Frequently Asked Questions: employment, wages, and establishment counts in hurricane flood zones
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1. What are the hurricane flood zone maps and tables?
These maps have been created for every county along or inland from the Gulf and Atlantic Coasts that have hurricane flood zones as determined by the U. S. Army Corps of Engineers or State emergency management agencies. Flood zones are defined based on the severity of a hurricane. For most states, severity is labeled using Saffir-Simpson Hurricane Scale categories I, II, III, or IV. The remaining states use an A through D severity range.
Matched to these zones are employer reports from the BLS Quarterly Census of Employment and Wages (QCEW) program. Totals for employment, wages, and establishment counts are created using reports from every employing establishment located in each flood zone. When permitted by the states’ interpretation of their confidentiality laws, the location and concentration of employing establishments is depicted on the maps using dots.
2. How were the maps and tables created?
BLS obtained flood zone mapping files from the U. S. Army Corps of Engineers or alternatively State emergency management agencies. These files (known as shape files) define the set of latitude and longitude values included within a particular zone. Addresses on the QCEW employer file are converted to latitude and longitude values and then matched against the shape files to assign zone codes to those establishments. The data from these establishments is summarized and displayed on the tables embedded in the maps. When permitted by State law, the establishment latitude and longitude is used to place dots on the maps.
3. Why were they created?
The QCEW program has been creating maps of employment in disaster areas for many years, starting with a map of lower Manhattan following the events of September 11, 2001. This map and those that have followed were typically created after an event, using ad-hoc approaches and were commonly not available immediately after an event.
As part of QCEW work performed in response to Hurricane Sandy, BLS learned that the U.S. Army Corps of Engineers had created and was maintaining a set of flood zone definitions. This presented BLS with the opportunity to create a set of maps and tables that would be available before an event.
4. How can they be used?
The summarized data are presented for each zone. If a Category I storm is experienced, the subject establishments are represented by the zone 1 sums. The zone 2 sums represent the establishments which are not affected by Category I storms, but are affected by Category II storms. To get a total for establishments subject to a Category II storm, users should add the zone 1 and zone 2 values. The zone 3 sums represent the establishments which are not affected by Category I or II storms, but are affected by Category III storms. To get a total for establishments subject to a Category III storm, users should add the zone 1, 2, and 3 values. The zone 4 sums represent the establishments which are not affected by Category I, II, or III storms, but are affected by Category IV storms. To get a total for establishments subject to a Category IV storm, users should add the zone 1, 2, 3, and 4 values.
Using the data provided in the flood zone tables, the proportion of a county’s establishments, employment, and wages in all flood zones can be calculated by dividing the “Total Included” by the sum of the values in the “Total Included” and the “Unincluded” categories. This proportion can be applied to newer county data to develop an approximation of updated data. This application assumes that the distribution of industry and geographic employment hasn’t changed since the maps and data were created. For example, about 6 percent of Harris County, Texas (Houston) employment during the third quarter of 2016 was located in establishments which would have been in evacuation zones if a Category IV hurricane occurred. BLS is working to compare the 2012 and 2016 distributions.
In addition to the maps and tables presented on the BLS web site, BLS provided establishment identifying files coded by flood zone to each State on the Gulf and Atlantic coasts. States may choose to use these files to create their own data products or to share the coded employer files with their emergency management agencies. These files are also provided to BLS business surveys for the determination of possible effects on data collection. These files are not available to the public.
5. What are the underlying sources of data?
Hurricane flood zones are produced for most States by the U. S. Army Corps of Engineers. Florida, North Carolina, and Texas produced their own flood zone definitions. Please note that these definitions were not produced for some Texas counties and Louisiana parishes.
6. Why are some observations not published?
Much of the data in the QCEW program is collected under a pledge of statistical confidentiality. That pledge, along with a variety of other federal laws and regulations both forces and allows BLS to protect the confidentiality of employer reports. In addition, there are applicable state laws that limit what can be disclosed. Table entries are withheld when they might compromise employer confidentiality.
7. Why do some maps have dots and not others?
Some hurricane flood zones dots as possibly revealing sensitive information regarding business or government establishments. Other States do not see the use of dots as a confidentiality issue.
8. Why do some maps have undesignated zones?
Some shape files provided to QCEW contained areas for which there were no assigned flood zones, but were still located in areas that are affected by storms. Instead of portraying these areas as no flooding areas, the QCEW program made the decision to refer to these areas as undesignated zones.
9. What QCEW data are used for the Hurricane Maps project?
Data for third quarter is used because it includes the months of July, August, and September--the peak months of hurricane season. The hurricane season starts on June 1st and runs through the end of November. The source data for this project is the Quarterly Census of Employment and Wages, which is published about six months after the end of the reference quarter. Due to that lag, the most recent comparable data available during hurricane season is the data for the third quarter of the preceding year. The maps and data presented here are based on the final QCEW publication of the third quarter of 2019.
Last Modified Date: March 17, 2020