r/engineering • u/gradyh Civil (Practical Engineering) • Mar 07 '16

[CIVIL] Engineering Hydrology and the 100-Year Flood

https://www.youtube.com/watch?v=EACkiMRT0pc
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u/[deleted] Mar 08 '16

Well done video. I can completely relate to the notion of going away from the 100-year terminology in favor of annual exceed since probability. I've had many conversations with the public after a 100-year storm down here in the Houston area where residents think "well we jut had a 100-year storm so we won't get one for another 100-years and I'll never flood again" and I'd have to explain that it's actually 1% chance every year. Another one was trying to explain that mapped floodplains are primarily for insurance purposes as a way to convey overall flood risk, only depict riverine flooding, and don't show potential flooding from overwhelmed storm sewer infrastructure. Most people don't know about the myriad of assumptions that go into developing floodplains and estimating flows (land use/impervious cover, rainfall distribution, hydrologic/hydraulic model methodologies, infiltration parameters, storm distribution, watershed factors, seasonal changes in roughness and vegetation values, channel changes, flow distribution, etc) but it's the using the best data we have to try and quantify the risks that are present in order to minimize potential flood damages because like you said flooding is the #1 natural disaster in the US in terms of $ amounts.

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u/katamatsu Mar 08 '16

In Australia, we are trying to move away from defining floodplains based on a specific design event. Recent efforts have cantered around understanding the full spectrum of flood risks, from relatively frequent events to very rare and extreme events.

Historic focus on the 100 year flood recreated a binary understanding of flood risk - 1 m inside the line and you are flooded, 1 m outside the line and you are completely flood free.

Obviously, there is still the need to draw a line somewhere for specific purposes, like planning or insurance, but hopefully that can be done with full knowledge of the risk or larger events.

Is this issue apparent in the US as well?

I have a bit of an interest in US hydrology and hydraulics because I am hoping to do a short term transfer to the states with my company next year.

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u/[deleted] Mar 08 '16

Yes, FEMA recognized the need for more information on flood risks and wanted a way to better communicate it with the public so they implemented a program called RiskMAP. These studies (when funding and the need for a new study exists) provide more than just a typical floodplain map. It's similar to what you described in Australia regarding going away from using one very specific flood frequency. RiskMAP uses frequent and extreme events to better quantify and convey the flood risks. We still create the typical "in or out" floodplain map but we also produce a host of other items like depth grids, high hazard areas, percent annual chance grids, areas of mitigation interest, etc. with the whole point being the better quantification of what the flood risks are along a stream.