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Flood Forecast Mapping, Tar River Basin, North Carolina
Frequently Asked Questions (FAQ) Regarding Flood Forecast MappingClick on a question to access the answer. How accurate are flood forecast maps? Why do we need flood forecast mapping, and who will use this information? How do forecast maps differ from traditional river forecasts? Where can I find some definitions for hydrology terminology? How does the U.S. Geological Survey (USGS) collect stream flow data? What is the history of the National Weather Service's flood forecasting capability? Question: What is the difference between FEMA Digital Flood Insurance Rate Maps (DFIRMs) and NWS Flood Forecast Maps? Answer: Flood forecast maps and DFIRMs are developed for two different purposes. DFIRMs map a statistically derived flood inundation for a specific return period (i.e. 100-year or 500-year chance of flooding) for insurance and floodplain management issues. Flood forecast maps show potential flooding based on current conditions and future rainfall potential over a period of time (24, 48, 72 hours into the future) acting as an early warning system for emergency planning and response. Several important points to consider are that
More Detail: A FEMA Flood Insurance Rate Map (FIRM) is an insurance and floodplain management map issued by FEMA that identifies areas of 100-year flood hazard in a community. In some areas, the map also shows base flood elevations and the 500-year floodplain boundaries and, occasionally, regulatory floodway boundaries. The 100-year or base flood is defined as the flood having a 1 percent probability of being equaled or exceeded in any given year. A common misconception about the 100-year flood is that it represents the peak flow from historical records, or that it will occur once every 100 years. In fact, A 100-year flood has a 26 percent chance of occurring during a 30 year period, the length of many mortgages. The 100-year flood is a statistically derived regulatory standard used by federal agencies, and most states, to administer floodplain management programs. (adopted from FEMA publication 258, May 1988). FIRMs are used by private citizens, insurance agents, mortgage brokers, lenders, community officials, and emergency managers to determine a well-defined boundary of where a flood is most likely to occur. Knowledge of these areas is used to administer floodplain management regulations, mitigate (prevent) flood damage, and ascertain whether flood insurance is required when making loans or providing grants for the purchase or construction of buildings. Digital versions of these maps, otherwise known as the Digital Flood Insurance Rate Maps (DFIRM), have been created for the state of North Carolina and are being made available to the public. DFIRMs show the same information as the FIRMs, but in a digital format that is available in geographic information system (GIS) layout. In other words, the FIRM information is georectified and can be overlaid on orthophotography, roads, and other infrastructure in a GIS environment. More Information Flood Insurance Rate Map Tutorial A flood forecast map shows areas that are likely to flood based on forecasted rainfall and river conditions for up to a 72 hour forecast period (present time to three days in the future). A flood forecast map is intended for planning and preparedness activities for a potential/predicted flooding event, such as determining which roads will be flooded and must be closed. Digital versions of these maps will be generated at selected points on the Tar River in North Carolina and made available to the public. This digital format will be available in a GIS format. Question: How accurate are flood forecast maps? Answer:
More Information LIDAR elevation accuracy in North Carolina Question: What are LIDAR-derived elevation data, and how are they being used in North Carolina? Answer: Light Detection and Ranging (LIDAR) is a remote sensing technology used to collect topographic data. These data are collected with aircraft-mounted lasers capable of recording elevation measurements at a rate of 2,000 to 5,000 observations per second. This enables dense coverage of the ground surface and thus captures subtle changes in topography. The state of North Carolina contracted for the acquisition of elevation data gathered using LIDAR to help update and develop new Digital FIRMs (DFIRMs). Elevation data from LIDAR can be collected and processed relatively quickly, can be very accurate (up to 20 centimeters), and, unlike other methods of developing elevation, do not require daylight for acquisition or extensive manual interpretation. Individual LIDAR points (known as mass points) and breaks in elevation near streams and roads (known as break lines) are used to create a Triangular Irregular Network (TIN) using geographic information system (GIS) software. This TIN is a three-dimensional representation of the ground surface, and this surface is used to map flood inundation. Flood forecasts are issued in terms of height relative to a U.S. Geological Survey (USGS) stream gage. Each USGS stream gage has a known, surveyed datum. Therefore an actual elevation in the North American Vertical Datum of 1988 (NAVD88) is derived and compared to the TIN surface. The resulting comparison is a polygon or area that depicts water coverage based on the flood forecast. More Information Floyd's Aftermath, Remapping North Carolina floodplains with LIDAR Question: Why do we need flood forecast mapping, and who will use this information? Answer: In a word, flooding. It is important to know when a flood crest will occur and how high it will be, especially if it is coming toward people and their property. The ability to provide a map that shows people the areas that could potentially flood can help the general public make preparations and/or make plans for evacuation. Many federal, state, and local planners and emergency managers need to know the same information to make crucial decisions on such things as road closures, evacuations, and infrastructure impacts. Forecast maps will be used as a tool for planning and for estimating potential areas that will be impacted by flood waters. Anyone who will be impacted by flooding will benefit from these products. Question: How do forecast maps differ from traditional river forecasts? Answer: Traditional river forecasts are provided in an ASCII text format, much like the following.
This forecast is for one location along a river reach (length of river) and gives a forecast stage (height) in relation to the USGS gage that is measuring the river level at that location. Flood forecasts in this format are of limited use, because it isn't easy to picture what these numbers mean (i.e., how high is the water and where will it flood?) or how flooding will impact areas in between these measurement points. Impacts to nearby towns or communities can only be extrapolated and many times emergency managers and local governments do not know what the river stage, relative to gage height, means in terms of the area that will be impacted by flooding. It simply is not financially feasible to have continuous gages along every point of the river. The installation and monitoring of such sites is expensive. Flood forecast mapping takes the above text information and converts it to an actual elevation, relative to a vertical datum (NAVD88). This elevation is then mapped on a topographic surface, in this case a LIDAR-derived topographic surface. In addition, areas between current flood forecast points are mapped based on hydraulic models that use river cross-section data to derive a water surface elevation for an entire reach of the river. The result is a product that is much easier to read and understand and gets the message across to people who will use this information to make better-informed decisions. See an example forecast map below.
Question: Where can I find some definitions for hydrology terminology? Answer: USGS Water Science Glossary of Terms Question: How does the U.S. Geological Survey (USGS) collect stream flow data? Answer: Questions: What is the history of the National Weather Service's flood forecasting capability? What is the current and future infrastructure of the National Weather Service's flood forecasting? What tools and models are the National Weather Service using for river forecasting? How is river forecasting tied into the National Weather Service's Advanced Weather Interactive Processing System (AWIPS). Answer: Modernization of National Weather Service River Forecasting (PDF, 81 KB) |
