Technical Information

What is a CLOMR?

A Conditional Letter of Map Revision, or CLOMR, is FEMA’s comment on a proposed project that would affect the hydrologic or hydraulic characteristics of a flooding source.

• A CLOMR does not revise the current National Flood Insurance Program map (NFIP). It just outlines whether the project, if built as proposed, would be recognized by FEMA.
• Building permits cannot be issued based on a CLOMR, because a CLOMR does not change the NFIP map.
• Once a project has been completed, the community must request a revision to the Flood Insurance Rate Map (FIRM) to reflect the project. “As-built” certification and other data must be submitted to support the revision request.

What happens after a CLOMR?

• The Dam 101 Project is tentatively scheduled to start construction in Fall of 2022 and be operational in Late Fall of 2023.
• Once it’s ready, the District will submit a LOMR to FEMA to amend the FEMA flood map to reflect the changes made by the new dam.

Does Dam 101 include Atlas Rainfall?

Dam 101 has been modeled using three sets of data:      1. The Probable Maximum Precipitation as dictated by TCEQ Dam Safety,  2. The current FEMA rainfall in our area (2010 USGS) as required, and 3. NOAA Atlas 14 rainfall. In 2019 the District commissioned a study to see if the Project could be amended to address the revised impacts of the new rainfall data. The conclusion of the study revealed that the dam was already performing the maximum extent of protections practicable.

How does Atlas 14 affect Flood Risk?

Atlas 14 rainfall data reflects the current flood risks since it has the newest collection of rainfall data. Although mapping of the projected changes has not been undertaken, the data and modeling clearly show that more people and roadways are benefitted by the construction and operation of Dam 101 than with the older USGS rainfall.

What is Atlas 14 Rainfall Data ?

In late September of 2018, the NOAA updated rainfall data for use in floodplain modeling in Texas. https://www.noaa.gov/media-release/noaa-updates-texas-rainfall-frequency-values In some cases, this data replaces data from over 40 years ago. In our area, the rainfall was updated with the recent FEMA flood-maps to leverage the 2010 USGS rainfall. 

Dam 101 has been modeled using three sets of data ;the Probable Maximum Precipitation as dictated by TCEQ Dam Safety ,the current FEMA rainfall in our area (2010 USGS) as required and NOAA Atlas 14 rainfall.

Atlas 14 Rainfall Data

No mapping of Atlas 14 rainfall data has been completed yet. Williamson County is working on a study of the watershed that could ultimately lead to mapping. 

In the meantime each governmental entity has made some decisions about how they are handling the data when it comes to permitting new development. The District has no authority over development, building or floodplain permits. 

City of Round Rock

• Rainfall Application Instructions for Hydrologic Analyses and Designs (Round Rock RAIn)
• Development Services Atlas-14 Scenarios

Permitting and Models

If you need hydraulic models please contact the appropriate floodplain administrator based on whether the property is located within the City of Round Rock or Williamson County.

For Easement Encroachment permits please see our permitting page. You can email permitting@ubcdams.org to contact us directly.

Floodplain permitting is handled by the applicable floodplain administrator.

Development permitting is handled through the County or City.