Cities along Clear Creek look at new rainfall intensity data

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Redefining the 100-year storm
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Wading through rainfall data
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Cooperation along Clear Creek
Atlas 14, a National Oceanic and Atmospheric Administration study of rainfall data released in September 2018, shows for the first time since the 1990s that cities along Clear Creek, including League City, deal with some of the hardest-hitting storms in the state. These municipalities have a 1% chance per year of receiving 17-18 inches of rain in a 24-hour period.

In mid-August, the Clear Creek Watershed Steering Committee, which is made up of 20 municipalities along the creek, voted to continue researching the adoption of Atlas 14, which could lead to stricter development standards to prevent flooding and drainage issues.

“It’s going to rain as it’s going to rain. We just have a better understanding now,” said Craig Maske, Harris County Flood Control District’s chief planning officer.

Meanwhile, the Federal Emergency Management Agency released updated flood plain maps based on data prior to Atlas 14, which means they could be updated further if cities adopt Atlas 14. League City on Aug. 15 adopted the maps, which officials use to set development standards.

Under the previous maps, which came out in 1999, 1,265 homes were in the 100-year flood plain, which has a 1% chance of flooding annually, and 4,245 homes were in the 500-year flood plain, which has a 0.2% chance of flooding annually. Under the new maps, 3,730 homes are in the 100-year flood plain, and 7,378 are in the 500-year flood plain.

“This is a big change,” League City Engineering Director Chris Sims said.

The data in Atlas 14 could further alter flood maps and regulations for new development—should cities choose to accept it. Cities in the Clear Creek Watershed are grappling to understand Atlas 14 and its effect on development, with the goal of every entity having up-to-date regulations.

UNDERSTANDING ATLAS 14


More rain is falling more frequently, and new data is needed to accurately reflect this, Maske said.
According to Atlas 14, a 100-year storm in League City is now 18 inches of rain in a 24-hour period compared to 13.5 inches previously. The committee can adopt Atlas 14 rainfall data as presented or look into studying it to make the data more specific to the Clear Creek Watershed’s entities, Sims said.

“The more precise data you have, the better decisions you can make from a planning standpoint, from a construction standpoint,” he said.

However, it may not be worth the millions of dollars it could cost to further study rainfall data to find out it is only slightly different than what is in Atlas 14. The steering committee is investigating to see what decision makes the most sense, Sims said.

Friendswood City Manager Morad Kabiri wants to adopt Atlas 14.

“For myself, I would prefer to adopt it. It’s the latest, best information,” Kabiri said.

Sims said he is not sure League City should accept Atlas 14 without refining it. Sims would not want the new standards to affect how much water is held back at new development, adversely affecting how much surrounding neighborhoods flood, he said.

Pearland is also hesitant to adopt Atlas 14, Pearland Director of Engineering Robert Upton said, as the city already operates with stringent drainage and development codes.

Harris County decided to adopt Atlas 14 on July 9, which means its drainage regulations could be different than surrounding municipalities if they do not adopt Atlas 14. The county’s decision was made from a public safety standpoint, Maske said.

“New development and the new projects we design and build are designed with the best available data,” Maske said.

Kabiri believes it is important for the watershed to make the decision together rather than Friendswood make its own decision.

The goal is for all of the steering committee’s 20 entities to be on the same page with Atlas 14 so one municipality is not creating development or drainage standards based on rainfall data that does not align with neighboring communities, Kabiri said.

NEW-DEVELOPMENT CODES


New structures have to be built to a standard that will prevent flooding in both that structure and existing structures. As Atlas 14 shows cities along Clear Creek are receiving more rain each year, building codes will need to be updated to keep water out of developments without sending it to nearby structures or cities.

“… You are going to have to provide larger facilities to mitigate [flooding] in terms of construction,” Kabiri said.

Larry Millican, a League City City Council member on the steering committee, said Clear Creek Watershed municipalities that adopt Atlas 14 would need to update engineering standards based on the new data. Examples include deepening detention ponds, adding more street gutters and installing larger water pipes so municipalities could handle the expected increased amount of rainfall during 100-year storms, Millican said.

Prior to Atlas 14, League City was proactive in using FEMA flood plain maps to create standards for new development. Instead of requiring new development be built 18 inches above the 100-year flood plain, League City City Council in July 2018 voted to increase the standards to 24 inches above the 100-year flood plain or 3 inches above the nearest 500-year flood plain. This measure was taken to prepare for the Atlas 14 data and new FEMA flood plain maps council and staff knew were coming, Sims said.

“I wouldn’t say it’s perfect yet, but we feel pretty good with what we’ve got right now, and we’ll continue to make tweaks on it,” he said.

LOOKING AT EXISTING DEVELOPMENT


As the steering committee looks deeper into Atlas 14, a technical committee formed within the steering committee suggested updating the existing policy and criteria matrix chart for the organization, which is a compilation of the flooding standards for the municipalities within the watershed.

One reason for this is to make sure everyone is following the same regulations, League City City Engineer Jack Murphy said.

“The Atlas 14 is kind of an interesting study because it has come out and unfortunately none of our formats use those rain intensities to define those levels,” Murphy said. “All of those maps we are working from were all created before this new Atlas 14 information is available.”

For example, Galveston County and League City in August updated their flood maps to comply with FEMA requirements, and many residents who were not in the flood plain now are. Harris County is in the process of doing this as well.

League City resident Marika Fuller lives along Marlin Court. She was not in a flood plain before, but under the new FEMA maps, she is. The new maps are affecting everything from home values to potential increases in flood insurance rates, she said.

Fuller’s neighborhood flooded during Hurricane Harvey, and many residents are trying to move out. Some are afraid the new maps mean their houses will stay on the market longer or sell at a lower price, Fuller said.

The FEMA maps have the latest information for windstorm damage but include rainfall data that came out prior to Atlas 14, meaning the maps are already potentially outdated. Creating new maps will take time and money, which Murphy said he hopes the watershed can help with.

Until then, cities will continue to evaluate their regulations.

“That’s one of the things that the steering committee needs to be bringing up and accomplish because that’s really what the organization is going to be good for,” Murphy said.
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