The Tidal Wetland Project hosted a pair of trips aboard the Elkhorn Slough Safari to visit the Parsons Slough Sill during high energy conditions. The purpose of the trip was to answer questions and hear the impression viewing the sill “in action” made on viewers.
Participants were asked:
- Does the sill look the way you expected?
- Having seen the sill in action, are there new questions that come to mind?
- Does a sill look like a strategy that is suitable for the mouth, at the Highway 1 Bridge?
Notes from the trip
The notes include individual comments and an overview of what participants said in response to these questions:
Major Themes of Participants’ Comments
On the Parsons Slough Sill
- The turbulence across the sill was greater than expected.
- The sill appears to be working: holding back the tidal flow to reduce current speeds.
- The jet of water flowing from the sill across the channel appears likely to erode the opposite bank, which was surprising to many participants.
- The quantity of turbulence appears unnatural and is not aesthetically pleasing.
- The quantity of turbulence may attract recreational thrill seekers.
- The turbulence makes the area near the sill unsafe for boating.
- Many people were curious about whether impacts to fish and wildlife occurred given the turbulence and the size of the structure. [Biological monitoring has found no impacts to date, but is only sufficiently powerful to detect large impacts on this short time scale.-Nina D’Amore]
- Long term monitoring of the sill will be important to determine whether it provides the anticipated benefits and/or results in impacts. This monitoring will provide valuable information to inform the decision whether to pursue a sill at the Highway 1 Bridge.
On the concept of a Sill at Highway 1
- A sill at the Highway 1 Bridge would reduce recreational access to the slough and would adversely affect recreation-related businesses.
- A sill at the Highway 1 Bridge could impact navigation in Moss Landing Harbor.
- A sill at the Highway 1 Bridge would have significant impacts.
- A sill at the Highway 1 Bridge would provide significant benefits
- The benefits of a sill at the Highway 1 Bridge would be less obvious than the impacts.
- Tidal electrical power generation would be one approach to reduce tidal energy.
Responses to Frequent Questions
- The top of the sill is two feet under water at a regular low tide, except in the middle where it is five feet under water.
- The flow across the channel and its effect on the opposite bank was anticipated, but was predicted to not be significant.
- Ongoing monitoring has found evidence of erosion of the bank opposite the sill, however, erosion in that area has been high for many years, and we don’t have sufficient pre-project data to determine whether this erosion is an increase over the baseline condition at this location. More time is needed to make that determination. If the erosion appears to damage the marsh there, action may be taken to reduce that erosion, while being careful not to rely excessively on structural approaches. For example, bank armor would likely be avoided.
- Ongoing monitoring has found no evidence of impacts from the sill on marine mammals, fish or birds.
- We conducted a feasibility assessment which estimated that a sill at the Highway 1 Bridge would take about 20 years to plan, permit and construct. It would involve extensive collaboration with the Army Corps of Engineers, and the appropriation of funds by the US Congress.