The reason Houston adopted the new levels, as did Harris County is that NOAA is in the process of finalizing their new rainfall estimates for Texas.
The old rainfall estimates for Houston for the 24-hour event are 100-year are 12.4-13.5 inches. The old 500-year estimates are 17.7 to 19.3 inches)
NOAA is suggesting that the new 24-hour 100-year rainfall event is in the range of 18.5 inches.
This means that the current 500-year water surfaces, will be in a few years (maybe 4) approximately the regulatory 100-year water surfaces. Therefore the new regulation means Houston is a little forward looking, and is currently regulating to the future 100-year event in response to the better data provided by NOAA. So, Houston is regulating to the future 100-year water surface plus 2 feet of freeboard.
I think the math will not totally support the assumption that 18.5 inches using 2001 channel and detention configuration will equal 18.5 inches using 2018 channel and detention configuration, but it will be closer to the future modeling answers than assuming 13.5 inches using the 2001 channel and detention configuration will equal 18.5 inches of rainfall using the 2018 channel and detention configuration.
It is politically much easier to increase design criteria after a big event than it is to increase the criteria in the middle of dry period. The Mayor, and County Judge are both making the assumption that the new water surfaces are increasing as a result of the future 100-year rainfall event data when they get remapped. And we almost always get remapped after a big storm because mapping money generally comes after a storm. Its not a bad bet since the Mayor is probably still going to be here when the new maps start getting published in 2020 or so. Its easier to decrease to the newly flood elevation that is less than we are currently regulating to than it is to increase it during a big dry-snap.
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Dwayne Culp, Ph.D., P.E., P.Eng, M.ASCE
Culp Engineering, LLC
Richmond TX
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Original Message:
Sent: 04-14-2018 10:22
From: Morgan Byars
Subject: Houston's New Floodplain Regulation
The frequency is only associated the 500-year event (plus the 2 ft of freeboard) and nothing else. Are you trying to ask what flood recurrence interval would result in a flood elevation that is 2 feet higher than the 500 year? That is completely location/topography dependent and not terribly relevant. Regardless it depends on the floodplain relief where wide flat areas would require much greater increases in rainfall (less frequent storm) to result in the same increase in flood depth as compared to a more confined valley. They implemented these rules advised by the new NOAA Atlas 14 statistics where it just so happens the updated 100-year is close to the existing 500 year. Using a standard freeboard height is common in design and the amount varies across applications, but it's not as common to associate another storm event with the top of freeboard. I'm not sure how terribly useful that would be and in addition once you get beyond the 500 year (.002 probability) the statistics become highly volatile and the values less reliable.
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Morgan Byars P.E., M.ASCE
Consulting Engineer
Buda TX
Original Message:
Sent: 04-13-2018 12:06
From: Alan Shaw
Subject: Houston's New Floodplain Regulation
As a lot of you are probably aware the Houston Department of Public Works approved new building regulations that all new structures in Houston are to be built two feet above the 500-year floodplain. What frequency does this make their regulations considering the 2 additional feet?
Alan Shaw
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Alan Shaw P.E., M.ASCE
Belgrade MT
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