Web Exclusive! The North Carolina Floodplain Mapping Program Delivers Impressive Results
Three years after Hurricane Floyd generated $3.5 billion worth of damage on North Carolina and two years after becoming the nation’s first Cooperating Technical State (CTS), the North Carolina Floodplain Mapping Program (NCFMP), has delivered the following impressive results:
- Aerially mapped six river basins (totaling 24,000 square miles) with LIght Detection And Ranging (LIDAR) by March 2001.
- Performed detailed hydrologic and hydraulic modeling (H&H) using the LIDAR data.
- Released the first set of digital Flood Insurance Rate Maps (DFIRMs) in January 2002; and have since released DFIRMs to five other counties totaling approximately 600 DFIRM panels. Note: In previous years, FEMA was only able to conduct one updated countywide flood insurance study in North Carolina per average year due to its limited budget.
- Launched the North Carolina Floodplain Mapping Program website (www.ncfloodmaps.com/) in January 2002.
- Launched the online, user-friendly interface to the Floodplain Mapping Information System (FMIS) in May 2002. The FMIS is the NCFMP’s dynamic, 10-terabyte size information technology (IT) infrastructure that analyzes, maintains, archives, and disseminates flood maps and associated data.
LogisticsThe NCFMP was able to deliver these results in such a short time frame by dividing the state into three manageable regions, Phases I – III. Phase I included the eastern river basins of Cape Fear, Lumber, Neuse, Pasquotank and the Tar-Pamlico; Phase II included the central river basins of Catawba, Chowan, New, Roanoke and Yadkin; and Phase III included the western river basins of Broad, French Broad, Hiwassee, Little Tennessee, Savannah and Watauga. The project then proceeded by working on areas (varying in size from the entire area of Phase I to specific river basins down to individual DFIRM panels) through the following five main steps (Figure 1):
- River Basin Funding
- Acquisition of New LIDAR Derived Topographic Data
- Hydrologic and Hydraulic Modeling (H&H)
- Preliminary DFIRM Preparation
- Public Review
Phase I was primarily funded by the North Carolina executive and legislative branches reallocating funds from the Hurricane Floyd funds in August 2000 and by a grant from the Federal Emergency Management Agency (FEMA). Additional funding and support came from the CTS partner consortium of over 30 federal, state and local agencies.
Aerial flights for the collection of new topographic data with LIDAR began during the following leaf-off season from January 2001 through March 2001. The raw LIDAR data was then post-processed to remove any non-bare earth features (e.g. buildings, dense vegetation and trees), which accounted from 60 to 80 percent of the total LIDAR mapping costs depending on terrain, vegetative cover and development. Independent surveyors then checked the post-processed LIDAR data for accuracy through a rigorous Quality Assurance/Quality Control (QA/QC) process. (For individual QC assessment reports, visit the www.ncgs.state.nc.us/floodmap.html web page). The entire area of Phase I passed the QA/QC stage of Step 2, except for a small portion of the Cape Fear River Basin (see Figure 2).
The LIDAR data was then used to conduct H&H to determine flood hazard area boundaries. The hydrologic component of H&H was performed on the LIDAR data along streams and rivers to calculate how much water would flow downstream during peak discharges of standard flood events (10-, 50-, 100- and 500-year floods). The hydraulic component of H&H determined the flood profiles and boundaries of these standard flood events based on:
- Discharges predicted from the hydrologic modeling
- Floodplain and stream channel cross-sections
- Floodplain structures (e.g. bridges, culverts, dams and levees)
H&H has been completed for 90 percent of the Lumber River Basin, 75 percent of the Tar-Pamlico River Basin, and the entire White Oak River Basin. H&H has been initiated for the entire Neuse River Basin and the remaining portions of the Lumber, Tar-Pamlico and White Oak River basins (see Figure 2).
Based on the H&H results, the areas that would be flooded during the 100-year and 500-year flood events were then mapped onto preliminary DFIRMs. This draft mapping stage has been completed for 40 percent of the Tar-Pamlico River Basin, the upper 10 percent of the Lumber River Basin and the entire White Oak River Basin (see Figure 2).
The areas that passed the preliminary DFIRM step were then submitted for public review to:
- Announce that new DFIRMs have been released for a particular area
- Explain the mapping process
- Allow the public to dispute during a 90-day appeal period any proposed change in flood elevation from a previous Flood Insurance Rate Map (FIRM) as well as the accuracy of any new study
The NCFMP conducts these public reviews by holding public meetings that are announced by legal advertisements in two issues of local newspapers with the 90-day appeal period officially beginning after the second announcement advertisement. This last step has been completed for the entire White Oak River Basin and initiated in 40percent of the Tar-Pamlico River Basin.
The User-Friendly NCFMP Website (www.ncfloodmaps.com/)The NCFMP website opens with a Macromedia Flash screen, which draws visitors’ attention to the site navigation panel on the right side of the screen and welcomes visitors with a scrolling marquee at the top. Users can navigate to the site’s five main areas (Digital Flood Maps, Public Documents, News, Status, and Links & Resources) by clicking on the compass icons that enlarge with cursor rollover.
Digital Flood Maps“Welcome to the North Carolina Floodplain Mapping Information System” is the greeting that users receive on the Digital Flood Maps introduction page. The page’s next statement,
“The North Carolina Floodplain Mapping Information System (FMIS) is an integral part of the North Carolina Floodplain Mapping Program. Using a map-based interface, users have three sets of web-based applications available for viewing, downloading, and determining address/flood boundary information throughout the State of North Carolina.”
and the directions that follow show this site’s powerful, value-added information. Instead of handing users massive amounts of raw, incomprehensible LIDAR data, they have provided a user-friendly interface (see Figure 2) for people to get the flood information they need and can understand.
The interface’s three main functions (viewing data, locating addresses and downloading data) are handled by collections of tools, except for the function of locating addresses which is handled by the address locator tool alone.
Data Viewer Tool CollectionThe data viewer tool collection (Table 1) allows users to browse, navigate and display data. It also provides users with information on what flood insurance rate maps and reports are available for download. Please note that depending on the map scale, layers become automatically visible for viewing and downloading.
Table 1. The data viewer tool collection on the North Carolina Floodplain Mapping Information System (FMIS) interface.
Tool Name Explanation Toggle between Legend and Layerlist Toggles between the legend and layer list displayed in the Layers/Legend Window. Toggle Overview Map Turns the locator view window on or off. Zoom In Zooms in on a user selected center point or drawn rectangle. Zoom Out Zooms out on a user selected center point. Zoom to Full Extent Zooms out to the full extent of the entire data set. Zoom to Active Layer Zooms in or out to the extent of the active layer. Back to Last Extent Zooms in or out to the previously viewed extent. Pan Pans the current view. Identify Displays the active layer’s attribute information in the Information/Analysis Window for a user selected feature. Query Opens a query builder in the Information/Analysis Window for a user to define a query based on the active layer’s attributes to search for specific features. Find Opens a search input field in the analysis window for the user to enter the name of a specific feature to find. Please note that numeric fields are not searchable. Measure Measures the distance of a user defined line that can have multiple line segments. Set Units Opens an input field for the user to set the map units to either feet, miles, meters, or kilometers. Buffer Displays attribute information from an indicated layer that are within a specified distance of a selected feature(s) from the active layer. Select by Rectangle Selects features of the active layer that are within a user a drawn rectangle. Select by Line/Polygon Selects features of the active layer that intersect the user’s drawn line or are within the user’s drawn polygon. Clear Selection Clears the currently selected features of the active layer. Prints the current map extent in a predefined frame with a user-defined title and system date stamp.
Address Locator ToolThe address locator tool (Table 2) allows a user to enter a street address to determine if the generated *approximate location on the DFIRM (shown labeled over an aerial image background with color-coded floodzones) is either inside the floodplain or outside the floodplain. If the generated location is outside the floodplain, the user can use the Measure Tool to determine the location’s approximate distance to the floodplain.
Tool Name Explanation Address Locator Tool Locates a user-specified address and displays it labeled on the map. Note: The displayed location is an *approximation (not an exact match) of the address.
*The Address Locator Tool generates approximate locations by geocoding, which is subject to positional errors dependent on the street segment length and the number of addresses per street segment.
Data Download Tool CollectionThe data download tool collection (Table 3) allows users to download their selected FMIS data onto their hard drives for further use.
Table 3. The data download tool collection on the North Carolina Floodplain Mapping Information System (FMIS) interface.
Tool Name Explanation Download Vector Data Downloads a predefined set of displayable vector and point layers, which are restricted to the currently displayed map extent, along with encompassing polygon layers (i.e. county boundaries, FIRM panels, flood hazard areas, municipal boundaries, etc.) to the local hard disk drive as a set of zipped (*.zip) shapefiles. Download DFIRM Downloads a Digital Flood Insurance Rate Map (DFIRM) as an Adobe (*.pdf) file by the user selected FIRM panel. Download FIS Downloads the selected county’s Flood Insurance Study (FIS) as an Adobe (*.pdf) file. Download GeoTiff Image Downloads the aerial photography used for the DFIRM map (*.tif file with accompanying world file) by the user selecting a specific DFIRM panel. Download Elevation Data Downloads elevation data by the user selecting a specific panel on the DEM download grid. The elevation data are delivered as compressed (*..zip) file archives grouped by data type (Bare earth mass points elevation files, 20 Ft. DEM files, and 50 ft. DEM files).
Example: Determining Flood Hazards for a U.S. Army Reserve CenterLet’s say that you are the base commander at the U.S. Army Reserve (USAR) Center at the Pitt-Greenville Airport and you need to know the flood hazards for the base in order to develop a flood preparedness plan. You can either spend an afternoon wading through outdated, paper FIRM maps or quickly get the most up-to-date, digital information from the FMIS interface. The first option is no longer feasible, and you are rapidly online at the FMIS.
Directions (Figure 3):
- Press the [Address Locator Tool].
- Type in the base’s street address, city and zip information in the appropriate fields.
- Press the [Locate] button.
- Select the base’s address from the “Locate Results” list.
Address Locator Results:Your desktop now has the address locator results (Figure 4), which clearly show the base in the 100-year Flooding (AE) zone bordered to the south by the 100-year floodway (AEFW) zone. You can then print out this view using the Print Tool.
In addition to printing out this view, you can use the following download tools to get the pertinent digital data that you will need to develop your flood preparedness plan:
Icon Tool Name Vector Data DFIRMs Flood Insurance Study Flood Insurance Study Elevation Data
Editor’s Note: To review the original articles on the North Carolina floodplain recovery efforts, visit Floyd's Aftermath part one and part two.