Hydrographs prediction
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The document discusses a collaboration between the USGS and National Weather Service to develop flood inundation maps based on flood forecasts. The maps show predicted flooded areas, depths, and times of arrival for an approaching storm. This helps fill an important information gap by showing where floodwaters will impact beyond just forecast points. New technologies like LIDAR and modeling software allow the USGS to quickly create detailed maps and make them available online for communities threatened by the predicted flood.
Hydrographs prediction
- 1. U.S. GEOLOGICAL SURVEY Fact Sheet 2004-3060 Mapping a Flood...Before It Happens By J. L. Jones What's missing from flood forecasts? Maps—The only maps generally available today are maps used for planning. They are maps of theoretical floods, not maps of flooding forecast for an approaching storm. The U.S. Geological Survey (USGS) and the National Weather Service (NWS) have developed a way to bring flood forecasting and flood mapping together, producing flood maps for tomorrow's flood today...and getting them on the Internet in time for those in harm's way to react. Figure 1. Time-to-arrival for flood, November 23, 1986, Snoqualmie River, Washington Extending the National Weather Service Forecast Flood Most flood maps today are not maps of real-life floods; they are maps of an imaginary flood used to help communities get an idea of where especially flood prone areas probably are. Sometimes these are called "100-year flood maps," although that phrase is a little misleading because it is based on statistical probabilities for some specific location, not for a region. There's a good chance that a "100-year flood" will occur somewhere in your state every year. Figure 2. Flood waters washing out bridge. Flood forecasts (like the ones seen on TV newscasts) are made by the National Weather Service for storms days in advance of the actual
- 2. flooding. These forecasts estimate the highest level the river will get, based mainly on how much rain is expected. Unfortunately, the forecasts are made only for a few specific places; they don't predict flood levels for anywhere except those specific places. So, unless you live near one of these "forecast points," the predictions are good only as a rough measure of how large the flood is predicted to be. They don't tell you whether your house, or a school, or your local sewage treatment plant is in danger of being flooded. Even if you do live near a forecast point, the forecast is still only an elevation describing the highest expected river level. It doesn't mean a lot to you unless you know your elevation compared to the reference elevation, or "datum," of the forecast point. What you want if you live in a floodplain, whether you live near a forecast point or not, is a map that shows where flooding is expected. When the NWS issues a forecast flood hydrograph (plot of flow vs. time) for a particular location, the USGS uses the hydrograph to create a flood- inundation map. The USGS has developed methods to make maps of these forecast floods for long stretches of a river near forecast points. These maps fill a critical information gap in flood forecasting: information on when and where floodwater from an approaching storm will arrive. The maps also show how deep the water is expected to be all across the floodplain. In essence, these methods combine to make a somewhat obscure "peak forecast elevation" into a map with flooded areas, times-of- arrival, and flood depths. A number of new technologies and methods make the creation of flood forecast maps possible. First is the ability to get very accurate elevations throughout the floodplain quickly and affordably. This is done with "LIDAR" technology (see more below). Second is a computer program (TRIMR2D) that can simulate flood flows all across the floodplain and many, many miles downstream from the forecast point. Third is spatial analysis software (GIS) that turns the model results into maps and overlays them on other maps, like a map of a neighborhood, or even onto an aerial photograph. Last is software (IMS) that makes the maps available on the Internet in a flexible and user-friendly way. (You can see a demonstration of the system on the Web at http://wa.water.usgs.gov/projects/pugethazards/urbanhaz/MappingNWS. htm.)