Executive Summary

Introduction

The historic Happy Isles Gauging Station Bridge (also known as the Happy Isles footbridge), which spans the Merced Wild and Scenic River, is a reinforced-concrete girder bridge that was originally constructed in 1921. The bridge sustained substantial damage due to natural catastrophic events in recent years. A massive rockfall and windblast in the Happy Isles area damaged the bridge in July 1996. A major flood in January 1997 caused extensive damage to human-made structures along the main stem of the Merced River, including the Happy Isles Gauging Station Bridge. The bridge remained in use until it was inspected in July 1997 by representatives of the Federal Highway Administration who recommended its closure. Based on these findings, the National Park Service closed the bridge for safety reasons in July 1997. Since that time, the bridge has continued to deteriorate and is currently showing signs of imminent failure, with a large sinkhole appearing on the river-left abutment. Due to the threat to public health and safety, the bridge needs to be removed before it collapses of its own accord. The river-right abutment of the bridge would be retained in order to protect the historic Happy Isles Gauging Station and streamflow gauge, which is operated by the United States Geological Survey. In the future, the National Park Service may relocate the Happy Isles Gauging Station. This action is described in Appendix A as a reasonably foreseeable future project.

The National Park Service plans to eventually replace the Happy Isles Gauging Station Bridge as a separate project, an action that was identified in the Yosemite Valley Plan.

Purpose and Need

The purpose of the Happy Isles Gauging Station Bridge Removal Project is to:

·         Protect visitor health and safety by eliminating the threat posed by the failing concrete bridge.

·         Protect park resources from localized flooding that could result from uncontrolled bridge collapse during a high water period.

·         Prevent the difficult and potentially dangerous removal of bridge debris from the river that would be required if the bridge collapsed.

·         Protect park infrastructure (e.g., the historic Happy Isles Gauging Station and Happy Isles Vehicle Bridge, as well as water supply lines) from possible damage due to uncontrolled bridge collapse by removing the bridge before it collapses of its own accord.

·         Protect Happy Isles Gauging Station from gauge calibration distortion caused by an uncontrolled bridge collapse.

·         Enhance the free-flowing condition of the Merced River.

The need for the proposed project arose as a result of the 1997 flood damage to the Happy Isles Gauging Station Bridge. Structural damage sustained during the flood, as well as subsequent deterioration and structural failure (including a large sinkhole on the river-left abutment), have resulted in the need to remove the failing bridge. The National Park Service intends to remove the failing structure, which has been closed to the public since 1997.

Relationship to Other Plans

The Yosemite National Park 1980 General Management Plan, Merced Wild and Scenic River Comprehensive Management Plan (Merced River Plan), and Yosemite Valley Plan are the guiding documents for the Happy Isles Gauging Station Bridge Removal Project, which is located within the Wild and Scenic River boundaries of the Merced River. The 1980 General Management Plan is the overall guiding document for planning in Yosemite National Park. In designating the Merced as a Wild and Scenic River, Congress authorized the National Park Service to prepare its management plan for the river by making appropriate revisions to the park’s 1980 General Management Plan (16 USC 1274[a][62]). The Merced River Plan, which is a programmatic plan that derives its authority from the Wild and Scenic Rivers Act, made certain revisions to the General Management Plan to further the protection of the Merced River. The Yosemite Valley Plan implements many of the Yosemite Valley provisions found in the 1980 General Management Plan, and modifies other provisions due to new and more current information. Actions proposed by this project are consistent with guidance set forth by the General Management Plan, the Merced Wild and Scenic River Comprehensive Management Plan, and the Yosemite Valley Plan.

Overview of the Alternatives and Environmental Assessment

The Happy Isles Gauging Station Bridge Removal Project Environmental Assessment presents and analyzes two alternatives. The National Park Service has identified Alternative 2, Controlled Demolition, as the Preferred Alternative. The alternatives are described briefly below and in detail in Chapter II, Alternatives.

Chapter III, Affected Environment, describes the setting and condition of the areas affected by the Happy Isles Gauging Station Bridge Removal Project. Chapter IV, Environmental Consequences, analyzes the environmental impacts associated with each of the alternatives. A summary of the environmental consequences is provided below.

Alternative 1: No Action

The No Action Alternative represents conditions and management practices as they currently exist for the Happy Isles Gauging Station Bridge. It provides the basis for comparison of each action alternative.

Under the No Action Alternative, Happy Isles Gauging Station Bridge would continue to degrade and would eventually fail. Bridge collapse would likely occur during high flow conditions. Bridge-related debris would be deposited downstream, possibly damaging the historic Happy Isles Gauging Station and disrupting the validity of measurements from the gauging station pool. Deposition of bridge-related debris would also adversely affect natural, cultural, and scenic resources and recreation. Sudden collapse of the bridge could result in serious injuries and/or fatalities to recreational users of the river. The National Park Service would remove bridge debris from the river as soon as possible following bridge collapse, although the retrieval effort may be delayed by several months, since debris removal activities would need to occur during periods of low flow. Diverted river flows and erosion could result in adverse impacts to vegetation, soils, and cultural resources along the riverbanks. Over the long term, uncontrolled failure of the bridge largely would restore free flow of the Merced River at this location.

Alternative 2: Controlled Demolition

Alternative 2, Controlled Demolition, would partially remove the Happy Isles Gauging Station Bridge. The river-right abutment, which is located on a large boulder on the riverbank, would be retained to protect the historic Happy Isles Gauging Station. Alternative 2 would involve separating the bridge into liftable segments and removing the segments using equipment located on the riverbank. A temporary containment system (e.g., a reinforced tarp, netting, cage, or floating barge) would be installed beneath the Happy Isles Gauging Station Bridge to catch small amounts of debris during partial bridge removal. This containment system would prevent slurry from concrete saws, as well as small debris, from falling into the Merced River.  However, not all demolition debris would be prevented from falling into the river; masonry debris greater than 2 inches in any dimension and all metal debris would be removed from the river. A temporary structural support system (e.g., scaffolding, jacks, or mechanical lifts) may be installed to prevent uncontrolled collapse of the bridge structure during demolition or to anchor the containment system.

Removal of Happy Isles Gauging Station Bridge would have short-term demolition-related impacts on natural, cultural, and social resources. Because demolition would occur in a controlled manner (e.g., within a delineated work area, during low-flow conditions, with the application of Best Management Practices), Alternative 2 would avoid the more pronounced adverse effects of uncontrolled bridge failure and debris retrieval activities described under Alternative 1. Demolition-related impacts would be reduced by application of Best Management Practices and resource-specific mitigation measures (refer to Chapter II). Minor regrading and revegetation would increase bank integrity resulting in beneficial effects on soils, water quality, cultural resources, and biological resources. Similar to Alternative 1, controlled bridge removal largely would restore the free-flowing condition of the Merced River and return this portion of the river to a more natural condition, thereby enhancing its biological and hydrologic integrity. Alternative 2 would have a long-term, beneficial effect on natural and scenic resources because it would return a portion of the riverbank to a more natural state (benefiting wetland and aquatic resources), restore the active flood regime and fluvial processes, and improve views from the riverbank. Retention of the river-right abutment would have minimal impacts to river flow because the abutment is anchored on a large boulder that controls the majority of flow around the abutment.

Environmentally Preferable Alternative

The CEQ Regulations implementing the National Environmental Policy Act (NEPA) and the National Park Service NEPA guidelines require that “the alternative or alternatives which were considered to be environmentally preferable” be identified (CEQ Regulations, Section 1505.2). Environmentally preferable is defined as “the alternative that will promote the national environmental policy as expressed in the National Environmental Policy Act’s Section 101. Ordinarily, this means the alternative that causes the least damage to the biological and physical environment; it also means the alternative that best protects, preserves, and enhances historic, cultural, and natural resources” (“Forty Most Asked Questions Concerning Council on Environmental Quality’s [CEQ] National Environmental Policy Act Regulations,” 1981).

Section 101 of the National Environmental Policy Act states that “… it is the continuing responsibility of the Federal Government to … (1) fulfill the responsibilities of each generation as trustee of the environment for succeeding generations; (2) assure for all Americans safe, healthful, productive, and aesthetically and culturally pleasing surroundings; (3) attain the widest range of beneficial uses of the environment without degradation, risk to health or safety, or other undesirable and unintended consequences; (4) preserve important historic, cultural, and natural aspects of our national heritage, and maintain, wherever possible, an environment which supports diversity, and variety of individual choice; (5) achieve a balance between population and resource use which will permit high standards of living and a wide sharing of life’s amenities; and (6) enhance the quality of renewable resources and approach the maximum attainable recycling of depletable resources.” The environmentally preferable alternative for the Happy Isles Gauging Station Bridge Removal Project is based on these national environmental policy goals.

Alternative 1: No Action

The No Action Alternative represents conditions and management practices as they currently exist for the Happy Isles Gauging Station Bridge. The provision of productive, and aesthetically and culturally pleasing surroundings (provision 2 of the national environmental policy goals) would be adversely affected due to uncontrolled collapse of the bridge that would result in visually intrusive bank erosion and debris within the channel of the Merced River, and endanger the historic Happy Isles Gauging Station and other sensitive cultural resources in the vicinity. Alternative 1 would not fulfill provision 3 of the national environmental policy goals because risks to public health and safety would worsen under this alternative due to the uncontrolled failure of the bridge. Alternative 1 would not preserve natural resources as required under provision 4 of the national environmental policy goals. Eventual bridge failure would lead to sudden bank erosion that would affect soils, water quality, and biological resources such as vegetation and special-status species.

Alternative 2: Controlled Demolition

Alternative 2 includes controlled demolition and partial removal of the Happy Isles Gauging Station Bridge. Because demolition would occur in a controlled manner (e.g., within a delineated work area, during low-flow conditions, with the application of Best Management Practices), Alternative 2 would avoid the more pronounced adverse effects of uncontrolled bridge failure and debris retrieval activities described under Alternative 1. The application of mitigation measures described in Chapter II would further reduce the potential adverse impacts. The provision of aesthetically pleasing surroundings (provision 2 of the national environmental policy goals) would be improved because of the inclusion of site restoration. Alternative 2 would fulfill provision 3 of the national environmental policy goals by reducing risks to public health and safety through the controlled demolition of the bridge and application of mitigation measures to reduce hazards to visitors. Alternative 2 would preserve natural and cultural resources as required under provision 4 of the national environmental policy goals. This alternative would implement measures to reduce adverse effects related to demolition activities (e.g., Best Management Practices) and includes site restoration to increase site stability and biological integrity. Alternative 2 also would ensure protection of the historic Happy Isles Gauging Station and other sensitive cultural resources in the vicinity by removing the bridge in a controlled manner and avoiding the adverse effects of bank erosion that would occur under Alternative 1.

Environmentally Preferable Alternative

The environmentally preferable alternative is Alternative 2 because, of the alternatives considered in detail, it most fully satisfies the national environmental policy goals as stated in Section 101. Alternative 2 would (1) provide a high level of protection of natural and cultural resources while concurrently attaining the widest range of beneficial uses of the environment without degradation; (2) reduce risks to public health and safety; and (3) provide an aesthetically pleasing surrounding.