Lessons from heavy flooding in Friendswood - when to evacuate?

Hurricane Harvey caused widespread damage in Friendswood, a city that straddles the boundary between Harris and Galveston counties about 20 miles southeast of downtown Houston.

Friendswood was featured in a February 4, 2018 Houston Chronicle article entitled "After Harvey's floods, Friendswood will allow residents to build at ground level - Effort helps 300 homeowners but puts other at risk."

https://www.houstonchronicle.com/news/houston-texas/houston/article/After-Harvey-s-floods-Houston-area-city-will-12540769.php?cmpid=gsa-chron-result

The main point of the article is that city officials decided to use an older version of the Flood Insurance Rate Maps (FIRM) to define the Special Flood Hazard Areas (SFHA) where regulations for repair and new construction are in effect.  The older maps are from 1999 and have smaller SFHAs compared to the current (2006) maps.

That decision raises many issues that will be discussed in future blog posts.  In addition to presenting the implications of using outdated floodplain maps, the article listed many interesting statistics from Harvey flooding  reported by the City of Friendswood.  Data available in Geographical Information Systems (GIS) format on ArcGIS Online is particularly useful for illustration in this blog.

The Houston Chronicle article reported that approximately 2400 homes were damaged, with 1074 experiencing major damage (24-48" of water) and 194 being destroyed (>48" of water).  Neighborhoods in northern Friendswood near the intersections of Cowart and Chigger Creeks with Clear Creek suffered particularly heavy damage (see Map #1).

Approximately 22% of all damaged properties are located outside of any mapped floodplains, particularly in the Katies Park, Imperial Gardens, and Orchard neighborhoods which are located on the Galveston County side of Clear Creek.   The floodplain maps for the two counties were prepared at different times using different modeling assumptions and it appears that the map is more accurate on the Harris County side.

The reported high water rescues and citizen assistance calls are also worth examination.  The city reported 389 of these incidents.  Ninety-five (24%) occurred at properties outside of mapped floodplains (see Map #2).  This is mostly due to the inaccuracy of the floodplain map on the Galveston County side as mentioned above.  

However, the sheer number of these incidents raises other points for consideration listed below:

1.  Were residents unaware of the amount of floodwater likely to accumulate at their location?  This in part goes back to the accuracy of the floodplain maps but also to resident knowledge.   For example, Map #3 below shows that fourteen properties at which rescues or assistance were required are actually mapped as within the Clear Creek floodway.

 2.  Another factor determining the need for citizen rescue or assistance is whether or not residents chose to evacuate.  Weather forecasts in the beginning stages of Harvey certainly called for much greater rainfall than the 100-yr storm on which Special Flood Hazard areas are based (12-14" in a 24-hr period).  However, at some point during the storm potential evacuation routes were closed off by high water. 

Did the Harvey experience cause you to consider evacuation during future severe storm events?  If so what factors will you consider in making that decision?  Are you aware of the real potential for damage at your location both as predicted by floodplain maps and by actual Harvey flooding?