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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 collected and analyzed data, developed models for predicting how dike configuration scenarios affect hydrology at the site, presented their initial findings to stakeholders, and performed supplemental analyses to answer stakeholder questions and extend the model.  PGG and ESA analysis is now complete. 

The project will now transition from hydrological risk assessment to working with stakeholders in finer-scale assessment of goals, alternatives and potential effects – ultimately leading to a decision on the future of the site. 

Current Updates
Pacific Groundwater Group (PGG) and Environmental Science Associates (ESA) completed their analysis in May 2019.  Their full technical report, an abridged/non-technical report, and a 6-page executive summary are all available for download below. 

A public meeting will be held from 1:30 to 3:00 PM at the Kiawanda Community Center in Pacific City, Oregon on June 27, 2019.  This public meeting will provide an opportunity for stakeholders to view a presentation summarizing study findings and ask questions regarding the study specifics.  The meeting will also provide OPRD discussion of the steps to come in the process and timeline for determining a course of action.

Summary of results from primary modeling scenarios:

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.
Topics of supplemental analysis:

Based on consultant recommendations and stakeholder review, 7 items of supplemental analysis were undertaken after the presentation of initial findings:  
1.    Evaluation of the feasibility of constructing a setback dike located south of Reneke and Beltz Creeks that would allow for full tidal reconnection of the majority of Beltz Marsh while maintaining muted tides in the area of the marsh closest to TDM. 
2.    Characterization of the effect of surface water “backing up” upstream of the Sand Lake Road culvert in the East Marsh, or in the TDM east ditch along Sand Lake Road when water is high in Beltz Marsh
3.    Characterization of  the protective value of an overtopped dike for its ability to reduce peak water levels or delay rising water levels upstream of Beltz Dike to overtopping.
4.    Assessment of how inclusion (or exclusion) of inundation accumulated behind the beaver dam affects surface-water model results (i.e., if substantial additional storage volume could be achieved by removing the beaver dam). 5.    Surface-water and groundwater modeling for an 8-foot modern tide gate closure setting instead of the previously-modeled 7.0 foot setting
6.    Assessment of groundwater model sensitivity to hydraulic connectivity between the Shallow Aquifer beneath TDM and Beltz Marsh, achieved by increasing the local transmissivity of the Shallow Aquifer beneath Beltz Marsh and the hydraulic conductivity of the “skin” sediments on the marsh floor.  
7.    Assessment whether compaction of soils beneath Sand Lake Road is likely to affect the transmissivity of the underlying Shallow Aquifer.

These supplemental analyses are described in the reports and summaries for which links are provided below.  None of the analyses significantly change the initial findings. 

These supplemental analyses did, however, indicate that newly identified options of a setback dike and an 8.0 foot closure setting on a modern tide gate are feasible and would not result in higher surface water levels or larger groundwater effects than the previously assessed scenarios.

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

June 2019:
Hydrology study conclusions and public information meeting 

July 2019 – January 2020:
Stakeholder meetings and further investigation of site goals
Development of preferred alternatives
Public meeting to review preferred alternatives
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 review

June 2019 Executive Summary (6 pages, 300 KB PDF)

Abridged, non-technical version of the 6/5/19 Project Report with selected figures (32 pages, 4 MB)

Full technical version of the 6/5/19 Project Report (55 pages, 600KB)

Report Figures and Tables (78 pages, 21MB)

Report Appendices (92 pages, 21 MB)
    

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)


Older Materials

February 2019 Interim Executive Summary (2 pages, 200 KB PDF)

February 2019 Interim Abridged, non-technical version of the 1/31/19 Project Report with selected figures (24 pages, 3MB)

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)

Beltz hydrology-OPRD introductory powerpoint (PDF)

PGG/ESA powerpoint (PDF)


Please send any comments or questions on this material to
noel.bacheller@oregon.gov