAL BENEFITS NET ANNUALIZED BENEFITS BENEFIT-TO-COST RATIO $ 682,000 $ 285, Construction first costs includes contingency (25%), engineering and design (8%), and supervision and administration (7%). The construction first costs were adjusted from 1990 price levels using a factor of (Civil Works Construction Index System). The above costs do not include habitat mitigation costs. See paragraph 8(d) and (e) of the letter report for discussion of mitigation considerations. -12-