Cost-Benefit Model Evaluation – Mud Bay Bridge, Puget Sound, Washington

The Washington State Department of Transportation (WSDOT) tested a National Cooperative Highway Research Program (NCHRP) cost-benefit model by evaluating adaptation options for replacing Mud Bay Bridge on SR101 in Olympia. The bridge serves the community as a major corridor through Olympia and provides access to Interstate-5, the main north-south freeway in Puget Sound. Depending on the rate of sea-level rise in the region under various climate change scenarios, it is anticipated that Mud Bay Bridge will become inundated before 2100. Currently, at high tide, water levels reach within three feet of the bridge deck and on the south side of the bridge.

The benefit-cost analysis (BCA) compared the cost of replacing the bridge without adaptation measures, versus with adaptation measures involving raising the height of the bridge. The analysis assumed a maximum potential sea-level rise of just over two feet by 2050, meaning that the bridge could require replacement around that time due to salt water corrosion that would occur at mean higher high water (the average of the high water heights observed over each tidal day). The cost of replacing the bridge without adaptation was estimated at $3.2 million, while the cost to replace with adaptation was estimated at $3.8 million. Thus, the adaptation measures were anticipated to cost an additional $0.6 million primarily from the costs of additional materials and construction requirements for augmenting pier walls and abutments. Two feet of sea-level rise was at the higher end of the projections for the period ending in 2050, so the BCA assumed a low probability of bridge failure in final six years of the period (years 2045 – 2050).

Despite the higher cost of replacing the bridge with adaptation, the benefit to cost ratio (b/c ratio) was still greater than 1.  This was primarily due to traffic impacts in the event of bridge failure. If the bridge were to fail prior to replacement, drivers would have to use an expected detour of over sixty miles because nearby alternative routes and bridges would likely be similarly affected by sea level rise.  Under this scenario, the discounted benefit for a preemptive bridge replacement with adaptation measures was calculated to be $518 million because the alternative, repair after bridge failure, would entail very high user costs for riders from the extensive detours.  However, the more likely scenario would be a phased bridge repair/replacement towards the end of the bridge’s life cycle, which would also be occurring as sea levels rise but before they reach the two-foot projected levels that would require immediate replacement.  Under this scenario, the b/c ratio for replacing the bridge with adaptation was ill found to be greater than 1.  The analysis included various model inputs and assumptions for 2050, such as annual average daily traffic, detour length and time, the time required to complete construction, construction and maintenance costs, resiliency, and other factors. The BCA model used a discount rate of 2.7 percent.

The model evaluation was part of a NCHRP project that involved testing a diagnostic framework intended to help transportation officials in assessing the costs and benefits of adaptation actions compared to no action. The study team that developed the framework was asked to test the diagnostic framework on WSDOT infrastructure. The assessment used climate change data assembled by the University of Washington Climate Impacts Group (CIG), global sea-level rise projections from Vermeer and Rahmstorf (2009), and climate change projections from global circulation models in the 2007 Fourth Assessment Report of the IPCC. The global projections were regionally downscaled for Washington and used low and moderate emissions scenarios. For the purpose of testing the diagnostic framework, the sea-level rise projections were the most relevant in looking at impacts on WSDOT infrastructure. WSDOT used CIG sea-level rise projections, which were generalized into two- and four-foot scenarios, and they looked at the cumulative effects of sea-level rise and more intense precipitation, runoff, and storm surge.  

From the Mud Bay Bridge case study, the researchers proposed revisions to the BCA model. The BCA model is designed to help planners answer the question of whether adapting an asset makes financial sense in the current year - and is therefore appropriately used in two circumstances: where an asset is at the end of its lifecycle and replacement is imminent, or where preemptive replacement may be appropriate given potential climate impacts. Researchers suggest, however, that the model could be repurposed to help planners with longer-term planning by determining the “threshold year” when it would make financial sense to adapt an asset. This would require adjusting the model to allow for consideration of future years (rather than the current year). To test the framework, WSDOT recommended focusing on the potential impact of sea-level rise on WSDOT infrastructure in Puget Sound, and more specifically using Mud Bay Bridge as a case study to evaluate the cost-benefit model because of its location and importance for travelers, as well as its potential to be impacted. The cost-benefit model evaluation demonstrated that such a tool can provide valuable information in making climate adaptation decisions about transportation infrastructure, even though methodologies may vary by state and by agency.


This Adaptation Clearinghouse entry was prepared with support from the Federal Highway Administration. This entry was last updated on January 30, 2015.



Publication Date: 2013

Related Organizations:

  • Washington State Department of Transportation


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Resource Types:

  • Best practice
  • Case study

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