top of page

2022 Design Options

Hand-placed LWM structures (2022 design option 2)

This option would provide the least sediment capture and erosion control and has the shortest lifecycle. This option includes approximately 17 in-stream LWM structures throughout the project area. Due to the smaller nature of the hand-placed LWM structures, less sediment would be captured in the canyon – SPU estimates only one-third of the sediment retention of machine placed structures. Timber frames may be included but would be redesigned and smaller so they could be anchored by hand rather than by machine. This option would require approximately 2 months of construction per year, and 2 to 3 years to complete.

​

A hand-placed option would require more regular maintenance because the LWM structures would be smaller and at risk for movement due to high creek flow and landslides. Maintenance could include replenishment of wood or adjustment of logs within each LWM structure to maintain fish passages, and the need to add more LWM structures to continue capturing sediment. Hand placing would not require machinery or an access road, except for a small path from Holyoke Way S to the creek for material import. We anticipate no trees would need to be removed for this option. We would not replace the current boardwalk with a permanent gravel trail because equipment and machinery would not be required for construction.

Benefits

  • Approximately 1000 cubic yards of sediment retained

  • Reduced connection to the surrounding floodplain

  • Moderate erosion mitigation

  • No access road or machinery required

  • No tree removal required

Impacts

  • Requires regular maintenance, causing interruptions in use of trails and more cost over time

  • No improvements to boardwalk

  • Longest construction timeline

  • No sewer improvements possible

Hybrid machine- and hand-placed LWM structures (2022 design option 3)

This option is a combination of machine- and hand-placed large woody material structures. It has fewer sediment retention benefits than entirely machine-placed, but retains more sediment than entirely hand-placed. Machine-placed LWM structures and a temporary access road would be placed in the lower half of the project area where there is the most significant erosion. Hand-placed LWM structures would be placed in the upper half of the project area. This option would require 4 months of construction per year, and 2 years to complete.

​

Impacts due to machine required access would be realized in the lower half of the canyon and risks present with the hand-placed LWM structures would apply in the upper half of the canyon. The hybrid option includes the ability to provide some additional sewer access points (e.g. maintenance hole or clean outs) during road construction which would help SPU improve sewer maintenance. Elimination of the boardwalk and replacement with a trail may or may not be possible under this option, depending on the length of the temporary access road.

Benefits

  • Moderate sediment retention (less effective than machine placed, but more effective than hand placed)

  • Significantly reduced erosion in lower canyon area due to installation of larger machine-placed LWM structures; moderately reduced erosion in upper canyon area due to installation of smaller hand-placed LWM structures

  • Fewer trees would be removed than machine placed

  • Portions of the deteriorating boardwalk would be replaced with a gravel trail

  • Some sewer access improvements would be completed

Impacts

  • Requires machinery and road access in the lower canyon area

  • Some trees would be removed in the lower canyon area

  • Deteriorating boardwalk may not be replaced with a gravel trail

Machine-placed LWM structures (2022 design option 1)

Option 1 is no longer being considered due to community feedback in response to the undesired effects of the proposed temporary access road in this design option.

 

This option would provide the most robust sediment management and long-term benefits to the creek and canyon. 23 in-stream LWM structures and 102 timber frames would be installed to trap sediment coming down the canyon and prevent erosion by reinforcing the stream banks. Typically,4 timber frames are associated with each large woody material structure, with 2 on each side. This option would require approximately 4 months of construction per year over 2 years.

Benefits

  • Approximately 3200 cubic yards of sediment retained (most effective option)

  • Increased sediment depth (~5 feet) along canyon floor, raising the creek bed by 5 feet

  • Reconnecting the creek with the surrounding floodplain, leading to a healthier ecosystem, reduced flooding, and increased habitat

  • Reducing the channel slope over time, thereby slowing the speed of the creek and reducing erosion

  • Structures are designed with low-flow fish passages at all stages of the structure lifecycle

  • Reduced erosion resulting from bank stabilization and increased sediment retention in the canyon

  • Replacement of the deteriorating boardwalk along the west canyon wall with a permanent fill wall and gravel trail

  • Improve SPU sewer maintenance by providing additional sewer access points (e.g. maintenance holes or clean-outs)

Impacts

  • Requires machinery to bring in and place the largest wood materials

  • Requires the construction of a temporary, 12-foot-wide access road along the existing trail that would be removed once work is complete. Impacts from the temporary access road include:

    • Removing approximately 104 trees over 6 inches diameter at breast height (DBH)

    • Removing an unknown number of trees under 6 inches diameter at breast height (DBH). We are currently surveying trees within the work area that are under 6-inches in diameter and will be able to share more information later this year.

bottom of page