Mimicking Mother Nature

Public Roads, Jan-Feb, 2006 by Megan Hall, Steve Moler

Prior to construction of the logjams, a risk assessment was conducted to estimate the maximum scour depths during a 100-year flood. Designing and building for a 100-year flood is the current engineering standard because building for more than that would entail great additional cost. The WSDOT design team calculated contraction scour, abutment scour, and local pier scour to estimate total scour for all the ELJ structures based on an equivalent bridge pier width. It was found that during a 100-year flood, scour could possibly extend 9 meters (30 feet) below the river bottom. Therefore, the WSDOT design team and contractor considered several alternatives to prevent undercutting, such as extending the ELJs deeper, constructing a scour apron of large rock beyond the perimeter of the ELJs, or extending a scour curtain of steel sheet piles beneath the perimeter of the ELJs to create a continuous rigid barrier that would prevent undercutting of the structure. The WSDOT designers chose the sheet pile scour curtain, deciding that it would create the least disturbance and highest level of protection for the cost.

The construction crew installed the scour curtain at each of the four midchannel logjams in the shape of a U, with the open ends downstream. To stabilize the scour curtain, the sheet pile extends below the depth of maximum expected scour and upward into the base of the ELJ structure. The curtain is three-sided and interlocking, so if the river undercuts the ELJs to expose the scour curtain, racked logs in the ELJs are expected to settle into the scour hole, thereby providing further protection to the scour curtain.

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The design team used one-dimensional (depth- and width-averaged flow) and two-dimensional (depth-averaged flow) numerical models to estimate the hydraulic conditions the ELJs would be subjected to during flood events. The models helped in determining critical design factors such as the location and size of the ELJs, pile size and depth, the size and number of logs, and backfill material. Modeling also was a key tool in evaluating the river's response to the proposed structures, particularly with regard to erosion and flood hazards. The ELJs were ultimately designed to handle flows exceeding a 100-year flood of 2,067 cubic meters (73,000 cubic feet) of water per second. "These logjams were designed to last a long, long time," says Abbe, the geomorphologist. "We engineered them with the same considerations and detail we would apply to designing a bridge with a design life of 75-100 years. They have to be built using sound engineering standards and factors of safety to protect human life and property."

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The design team conducted an extensive analysis of historical channel migration patterns to predict future changes in the river and the implications for the project. The analyses indicated that the river would continue to shift throughout its channel migration zone, so the ELJ structures were designed to accommodate changes in the river's location, including a 180-degree change in the channel approach. The designers also estimated the sediment transport likely to result from the proposed design, particularly the new diversion channel and its potential downstream effects.