The City of Austin is in the process of updating flood maps as a result of studies regarding the following creeks: Boggy, Bull and West Bull, Carson, Cottonmouth, Dry Creek East, Fort Branch, Shoal, and Tannehill. The City website states that the new, more accurate maps will remove approximately 2,400 properties from the flood plain. On the other hand, roughly an additional 2,400 other properties will be soon considered to be part of the flood plain. The City website states that the new maps should become official next January, but are already in use by the City of Austin for controlling future development.
At a recent Stanberry & Associates meeting, we were fortunate enough to have Wes Birdwell, P.E. speak to us about how the flood maps are created. Mr. Birdwell, the Water Resources Planning Director of Halff and Associates, Inc., has coordinated many Central Texas flood studies in recent years. A hydrologist, Birdwell was the LCRA River Operations Manager until he retired and began a career with Halff and Associates.
Birdwell explained that old flood maps usually need to be revised for two reasons: first, technology has been vastly improved the accuracy of topography mapping with Light Detection and Ranging (or LIDAR as it is usually known); and second, the addition of new roads and other development greatly alter the runoff and usually result in higher flood levels.
However, flood plains do not always only go up. Sometimes problem areas can be improved with mitigation efforts. He gave some examples of areas such as upper Shoal Creek, which have had major drainage projects and have been greatly improved by channelization and detention ponds. In other words, it is sometimes possible to lessen flooding problems with corrective measures, which may explain some of the 2,400 properties that will no longer be considered in the flood plain.
A small preview of the proposed Austin floodplain Maps