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Sitka Sedge State Natural Area Hydrology Modeling
The Park

Sitka Sedge State Natural Area is a 357-acre park in Tillamook County.  This ecologically diverse park contains ocean beach, dunes, forest, tidal marsh, freshwater marsh, and mudflats, and supports an array of important and rare plants and wildlife. 

The Dike
The park contains a half-mile dike and tide gate that was installed in the 1930s.  The purpose of the dike was to hold back tidewater and drain the area behind the dike.  The dike has also had the effect of holding back native fish — including coho, chum, steelhead and coastal cutthroat trout — from migrating upstream to historic spawning and rearing areas. In the past 30 years, the tide gate has deteriorated significantly.  It is now leaky, and no longer completely blocks incoming tides – it just slows their entry and functions like an hour glass for tide water… tide water comes in slowly, reaches a lower total tide height inside the dike than outside, and then is released slowly.  The tide gate will continue to deteriorate, and will eventually fail if not repaired or replaced.  Any repair or replacement options will ultimately need to address fish passage concerns. The potential impacts of tide gate failure or dike modification on flooding and groundwater in the adjacent community of Tierra del Mar (which is behind the dike) is a topic of concern.

The Project
Oregon Parks and Recreation Department (OPRD), along with stakeholders and partners, is studying the hydrology of the park and the adjacent Tierra del Mar community to determine dike modification options that would allow fish passage and habitat improvement without increasing flood risk to Tierra del Mar (TDM).  OPRD hired Pacific Groundwater Group (PGG) and Environmental Science Associates (ESA) to create a model that predicts and compares groundwater levels during both average conditions and during extreme tides and precipitation events according to different dike configuration scenarios.  Over the past year and a half, PGG and ESA have been collecting and analyzing data, and have now completed development of their model for predicting how dike configuration scenarios affect hydrology at the site.  
The consultants will present tidal surface water and TDM groundwater predictions for three initial dike scenarios at a public meeting in February 2019. If more information is needed, the consultant team, OPRD, and stakeholders may collaboratively develop additional scenarios. Ultimately, OPRD will make a decision based on available information and stakeholder input. On-the-ground work would not start until the 2021-22 biennium.

Current Updates
Pacific Groundwater Group (PGG) and Environmental Science Associates (ESA) completed work on initial surface and groundwater modeling in January 2019.  Their full technical report and an abridged version are available for download below.  A public meeting was held on February 27 from 2:00 to 4:00 PM at the Kiawanda Community Center in Pacific City, Oregon.  This public meeting provided an opportunity for stakeholders to view a presentation on the modeling process and results, to ask questions regarding the study specifics, and to provide input toward further modeling scenarios.   OPRD collected comments and suggestions at the meeting and requested that additional public comments and modeling scenario be sent to OPRD by March 15 to start the next phase of modeling and refinement work.  OPRD, PGG, and ESA collectively worked to address the comments received and developed study refinement tasks in April.  PGG and ESA began work on new tasks resulting from the meeting and public comment in late April.  A document  that summarizes comments and suggestions received along with corresponding joint OPRD/PGG/ESA response and course of action for each comment is available for download below.

PGG and ESA are projected to have completed the final phase of modeling tasks in early May, and will have completed additions and edits to the study report by June.  OPRD hopes to hold a final public meeting to present the final results in late June in the Pacific City/ Hebo/Tillamook area.  OPRD will create a survey to find a date for the final meeting in early May.  A survey link will be sent to stakeholders for whom OPRD has email information.  To add your contact information to the survey list, please email noel.bacheller@oregon.gov and request that your email address be added.

Summary of results from initial, modeling scenarios:

Current results from the groundwater portion of the study predict marsh surface-water and Tierra Del Mar groundwater levels under a range of hydrologic conditions chosen for their potential to cause high groundwater levels beneath TDM, including average winter conditions and combined significant winter storm and king-tide events. These hydrologic conditions were applied to three initial dike configurations: 1) the existing, malfunctioning tide gate; 2) replacing the existing tide gate with a modern muted tide gate that meets fish passage regulation requirements; and, 3) a dike breach sized to pass enough tidal water so that water-surface elevations in Beltz Marsh would match water-surface elevations outside the dike in Sand Lake.  The goal of running these initial model scenarios was to use them to assess extreme effects and then re-engage stakeholders in potentially refining options for future scenarios.  The predicted effects of these initial scenarios are briefly summarized below:

  • Existing tide gate: Groundwater and surface water unchanged.  Dike continues to back up stormwater during significant storm events, with associated existing effects on groundwater.
  • Breached dike: Under storm conditions, groundwater levels beneath Tierra del Mar decrease compared to existing tide gate because the breach eliminates the current tidegate’s prolonged retention of stormwater due to restricted outflow.  Under average winter conditions there is an insignificant groundwater increase (¼ inch), and only in areas very close to Beltz Marsh.  There would be increased frequency of inundation of upper elevations of the marsh, but decreased duration of inundation during storms.
  • Modern Tide Gate: Under storm conditions, groundwater level beneath Tierra del Mar would be decreased compared to the existing tide gate due to improved elimination of stormwater accumulation behind the dike.  Under average winter precipitation and tide conditions, there is an insignificant reduction (1/8 inch) of groundwater levels beneath TDM compared to the existing condition.  Frequency of surface-water inundation of upper elevations of Beltz Marsh would be decreased compared to existing condition due to more effective blocking of tides over 7.0 feet inside the dike.

More detailed information on the hydrological study work completed to date can be found in the form of a short executive summary document, the abridged, non-technical version of the project report, and the full technical report – all of which can be downloaded using the links below.  Additionally there is a link below for a comparative video simulation of surface-water patterns in Beltz Marsh predicted by the study’s surface-water model.

Draft Timeline for Beltz Dike Assessments, Options Scoping, and Implementation

February – August 2019:
Hydrology study conclusions and public information meetings 
Development of preferred alternatives
Public meeting to review preferred alternatives

August 2019 – January 2020:
Complete habitat assessments of the site – create an Aquatic Habitat Enhancement Plan
Host a public meeting to present our final preferred alternative

January 2020 – September 2020:
Develop preliminary Habitat Restoration Project Design and funding strategy for Beltz Marsh, Reneke Creek and Beltz Creek and adjacent areas.

October 2020 – October 2021
Secure funding and develop construction specifications
Project construction

Current reports and materials for reviewExecutive Summary (2 pages, 200 KB PDF)

Abridged, non-technical version of the 1/31/19 Project Report with selected figures (24 pages, 3MB)

Full technical version of the 1/31/19 Project Report (50 pages, 600KB)
     Report Figures and Tables (75 pages, 18MB)
     Report Appendices (64 pages, 20MB)

Surface water model: scenario animation video for average tides and precipitation (3MB)

Surface water model: scenario animation video for 50-year storm and king tides (4.5MB)

Meeting 2 ESA Surface water presentation Feb 27 (1 MB PDF)

Meeting 2 PGG Groundwater presentation Feb 27 (2 MB PDF)

Meeting 2 OPRD Intro and Wrap up presentation Feb 27 (PDF)

Sitka Sedge SNA Hydrology Study Meeting 2 QA Notes Feb 27 (PDF)

Response to public comments and modeling recommendations for further hydrological modeling Apr 30 (PDF)

Older MaterialsBeltz hydrology-OPRD introductory powerpoint (PDF)

PGG/ESA powerpoint (PDF)

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