Michigan Cost-Benefit Model Evaluation of M222 Slope Stabilization
The Michigan Department of Transportation (MDOT) tested a National Cooperative Highway Research Program (NCHRP) cost-benefit model to evaluate the cost-effectiveness of a roadside slope stabilization project in light of future temperature and precipitation projections. The road, which is located on a portion of route M-222 along the Kalamazoo River in the City of Allegan, has a slope that has already experienced erosion caused by intense precipitation and flooding. MDOT was in the process of stabilizing the slope using structural measures, rather than vegetative or other “soft” approaches. MDOT used the cost-benefit model to determine if the project was cost-effective given climate change projections for the state.
Michigan is projected to experience more intense precipitation and flooding under climate change scenarios, which in the future may lead to erosion and structural instability along roadside slopes in other areas of the state. Accordingly, MDOT decided to evaluate the cost-effectiveness of this project, which was already underway, because it is likely to be representative of future projects that may be warranted. For this study, the researchers developed and used more localized precipitation and temperature projections under climate change models to evaluate the cost-effectiveness of the slope stabilization project. Based on these projections and using the cost-benefit model, MDOT’s analysis determined that the project was a cost-effective adaptation measure for avoiding instability or failure of route M-222 under future conditions.
The cost-benefit model helps transportation agencies calculate the benefit-to-cost ratio of a proposed adaptation strategy for vulnerable infrastructure, based on the likelihood of infrastructure failure. The risk of infrastructure failure is determined by considering the probabilities of certain events occurring and of the infrastructure’s ability to withstand those events, both without and with adaptation measures implemented. In this study, MDOT evaluated the benefit-to-cost ratio of the slope stabilization project and the project’s effect on the probability of road failure under future scenarios with higher precipitation and peak river flow.
The slope involved in this study had already experienced movement and severe erosion, leading to over-steepened conditions, loss of vegetation, toppling of trees, and instability of a residence nearby. The slope is threatened by two precipitation-related risks that affect stability: erosion caused by excessive soil moisture and scouring of the base caused by high peak flows in the Kalamazoo River below. If allowed to continue, the erosion on the slope and at the base of the slope would eventually affect the stability of the M-222 roadway above.
To evaluate the cost-benefit model, the researchers selected 25-year and 100-year events for the month of April as proxies for events that could cause slope failure and resulting long-term road closure. Increases in these magnitude rainfall events were modeled out to 2050 using a 30-year historical period for baseline data. The estimated project cost for stabilizing the slope was $9 million. Compared to an estimated $16 million discounted benefit of stabilizing the slope and thereby avoiding road closure and resulting delays and detours, the benefit-to-cost ratio was determined to be 1.56 based on an average projection for a 25-year precipitation event.
Prior to calculating the benefit-cost ratio, the researchers used the SimCLIM tool to project temperature and precipitation changes out to 2050. Changes in temperature and precipitation will alter the timing and intensity of snowmelt, which is a major factor affecting flooding potential in the river. Projections developed included:
- Change in average precipitation from November through April, the months indicating the size of snowpack;
- Change in March and April precipitation individually, which primarily determine the peak river flow;
- Change in temperature for November through April, which affects total size of snowpack;
- Change in temperature for March and April individually, which determine timing of snowmelt;
- Change in intense precipitation (25-year and 100-year events) in April, which can help estimate flooding potential; and
- Change in intense precipitation (25-year and 100-year events) annually.
By 2050, intense precipitation is projected to have increased by approximately 5.7% for 25-year events and approximately 4% for 100-year events, both annually and in April. Average temperature and precipitation are also projected to increase by 2050 for the period November through April and in March and April individually. The precipitation projections indicate that soil saturation and average flow in the Kalamazoo River will likely increase. The projected temperature increases further add to flood risk by affecting the timing of peak snowmelt and resulting peak river flow.
The cost-benefit model tested in this study was part of a larger project related to the impacts of climate change and extreme weather events on transportation infrastructure. The project produced a finalized practitioner’s guide and research report, which includes information about the benefit-cost methodology in Part I, Appendix B.
This Adaptation Clearinghouse entry was prepared with support from the Federal Highway Administration. This entry was last updated on February 6, 2015.
Publication Date: 2011
Related Organizations:
- Michigan Department of Transportation
Sectors:
Resource Category:
Resource Types:
- Best practice
- Case study
States Affected:
Impacts:
- Flooding
- Air temperature
- Precipitation changes
