Tropical Storm Harvey and Houston Floods

Most of yesterday, I was following the flooding in Houston via online news sites and twitter.  The misery and destruction that Tropical Storm Harvey has brought there has been incredible.  With up to 50 inches of rain expected by the end of this storm, it has created a deluge in parts of Houston that has been described by the National Weather Service as “unprecedented” and having impacts that are “unknown and beyond anything experienced.”

I have a personal interest in the events since my parents live right in the flooded areas in the Southwest part of Houston.  Yesterday, their neighborhood was completely inundated, with all the streets looking like canals.  TV news were showing people getting rescued from homes many feet under water and people walking through chest-high water to get to dry ground.  Fortunately for my parents, the waters stopped rising just after the water had started entering their house.  Hopefully, the flood damage at this point is not too bad.  However, more rain is expected over the next few days.

In looking for information about conditions in the area, I came across the stream level gauges that the Harris County Flood Control District operates in the Houston area.  My parents live not too far, maybe just a mile, from the Brays Bayou, which overflowed yesterday. I think this may have been the first time that has happened.  In almost 30 years that they have lived there, they have never seen floods coming into their house.  However, the data from that gauge (Brays Bayou at Stella Link) was fascinating.

Braes Bayou at Stella Link 8-28-2017

When the flooding was worst yesterday (Sunday), the water was about a foot over the banks of the stream, at a stream level of almost 50 ft.  It then started to recede by a few feet, probably because the rains did not come down as heavily in their area. Today, it appears to be rising again.

Braes Bayou at Stella Link pt2 8-28-2017

If one looks at yesterday’s stream level and the historical information, one sees something remarkable.  Yesterday’s top stream level of 49.87” rose to that of a 100-year flood (49.60”).  (The stream bank stands at 48.40”, which means that at its highest level, the stream was overflowing its banks by about a foot.)  When FEMA designates 49.60” as a 100 year flood level, it expects such a deluge to happen, on average, only once every hundred years.

So, Ok – Harvey has been said to be unprecedented in its impact.  And a 100-year flood might not be so unexpected.  (Others have thrown around the comparison to a 500-year flood.)  But past data show that stream elevation was at 48.38” in 2001 (during Hurricane Allison, another really bad event that flooded most of the same parts of Houston) and at 48.30” in 2015, just a couple of year ago.  (Both times, my parents were lucky and were pretty much unaffected.) Both of those levels exceed the 50 year flood levels, 47.50”.

Taken together, this means that over the last 16 year, Houston (and maybe more specifically, the area where my parents live) has had two 50-year flood and one 100-year flood level events!  (And stream levels have reached 10-year flood levels in 5 of the last 10 years — an event that that has been calculated to have a 10% probability has occurred 50% of the time over the last 10 year.)  Hmmm.

Of course, these data points are very specific to one area, and one would need much more data to draw broader conclusions.  (And in any event, this storm and flood is not over, yet.)  But the events are also consistent with the greater variations in weather and higher frequency of severe storm events that scientists have predicted as an effect of climate change.  If this is a sign of what’s to come, as greenhouse gas concentration in the atmosphere rise, we are in a lot of trouble.

I sure hope that I am wrong.  Either way, this is one more example of what is at stake in the effort to bend the curve on greenhouse gas emission.


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