Engineering Solutions for the Creek

Watch our Engineering Solutions video!

What are Large Woody Material (LWM) structures?

When describing the science behind the project, we noted how large woody material (LWM) structures within and along the creek help trap sediment and curb erosion and flooding. Natural LWM are trees, branches, limbs and logs located within a creek or riverine corridor that have been left there over time by flooding, landslides, or simply dying and falling into the creek. Natural LWM plays a critical role in many Washington streams through its influence on aquatic habitat and stream geomorphic processes. In many forested streams, wood is a fundamental driver of fluvial geomorphology—the shape of the stream channel and how it changes over time. Human alteration to natural LWM in creeks like Taylor Creek has led to unintentional damage to the natural habit and structure of the creek. Human-placed LWM structures are engineered for a particular creek or riverine corridor to mimic the positive effects of natural LWM.

Above: Example Large Woody Material structures

How do Large Woody Material structures work?

LWM structures store sediment, resulting in a rise and widening of the creek bed, replicating natural conditions. As the large wood structures fill with coarse sediment, they create steps that reduce the overall slope of the streambed. The reduced slope and barriers of large wood slow the velocity of the water and reduce the erosive energy during storm events. This has

Above: LWM structures in profile.

Below: Cross-section of a floodplain.

compounding positive effects as the streambed begins to fill the channel and provides additional support for the creek banks, which also helps to reduce erosion.

When the streambed becomes a deep layer of sand and gravel, flowing water will widen the channel and connect more floodplain. A floodplain is a wider, flatter area next to the channel that receives water during times of high flow and serves as an important storage area for sediments.

Reducing flow velocity and erosive force will lead to the Taylor Creek system retaining more landslide materials. Each LWM structure is paired with a timber frame to provide initial stability until installed native plants are more established for long-term stability. This may address potential accelerated bank erosion and landslides.

Research on incised stream channels shows that altering the long-term rate of sediment movement to create a net accumulation in a canyon is possible, even if some extreme events will still cause sediment to be lost downstream

Above: Timber frames in creek bank.

along the way. We plan to monitor the structures to understand how continued landslides affect the sediment movement and how the structures fill and release sediment over time. In particular, when the structures are near capacity and become part of the new valley floor, we will be monitoring flow and sediment to study the amounts and the mixture of grain sizes of the sediment transported to the delta. At that stage, we expect the fine-grained sediment (sand) to continue to make it downstream to the mouth and delta where it will continue to provide valuable refuge and rearing habitat for juvenile salmon.

Additional Resources:

Washington State Department of Transportation’s guidance for designing Woody Material in the Riparian Zone describes LWM structures and their design in more depth.
Taylor Creek Restoration: Sediment Management Options Evaluation provides more details on SPU’s research on erosion in Dead Horse Canyon and sediment management strategies.

How are LWM structures installed?

There are two primary methods for installing the LWM structures. The method used would have a significant impact on the sediment retention capacity and effectiveness of the structures.

Machine placed: when machines are used to place the LWM structures, we can use larger logs and secure them better in the bank.
Hand placed: when human labor is used to place the LWM structures, there are limitations to the size of the logs that we can use and the depth at which we can secure them in the bank.

Please note that the method of installation is not the only factor to consider, but it has a high impact on the success of the project. How we deliver the machines, people, and materials into the creek for construction is a separate, significant project design issue that the project team is still working to solve.

How does capturing sediment affect the delta?

The delta at the mouth of Taylor Creek in Lake Washington has expanded over time as coarse sediment from the upper watershed in the canyon becomes thicker. While finer materials such as sand and silt are often dispersed during storms and via waves, coarser sediment is not easily moved and prevents these naturally occurring forces from reshaping or halting the growth of the delta.

1935

1993

2010

The installation of sediment retaining structures would facilitate most of the fine sediment to be transported downstream and deposited in the delta, while retaining coarser material. The fine sediment will help to improve and sustain salmon habitat, whereas the large sediment deposited in the delta blocks access to the lower creek where juvenile salmon can thrive as they grow before migrating to Puget Sound. We’ll talk more about how salmon use the lower creek and delta for habitat in the next section.