Using GIS for Floodplain Management
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Geographic Information System (GIS) is a tool that can assist floodplain managers in identifying floodprone areas in their community. With GIS, geographical information is stored in a database that can be queried and graphically displayed for analysis. By overlaying or intersecting different geographical layers, floodprone areas can be identified and targeted for mitigation or stricter floodplain management practices.
Like the pages of a book, the topmost layer in GIS will cover the lower layers. Typically, point features are the topmost layer followed by lines, then polygons, and finally raster data such as digital images. When different layers of geographical data are organized in a certain sequence, a floodplain analysis can be performed.
There are many different types of GIS data layers that can be used for floodplain management purposes. The main data layer used is the Federal Emergency Management Agency (FEMA) Q3 flood hazard data, or Digital Flood Insurance Rate Map (DFIRM) depending upon in which county you live.
The Q3 is a digital scanned copy of the paper Flood Insurance Rate Map (FIRM), but contains only a small portion of information compared to the paper map. The Q3 only has the various floodplain zones shown on the paper map. Unlike the paper map, streets, water bodies, survey benchmarks and labels are not stored in the Q3 database. The Q3 is for planning purposes only and cannot be used for permitting or regulating development due to the inconsistency of the data. Figure 1 is an example of the Q3.
The DFIRM contains more information then the Q3 and is much more accurate on a horizontal and vertical level. The DFIRM is the result of a refined analysis of the floodplain using better topographical data. Unlike the FIRM, the DFIRM can be used for planning, permitting and regulatory purposes. Figure 2 is an example of the DFIRM for the same area as Figure 1. Notice how the DFIRM contains more detail then the Q3.
Maryland Department of the Environment (MDE) is currently working with FEMA and local governments to update all of the paper FIRMs in Maryland and to develop a DFIRM in most of Maryland.
Other types of GIS data layers that can be used for floodplain management include property parcel data, road and stream centerlines, and imagery such as Digital Orthophoto Quarter Quadrangles (DOQQs). Figure 3 is an example of incorporating the different data layers together.
GIS can be used to map repetitively flooded properties. Repetitive losses, as they are commonly referred to, indicate the need for FIRM updates, stricter floodplain management practices, or mitigation. A repetitive loss property is a property that has a flood insurance policy with the National Flood Insurance Program (NFIP) and has flooded on two or more occasions within a ten-year period with claims totaling $1000 or more on each loss. The NFIP produces a detailed list of repetitive loss properties including information such as the property address, the name of the person who made the claim, amounts of losses and dates of losses among other things.
In 2002, MDE completed an extensive project to identify all of the repetitive losses in Maryland (See Figure 4 ). A site visit was made to each repetitive loss property, and data was collected and stored in a GIS database. Using a Global Positioning System (GPS), coordinates, first-floor elevations and lowest adjacent grades were collected. In addition, structural information was gathered and digital photographs were taken for each structure. If available, property owners were interviewed to get a better understanding of the extent of flooding and damage that occurred. MDE is currently working on an update of the Repetitive Loss Data Base (2007).
Collecting vertical elevations and horizontal coordinates was made possible using a survey-grade Corvallis Microtechnology (CMT) Z33 GPS fitted with a Laser Technology, Inc. laser rangefinder for distance and elevation measurements, and a magnetic compass for horizontal positioning of offset points.
After all of the information was collected, it was organized in a GIS database and a property information sheet was created for each repetitive loss property. The property sheet included a summary table of flood-related information, a digital photograph, a GIS map showing the location of the structure in relation to the 100-year (or 1% chance) floodplain, and recommendations for mitigation (Figure 5). The information was shared with FEMA, the Maryland Emergency Management Agency (MEMA), and with representatives of each county to include in their local hazard mitigation plans. The overall goal of the project was to identify the repetitive losses in Maryland and create a prioritized list so that as mitigation funds become available, structures with the most severe flooding can be addressed first.
For more information about repetitive loss property data, please call John Joyce at 410-537-3914.
- To calculate the percentage of land in the floodplain by intersecting the county boundary with the FEMA Q3. See Table 1. (.pdf)
- To assist municipalities without GIS mapping capabilities by providing them with a floodplain map that can be used for floodplain management and planning purposes.
- To identify potential problems with Flood Insurance Rate Maps (FIRMs) and target areas for remapping.
- To interface with the FEMA HAZUS-MH flood hazard analysis program to model a 100-year flood and perform damage estimates.
In conclusion, GIS can be a useful tool for floodplain management and planning, MDE is continuously finding new uses for GIS and hopes to add additional data layers and expand its capabilities in the future.
Updated July 8, 2004
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