6. GEOTECHNICAL, SEA LEVEL RISE AND SHORELINE IMPROVEMENTS

6.1 GEOTECHNICAL DOCUMENTS 233 6.2 TREASURE ISLAND AND CAUSEWAY GEOTECHNICAL IMPROVEMENTS 234 6.3 YERBA BUENA ISLAND GEOTECHNICAL IMPROVEMENTS 238 6.4 SEA LEVEL RISE STRATEGY AND SHORELINE IMPROVEMENTS 240

TREASURE ISLAND & YERBA BUENA ISLAND MAJOR PHASE 1 APPLICATION 6 - GEOTECHNICAL AND SHORELINE IMPROVEMENTS 231 6.1 GEOTECHNICAL DOCUMENTS

The documents noted below were separately distributed to agency representatives from the Department of Public Works (DPW) and the Department of Building Inspection (DBI) on February, 3, 2015, and they are also included herein as Appendix E.

1. Treasure Island Geotechnical Conceptual Design Report, February 2, 2009 2. Treasure Island Geotechnical Conceptual Design Report Appendix 4, February 2, 2009 3. Treasure Island Sub-phase 1A Geotechnical Data Report; Draft, December 31, 2014 4. Technical Memorandum 1, Preliminary Foundation Design Parameters Treasure Island Ferry Terminal Improvements, January 2, 2015 5. Technical Memorandum 2, Preliminary Geotechnical Design for Sub-Phase 1A Shoreline Stabilization, January 2, 2015 6. Treasure Island Sub-phase 1A Interim Geotechnical Characterization Report; Draft, January 5, 2015

TREASURE ISLAND & YERBA BUENA ISLAND MAJOR PHASE 1 APPLICATION 6 - GEOTECHNICAL AND SHORELINE IMPROVEMENTS 233 6.2 TREASURE ISLAND AND CAUSEWAY GEOTECHNICAL IMPROVEMENTS

GEOLOGIC SETTING AND DEPOSITIONAL HISTORY into the Bay. The grain-size distribution of windblown sands on Yerba Buena Island is essentially the same as fine silty sands The San Francisco Bay around Treasure Island is underlain interbedded with Young Bay Mud below Treasure Island. The by rocks of the Franciscan Complex of the Alcatraz Terrain, erosion of the windblown sand from Yerba Buena Island and consisting mainly of interbedded greywacke sandstone and surrounding areas is likely the source for both the historic sandy shale. Under Treasure Island, the Franciscan Complex bedrock shoal deposits, and the fine silty sands interbedded with Young is covered by Quaternary sediments and fill to depths ranging Bay Mud deposits. from 180 feet under the causeway to as deep as 280 feet near the north end of the island. Interpretations of the subsurface stratigraphy across the entire island along the north-south direction are illustrated on Figure The Quaternary sediments at Treasure Island can generally be 6.1 divided into older, Pleistocene-age marine and alluvial deposits (“Older Bay Deposits”), young Holocene-age marine clay and sand deposits (“Young Bay Mud”), native sandy shoal deposits, and hydraulic sand fills. Under Treasure Island, the Young Bay Mud varies greatly in thickness from about 20 feet near the causeway to more than 120 feet near the northwest corner of the island. The Young Bay Mud varies in thickness because it was deposited on an eroded surface of Older Bay Deposits as sea level rose over the last 12,000 years. Under the southern portions of the island, the Young Bay Mud contains many interbedded layers of fine silty sand making up as much as one-third to one-half of the thickness of the deposit. The sand lenses thin and decreases in number to the north. Near the north end of the island, sand lenses within the Young Bay Mud are very thin or absent.

Extensive windblown sand deposits are believed to have formed across the bottom of San Francisco Bay when it was exposed during low stands of sea level. Just south of Clipper Cove, Yerba Buena Island is mantled with thick (100 to 120 feet) deposits of uniform fine silty sand interpreted to be windblown deposits. The thick sand deposit has been extensively eroded by wave action at the north side of Yerba Buena Island, forming a steep sand bluff FIGURE 6.1 TREASURE ISLAND GEOLOGICAL NORTH-SOUTH CROSS SECTION that is over 200 feet high and is still periodically shedding sand

TREASURE ISLAND & YERBA BUENA ISLAND MAJOR PHASE 1 APPLICATION 6 - GEOTECHNICAL AND SHORELINE IMPROVEMENTS 234 TREASURE ISLAND CONSTRUCTION The combined thickness of the sand fill and shoal sands varies Older Bay Deposits between approximately 30 and 50 feet. Older Bay deposits encountered below the Young Bay Mud 6.2 TREASURE ISLAND AND CAUSEWAY Treasure Island and the causeway that connects it to Yerba deposits consist of interbedded very stiff to hard, low to high Buena Island were constructed in the late 1930s by placing over Young Bay Mud: Onshore Fine-Grained Deposits plasticity clays, silts, and dense to very dense fine silty and 29 million cubic yards of fine- to medium-grained sand and silty The sandy fill and shoal materials is underlain by Young Bay clayey sands. In many borings, the deep stiff/dense deposits are GEOTECHNICAL IMPROVEMENTS sand over a natural sand shoal and a layer of weak, compressible Mud consisting of soft to stiff silty clay deposits with occasional described as containing shell fragments or peat, suggesting that clay (locally known as Young Bay Mud). The sand was dredged interbedded sand layers. The Young Bay Mud thickness varies they are mainly old Bay or Bay margin deposits. Many borings from the shoals south of the island and from other shoals to the from 20 to 120 feet with the greatest thickness occurring under note color changes from gray or dark gray to light greenish gray or east. The dredged sand fill was placed hydraulically. Where the the northwest corner of the island. The Young Bay Mud is also brown or note mottling suggestive of oxidation and weathering. Bay floor was lower than approximately Elevation -6 feet NAVD, deeper under the southeast corner of the island. The Bay Mud Borings just east of Sub-phase 1A note thick layers of brown a bed of hydraulic fill was placed to raise the Bay floor elevation is generally normally consolidated and moderately compressible. gravelly sand and clay that may be of alluvial origin. The to -6 feet. A rock dike was then constructed with crest elevations Where the Young Bay Mud has been consolidated under the variation in thickness of these older sediments is not generally between 2 and 6 feet MLLW, and sand fill was deposited in place weight of the existing fill, it has moderate shear strength. known because only a few exploratory borings on the island have until the elevation reached the top of the dike. Another rock penetrated to bedrock. dike was placed on the previously constructed dike and filling Young Bay Mud: Soft Offshore Deposits continued. This process was repeated until the interior elevation The Young Bay Mud deposits encountered in offshore borings are Bedrock reached approximately Elevation 13 feet NAVD. Filling started very soft to soft. The difference between onshore and offshore The Franciscan-Formation bedrock encountered in deep borings at the southwest corner and progressively proceeded to the east Young Bay Mud may be due to consolidation effects from the has been described as moderately weathered dark gray sandstone and north. The rock dikes were faced with riprap constructed shoal sands and from consolidation due to placement of the and shale. Bedrock was encountered under the south end of the with an outboard slope of 1:1 and extended to a final grade island hydraulic fills. Comparison of 1926 to modern bathymetry causeway at an elevation of -10 feet NAVD and at an elevation of elevation of approximately Elevation 14 feet NAVD. shows that areas immediately offshore along the west margin -180 feet NAVD near the middle of the causeway. Under Treasure of Treasure Island are at nearly the same elevation as 1926. Island, bedrock was encountered at an elevation of -255 NAVD. SUBSURFACE STRATIGRAPHY Somewhat further offshore, up to approximately 25 feet of recent Under the middle of the island bedrock was encountered at an sedimentation may have occurred. In Clipper Cove, soft organic elevation of -271 NAVD. Subsurface materials at Treasure Island can generally be clays up to 20 feet thick have accumulated since 1949. divided into five geologic units: Sand Fill, Shoal Sands, Young Bay Mud, Older Bay Deposits, and Franciscan Bedrock.

Sand Fill and Shoal Sands As described above, the hydraulic sand fills were deposited directly on native sandy shoals across most of the island footprint. In many Cone Penetration Test (CPT) probes, the contact between the base of fill and top of shoal sand can be approximately distinguished by an increase in interbedded clays and silts. Determination of the base of fill deposits is difficult in many borings. The approximate base of the hydraulic fills can be estimated from the pre-filling bathymetry, plotted on the Cross Sections. The hydraulic fill and shoal sands both consist of loose to medium-dense silty to clayey fine sand with variable fines contents. The base of sand shoal deposits was selected as the contact between loose to medium-dense sand and soft clay or denser sand deposits interbedded with the younger Bay Mud. HISTORIC PHOTO OF TREASURE ISLAND CONSTRUCTION

TREASURE ISLAND & YERBA BUENA ISLAND MAJOR PHASE 1 APPLICATION 6 - GEOTECHNICAL AND SHORELINE IMPROVEMENTS 235 GEOTECHNICAL CONCERNS GEOTECHNICAL MITIGATION

There are three primary geotechnical issues that influence Mitigation of Liquefaction and Lateral Spreading shoreline and site improvements at Treasure Island: liquefaction, Numerous ground improvement techniques are available to settlement, and seismic stability. mitigate the potential for liquefaction and its consequences. Some of these techniques considered in Major Phase 1 include Treasure Island was constructed in the late 1930s by placing Vibro-compaction, Vibro-replacement such as “stone columns”, dredged sand fill over a sand shoal located north of Yerba Buena and Deep Soil Mixing Island. From a geotechnical perspective, there are three primary issues for any new development at Treasure Island: liquefaction Mitigation of Young Bay Mud Consolidation Settlements settlement of the Sand Layers, settlements of the Young Bay Surcharging or preloading can be used both to speed primary Mud, and seismic stability of the perimeter and the causeway. consolidation under the weight of additional fill and to reduce the settlement caused by subsequent building loads. Surcharging Liquefaction of Sand Layers is often coupled with the installation of pre-fabricated vertical The combined thickness of the sand shoal and the dredged sand drains, commonly known as wick drains, which allow excess fill ranges from about 30 to 45 feet. These sands are generally pore pressures to drain laterally, shortening the drainage path loose to medium dense and are susceptible to liquefaction and and taking advantage of the fact that the horizontal permeability seismic recompression settlement. Liquefaction is the reduction of soils is normally much greater than the vertical permeability. of the soil’s strength and stiffness by earthquake shaking. The rate of consolidation can be controlled by selecting the type of drain and the spacing between the drains. A horizontal Settlement of Young Bay Mud drainage system can also be installed at the ground surface to Beneath the sands are layers of compressible Young Bay Mud collect and divert water expelled from the wicks. Wick drain and that ranges in thickness across the site from approximately 20 to surcharge test sections can be used to confirm and refine the 140 feet. The Young Bay Mud is generally normally consolidated surcharge design. and the settlement rate due to the weight of the dredged sand fill is now small. However, increases in loads due to placement of Shoreline Stabilization new fill or the construction of buildings will initiate a new cycle As discussed previously, the shoreline may be susceptible to of consolidation settlements. The Young Bay Mud is underlain by earthquake-induced deformation and, possibly, deep-seated dense to very dense sands and stiff to hard clays, which extend slope failures in areas of deep Young Bay Mud. Lateral spreading to bedrock at depths of 180 to 270 feet. of the island perimeter can be mitigated using vibro replacement methods, or deep soil mixing to improve approximately a zone Seismic Stability of Perimeter and Causeway around the island perimeter. The perimeter of the island and the causeway connecting Treasure Island to Yerba Buena Island are susceptible to Causeway Stabilization earthquake-induced lateral spreading due to liquefaction of the The issues potentially affecting the causeway are generally fill and shoal sands. In addition, deeper lateral deformations are similar to those impacting the island perimeter. Lateral spreading expected within the underlying Young Bay Mud layer. can be mitigated using vibro-compaction or vibro-replacement methods, or deep soil mixing.

Based on historic and recent geotechnical field investigation, a conceptual geotechnical mitigation plan for Treasure Island Major Phase 1 is illustrated in Figure 6.2.

TREASURE ISLAND & YERBA BUENA ISLAND MAJOR PHASE 1 APPLICATION 6 - GEOTECHNICAL AND SHORELINE IMPROVEMENTS 236 FIGURE 6.2 MAJOR PHASE 1 TREASURE ISLAND CONCEPTUAL GEOTECHINCAL MITIGATION

TREASURE ISLAND & YERBA BUENA ISLAND MAJOR PHASE 1 APPLICATION 6 - GEOTECHNICAL AND SHORELINE IMPROVEMENTS 237 6.3 YERBA BUENA ISLAND GEOTECHNICAL IMPROVEMENTS

EXISTING GEOLOGIC CONDITIONS The abutments for the eastern and western spans of the San of the eastern span of the Bay Bridge. Construction of access Francisco-Oakland Bay Bridge (Bay Bridge) occupy considerable ramps for the eastern span of the Bay Bridge is currently in Yerba Buena Island is underlain by Franciscan rock. On the portions of the island adjacent to the Yerba Buena Tunnel, which progress by Caltrans. island, the majority of the Franciscan Complex bedrock is covered passes through the southeastern portion of the island. Access with unconsolidated windblown sand and alluvial deposits, along around Yerba Buena Island is provided by perimeter roadways, The western portion of Treasure Island Drive is supported on the with localized areas of artificial fill. Windblown sand deposits in including Macalla Road, Treasure Island Drive, and Hillcrest Yerba Buena Island Viaduct, which is a complex of cantilevered the Bay Area are typically of Pleistocene to Holocene age, and Road. Secondary roads provide access to the residential area steel structures, concrete spans and retaining walls. The viaduct are believed to have formed by Aeolian (windblown) re-working on the central portion of the island and to the western abutment was originally constructed in the late 1930s to provide access of sandy shoals exposed during low stands of sea-level. The most extensive deposits of unconsolidated sandy soils are found on the windward north and northwest sides of the Yerba Buena Island.

The steep perimeter slopes appear to be the result of a combination of wave erosion and mass wasting, including landslides, slope creep and surficial erosion. Along the western and southern perimeter of the island, the lower slopes are steep, wave-cut bluffs exposing bedrock. The highest steep perimeter slopes occur below Macalla Road, where a 1.5:1 (h: v) slope as high as 240 feet has been eroded into deep deposits of windblown sand, colluvium and landslide debris. Bedrock is exposed only sporadically at the base of this slope, suggesting that unconsolidated deposits may locally extend below sea level. The original island topography has been altered by man-made excavations for roadways and building pads.

CONSTRUCTION HISTORY

Construction of United States Naval facilities began on Yerba Buena Island in the late 1800s and continued through the 1960s. Existing improvements include Coast Guard facilities on the east side of the island, residential units, and utility infrastructure. There are numerous underground utilities, many of which have been abandoned in place, as well as two existing above-ground water storage tanks and two below-grade water reservoirs.

FIGURE 6.3 YERBA BUENA ISLAND GEOLOGIC MAP

TREASURE ISLAND & YERBA BUENA ISLAND MAJOR PHASE 1 APPLICATION 6 - GEOTECHNICAL AND SHORELINE IMPROVEMENTS 238 from the eastbound Bay Bridge to Yerba Buena Treasure Islands. have encountered colluvium and silty sand to depths of up to 66 slope exposures, the Franciscan bedrock is moderately to slightly Portions of the original structures were removed and replaced feet. Surface exposures of Qs deposits typically consist of loose weathered and appears relatively stable at inclination of as steep in the 1960s, 1970s and 1990s. In 2011, the San Francisco light brown silty sand locally overlain by thin organic surface as ½:1. In many borings directly below surficial deposits, the Metropolitan Transportation Agency (SFMTA) started the design soils. Test pits and borings typically encountered denser, upper portions of the bedrock are highly weathered. for the seismic retrofit of the viaduct structures. Construction of reddish-brown silty sand with weak clay cementation below the the viaduct retrofit is anticipated to commence in 2017. surficial sand layers. These clay-cemented layers may represent GEOTECHNICAL CONCERNS local development of weathering profiles. Deeper borings in Qs Level areas for roadways and building pads were constructed deposits did not encounter uniform cementation in Qs deposits. Geotechnical concerns at Yerba Buena Island are associated by making side hill cuts and fills. Side hill cut slopes are The consistency of Qs sands ranges from loose near the surface to with: typically inclined between 1:1 and 2:1 (h: v). Many side hill cut very dense in deep deposits; however, along portions of Macalla • Loose surficial soils and undocumented existing excavations are supported by retaining walls that are typically Road, medium dense sands extend to depths of 50 to 100 feet. fills between 5 and 15 feet high, but are locally as high as 20 Where borings have penetrated to bedrock, a layer of stiff rocky • Slope stability issues along the existing steep feet. The majority of existing walls are cast-in-place concrete clay has locally been encountered at the base of the Qs deposits. perimeter slopes structures or concrete crib walls. On the southwest slopes of the Island, between Yerba Buena Road and Treasure Island Drive, Landslides and Colluvium GEOTECHNICAL IMPROVEMENTS there is an area of terraced slopes and abandoned foundations The steep island perimeter slopes are mantled with sandy associated with past site uses. colluvium and landslide deposits, as shown on Figure 6.4. The Mitigation measures for the above geotechnical concerns may mapped landslides include relatively shallow debris flows and require re-grading to remove weak soils. Potential slope-stability SUBSURFACE CONDITIONS more deep-seated debris slides triggered by a combination of hazards adjacent to the steep northern perimeter slopes can be natural processes such as over-steepening of coastal bluffs by addressed with combinations of re-grading to remove weak soils Areas of artificial fill were created by grading associated with erosion, seasonal rainfall, poor drainage associated with runoff and using setbacks from the existing top of slope when siting past development activities on Yerba Buena Island. Artificial from roadways and unstable fills associated with roadway new improvements. fills on much of the island consist of sliver fills under building grading. Shallow debris flows have also locally occurred in overly pads and roadways. Bay margin fills underlie the Treasure steep cut slopes associated with roadways and building pads in Macalla Road Island Causeway and Coast Guard facilities on the east side of the residential areas of the island. Existing improvements such A segment of Macalla Road will be realigned due to its proximity the island. On the upper portions of the island, the existing as roadways and retaining walls around the island perimeter to steep perimeter slopes with potential instability concerns. The fills appear to consist of sand and mixtures of sand and rock have been damaged by slope movements several times since new road alignment will be set back from the top of the perimeter fragments, derived from local cuts. The Bay margin fills were the late 1940s. The most extensive area of landslide deposits slopes by constructing a retaining wall into the existing hillside, reportedly constructed with a combination of dredge spoils and and colluvium occurs along the north island perimeter below removing all the weak soils will be removed and constructing possibly excavated material from the Bay Bridge tunnel. Macalla Road, where there are several steep arcuate scarps the roadway will be constructed as engineered fill to provide interpreted to be the heads of older landslides (See Figures 6.2 vehicular access to the Bay Bridge, and stable ground conditions Windblown Sand and 6.3). Landslide activity has also occurred below portions of for the new mainline utilities. The bedrock across most of the island is mantled by uncemented the Viaduct and on slopes below Hillcrest Drive. to partially cemented silty to clayey sand, (Qs) interpreted to Water Tanks be a combination of wind-blown sand, alluvium and marine Franciscan Formation The three new water tanks will be located on the western terrace deposits (Wagner, 1991, Blake, et al., 2000). Within the The Franciscan Complex bedrock consists of graywacke facing slopes of YBI, at a super-pad that will be constructed proposed development area the Qs deposits consist of relatively sandstone, interbedded with siltstone and shale in varying by cutting a retaining wall into the existing hillside. During the uniformly-graded silty sand that appears to be of windblown proportions (Blake, et al, 2000). The rocks are slightly grading of the pad, all weak soils will be removed and placed origin. The thickest deposits of Qs occur on the north side of metamorphosed, with localized intra-formational tectonic back as engineered fill. The major considerations in foundation the island along Macalla Road, where previous explorations shearing. According to regional and local geologic mapping, design for the water tanks will include the effects of potential and this investigation have encountered up to 120 feet of silty the bedrock layering exhibits a relatively consistent northwest differential movement of on-site soils as a result of their shrink- sand. Geotechnical explorations along the Viaduct alignment strike and typically dips northeast at 20 to 80 degrees. In cut swell characteristics, settlement associated with deep fills, and the distance of the tanks from the top of slopes.

TREASURE ISLAND & YERBA BUENA ISLAND MAJOR PHASE 1 APPLICATION 6 - GEOTECHNICAL AND SHORELINE IMPROVEMENTS 239 6.4 SEA LEVEL RISE STRATEGY AND SHORELINE IMPROVEMENTS

SHORELINE PROTECTION rise will be built into the initial construction, and the design mains, lift stations, pump stations and the reconstruction of will be adaptable to higher levels of sea level rise by leaving a existing outfalls. Refer to Section 5.2, Storm Drain, for more The existing rock slope at the shoreline will be augmented with significant development setback such that improvements can be information about this system. additional rock to create elevated protection. The higher berm made. crest will prevent excessive wave overtopping. Setbacks will Existing outfalls will be replaced, renovated or abandoned in allow future rock to be added to raise the elevation as sea level The following table show the results of the runup analysis for place during each of the four Major Phases. A total of 14 outfalls rise requires. present conditions and with 16 inches of sea level rise. will be replaced or renovated from existing outfalls on Treasure Island and Yerba Buena Island. The outfalls will penetrate the The existing perimeter protection was analyzed for protection rock slope into the Bay water. Recommended Crest Elevation for Existing against coastal flooding. The recommended perimeter berm crest Runup associated with 1% event + Location Elevation at different locations around Treasure Island, prior to adding minimal overtopping + 16” of SLR allowance for sea level rise, is the highest of the following three (feet, NAVD88) (feet, NAVD88) SEA LEVEL RISE STRATEGY cases: Southwest 10 to 13 14.3 West 10 to 11 14.6 • The one percent annual chance runup elevation; Estimates of sea-level rise (SLR) in literature vary widely based Northwest 11 to 13 15.1 on the methods used and it is clear that the science of climate • The maximum elevation at which the one percent North 12 16.8 change and sea level rise is evolving. It is prudent to develop annual chance overtopping rate is not dangerous Northeast 10 to 11 13.3 community designs that can accommodate various levels of SLR to pedestrians walking along the perimeter; East 11 to 13 11.7 • Two feet above the ten percent annual chance South 11 to 13 11.7 over the planning horizon rather than design to a specific report still water level allowing for vessel wakes, (Proposed embankment sliope = 3H:1V above +12' NAVD and 2H:IV below +12') or estimate. Features that have been incorporated into project Table 6.1 - Recommended Perimeter Crest Elevations planning documents and design criteria are summarized below. which are of short duration and are not required FIGURE 6.1 RECOMMENDED PERIMETER CREST ELEVATIONS to be included in the coastal flooding analysis, but which could cause nuisance flooding. • Given the relatively high elevation of over half ISLAND CENTER the island, the decision was made to raise the development The wave runup analysis was performed around the perimeter footprint to be higher than extreme Bay water levels. All streets of Treasure Island with the following criteria for a compound Existing elevations within the Island Center are among the and entrances to subterranean parking will be set at an elevation shoreward slope: highest on the island and will allow the historic buildings within that is 36 inches higher than the Base Flood Elevation (BFE, • 2H:1V below +12 feet NAVD88, the district to remain in use through the time frames cited above. current 100-yr return period water level). All buildings will have • 3H:1V slope above +12 feet, NAVD88. a finished floor elevation that is 42 inches higher than the BFE STORM DRAIN OUTFALLS (includes a 6-inch freeboard). This will ensure that even in the The resulting crest elevations for specific reaches around TI are event of deficiencies along the perimeter, habitable structures shown in Table 6.1. Stormwater runoff from streets and paved areas on Treasure will not be flooded for water levels 42 inches higher than the Island and Yerba Buena Island is currently discharged untreated BFE. This will put it beyond the 2080 time frame according It is not practical to build a high wall around the project for directly to the Bay through 31 outfalls around the perimeter to the most aggressive sea level rise, and close to or beyond a design condition that may not happen for several decades, of Treasure Island and 32 outfalls from Yerba Buena Island. 2100 according to National Research Council (NRC) and because it will pose a visual obstruction and severely limit The existing stormwater system will be replaced with a new Intergovernmental Panel on Climate Change (IPCC) projections. public access. Therefore, an allowance of 16-inches of sea level collection system, which will include gravity pipelines, force

TREASURE ISLAND & YERBA BUENA ISLAND MAJOR PHASE 1 APPLICATION 6 - GEOTECHNICAL AND SHORELINE IMPROVEMENTS 240 • The perimeter of the island will be geotechnically ADAPTIVE MANAGEMENT relatively easy adaptations in the future, for example by raising improved such that existing coastal flooding from various the perimeter. Development setbacks are included along the combinations of tides, waves, surges and tsunamis will be Future SLR will be accommodated within each of the above perimeter to allow these future improvements to be constructed mitigated. The crest elevation of shoreline structures will be set proposed improvements, such that future improvements will be within the island footprint rather than encroaching into the Bay. at an elevation that is 16 inches higher than what will be needed necessary only after a significant amount of SLR has occurred Funding at the present time, and the design will be adaptable to higher in the Bay. Due to differences in adaptive capacity, different levels of SLR by leaving a significant development setback such planning horizons were adopted for the various elements of the A stream of funding will be set up for the community to construct that improvements can be made. This will ensure that visual project. these improvements as part of an Adaptive Management Plan. obstructions and public access limitations do not occur for a The Adaptive Management Plan is a project-specific Plan that condition that will not happen for several decades in the future. Low Adaptive Capacity identifies stakeholders, provides guidance, defines appropriate Areas that cannot tolerate flooding (low adaptive capacity such as triggers and management actions, and establishes long-term • The storm drain system will be improved such that urban promenades, building pads, City parks) will accommodate specific funding mechanism. The Plan will be administered by runoff from the island will flow out to the Bay under gravity and a higher amount of SLR because building structures for example Treasure Island Development Authority which will have taxing not result in flooding of the development parcels. The gravity- are generally “immovable.” authority and funding responsibility. It will lay out the elements drained storm drain system will be constructed with an initial of a monitoring program for incorporating ongoing measurements SLR allowance of 16 to 24 inches, which will be adaptable High Adaptive Capacity of SLR from the scientific community into periodic perimeter to higher levels of SLR with minimal intervention. It will thus Passive use, open space areas where infrequent flooding can system inspection reports that will guide the decision making function as a gravity-drained system for several decades, beyond be tolerated (high adaptive capacity such as natural areas process for future improvements. The Plan will also define which adaptations will be implemented consisting of installing oriented to habitat creation and passive open space uses) specific triggers for action, based on observed changes in sea storm drain pumps. will accommodate a lower amount of SLR, with provisions for level.

TREASURE ISLAND & YERBA BUENA ISLAND MAJOR PHASE 1 APPLICATION 6 - GEOTECHNICAL AND SHORELINE IMPROVEMENTS 241 SEA LEVEL RISE STRATEGY

C B2

C

A2

Yerba Buena Island elevations preclude the need for sea level rise adapations.

C B1 A1

KEY Elevated Development Lands Island Core (Existing Grades) Major Phase 1 Boundary Clipper Cove - Ferry Terminal Shoreline Park - Recreational Areas N Northern Shoreline Park - Wilds - Adaptive Habitat Areas 1000’ 2000’

FIGURE 6.4 SEA LEVEL RISE STRATEGY SITE PLAN DIAGRAM

TREASURE ISLAND & YERBA BUENA ISLAND MAJOR PHASE 1 APPLICATION 6 - GEOTECHNICAL AND SHORELINE IMPROVEMENTS 243 SEA LEVEL RISE STRATEGY

SECTION A1 FERRY TERMINAL SECTION B1 CITYSIDE WATERFRONT PARK TYPICAL CONDITION

EXISTING GRADE EXISTING GRADE

WRU WRU SLR SLR BFE BFE MHW MHW

SECTION A2 CLIPPER COVE PROMENADE SECTION B2 EASTERN SHORELINE PARK

EXISTING GRADE EXISTING GRADE

WRU SLR WRU BFE SLR MHW BFE MHW

FIGURE 6.5 SEA LEVEL RISE SECTIONS A1, A2, B1 AND B2

LEGEND WRU Wave Run-Up SLR Sea Level Rise BFE Base Flood Elevation MHW Mean High Water

TREASURE ISLAND & YERBA BUENA ISLAND MAJOR PHASE 1 APPLICATION 6 - GEOTECHNICAL AND SHORELINE IMPROVEMENTS 244 SEA LEVEL RISE STRATEGY

SECTION C NORTHERN SHORELINE PARK - OPTIONS

WRU SLR BFE MHW OPTION A PROTECT

WRU SLR BFE MHW OPTION B CENTRAL MARSH

WRU SLR BFE MHW OPTION C TIDAL MARSH

FIGURE 6.6 SEA LEVEL RISE SECTION C

TREASURE ISLAND & YERBA BUENA ISLAND MAJOR PHASE 1 APPLICATION 6 - GEOTECHNICAL AND SHORELINE IMPROVEMENTS 245 TREASURE ISLAND & YERBA BUENA ISLAND MAJOR PHASE 1 APPLICATION 7 - LAND TRANSFER AND IMPLEMENTATION 246