COTTONWOOD DRIVE STORMWATER IMPROVEMENT PROJECT
Contact Kraig Olason, Senior Planner at Whatcom County Public Works with questions at (360) 778-6301.
Regular flooding occurs throughout the wet season. The existing drainage inlet into the marine outfall at Cottonwood Drive is easily clogged with debris. Flooding occurs when flow entering the outfall is restricted during large rainfall events. Drainage is poor between Birch Bay Drive and the escarpment located just water-ward of Morgan Drive. Floodwater collects in this area and sits for much of the winter. Frequent flooding from the poorly-designed inlet creates ponding and hardship for some landowners.
The following elements were considered when developing the proposed project:
- Minimize flooding in the Cottonwood South Neighborhood by improving inlet performance
- Realign current stream channel to limit bank erosion and provide improved entry into the inlet
- Acquire drainage easement and improve access to new inlet for maintenance needs
The current inlet alignment is not optimal for efficient flow, and erosion of the east bank is undermining the steep slope and overlying vegetation. A new stormwater inlet will be installed just upstream of the current inlet, which includes an emergency overflow. Drainage water will be conveyed into the outfall pipe if the main inlet becomes plugged or congested. Additional improvements include re-aligning the stream and providing rock armoring on the east bank to improve its stability. Native plantings will be installed following the construction project and maintained by Whatcom County until fully established.
Project is complete! A trash rack and native vegetation, including snowberry, salmonberry and pacific nine bark were installed at the end of October. See below for a photo of the finished site with a fresh dusting of snow.
All of the heavy lifting is over and the Cottonwood Drive Stormwater Improvements project is nearly complete. A berm with a spillway was built around the overflow structure and a quarry spall was placed on and around the berm for added protection during high flow events. The hillslope adjacent to the stream was seeded and covered with a biodegradable erosion control blanket. Native plants will also be installed in this area in the near future. Rock was placed along the east bank and coir log on the west bank to armor against scour and prevent erosion. The final touches included a split rail fence along a portion of Cottonwood Dr., fresh asphalt and landscape cobbles to tie it all together.
Two weeks down and about a week left on the Cottonwood Drive Stormwater Improvements project. Last week, we focused on establishing the new, wider stream channel. This week we removed a section of the old outlet pipe and put the new structures in place, including a new inlet pipe, catch basin and outlet pipe that connected to the existing storm main. The installation went smoothly and Iverson Earth Works, LLC worked with care as they moved the large concrete structures into place. You may notice the material covering the slopes in some of the pictures; this was in place to protect the exposed soil from erosion during the rain we had on Tuesday. Once the new structures were installed, we began backfilling and compacting the material around the pipes. Next week, we will focus on building a berm around the overflow structure, seeding the hillslope and cleaning up the surrounding facilities. Stay tuned for more progress pictures!
The Cottonwood Drive Stormwater Improvements project is underway and on schedule. Iverson Earthworks, LLC has been hard at work excavating the area for the new stream channel, which meets up with the existing channel, and grading the surrounding slopes. Geotextile fabric was placed within the channel and topped with streambed gravel. An area was excavated and prepared for the placement of the inlet pipe and catch basin structure. Next week, we will be placing the catch basin and inlet pipe. The next step will be to remove a section of the existing pipe and add a new section for the outlet that will tie into the catch basin.
Project update postcard sent to nearby residents in July 2017