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 recently prepared the Yosemite Valley Plan to provide for managing natural and cultural resources, park facilities, and visitor experiences in the Valley. Implementation of actions presented in the Yosemite Valley Plan will further the park’s goals of restoring, protecting, and enhancing the resources of the Yosemite Valley; providing opportunities for high-quality, resource-based visitor experiences; reducing traffic congestion; and providing effective park operations to meet the park’s mission. Many of the over 250 actions will result in restoration of developed and disturbed areas to natural conditions, redevelopment of some areas, and development of other areas to accommodate visitor and employee services. During development of the Yosemite Valley Plan, the need to remove utility infrastructure from environmentally sensitive areas and to consolidate utilities into environmentally preferable areas was identified.

The National Park Service has prepared the East Yosemite Valley Utilities Improvement Plan to provide a roadmap for the development of consolidated utility facilities within Yosemite Valley. Implementation of the East Yosemite Valley Utilities Improvement Plan will result in a utility system that maximizes the efficiency of utility operations and maintenance and minimizes the potential for future environmental impacts. There is a need for a utility plan that will meet the following goals.

§         Ensure adequate service to facilities relocated or developed under the Yosemite Valley Plan

§         Implement upgrades needed to address previously identified utility condition and capacity issues

§         Maximize use of existing transportation and utility corridors and proposed new transportation corridors

§         Minimize potential future impacts to environmentally sensitive areas

§         Protect and preserve the Merced Wild and Scenic River as called for in the Merced Wild and Scenic River Comprehensive Plan (Merced River Plan) (NPS 2001)

The purpose of this Floodplain Statement of Findings is to review the East Yosemite Valley Utilities Improvement Plan 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 Proposed Action

The East Yosemite Valley Utilities Improvement Plan is designed to develop consolidated utility corridors, reduce utility infrastructure within the Merced River and its tributaries, reduce utility infrastructure within the Merced River floodplain, remove utility infrastructure from areas identified for ecological restoration and concentrate utility infrastructure into corridors in environmentally preferred areas. Although the proposed East Yosemite Valley Utilities Improvement Plan would reduce utility infrastructure in the floodplain, it would not remove utility infrastructure completely. Those utility corridors and associated utility infrastructure (such as sanitary sewer lift stations and electric transformers) proposed to remain or to be constructed within the floodplain would be designed to be submersible and to reduce the potential for damage from flood events.

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.

All of the actions proposed under the East Yosemite Valley Utilities Improvement Plan area considered Class I actions. The regulatory floodplain for Class I actions is the 100-year floodplain.

Proposed Actions

Under the Proposed Alternative in the East Yosemite Valley Utilities Improvement Plan, utility infrastructure within the Merced River floodplain would be reduced. Existing utility river crossings would be reduced from 13 to three consolidated crossings. Two consolidated utility river crossings would replace existing utility crossings at Clark’s Bridge and Ahwahnee Bridge. One new consolidated utility river crossing would be constructed between Housekeeping Camp and Camp 6. The proposed plan would remove all Tenaya Creek utility crossings and remove one utility crossing from Yosemite Creek.

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).

Justification for Use of the Floodplain

Yosemite Valley is the most visited area in Yosemite National Park. The Yosemite Valley Plan provides a framework for managing natural and cultural resources, park facilities, and visitor experiences in the Valley. The utility infrastructure within Yosemite Valley must be designed to efficiently serve the areas identified for redevelopment and new development under the Yosemite Valley Plan, while minimizing impacts on highly valued natural and cultural resources. The East Yosemite Valley Utilities Improvement Plan provides for the development of consolidated utility corridors primarily located in environmentally preferable areas and the reduction of utility infrastructure within the floodplain and other environmentally sensitive areas. The utility facilities proposed in the East Yosemite Valley Utilities Improvement Plan are designed to be submersible and will be placed underground to minimize the potential for damage to these facilities from flood events.

Description of Site-Specific Flood Risk

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.

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. In particular, the utility facilities proposed in the East Yosemite Valley Utilities Improvement Plan are designed to be submerged and will be placed underground to minimize the potential for flood damage.

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 Valley’s natural and cultural resources will be minimized and mitigated.

Conclusion

The Preferred Alternative would substantially reduce potentially hazardous conditions associated with flooding by reducing the utility infrastructure within the floodplain in Yosemite Valley. In addition, the proposed utilities that would remain or be constructed within the floodplain are designed to be submersible and will be constructed underground to reduce the potential for flood damage from future flood events.

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.