News Articles
NWS Updates Flood Criteria
Posted on Aug 29th, 2018
National Weather Service Flood Criteria Changes for San Bernard River
Because of public comments and additional research and surveys, Katie Landry and the NWS team has requested FOR publicize these new alert levels to the river community. Emergency Management Coordinators for the City of Sweeny and Brazoria County have signed off on the newly proposed levels. (Compare with August 25 article levels below.)
- Action Stage: 7.0 FEET - 5 FEET Private docks near or upriver from the Phillips Terminal are covered
- Flood Stage: 10.0 FEET - 7 FEET Hanson Park begins to flood
- Moderate Stage: 16.0 FEET - 11 FEET An RV park on the left bank is threatened
- Major Stage: 19.0 FEET - 16 FEET
August 25th Article
A public information statement from the National Weather Service Houston/Galveston was published on Fri Aug 10th about moving forecast services from the Phillips 66 terminal on the San Bernard River to the automated gauge at Hwy 35, five miles up stream. The NWS has revised the flood designations (Action, Minor, Moderate, Major) at the new Hwy 35 location and asks for email comments to be sent to Katie.Landry@NOAA.gov
River resident Tom Ronayne has replied to Ms. Landry with suggested changes to the flood designations. Because water flows downhill, the Hwy 35 levels should be higher than the P66 Terminal levels, but he believes the NWS proposed flood designations are too high. His comments are summarized below. (All numbers are in feet above sea level.)
- P66 current designations: Action - 1.5 Minor - 5.5 Moderate - 6.5 Major - 9.5
>> NWS proposed for Hwy 35: Action - 7.0 Minor - 10.0 Moderate - 16.0 Major - 19.0
- Tom's proposed for Hwy 35: Action - 4.0 Minor - 7.0 Moderate - 13.0 Major - 16.0
The 30' crest at Hwy 35 during Harvey corresponded to a 23' crest at the Phillips terminal.
A 0' crest at Hwy 35 corresponds to a 0' crest above sea level at the terminal.
I've assumed that for any crest at Hwy 35 between 0' and 30' the crest at the terminal is a linear proportion of the 23'. I understand the relationship might not be perfectly linear but it's close enough for talking purposes and we've found the assumption works well for comparing stages at the terminal with residents' observations at a known distance downriver from it.
Using these assumptions I've taken the proposed flood criteria for Hwy 35 and calculated the corresponding stage at the terminal where we have historical comparisons.
Proposed action stage at Hwy 35: 7'
Corresponding stage at the P66 terminal: 7/30x23=5.4'
Comment: The current action stage at the terminal is 1.5' above sea level which clearly is too low. At 5.4' however, all private docks upriver from the terminal and for several miles downriver will be under water. 3' might be a more appropriate action level as that is when articles on the river bank subject to floating away or being damaged should be moved up. The Hwy 35 gauge corresponding to 3' at the terminal is 3x30/23=3.9' I would recommend 4' as the action level at Hwy 35.
Proposed minor flood stage at Hwy 35: 10'
Corresponding stage at the P66 terminal: 10/30x23=7.7'
Comment: At that level my boat would be trapped in the lift. Other boat lift owners might be affected at higher or lower levels depending on the size of their boat and/or height of their lift. The previous minor flood stage at the terminal was 5.5' above sea level and I believe that was appropriate. That translates to 7.2' at Hwy 35 (5.5x30/23). I recommend 7' as the minor flood stage at Hwy 35.
Proposed moderate flood stage at Hwy 35: 16' Corresponding stage at the P66 terminal: 16x23/30=12.3'
Comment: Including Hurricane Harvey, only five floods have reached this level going back to 1995. That would seem to be more appropriate for a major flood level. At 10' at the terminal, all boat lifts from the terminal northward must be vacated and a small number roadways, driveways or older low lying homes receive water. The corresponding stage at Hwy 35 would be 10x30/23=13'. I recommend 13' as the moderate stage at Hwy 35.
Proposed major flood stage at Hwy 35: 19'
Corresponding stage at the P66 terminal: 19x23/30=14.6'
Comment: Except for Hurricane Harvey, no storm in NWS records has exceed 13.4' above sea level at the terminal. Setting the major flood stage at a level that is never reached doesn't seem useful to the public. As stated above, 12.3' at the terminal has been reached five times since 1995, and corresponds to 16' at Hwy 35. I recommend 16' as the major stage at Hwy 35…”
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