ALDOT: Replacing Bayway to 100-year storm surge elevation is in public’s best interest

Mobile River Bridge

MOBILE, Ala. (WKRG) — The Alabama Department of Transportation says it is committed to providing safe and efficient transportation to the driving public. In order to do that and to comply with federal regulations, ALDOT says it is determined to replace the Bayway above the 100-year storm surge elevation. ALDOT also says this will be the most cost-effective solution.

ALDOT is working to develop the Mobile River Bridge and Bayway Project that will feature three key elements: the Mobile River Bridge, a new eight-lane Bayway above the 100-year storm surge level, and modifications to seven interchanges along I-10. ALDOT says many factors went into this decision to replace the Bayway with a new, higher structure rather than widening it. Those factors including storm surge analysis, federal regulations, ALDOT design standards, and cost.

The following are the federal regulations ALDOT has included:

Per 23 CFR 650.115, the design for an encroachment on a base (100-year) floodplain shall be supported by analyses of design alternatives with consideration given to capital costs and risks, and to other economic, engineering, social, and environmental concerns.
23 CFR 650.115(a)(2) states that the design flood for through lanes of interstate highways shall not be less than the flood with a 2-percent chance of being exceeded in any given year (also known as the 50-year storm).
23 CFR 650.115(a)(3) states that freeboard (clearance between the lowest member of the bridge and the top of the flood water) shall be provided, where practicable, to protect bridge structures from debris- and scour-related failure.
23 CFR 650.111 states that where FEMA Flood Insurance Rate Maps are available, their use is mandatory for highway agencies in determining whether a highway location alternative will include an encroachment on the 100-year (base) floodplain.
In December 2014, the FHWA issued Order 5520 Transportation System Preparedness and Resilience to Climate Change and Extreme Weather Events. This directive requires FHWA “to ensure that their programs, policies, and activities for which they are responsible integrate consideration of climate change and extreme weather event impacts and adaptation into its planning, operations, policies, and programs, in order to promote climate change and extreme weather event preparedness and resilience. Proactive management involves developing engineering solutions, operations and maintenance strategies, asset management plans, and transportation programs that address risk and promote resilience at both the project and systems levels.”
In a January 2017 memo put out by the Federal Highway Administration, which falls under the U.S. Department of Transportation, states the following:

The Fixing America’s Surface Transportation (FAST) Act, signed into law December 2015, requires agencies to take resiliency into consideration during transportation planning processes.

Following passage of the FAST Act, the Federal Highway Administration and the Federal Transit Administration updated the metropolitan and statewide transportation planning regulations to reflect these new requirements.

The transportation planning rule includes:

A new planning factor for states and metropolitan planning organizations (MPOs) to consider and implement: improving the resiliency and reliability of the transportation system (23 CFR 450.206(a)(9) and 23 CFR 450.306(b)(9)).

A recommendation for MPOs to consult with agencies and officials responsible for natural disaster risk reduction when developing a metropolitan transportation plan and the transportation improvement program (23 CFR 450.316(b)).

A requirement that the metropolitan transportation plan assess capital investment and other strategies that reduce the vulnerability of the existing transportation infrastructure to natural disasters (23 CFR 450.324(g)(7)).

FHWA provides designers with manuals to be used to design projects that comply with the Code of Federal Regulations. These manuals present methodologies that must be followed by designers to satisfy federal requirements and meet design standards. The manuals applicable to this project include:

 U.S. Department of Transportation, Federal Highway Administration’s Hydraulic Engineering Circular No. 25, 2nd Edition, Highways in the Coastal Environment, dated June 2008 (Applicability: For bridges in coastal areas, engineers must evaluate the risk of the bridge being impacted by storm surge and wave impacts and design appropriate alternatives to mitigate for those risks.)

 U.S. Department of Transportation, Federal Highway Administration’s Hydraulic Engineering Circular No. 17, 2nd Edition, Highways in the River Environment – Floodplains, Extreme Events, Risk, and Resilience, dated June 2016 (Applicability: Engineers must consider climate change and sea level rise in designing bridges to be resilient to climate change and extreme weather events.)

In addition to the above-listed circulars, FHWA requires the use of the 2008 AASHTO Guide Specifications for Bridges Vulnerable to Coastal Storms, which provides technical guidance and oversight for bridges vulnerable to coastal storms. These AASHTO specifications were developed by an FHWA-pooled fund following the wave induced damage experienced on a number of Gulf Coast bridges following Hurricanes Ivan, Rita, and Katrina. This document includes the following requirement.

ALDOT says, in summary, the bridges must be designed to avoid significant encroachments on the 100-year floodplain and to provide clearance about the 100-year floodplain unless it can be proven that it is not practical. The bridges must also be signed to withstand storm surge and wave action impact, along with being resilient to climate change and extreme weather events, something we face here often on the Gulf Coast.

ALDOT had to conduct a storm surge analysis to determine the flood height and wave impact during various storm events. The analysis confirmed that a 100-year-old storm event would catastrophically damage a major portion of the existing I-10 Bayway beyond repair. ALDOT used the I-10 bridge in Pensacola after Ivan and the I-10 and US-90 bridges in Louisiana and Mississippi following Katrina as examples.

ALDOT says most of the existing Bayway is well below the 100-year wave crest elevations, placing it well within the wave impact.

The Level I and III Storm Analyses also showed that due to the height of the existing Bayway, a 50-year storm could still damage over 50% of the Bayway. There is a 64% probability of a 50- year storm impacting the existing or new Bayway built at the same height during the 55-year Concession period.

ALDOT says it performed a structural analysis on the existing Bayway for a design of several retrofit options. The analysis revealed that even with the retrofit design, ” the uplift buoyant force from the waves for a 100-year storm damaged 75% of the bridge beyond repair.”

ALDOT evaluated multiple design alternatives in the document “Bayway Alternative Analysis Matrix 08 26 2019” on the project website:

ALDOT says it has also studied the economics of retrofitting and widening the existing Bayway. It says the estimated cost to retrofit the existing Bayway at the 100-year storm surge level would be $1.032 billion. In 20 years, ALDOT states that structure would need to be replaced, a cost estimated at $890 million. That would bring the total estimate for widening the current Bayway and replacing it in 20 years to $1.922 billion.

ALDOT says the cost to replace the Bayway, with eight-lanes above the 100-year storm surge level, is estimated at $886 million.

The Mobile River Bridge will have a design life of 100-years, as well, ALDOT states.

ALDOT also references a federal report done in 2016 that found the existing Bayway would be impacted if a storm similar to the strength of Hurricane Katrina hit Mobile directly. Both state and local governments would experience loss in potential tax revenue estimated at $7,150 and $23,310 per day due to bridge disruption.

ALDOT concludes the findings by saying it has heard the public’s concerns about replacing the Bayway rather than widening it, but finds that in order to comply with federal regulations, it has determined replacing the Bayway to the 100-year storm surge structure would be in the best interest for the safety of the traveling public and is the most cost-effective solution.

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