Wednesday, July 17, 2019

Mobile River Bridge Plan is Fatally Flawed: Reason 5


I submitted an independent review of the Supplemental Draft Environmental Impact Statement (SDEIS) to the Alabama Department of Transportation (ALDOT) on July 10, 2019. As of this writing, ALDOT has not acknowledged receipt of my report, containing 19 specific issues with the project's decision document. The SDEIS is the most important document pertaining to the project. The bridge and bayway replacement cannot go forward without it.

The document is fatally flawed.

Since ALDOT has neither publicly nor even privately acknowledged that they are aware of these 19 specific problems with their decision document, I am sharing them with you, one at a time.

Comment 5: The evaluation of potential storm surge risk to the existing Bayway in Appendix G is incomplete without a consequence analysis, and storm surge risk is a key factor in the decision to examine only replacement alternatives for the Bayway.

Basis: Rather than focusing only on Bayway replacement, additional alternatives that include armoring and retrofit of an existing and storm-tested structure are required.

The mere existence of probability that a tropical storm or hurricane could affect the existing Bayway is insufficient justification for forgoing evaluation of alternatives that add capacity to the structure. While the coastal engineering analyses in Appendix G show that there is a chance that the Bayway will be overtopped or inundated, there is no discussion of the consequences of inundating events. Merely stating that the existing Bayway is vulnerable to storm urge is not enough to demand its replacement.

Even a storm with the intensity and surge of Hurricane Katrina could leave the unmodified but well-maintained structure intact or cause a damage level that is acceptable given such storms’ very low frequency of occurrence. Damage consequences must be considered in tandem with damage frequency. We cannot design or build away 100% of risk. The SDEIS fails to address the tandem relationship between storm event probability and consequence, and risk cannot be communicated without it.

This risk analysis cannot be done reliably in a qualitative manner. Explicit return frequencies of potentially damaging storms must be disclosed. The potential damages to the affected structures must be estimated, and the full life-cycle cost of the storm damage risk must be shown. Since 100% of risk avoidance is unattainable, some measurable level of risk must be identified as acceptable and it must be made explicit. If the SDEIS states that a structure must be designed to withstand the 100-year event, it must explain why that level of protection is necessary.

Residual risk must also be communicated under current NEPA guidance. The SDEIS does not state the risk that will remain in the as-built condition. There is no discussion of expected project performance. Given the uncertainties associated with a 100-year event, a 100-year storm would still have a probability of causing damage and disruptive effects. These are not acknowledged or discussed. The project performance in the 250- and 500-year events’ risk are not discussed.

Appendix G is thus incomplete and represents only one half of the risk analysis.

Without resolution, this issue should halt finalization of the SDEIS.

1 comments :

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