Geomorphic Assessment and Restoration Alternatives Development for the Merced River in East Yosemite Valley, Yosemite National Park

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The National Park Service (NPS) and USGS jointly will investigate impacts to the Merced River due to infrastructure and historic channel widening in east Yosemite Valley with a goal of constructing river management alternatives. Past management actions, such as gravel mining, channel riprap, and the construction of bridges that do not accommodate flood flows, have had long-lasting impacts to the river in east Yosemite Valley. Previous peer-reviewed studies have identified substantial channel changes over time in this area including substantial channel widening. Current and future management actions, such as management of rafting, manipulation of large wood, and location and design of bridges (and approaches) potentially have impacts to the river. This work is intended to complement the broader survey of river condition by focusing on areas of immediate management concern.

Map of California showing outlines of Yosemite and Sequoia & Kings Canyon National Parks.

Map of California showing outlines of Yosemite and Sequoia & Kings Canyon National Parks. (Public domain.)

The objectives of the Yosemite National Park Flow Modeling Study (FMS) are to create a 2D+ flow model of the Merced River from the Happy Isles USGS gage to Sentinel Bridge. The flow model will then be used to evaluate several scenarios including removal or raising of bridges or other restoration actions, such as channel narrowing and bank reconstruction.

The location of the proposed FMS is the Merced River in Yosemite National Park, starting at the Happy Isles United States Geological Survey (USGS) gage and ending approximately 400m upstream of Sentinel Bridge. The reach of river includes seven bridges (six road bridges, 1 foot bridge) which are one of the main focuses of the flow model. 

The scope of work for the proposed study consists of four parts:

  1. Merging of existing topographic datasets to create a single floodplain – channel topographic dataset (includes merging of aerial LiDAR, terrestrial LiDAR and ground survey data). The existing topographic datasets will be provided by the National Park Service (NPS) to the Flow Modeling Team.
  2. Quality assessment of the topographic data, potentially including additional surveying of portions of the channel and/or resurveying control points as needed. Surveys will be performed using an RTK or total station (equipment provided by NPS).
  3. Creation of 2D+ flow model for the floodplain and channel of the , including bridges. The flow model proposed for use in this study is the USGS Multidimensional Surface Water Modeling System (MD-SWMS), which is a pre- and post-processing application for computationally modeling surface water hydraulics. As part of the model creation and calibration, some field work will be necessary including collection of water surface elevations and bed roughness estimates.
  4. Evaluation of scenarios using the created 2D+ flow model. The Flow Modeling Team will meet with NPS personnel to determine the scenarios used in the flow model. Likely scenarios will include modifications to existing bridges, and alterations of the bed and/or banks of the river.

It is expected that NPS will provide the topographic datasets to the Flow Modeling Team in a timely manner so that work may begin on the merging and quality assessments as soon as possible. NPS will provide ground survey equipment (RTK and/or total station), while USGS will provide equipment to measure water surface elevations and/or flow and channel characteristics. 

The Flow Modeling Team will meet with NPS personnel (2+ times) during the course of the study to review and set flow modeling scenarios. The Flow Modeling Team will also be expected to give presentations of modeling scenarios to NPS personnel to help guide the scenario selection process.