Water and Natural Resources Team Wraps Up Derry Dell Creek Restoration
September, 2020
Otak recently completed its work on the rehabilitation of the East Fork Derry Dell Creek in Tigard, Oregon. The City of Tigard hired Otak under its on-call contract.
Project manager and principal in charge Kevin Timmins, was joined by Scott Banker who is certified in erosion and sediment control, and Nick Cook, PhD, who provided engineering design for the project.
The creek, on private property, suffered from increased urban runoff that incised the channel and was causing severe erosion along the creek banks and scour around an existing stormwater outfall, so much so that the trunkline sanitary sewer and several laterals had been exposed. Otak was hired to reconstruct about 900 feet of the stream channel, rebury the sewer line, and restore the floodplain connectivity. 
Design and permitting for the project started in April 2018 and permits were received in October 2019. To protect fish species construction had to wait for the next state-approved in-water work window of Summer 2020. Construction began mid-summer and was completed in just four weeks.
The old stream channel had reached depths up to six feet due to the excessive erosion so it was filled in and a gravel streambed was built at the upstream end. The stream channel was relocated at the downstream end to what the engineers believe was the original location of the stream. A mix of rock was used in the new channel to resist erosion and the channel was built with variations to mimic a real streambed and create different habitat conditions. Three trees that had fallen across the creek were cut and reused in the floodplain and stream to provide additional habitat enhancements and channel stability.
The team specified the use of Beaver Dam Analogs (BDA), designed under the same concept as naturally occurring beaver dams, as a cost-effective and efficient means to slow the water and collect sediment to build back the stream bed elevation so it can better access the floodplain.
Otak’s expert water and natural resources team left the stream in a healthier condition while stabilizing the adjacent slopes and protecting the sanitary sewer.
With the stream completed, the City of Tigard is moving forward this coming fall with revegetation of the adjacent stream corridor and wetland areas.