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General Information

"Snow Dog Creek" Trail Bridge is a mountain biking and hiking bridge located on trail G4, 2.8 miles from Dogsled trailhead. The bridge is at longitude 90˚54'10"E, latitude 56˚34'05"N. The bridge crosses Snow Dog Creek and is a small, 25-foot-wide stream, which has fish spawning at the site. See the attached quad map with location marked in folder.

Site reconnaissance was done on June 25 by John Barger, Scott Groenier, and Rod Dell'andrea. Additional sites were reviewed upstream and downstream and no better locations were found within 300 yards. No ice damage was seen, but medium size debris was observed at the site. A small seep occurs just downstream of the structure on the right side facing downstream.

The project is scheduled for construction during the summer of 2005 by contract. The environmental assessment was completed in 2002 and a record of decision was made in May of 2003. Aerial infrared photos and photographs of the site are in the design folder.

Survey

The bridge was surveyed on July 26, 2004, by the Chugach Survey Crew, consisting of Zack Smith and Pete Doe. Two control points and a temporary benchmark were set for the survey. The designfolder contains copies of the survey notes, photographs, and a topographical survey map of the site. The survey points, established by a hand-held geographical positioning system unit, use the Universal Transverse Mercator (UTM) coordinates.

Hydraulics and Hydrology

The stream is classified as a Type B3 stream with gravel bottom and sides. Streamflow estimates were made using the regression equations for Alaska from the U.S. Geological Survey publication "Estimating Magnitude of Peak Flow for Alaska." The Q2 is estimated at 87 cubic feet per second and the Q100 is estimated to be 400 cubic feet per second.

A hydraulic survey of the stream was completed on July 26, 2004, by Bill Macfarlane and John Barger. The bankfull width is about 26 feet and the mean depth is about 0.8 feet. The velocity was 2.3 feet per second when the stream was running less than bankfull. The Mannings Number (0.38) was backcalculated from the survey. The slope of the stream is 0.5 percent obtained from water slope over 300 feet. High water marks were flagged in the field and included in the survey.

The stream was modeled using WinXSPRO. The Q100 elevation is estimated at 1.9 feet. The minimum freeboard should be 3 feet based on wood debris in the watershed.

The bridge is recommended to be placed on stable ground above elevation 96.00 and 3 feet away from the top of the bank.

Geotechnical Investigation

John Barger completed the soil investigation on July 28, 2004, using a Williamson Probe. The soil is 2 feet of organic material over glacial deposited cobble and gravel. The blow counts were 30 blows per foot into the cobble and gravel.

An allowable strength of 2,500 pounds per square foot should be used for this site following International Building Code recommendations for this type of soil. The recommended foundation is a spread footing placed down to frost depth.

Superstructure Design

The required bridge length is 30 feet center of bearing to center of bearing. Bridge width inside of rail to inside of rail should be 4 feet for hiking. The railing should be 42 inches high. The three rails should have a maximum opening of 15 inches.

Design loads for the structure will be 85 pounds per square foot for pedestrian load, 300 pounds per square foot for snow loads, and 90 miles per hour for wind loads.

The bridge will be a prefabricated structure and will be required to be stamped by a professional engineer. Plans and calculations will be added to the design folder when they become available.

Permits

The bridge will be outside of the flood plain. Snow Dog Creek is not a navigable water, so no permits are required for this structure. A set of plans will be sent to the U.S. Army Corps of Engineers to verify this determination.

Cost Estimate

The bridge superstructure is estimated at $25,000. The bridge will have to be flown to the site by helicopter. The site is remote. A base camp at the bridge site will be needed for construction. The total cost of the bridge superstructure and installation is estimated at $60,000.

Alternative Locations

No other sites were considered for this bridge.

Special Requirements

No instream construction will be allowed from June 15 through October 1 because fish spawn at the bridge site.