City studied eight watersheds, including West Bull, Bull creeks
The city of Austin recently updated its flood plain maps, and the changes affect more than 4,600 properties.
The city surveyed 154 miles in eight of the city's watersheds, which are areas of land that drain into one of the larger creeks. About 2,200 properties are no longer in the flood plain, but 2,400 additional properties are now considered to be in the flood plain. Kevin Shunk, supervising engineer at the Watershed Protection Department, said the changes affect property owners by identifying their risk of flooding.
"The flood risk was already there," he said. "The map doesn't create risk; it just identifies it. Now they can prepare for it."
Every few years, the city's Watershed Protection Department updates the maps to provide the most up-to-date information on residents' risk for flooding when their property is located in the 25- or 100-year flood plain. A 25-year flood plain means a property has a 4 percent chance of flooding in any given year, and a 100-year flood plain has a 1 percent chance of flooding in any given year.
In some cases, updates are needed because the data is outdated or was compiled using outdated technology and engineering models, Shunk said. New development can also affect the flood plains and how water flows.
"We base our models on what we know about historical rainfall, but if that was 30 years ago, we missed out on the last 30 years of rain," he said.
In other cases, the city did not have all the data on the entirety of a watershed, such as Bull Creek. The city had never surveyed the portion of Bull Creek that travels through the Great Hills neighborhood.
"We felt there were flood hazards that existed here that we felt people needed to know about," Shunk said.
In August, the city sent out 11,000 letters to affected property owners and tenants, even those whose properties are no longer in the flood plain, to let them know about the changes, which could affect their insurance premiums.
"There could be financial ramifications to that depending on the risk of flooding on the property. That [risk] determines how much flood insurance premiums they'll have," Shunk said.
Homeowners could also be affected by the changes if their home is now in a flood plain because the city uses the flood plain maps to regulate development, Shunk said. Even interior renovations could be limited or restricted depending on the project. The department reviews any building permit submittals for properties in the flood plain.
Making changes to the city's flood plains maps is a lengthy process. In 2009, city staff met with the Federal Emergency Management Agency to discuss a $1.1 million grant for this latest study that will cost $2.6 million. The remainder of the funds will come from the Watershed Protection Department, which is funded through the drainage utility fee that is applied to each resident's water bill.
Engineering work began on the existing survey in mid-2010. How a watershed is chosen for surveying depends on the age of the last study, how much development might have occurred since the last study, how many flood risks might be present and if there are any projects the city wants to pursue that would warrant a new study, Shunk said. The city surveys each watershed using aerial topography data, survey crews and bridge data.
Now that the city completed its recent survey, FEMA will approve the maps and print paper copies for the agency's Flood Insurance Rate Map on which FEMA bases insurance premiums.
Information about the changes may be found at www.austintexas.gov/floodplainchanges
What's next
Residents will have the opportunity to weigh in on the proposed changes in 2014 before the Federal Emergency Management Agency approves the new maps.
Early 2014: City staff will meet with communities affected by the changes to listen to their feedback.
Mid-2014: FEMA's 90-day appeal and comment period commences to allow residents officially to submit comments or appeals.
Mid- to late 2015: FEMA will approve the maps, and changes will affect flood insurance rates.
Other resources
Homeowners can use several resources to find out if their property or nearby roads are in a flood plain.
www.atxfloods.gov
This website lists all the existing road
closures because of flooding in the city.
www.atxfloodpro.com
Residents can find a map of all the existing flood plains in Austin.
www.floodsmart.gov
The National Flood Insurance Program offers flood insurance to homeowners because coverage is not included in standard homeowners insurance.
