Appendix N - Floodplain Statement
of Findings for the
Final Yosemite Valley Plan/SEIS

This Floodplain Statement of Findings is included in this document for public review to meet the obligations of Executive Order 11988 (Floodplain Management) and the NPS Floodplain Management Guideline 1993.

Introduction

The National Park Service has prepared the Final Yosemite Valley Plan/Supplemental Environmental Impact Statement (SEIS) to provide direction and propose specific actions to preserve Yosemite Valley’s natural, cultural, and scenic resources, and to provide opportunities for high-quality, resource-based experiences for visitors. It is based on the broad goals of the 1980 General Management Plan for Yosemite National Park. The purpose of this Floodplain Statement of Findings is to review the Final Yosemite Valley Plan/SEIS in sufficient detail to:

• Provide an accurate and complete description of the flood hazard assumed by implementation of the proposed action (without mitigation)
• Provide an analysis of the comparative flood risk among alternative sites
• Describe the effects on floodplain values associated with the proposed action
• Provide a thorough description and evaluation of mitigation measures developed to achieve compliance with Executive Order 11988 (Floodplain Management) and the NPS Floodplain Management Guideline 1993

Floodplain Extent

The best available data were used to determine the extent of existing floodplain boundaries and water surface characteristics of the Merced River. The Stantec (2000) and Cella Barr Associates (1998) model was used to analyze the extent of the 2-, 10-, 25-, and 100-year floodplains in the east end of Yosemite Valley between Happy Isles and the west end of Yosemite Lodge. The line that delineates the January 1997 flood extent was used to determine the 100-year floodplain in the west end of Yosemite Valley from the west end of Yosemite Lodge to Pohono Bridge. The U.S. Army Corps of Engineers Merced River study (1998) was used to determine the 100-year and 500-year floodplain in El Portal. The U.S. Army Corps of Engineers floodplain map (1981b) was used to determine the 100-year and 500-year floodplains in Wawona.

The Proposed Action

The Final Yosemite Valley Plan/SEIS aims to restore degraded areas and reduce development within the Merced River ecosystem and other highly valued natural and cultural resource environments. It strives to reduce traffic congestion and supports the use of alternative fuels to reduce mobile sources of air pollution. It presents alternatives to expand orientation and interpretation services. It proposes to move nonessential housing, administrative headquarters, offices, and other functions out of Yosemite Valley. Many of these functions would move to the El Portal Administrative Site on the western boundary of the park. The plan proposes options for the size and placement of parking areas, both within and outside of Yosemite Valley. Attachment A of this Statement of Findings lists the nonexempted actions in the floodplain that are proposed in the Final Yosemite Valley Plan/SEIS.

Existing Structures in the Floodplain

The NPS Floodplain Management Guideline 1993 divides actions into the following three groups:

• Class I Actions — include administrative, residential, warehouse and maintenance buildings, and nonexempted (overnight) parking lots
• Class II Actions — those that would create "an added disastrous dimension to the flood event." Class II actions include schools, clinics, emergency services, fuel storage facilities, large sewage treatment plants, and structures such as museums that store irreplaceable records and artifacts.
• Class III Actions — Class I or Class II Actions that are located in high hazard areas such as those subject to flash flooding.

The regulatory floodplain for Class I actions is the 100-year floodplain. The regulatory floodplain for Class II Actions is the 500-year floodplain. There are no Class III actions in the project area.

Most existing structures in the regulatory floodplain in Yosemite Valley are Class I actions. These structures include five motel units at Yosemite Lodge, the Concessioner Headquarters Building at Yosemite Village, Indian Creek employee housing, 248 units at Housekeeping Camp, the Superintendent’s House (Residence 1), and the concessioner stable. The regulatory floodplain for the museum collection, a Class II action, is the 500-year floodplain. The museum collection is currently housed outside of the 100-year floodplain in Yosemite Village in Yosemite Valley. It may be in the 500-year floodplain, though current data are not available.

The Cascades Diversion Dam is located west of Pohono Bridge in Yosemite Valley. This dam was constructed as part of a small hydroelectric plant. Though the plant is no longer functional, the dam remains in place. Safety engineers have classified the dam as a "high hazard potential structure."

Most existing structures that are found in the regulatory floodplain in El Portal are Class I actions. These structures include the El Portal Market, the Motor Inn (employee housing), the warehouse complex, and the ranger station. There are two existing Class II actions in the regulatory floodplain in El Portal: the gas station (currently not in use) and the bulk fuel storage facility.

In Wawona, parts of the Pioneer Yosemite History Center are in the regulatory floodplain.

Proposed Actions

Under the Preferred Alternative in the Final Yosemite Valley Plan/SEIS, all existing nonexempted facilities in Yosemite Valley would be removed from the floodplain except for the following:

• New overnight parking at Yosemite Lodge
• Three Ahwahnee Row houses (all of the Ahwahnee Row houses would remain, but only three are within the regulatory floodplain)
• New visitor services and a transit center at Yosemite Village in the vicinity of the Concessioner Headquarters Building
• 84 units at Housekeeping Camp
• The Yosemite Museum collection

Under the Preferred Alternative in the Final Yosemite Valley Plan/SEIS, the following facilities would remain or could be placed in the floodplain in El Portal:

• Hennessey’s Ranch — Mobile homes would be removed from the site and replaced with high-density housing.
• Village Center — This area has been designated for necessary support facilities and commercial services and could also support parking. Parts of this area are in the regulatory floodplain. The exact placement of new and replacement facilities will be determined in subsequent site design. New development could include a community center, post office, enlarged grocery store/deli, laundry, recreation facilities, hair salon, medical clinic, and office spaces.
• The NPS warehouse at Railroad Flat
• The gas station

In Wawona, parts of the Pioneer Yosemite History Center would remain in the regulatory floodplain.

General Characteristics of Flooding in the Area

Floods on the Merced River are of two general types: those that occur during the late fall and winter (November through March) primarily as the result of intense rainfall, and those that occur during the spring and early summer resulting from snowmelt. At the beginning of the wet season the ground is extremely dry, and about 3 to 5 inches of precipitation is required to satisfy the retention storage capacity of the soil before any significant runoff occurs. Later in the season, when the ground may be very wet and there may be a moderate snow cover at the higher elevations, heavy rainfall over the basin causes large flood runoff. An intense storm with a high freezing level may result in flood runoff from almost the entire basin, with as much as 2 inches of snowmelt augmenting the rainfall. Most of the runoff from the Merced River basin occurs from November through July.

Yosemite Valley has a well-developed, relatively wide floodplain that is confined by steep valley walls. The Merced River in Yosemite Valley has a relatively mild slope, with an average of 0.1%. In the middle reach of the Merced River in Yosemite Valley, downstream of Clark’s Bridge to the El Capitan moraine, the river flows through a shallow channel approximately 100 to 300 feet wide. Typically, the main channel in this reach has the capacity to convey between 2- and 5-year flow events within the existing channel banks (Stantec 2000). Historic discharge in the river, measured at the Pohono Bridge gauging station, has ranged from a high of about 25,000 cubic feet per second to a low of less than 10 cubic feet per second. The mean daily discharge rate is about 600 cubic feet per second.

The low flow channel in the middle reach of Yosemite Valley meanders across a broad floodplain and through a series of bends and divides. During 25- and 100-year floods, waters substantially overflow the meandering low-flow channel path and flow straight down the Valley (Stantec 2000). Near Yosemite Lodge and downstream to the El Capitan moraine, flood waters back up against the moraine and tend to be deep and slow. This backwater influence, which reduces flow velocities and increases flow depths, extends about 4.5 miles upstream of the El Capitan moraine past Sentinel Bridge (Stantec 2000). Flow velocities in this backwater area for 2- and 10-year events are actually higher than for 25- and 100-year events in both channel and overbank areas due to the backwater influence (Stantec 2000).

The river channel in El Portal is steep and flow velocities are high. Some lateral shifting can occur during large floods. Flow volumes are not available but should be slightly larger than those of the Pohono Bridge gauging station. The levee at the east edge of Hennessey’s Ranch (Trailer Village and Abbieville) prevents water from entering the site and is effective for containing floods that have less than a 100-year recurrence interval.

The floodplain in Wawona along the South Fork is an elongated alluvial valley. The river channel can shift laterally during large floods. In Wawona, upstream of the Big Creek confluence, the average annual flow was 174 cubic feet per second between 1958 and 1968, as measured at the Wawona gauging station, with an estimated maximum flow of 15,000 cubic feet per second in December 1955.

Justification for Use of the Floodplain

New Development

Overnight Parking at Yosemite Lodge. During site design planning, should no reasonable alternative be identified for overnight parking outside of flood limits for the Yosemite Lodge, overnight parking would be placed within the 100-year floodplain. The Yosemite Lodge area is constrained by natural boundaries as well as by development boundaries. If necessary, overnight parking would be placed in a previously developed area that once served as the site of concessioner employee dormitories. The dormitories were removed after the January 1997 flood because of extensive flood damage.

In high flood conditions similar to those of the January 1997 flood, there would be slow water movement in the potential parking area. The new parking would have a minimal effect on flood characteristics during high water levels in relation to previous dormitory structures. The new parking area would prevent the establishment of floodplain-related natural communities such as riparian areas, wetlands, and meadows.

Hennessey’s Ranch (Trailer Village and Abbieville). The National Park Service has determined that the El Portal Administrative Site will serve as the principal location for National Park Service employee housing in the Final Yosemite Valley Plan/SEIS. This decision was based on a thorough evaluation of potential environmental impacts, and on clear and overwhelming public comment and endorsement. The El Portal Administrative Site lies at the bottom of a steep river canyon. Available building space is at a premium due limits imposed by the steep terrain, flood dangers, and natural and cultural resources.

The National Park Service evaluated all potential building sites in El Portal and identified seven potential sites for employee housing: Hennessey’s Ranch, Hillside West, Hillside East, Village Center, Rancheria Flat, Old El Portal, and Riverside. In the Preferred Alternative of the Final Yosemite Valley Plan/SEIS, employee housing would be built at all of these sites except for Riverside. Riverside was removed from consideration for housing for the following reasons: the site contains significant cultural resources; a bridge would need to be constructed to access the site; evacuation would be difficult if the bridge failed during a flood; the site is subject to river erosion; potential impacts to threatened and endangered species could occur; and the site contains high-quality wildlife habitat. All of the remaining potential housing sites are necessary to support the large number of employee housing proposed in El Portal. There are no reasonable alternatives to these six remaining sites, including Hennessey’s Ranch.

Village Center. This area has been designated for necessary support facilities and commercial services and could also support parking. Parts of this area are in the floodplain. New development could include a community center, post office, enlarged grocery store/deli, laundry, recreation facilities, hair salon, medical clinic, office spaces, and a gas station. If day-visitor parking were to be developed, this action would be exempted from the NPS Floodplain Management Guideline (1993c). Should other facilities be developed in the regulatory floodplain, a subsequent Floodplain Statement of Findings would be developed as a part of future compliance.

Existing Development

Ahwahnee Row Houses (Three Houses in Floodplain). These houses would not be removed because they are important contributing elements to the Yosemite Valley cultural landscape.

Housekeeping Camp. Currently, 248 units at Housekeeping Camp are within the 100-year floodplain. These units are available seasonally, and the area is closed for overnight use in the winter. In the Preferred Alternative in the Final Yosemite Valley Plan/SEIS, 84 units at Housekeeping Camp would remain in the floodplain along with six miscellaneous structures (such as bathrooms and the store). The 164 units that are closest to the Merced River would be removed from the floodplain.

Housekeeping Camp has had a long history of traditional use. Housekeeping Camp provides a unique opportunity in Yosemite Valley for a rustic camping experience with "developed camping shelters" that eliminate the need to purchase large amount of camping equipment. Housekeeping Camp is the only place in Yosemite Valley where overnight visitors can cook their own food, other than the campgrounds.

Housekeeping Camp is closed during the winter, when most high-flow events have occurred. There would be sufficient time to evacuate visitors in the unlikely event that evacuation would be necessary. To preserve the floodplain values in areas close to the river while still preserving the unique visitor experience, the 164 units that are closest to the Merced River would be removed and 84 units would remain within the floodplain. The remaining units would have little effect on flood attributes.

The Yosemite Museum Collection. The museum collection would remain in Yosemite Village in proximity to museum exhibit space. This would allow staff to easily move collection objects to and from exhibit space, allow users of the research library to easily access the collection, and protect the collection from risks involved with transfer to another location.

Some parts of the museum collection, in particular the Native American collection, were collected, made, or are strongly associated with the Valley. These articles have more intrinsic value to American Indian and other groups when stored in the Valley.

The NPS Warehouse Complex at Railroad Flat in El Portal. When the first phases of this extensive warehouse/office complex were planned and constructed in 1994 and 1995, U.S. Army Corps of Engineers map data determined that most of the complex would be out of the floodplain. New information based on data from the January 1997 flood (USCOE 1998) has revised this determination to indicate that most of the complex is in the 100-year floodplain. A Statement of Findings would be developed as part of the El Portal design concept process to provide an accurate description of flood hazards at the site and identify necessary mitigation.

The Gas Station in El Portal. There is an immediate need for a gas station in El Portal to serve local residents (numbering about 1,000) and National Park Service and park partner employees who work in El Portal. The closest gas stations to El Portal are located in Midpines and Crane Flat, both of which are about a 30-minute drive from El Portal under good driving conditions. The location of the gas station would be re-evaluated during specific site design process for El Portal.

The Pioneer Yosemite History Center in Wawona. Parts of this interpretive site are located in the regulatory floodplain. Four buildings in the Pioneer Yosemite History Center are listed on the National Register of Historic Places (Jorgensen Studio, Hodgdon Homestead Cabin, Superintendent’s House (Residence 1), and Yosemite Transportation Company Office). Whether a historic building retains its original location is an important consideration when assessing its eligibiity for the National Register. The buildings would be left in their current locations because moving them would affect their historic integrity and possibly their National Register status.

Redevelopment

Visitor Services and Transit Center at Yosemite Village. Several areas in Yosemite Valley have been zoned to concentrate intensive visitor use in order to protect the renowned qualities of Yosemite Valley, while still providing services for the public. In the Preferred Alternative in the Final Yosemite Valley Plan/SEIS, Yosemite Village would be a major parking area and transportation hub as well as the site for the visitor center. The placement of the visitor center near the transportation hub would ensure that visitors have easy access to orientation tools to help them understand the park, as well as transportation connections to other areas.

Parts of the Yosemite Village area fall within floodplain boundaries, particularly in the vicinity of the existing Concessioner Headquarters Building. Visitor services and a transit center are proposed for redevelopment in this area. Site-specific design has not been completed, and the specific nonexempted actions that would fall within floodplain boundaries are not known. If day-visitor parking were developed, this action would be exempted from the NPS Floodplain Management Guideline (1993c). Should structures to support visitor services be proposed within the floodplain, a subsequent Floodplain Statement of Findings would be developed as a part of future compliance.

Description of Site-Specific Flood Risk

Yosemite Valley

Floods of consequence in Yosemite Valley always occur with some warning. It takes a prolonged period of intense rain for at least 24 hours to create flood conditions. Risks to humans can typically be mitigated by warning and evacuation.

In Yosemite Valley, the character of flooding varies in different locations because of local hydraulic controls. From Clark’s Bridge to Housekeeping Camp in the east Valley, the Merced River floods areas outside the main river channel with shallow, swift flows that cut across meander bends. Near Yosemite Lodge and downstream to the El Capitan moraine, flood waters back up against the moraine and dense vegetation. Flood waters in this area are of low velocity and significant depths. At Housekeeping Camp, velocities are relatively higher with lower depths.

The historic discharge in the river, measured at the Pohono Bridge gauging station, has ranged from a high of about 25,000 cubic feet per second to a low of less than 10 cubic feet per second. The mean daily discharge rate is about 600 cubic feet per second.

El Portal

The El Portal area is located in an extremely high energy, bedrock-controlled reach with little high floodplain suitable for development. Due to high flood velocities, infrastructure and developments must be located above flood levels or be massively armored. Evacuation of flood-prone areas should be mandatory during flood events of any appreciable size (National Park Service 1997g).

Wawona

Floods of consequence in Wawona always occur with some warning. It takes a prolonged period of intense rain for at least 24 hours to create flood conditions. Risks to humans can typically be mitigated by warning and evacuation.

Design Or Modifications To Minimize Harm To Floodplain Values Or Risks To Life And Property

General Mitigation

The design of all new structures would incorporate methods for minimizing flood damage, as contained in the National Flood Insurance Program "Floodplain Management Criteria for Flood-Prone Areas" (CFR 44, 60.3) and in accordance with any local, county, or state requirements for flood-prone areas. Furthermore, the park staff would maintain an active flood evacuation plan. The plan details responsibilities of individual park employees for advanced preparedness measures; removing or securing park property; records and utility systems; monitoring communication; and conducting rescue and salvage operations.

Impacts on the site’s resources would be minimized and mitigated. The design for impermeable areas would provide for appropriate drainage to ensure that natural resources are not further degraded by associated runoff following hydrologic events.

Site-Specific Mitigation — No Subsequent Statement of Findings Necessary

Housekeeping Camp, Ahwahnee Row Houses, and Ahwahnee Cottage

• Plans would be made for timely and safe evacuation of Housekeeping Camp and the Ahwahnee Row houses in times of rising water.

Site-Specific Mitigation — Subsequent Statement of Findings Necessary

Overnight Parking at Yosemite Lodge

• Site-specific design has not been completed for this area, and specific overnight parking needs have not been determined. If overnight parking were developed, a subsequent Floodplain Statement of Findings would be developed as a part of future compliance.
• Design of overnight parking would allow minimal resistance to flood waters to minimize impacts on natural flood processes.
• Plans would be made for timely evaluation of the lodge area in times of rising water. Evacuation routes would be outside the flood levels, along surfaced pedestrian/bicycle routes that are wide enough for vehicles.

Parking and Visitor Services at Yosemite Village

• Site-specific design has not been completed for this area, and it is not possible to determine the specific nonexempted actions that would fall within floodplain boundaries. If day-visitor parking were developed, this action would be exempted from the NPS Floodplain Management Guideline (1993c). Should structures to support visitor services be proposed within the floodplain, a subsequent Floodplain Statement of Findings would be developed as a part of future compliance.
• Plans would be made for timely and safe evaluation of the Yosemite Village area in times of rising water.

Yosemite Museum Collection

• The site-specific location and plans for the museum collection have not been determined. A subsequent Floodplain Statement of Findings would be developed as a part of future compliance.
• All museum storage facilities would have permanent foundations and finished floor elevations above the current 500-year flood high-water line and be engineered to withstand inundation.

The Village Center in El Portal

• Site-specific locations for necessary support facilities and commercial services have not been determined. A Floodplain Statement of Findings would be developed as part of future site design and compliance.

The Gas Station in El Portal

• The site-specific location for the gas station in El Portal has not been determined. A subsequent Floodplain Statement of Findings would be developed as a part of future site design and compliance.
• Facilities would be built to avoid or withstand the 500-year flood.

Hennessey’s Ranch

• The site-specific design for employee housing at Hennessey’s Ranch has not been developed. A subsequent Floodplain Statement of Findings would be developed as a part of future site design and compliance.
• As many structures as possible would be built on the high island in the center of the area outside of the 100-year floodplain.
• All dwellings would have permanent foundations and finished floor elevations above the current 100-year flood high-water line and be engineered to withstand inundation.
• The existing levee would be rebuilt to withstand the 100-year flood.
• A "community open space" or riparian buffer zone would be left adjacent to the river. This would retain more space for the Merced River to spread out horizontally, and the levee would not need to be as high.
• Along with raising the levee, appropriate measures would be taken to prevent flood waters from entering the area via the Highway 140 corridor below the highway bridge.
• An emergency evacuation plan would be developed. The plan would designate a specific river stage at which evacuation of people would begin.

The NPS Warehouse Complex at Railroad Flat

• New information developed after the complex was constructed shows that a large part of the complex is within the 100-year floodplain. A Statement of Findings would be developed as part of the El Portal design process to provide an accurate description of flood hazards at the site and identify necessary mitigation. Mitigation measures would be evaluated for this area, including raising the elevation of the building and constructing a floodwall.
• An emergency evacuation plan would be developed. The plan would designate a specific river stage at which evacuation of people would begin.

Conclusion

The Preferred Alternative would substantially reduce potentially hazardous conditions associated with flooding by relocating facilities out of the floodplain in Yosemite Valley. Facilities that would be removed from the floodplain include five motel units at Yosemite Lodge, Indian Creek employee housing, 164 units at Housekeeping Camp, the Superintendent’s House (Residence 1), three Ahwahnee Row houses, and the concessioner stable. The Preferred Alternative would have beneficial impacts on floodplain values by linking river-associated wetlands and meadows that have been degraded or fragmented into one large and dynamic, river-governed ecosystem. However, the National Park Service has determined that there is no practicable alternative to maintaining the following within the regulatory floodplain: overnight parking at Yosemite Lodge, three Ahwahnee Row houses, new visitor services and a transit center at Yosemite Village, and 84 units at Housekeeping Camp. In El Portal, there is no practicable alternative to high-density employee housing at Hennessey’s Ranch; support facilities, commercial services, and parking at Village Center; the National Park Service warehouse at Railroad Flat; and the gas station in the regulatory floodplain. These facilities are not within areas subject to frequent flooding, and with the early warning system and evacuation plan in use, the risk to human safety would be minimized.

The National Park Service concludes that the Preferred Alternative would reduce the impacts of potentially hazardous conditions associated with flooding in Yosemite Valley. Mitigation and compliance with regulations and policies to prevent impacts to water quality, floodplain values, and loss of property or human life would be strictly adhered to during and after the construction. Individual permits with other federal and cooperating state and local agencies would be obtained prior to construction activities. No long-term adverse impacts would occur from the proposed actions. Therefore, the National Park Service finds the Preferred Alternative to be acceptable under Executive Order 11988 for the protection of floodplains.

ATTACHMENT A — Current and Proposed Nonexempted Actions in the Regulatory Floodplain

Final Yosemite Valley Plan/SEIS

| Table of Contents | Appendix A | Appendix B | Appendix C | Appendix D | Appendix E | Appendix F | Appendix G |
| Appendix H | Appendix I | Appendix J | Appendix K | Appendix L | Appendix M | Appendix N |

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