NOTN- City officials and U.S. Army Corps of Engineers representatives outlined ongoing and proposed flood-mitigation work last night.
The Committee of the Whole work session focused on HESCO barriers and river armoring installed after the series of glacial lake outburst floods in recent years.
Mike Records, the Technical Lead for the Army Corp of Engineers, outlined the hydrology of the system, how the flood water moves.
His presentation estimated flood risk. those estimations will be used to design short, medium and long-term protections.
Engineers discussed “Annual Exceedance Probability” or AEP, which was once referred to as “100 Year Flood.”
This means the chance that a flood of a certain size will happen in any given year. For example, a “1% AEP” flood has a 1% chance of happening each year.
But according to officials, those odds add up over time. Even a rare flood becomes much more likely if you look over many years. A flood with a 1% yearly chance has about a 40% chance of happening at least once over 50 years.
“A 1% annual exceedance probability event would be the equivalent of a 100-year event, there is a low chance of that happening, but over time, those chances compound.” Records said, “The annual exceedance probability events that we developed are peak discharge events that we then used in hydraulic modeling to inform decision making on design. You’ll see that a lot of these events are significantly larger in magnitude than anything that’s been experienced at this point.”
In most places, engineers rely on decades of historical data to estimate flood risk, but in this case, there are only about three years of major data available.
“There’s flooding all over the country, it’s rare that it develops so suddenly like this.” Records said, “So normally, you might have like a 60 year period of record of flood events that you could use to develop your AEPS. In this case, there’s basically three years of full drainage events.”
Because of limited data and a changing environment, there is a lot of uncertainty in these estimates, so engineers are planning for the worst-case scenario.
According to officials, Suicide Basin is expanding. This is because the glacier is melting and retreating, which opens up more space for water to collect. Ice is breaking off (calving), melting, and adding to the lake, so overall the basin can hold more water than it used to. On the other side, the spillway,, the pathway where water drains out beneath or around the glacier, is also changing. As the glacier thins and melts, the outlet is getting lower, which can allow water to start draining sooner.
Cubic-feet-per-second or CFS, is the rate at which water flows. Last year’s flood reached about 50,000 CFS. Officials said work aims to reach a 63,500 CFS flood scenario threshold by mid-July.
Assembly Member Nano Brooks asked Records about the service life of the HESCO barriers, to which Records responded, “They have a warranty of five years. I think that’s primarily for fabric degradation. Of course, those parts can be replaced, but that is kind of the maximum expectation without significant ongoing maintenance.”
Three options were presented for the coming season: the status-quo-plus build, upgrades to protect to 63,500 CFS (the staff recommendation), or a larger, more costly build to protect to 90,000 CFS. According to officials, that type of flood has an 18% chance of occurring in the next 10 years.
Officials said the 63,500-CFS option would leave an estimated $3 million funding gap and the 90,000-CFS option about $8 million short of current funding.
The city is pursuing State Revolving Fund assistance that would forgive half the loan and could cover much of the cost if approved.
Ultimately The City and Borough Assembly voted 7-2 to authorize work to reinforce and raise HESCO flood barriers to protect against a 63,500 CFS event ahead of the 2026 flood season .
An ordinance to formalize the appropriation is expected for introduction May 18.
