Skip to main content

Sitka Sedge Hydrology

Member of the Tierra del Mar community?

Submit feedback on the dike options by February 21, 2020, via our online survey at https://www.surveymonkey.com/r/sitkadike

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, has completed study of the hydrology of the park and the adjacent Tierra del Mar community.  This study assessed whether dike modification options that span the range of potential tidal flow into Beltz Marsh could allow fish passage and habitat improvement without increasing flood risk or elevated groundwater levels in Tierra del Mar (TDM). OPRD hired Pacific Groundwater Group (PGG) and Environmental Science Associates (ESA) to model and compare surface-water and groundwater levels during both average conditions and during extreme tides and precipitation events according to different dike configuration scenarios. PGG and ESA analysis is now complete and indicates that TDM groundwater effects from dike alteration range from insignificant to beneficial depending on the dike alteration alternative.  Surface water characteristics are more complex and will be subject to continuing discussion as the project moves to a broader scoping of alternatives.  

After completing the hydrological analysis, OPRD convened a team of subject-matter experts and stakeholder representatives to look into dike alteration alternatives more closely and assess a range of potential effects in terms of feasibility, water quality, ecology, biology, finance, permitting, community, engineering, recreation, etc.  The team was selected to have the collective expertise and representation necessary to be able to explore the alternatives, understand potential effects, and help OPRD to make a well-informed decision.   The team met four times in the fall of 2019 to hear presentations from subject matter experts weighing differential effects across the alternatives.  Each team member scored and ranked alternatives and their effects.  Upon completion of the Technical Team’s evaluation, OPRD staff processed Technical Team scoring, assessed the pros and cons of the various alternatives, and summarized key criteria for decision making.  An OPRD decision-making team is scheduled to review the findings and make a decision in March, 2020.

Videos

Sitka Sedge hydrological risk assessment -- average tides and precipitation

Preliminary scenario animation video including an 8ft modern tide gate closure setting and setback dike for average tides and precipitation.

Sitka Sedge hydrological risk assessment - 100-year storm event

Preliminary scenario animation video including an 8ft modern tide gate closure setting and setback dike for a 100-year storm event, and extreme tides.

Your browser is out-of-date! It has known security flaws and may not display all features of this and other websites. Learn how

×