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Home , Flood, Flood forecasting, Stream

Gaging and Forecasting

A Partnership of the U.S. Geological Survey and the National Service

Chartered in 1879 by Congress to clas­ Watch sify the public and to examine the , Washingtonr D.C geologic structure, , 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 ..f Valtd USGS is the Nation's leading sci­ until 10 pm. EDT this evening,.,for a large part of western Virgmia...A southward moving ence information agency. As part of its will act to focus heavy producing showers and 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 and over the region and the added focus of the cold front; wul allow for widespread rain to that is useful for mitigation of develop. Saturated from recent will add to the, flooding problem, associated with and 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 and . 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 in terms of and warning for the USGS -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 forecasts and flood warnings. The more than 85 percent of the Nation's and 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 ­ 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 lost is 95, mostly as a forecasting of weather, the issue of 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 , 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 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 preparedness, which decreases 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 depth above some Per your letter request dated June 5, enclosed are updated rtitirigtables for thefollowing arbitrary datum, commonly measured in USGS streamflow gaging stations in Virginia: James River at Lick Run, Jackson River feet, and flow or , 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 and rating is well established, additional dis­ strong 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 . ously. As a matter of practicality, dis­ charge during inclement weather, day or charge is usually estimated from pre- night. 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 of sed­ enables the NWS to transform an obscure when the flow in the river is expected to iment or changes in streambed and 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 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, 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 - " 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. caused by the 1993 Midwest flood were a on small streams near urban areas that The river has reached its 15 foot flood stage financial disaster for the Nation, the loss have a history of flooding. In every case, and will crest between 16 and 18 feet over­ of human lives was relatively small; a records of river discharge must be avail­ night. smaller flood in 1903 claimed 100 rives. able so the NWS can develop a river These rises will continue to move down- Most of the savings in human lives can be model. An important hydraulic input to stream toward Fredericksburg. At 10:00 attributed directly to the early and accu­ these models is the USGS stage/discharge :pm the Fredericksburg gage was reading rate river forecasts that were made possi­ rating. The resulting model is rarely 6.4i$. A sharp rise and crest of around 20 ble by recent advances in remote stream- exact, but it provides estimates of river feel is expected Wednesday afternoon. gaging telemetry and data-intensive river- response to rainfall. Thereafter, when flow modeling, as well as to flood-control heavy rainfall is forecast for the river dams, locks, and levees. basin, those amounts are entered into the Even a well-calibrated model is an model, and the model estimates the river ephemeral commodity. Once a river stage and discharge that will result. As model is developed, changes in watershed River Flood Statement new river and rainfall data are collected characteristics, such as increasing urban­ National V^eather Service, during a storm, the new data are entered ization, drainage improvements, and con­ Washington, D.C , into the computer, and new river forecasts struction of dams and levees, can make 3:00 pm EDT Wednesday, June 2flt 1995 the model obsolete. A continuing cycle of are produced (fig. 5). "Atl :00 am the Rappahannock River at model calibration, collection of river-dis­ -, Remington river stage was 17.6 feet and { charge and rainfall data, and model recali- As forecasts are prepared, water that l rising. 1 bration is required to provide a current, ; * flows into large rivers from upstream useful, and accurate flood-forecasting 1 Stages are at hazardous levels throughout points and streams must be con­ tool. the Rappahannock basin. sidered; in fact, gaging important tribu­ tary streams is often needed even at Working Together locations where forecast services are not The Future provided. These points are used in the During a flood, the USGS and the forecast models as model control points. NWS work together to collect and use the In addition to their role in flood fore­ Because none of the models can predict most up-to-date hydrologic data. The casting, USGS stream-gaging stations exactly what will happen on a river, the USGS furnishes continuous information provide information that is useful for the use of river stages and the associated rat­ on river stage and discharge and provides design and operation of dams, levees, ing curve to reassess continuously how rating revisions to the NWS as they bridges, water- and wastewater-treatment much water is in every stream is a vital become available. The NWS uses its river facilities, and for other engineering part of the forecast process. models and hydrometeorological data works. The data also are used in the prep­ (and forecasts) to predict the discharge at aration of forecasts of public water sup­ ZW 5 each forecast service point and the most plies, monitoring of , and up-to-date stage/discharge rating to fore­ assessment of environmental regulation. cast how deep the water will get. In addition to their use by the USGS and The 1993 Midwest flood presented the NWS, USGS real-time streamflow both agencies with an unprecedented data are used by water management agen­ challenge. During the floods, USGS cies, such as the COE, the Bureau of Rec­ hydrographers made more than 2,000 vis­ lamation, the Tennessee its to stream-gaging stations in the flood- Authority, and the Natural Resources affected areas to verify that instruments Conservation Service, man­ were working and communicating prop­ agement officials, such as FEMA, and erly, to make repairs as needed, and to other Federal, State, and local agencies. measure river discharge. The NWS issued 1995 more than 135 river flood warnings and Of all USGS stream-gaging stations, 90 2,562 river flood statements from June 1 percent are operated by the USGS in Figure 5. Observed and computer model esti­ mates of river stage and selected model to August 15, 1993. Both agencies sup­ cooperation with other Federal, State, and updates for the Rappahannock River at Rem­ plied the U.S. Army Corps of Engineers local agencies. About 50 percent of the ington, Va., during the flood of June 1995. (COE), the Federal Emergency Manage­ stations are funded through cost-sharing vides one-half of the funds for the stations Demand for NWS river-forecast ser­ U.S. Geological Survey, 1995, Federal- and the cooperating agencies provide vices continues to grow owing to an State cooperative water-resources pro­ the other half. Another 40 percent of the expanding population, , and gram: U.S. Geological Survey Fact Sheet stations are funded entirely by the cooper­ economic growth NWS now provides FS-052-95,4 p. ating agency. However, the resulting forecast services at about 4,000 locations. streamflow data are available to all Although new radar technologies and van der Leeden, Frits, Troise, F.L., Todd, potential users through USGS data bases, computer visualization techniques hold D.K., 1990, The water encyclopedia (2d on the Internet, and through USGS significant promise for improving the ed.): Chelsea, , Lewis Publish­ publications. timeliness and accuracy of river forecasts ers, Inc., 808 p. and flood warnings, ground-based verifi­ cation will still be needed even after such Wahl, K.L., Thomas, W.O., Jr., and River Flood Statement technologies are in place. The need for Hirsch, R.M., 1995, The stream-gaging "- National Weather Service, real-time verification of river discharge program of the U.S. Geological Survey: Washington, D.C. and subsequent model adjustment is more U.S. Geological Survey Circular 1123, 4:26:am EDT TJmrsday, June 29, 1995 than a scientific quest for accuracy; it is 22 p. critically important to maintain model , River stage? are falling across the Rapidan accuracy to minimize economic damage 1995, An overview of the stream- and Rappahannock Rivers. At 3:45 amtlie and human suffering. The detail and time­ level on the Rappahannock River at Rem­ gaging program: U.S. Geological Survey liness of the required data can be fur­ Fact Sheet FS-066-95, 4 p. ington was 13,46 feet, well below its 15 nished only by on-site stream-gaging i foot flood stage. The Rappahannock River stations at Fredericksburg crested at around 25.1 For more information, contact feetat 2:30 am this morning. Robert R. Mason, Jr., any of the following: I This will be the last statement of this flood U.S. Geological Survey, and event. Benjamin A. Weiger, U.S. Geological Survey National Weather Service Office of The NWS has developed extensive 415 National Center river-forecasting services that are based References Reston, Virginia 22092 on access to USGS data. When cooperat­ (703) 648-5977 ing agencies have obtained the informa­ tion that they need from a stream-gaging Gilbert, B.K., 1995, Water data program: station, they usually discontinue funding U.S. Geological Survey Fact Sheet National Weather Service for that station. When either party (USGS FS--065-95,4 p. Office of or its cooperators) discontinues funding 1325 East-West Highway for a gage as a result of budget reductions Stallings, Eugene, 1991, Institutional and Silver , 20910 or for other reasons, the operation of the management aspects, flood forecasting (301) 713-0006 station must be discontinued. This and prediction by the National arrangement has an unintended conse­ Weather Service, in Paulson, R.W., Additional earth science information can quence for the NWS and the communities Chase, E.B., Roberts, R., and Moody, be obtained by accessing the USGS that depend on NWS river-forecast ser­ D.W., comps., 1991, National water sum­ "Home page" on the World Wide Web at vices; gaging stations that are critical to mary 1988-89-Hydrologic events and "http://www.usgs.gov" the forecast service may be discontinued floods and droughts: U.S. Geological Sur­ or the NWS "Home page" at owing to circumstances beyond the con­ vey Water-Supply Paper 2375, p. 117- "http://www.nws.noaa.gov." trol of the NWS or of its customers. Since 122. 1983, 57 river-forecast service points have been affected by closure of one or National Weather Service, 1994, The For more information on all USGS more USGS stream-gaging stations and : National Weather reports and products (including maps, the trend accelerated during the early Service, Survey Report, images, and computerized data), call 1990's. Washington, D.C., 281 p. 1-800-USA-MAPS.

August 1995 Fact Sheet FS 209-95