Atlas 14, which was released in 2017, is a study of rainfall data, including in Friendswood. As a result of Atlas 14, the flood maps that Friendswood and many other cities use have become outdated, as Atlas 14 shows that much more rain falls in the area than the maps indicate. The maps are in the process of being updated using Atlas 14 through the Federal Emergency Management Agency, though this process takes years, according to a city memo.
In the meantime, Harris County has adopted stricter development standards related to Atlas 14 data. The county is requiring other cities within the county, including Friendswood, to adopt the same standards; the county will no longer work on flood control projects with the cities that do not comply, Deputy Engineering Director Samantha Haritos said.
“What used to be the 100-year storm is no longer the 100-year storm, and unfortunately, it’s going to be a process to update the FEMA maps, so they had to come up with something ... in the interim.”
Under old flood maps, a 100-year storm was 13.5 inches in Friendswood. Under Atlas 14, it is now 18 inches, which means that when it rains 13.5 inches in Friendswood, it is now considered more common than a 100-year storm, Haritos said.
“Because the maps are showing that old 13.5-inch rainfall depth, we have to do something to address that, really, the 100-year rainfall is more than it is,” she said.
One of Harris County’s new standards is that cities must use Atlas 14, not old data, to calculate how much stormwater runs into drains and culverts, Haritos said.
Additionally, Friendswood will require 0.55 acre-feet of detention per 1 acre of new developments. This will not affect the city much because Friendswood already has similar detention rates in place, Haritos said.
One of the most significant changes is the requirement of no net fill in the 500-year flood plain. Right now, the city’s requirement is only the 100-year flood plain, she said.
“This one’s a big one,” she said. “That’s going to be a big adjustment for developers going to the 500-year flood plain.”
Finally, the county requires a minimum elevation be established at the 500-year flood plain for new developments. This also will not affect Friendswood much because many new developments are being built 24 inches above the 100-year flood plain, which, in some instances, is already higher than the 500-year flood plain, Haritos said.
All these changes are interim standards until FEMA releases new flood maps for 100- and 500-year storms, she said.
Haritos encouraged council to approve the adoption of Harris County’s standards. The city is working on drainage projects, and if the city were to not adopt the new standards, Harris County would pull support for them.
“This is something that needs to happen,” she said of the new standards.
Council voted 6-0 in favor, with one absent. The second and final reading will go before council in December.