Great Lakes

WI - Association of State Floodplain Managers Issues Public Comment on FEMA Notice

WASHINGTON, July 27 -- The Association of State Floodplain Managers, Madison, Wisconsin, has issued a public comment on the Federal Emergency Management Agency's notice entitled "Technical Mapping Advisory Council; Meeting".

The comment was posted on July 24, 2020:

The Association of State Floodplain Managers believes the following issues are critical input to developing a proposed vision statement for the flood mapping program and should be incorporated into the 2020 TMAC report or the 2021 work plan. Some of these have been addressed by TMAC in the past, but have not been implemented by FEMA. The TMAC may need to provide options or added information to assist FEMA in implementing these key issues, which are:

1. Atlas 14 updates. Accuracy of the flood maps is dependent on a few key inputs, such as expected rainfall precipitation for the 1% rainfall event, up-to-date LIDAR, and appropriate hydraulic models. One of these inputs is particularly troublesome, the expected 1% rainfall estimate, especially for urban areas. This estimate is most frequently obtained from Atlas 14 publications for watersheds across the nation--there are currently 11 Atlas 14 volumes covering different regions of the nation. The NOAA/National Weather Service publication is developed based on all the years of record so the calculated flood levels are as accurate as possible, but based on past rainfall data.

PROBLEM: A number of the Atlas volumes are far out of date and no volume has been developed for the 5 Northwest states, leaving them to use other NOAA precipitation data that is over 50-years old. Consequently, the rainfall estimates for these areas do not reflect the recent decades of increased rainfall intensity. As a result, flood maps consistently underestimate risk and do not properly show which properties need to buy flood insurance. It is noted that the Atlas 14 volume (No. 11) for Texas, including Houston was being worked on when Hurricane Harvey struck, and the updated 1% rainfall increased from 13" in 24 hours to about 18", a 38% increase.

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