Stream Gaging and Flood Forecasting

Stream Gaging and Flood Forecasting A Partnership of the U.S. Geological Survey and the National Weather Service Chartered in 1879 by Congress to clas Flash Flood Watch sify the public lands and to examine the National Weather Service, Washingtonr D.C geologic structure, mineral resources, and 4:30 am EDT Tuesday, June 27,1995 products of the national domain, the The National VJeather Service in Washington, D;C has issued a flash flood watch..f Valtd USGS is the Nation's leading earth sci until 10 pm. EDT this evening,.,for a large part of western Virgmia...A southward moving ence information agency. As part of its cold front will act to focus heavy rain producing showers and thunderstorms today, and mission, the USGS provides practical tonight in and close to the watch area, ^combination of a very moist air massmplace information about the Nation's rivers and over the region and the added focus of the cold front; wul allow for widespread rain to streams that is useful for mitigation of develop. Saturated soil from recent rains will add to the, flooding problem, hazards associated with floods and droughts and defines the hydrologic and \ Persons in flood prone areas should monitor rainfall today and have a plan to move to high hydraulic characteristics needed for the ^ground should persistent heavy rains occur. design and operation of engineering projects, such as dams and levees. The Floods are among the most frequent known as the Federal agency in charge of primary source of this information is the and costly natural disasters in terms of weather forecasting and warning for the USGS streamflow-gaging station net human hardship and economic loss. As Nation. Many people, however, are not work. much as 90 percent of the damage related aware that the NWS also is charged by to natural disasters (excluding droughts) law with the responsibility for issuing The USGS operates and maintains is caused by floods and associated mud river forecasts and flood warnings. The more than 85 percent of the Nation's and debris flows. Over the last 10 years National Weather Bureau Organic Act of stream-gaging stations, which includes (1985-94), floods have cost the Nation, 1890 (U.S. Code title 15, section 311) 98 percent of those that are used for real- on average, $3.1 billion annually in dam mandates that the National Weather Ser time river forecasting. Currently, this net ages. The long-term (1925-88) annual vice is the responsible agent for "***the work comprises 7,292 stations dispersed average of lives lost is 95, mostly as a forecasting of weather, the issue of storm throughout the Nation, 4,200 of which are result of flash floods. One has only to warnings, the display of weather and equipped with earth satellite radios that recall the flash flooding of the Big flood signals for the benefit of agricul provide real-time communications. The Thompson River in Colorado in 1976, ture***." The NWS uses many sources of NWS uses data from 3,971 of these sta which killed 139 people as it swept tions to forecast river depth and flow con through campgrounds and vacation data when developing its flood forecasts. ditions at 4,017 forecast-service locations homes nestled in a narrow canyon, to real The U.S. Geological Survey (USGS) is on major rivers and small streams in ize how unexpected and costly, in human the principal source of data on river depth life alone, such phenomena can be. and flow. urban areas (fig. 1). Important elements in the Nation's pro gram to reduce flood damages include flood warnings and river forecasts. Timely warnings and forecasts save lives and aid disaster preparedness, which decreases property damage by an esti mated $1 billion annually. Although the issuance of flood forecasts is now accepted as common and routine, their preparation is no minor feat. This techni cal achievement is made possible by the joint efforts of several Federal, State, and local agencies and many dedicated people across the Nation. A Partnership The National Weather Service (NWS), which is part of the National Oceanic and Figure 1. Locations of U.S. Geological Survey stream-gaging stations that are used by the Atmospheric Administration, is widely National Weather Service to develop river forecasts. Stream Gaging National Weather Service The two most fundamental items of State College, Pa hydrologic information about a river are stage, which is water depth above some Per your letter request dated June 5, enclosed are updated rtitirigtables for the following arbitrary datum, commonly measured in USGS streamflow gaging stations in Virginia: James River at Lick Run, Jackson River feet, and flow or discharge, which is the below Gathrighi Dam, Jackson River at Covmgtori, Maury River near Buena Vista, Rap- total volume of water that flows past a pahannock River near Fredericksburg, dndCraing Creek at Parr. Ratings at the other six point on the river for some period of time, sites are unchanged.,.,The low-end portion of the Gtithright Dam rating is currently usually measured in cubic feet per second Undergoing reassessment but the high flow portion of the rating is not expected to change or gallons per minute. These two key U.S. Geological Survey factors are measured at a location on the Richmond, Virginia river called a stream-gaging station (fig- 2). roughness alter the stage/discharge rela 10 By using automated equipment in the tion. Such changes are particularly preva Hi LU gaging station, river stage can be continu lent during floods. Occasionally, changes LL ? 5 Stage/discharge rating ously monitored and reported to an accu are so severe as to require development of after flood of 1985, racy of 1/8 of an inch. Linking battery- a new stage/discharge rating; this powered stage recorders with satellite occurred at the North River at Stokesville, radios enables transmission of stage data Va., as a result of a major flood in 1985 Stage/discharge rating to computers in USGS and NWS facilities (fig. 4). Thus, even after a stage/discharge before flood of 1985 even when extreme high waters and rating is well established, additional dis strong winds disrupt normal telephone charge measurements are required period 0.1 1 10 100 1,000 10,000 and power services. In this way, USGS ically to detect and track changes and to RIVER DISCHARGE, IN CUBIC FEET PER SECOND and NWS hydrologists know the river update the rating. Updated rating curves Figure 4. Stage/discharge relations at North stage at remote sites and how fast the are provided to the NWS. Because docu River near Stokesville, Va., before and after water is rising or falling. mentation of flood discharges is so impor the flood of 1985. tant, USGS field personnel are routinely It is much more difficult to measure deployed to stream-gaging stations during ily visualized and well-understood mea river discharge accurately and continu periods of high flow to measure river dis sure of public risk, the flood stage. ously. As a matter of practicality, dis charge during inclement weather, day or charge is usually estimated from pre- night. Flood Forecasting established stage/discharge relations, or rating curves. The rating curves are con By using an up-to-date stage/discharge River-flood forecasts are prepared by structed by USGS field personnel who rating and a river-stage reading, an accu 13 NWS river-forecast centers and dis periodically visit the gaging station to rate estimate of the river discharge can be seminated by NWS offices to the public. measure river discharge (fig.3). For more produced. An important characteristic of During periods of flooding, the NWS information about measurement of river a stage/discharge rating is that the process river-forecast centers issue forecasts for discharge see Wahl and others (1995). also works in reverse; given a discharge the height of the flood crest, the date and estimate, the corresponding river stage time when the river is expected to over Changes in river cross sections that can be determined. This functionality flow its banks, and the date and time result from the scour or deposition of sed enables the NWS to transform an obscure when the flow in the river is expected to iment or changes in streambed and bank river parameter, its discharge, into an eas- recede to within its banks. These fore casts are updated as new information is -Satellite acquired. radio antenna River Flood Warning National Weather Service). Washington, D.C. , 4:15 pm EDT Tuesday, June T7\ 1995 ! Heavy rain across the Rappahannock River basin in northern Virginia win cause sig nificant flooding. At 4:10 pm the Rappa-: hannock River at Remington was 12,4 feet and rising sharply: The river should reach its 15 foot flood stage tonight and crest Figure 3. Crane, current meter, and weight between 1$ and 20 feet early Wednesday Figure 2. Schematic of a stilling well and used for measuring the discharge of a river morning. shelter at a stream-gaging station. from a bridge. To develop flood forecasts, the NWS ment Agency (FEMA), and many State develops and calibrates complex mathe River Flood Statement and local agencies with a continuous matical models of how the Nation's rivers National Weather Service; stream of water-related information for and streams respond to rainfall and snow- " Washington, D.C their use in flood management and disas melt. These models are developed for i EDT TuesdayJune 27, 1995 ter mitigation. preselected forecast service points, which At W:OQ pm the Rappahannock River at Although the economic damages are usually located along major rivers or Remington waslSJQZfeet and still rising.

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