Skykomish River Assessment and Alternatives

PURPOSE: to review landowner comments on ways to protect their properties from increasing river migration and related bank erosion

The “Reach” refers to the river miles 10 to 13

A revetment is channel bank armoring with wood and/or rock.

Snohomish County Public Works January 8, 2017 We heard your comments and want to work together with you. We suggest creating solutions that are:

• LONG-TERM: effectively address the problem over the next 20 to 50 years as opposed to short term of 5 to 10 years

• REACH-SCALE: withstand shifting channels in a constantly building & changing river landscape

• FUNDABLE: provide a practical solution that can be permitted and funded at state and federal levels Skykomish River Assessment and Alternatives: Landowner Comments THE CHALLENGE: Increased sedimentation caused by decreasing river slope and widening of the valley (EXAMPLE: at river mile 15 and 22) A CHANNEL MIGRATION ZONE reflects the activity and locations of past channel migration and also depicts the potential area that the channels could occupy in the future Varying bank conditions within the Reach provide different degrees of RESISTANCE TO EROSION and include vegetated low banks reinforced by the roots of woody plants, historic riprap revetments, bedrock, and log jams A COMBINATION OF STRUCTURES with multiple functions would have a higher degree of success than revetment structures alone in reaches prone to lateral channel migration LARGE WOOD REVETMENT – provides direct bank protection ENGINEERED LOG JAMS – can slow the rate of channel migration FLOODPLAIN FENCES – reduce avulsion risk and overland flows A CONSTRUCTED SIDE CHANNEL would decrease the erosion in the existing side channel and provide a buffer along the field REPLACING COMPROMISED RIPRAP BANK with a large wood revetment would help slow the water. A COMBINATION OF STRUCTURES could reduce the amount of bed load sediment entering the side channel, reduce bank erosion locally, and address lateral channel migration CONCLUSIONS: • Rock and/or wood revetments alone of will not address the causes of bank erosions • A combination of structures could be used to address local bank erosion and accommodate lateral channel migration • Solutions should be long-term to avoid similar problems in the future • Solutions should be balanced throughout the Reach: • to slow the channel migrations rates, and • avoid increasing the erosion of adjacent properties • A practical solution would need to be permitted and funded at the state and federal level to be feasible