Importance: Index-Galena Road is the only year-round arterial that provides access for property owners with land holdings in the North Fork Skykomish River valley upstream from the town of Index, maintains vehicular access for emergency service providers to these properties, maintains public recreational access to the North Fork Skykomish River valley, and maintains administrative access for the U.S. Forest Service to manage their lands located in the Mt Baker Snoqualmie National Forest (MBSNF). Index-Galena Roads functionality is dependent upon three identified infrastructure components: 1) Howard Creek Bridge at MP 9.0; 2) repair of the washout between MP 6.4-6.9 and; 3) Trout Creek Bridge #494 at MP 6.045. Of these, Howard Creek Bridge was replaced and open to traffic in 2017. Washout repair will be undertaken in 2019-2021. USFS and Snohomish County are pursuing FLAP funds for the Trout Creek Bridge #494 replacement to complete the series of improvements required to ensure transportation continuity and resiliency along its entire length for all MBSNF users. Currently, the alternate access upstream from Trout Creek Bridge is an approximate 40 mile detour route to arrive at the upper end of the Index-Galena Road washout at MP 6.9. The detour is typically available only seasonally for five months (usually late May to early November) and provides the sole vehicular access for property owners, emergency service providers, recreational users to camp sites and trailheads, and U.S. Forest Service personnel.
Deficiencies: This three span bridge built in 1946 is functionally obsolete and scour critical. Its sufficiency rating is 46.14 out of 100. The two in stream piers are scour critical with a WSBID scour code of 3. The piers are supported by spread footings on the creek bed boulders at an unknown depth. This bridge requires yearly inspection to monitor the scour. A scour hole under the east pier was grouted in 2013. We expect scour holes to develop again as the creek continues to evolve. The bridge width (19.3 feet) is narrower than the width of the road approaching it at either end. The bridge is posted for load limitations. The west bridge approach exceeds 5% vertical grade.
Proposed Improvements: The proposed improvements will replace the existing 120 foot long three span bridge with a new longer single span bridge. Providing a single span bridge will allow us to remove the two scour critical piers constraining Trout Creek. A hydraulic study is required to determine the recommended hydraulic opening and bridge length. Approximately 100 feet of bridge approach on each side of the bridge will be rebuilt to reduce road grades and a vertical crest on the existing bridge. The proposed bridge will be 28 feet wide between rails matching other bridges along Index-Galena Road. The proposed bridge rail will be MASH or NCHRP 350 test level 2 or better. The bridge rail will be transitioned to Type 31 guardrail. The proposed bridge will be designed to current AASHTO standards including the HL-93 design live load.