Flash Flood Watch
National Weather Service, Washington, D.C.
4:30 am EDT Tuesday, June 27, 1995The National Weather Service in Washington, D.C. has issued a flash flood watch...Valid until 10 pm. EDT this evening...for a large part of western Virginia...A southward moving cold front will act to focus heavy rain producing showers and thunderstorms today, and tonight in and close to the watch area. A combination of a very moist air mass in place over the region and the added focus of the cold front will allow for widespread rain to develop. Saturated soil from recent rains will add to the flooding problem.
Persons in flood prone areas should monitor rainfall today and have a plan to move to high ground should persistent heavy rains occur.
Important elements in the Nation's program 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 estimated $1 billion annually. Although the issuance of flood forecasts is now accepted as common and routine, their preparation is no minor feat. This technical achievement is made possible by the joint efforts of several Federal, State, and local agencies and many dedicated people across the Nation.
Chartered in 1879 by Congress to classify the public lands and to examine the geologic structure, mineral resources, and products of the national domain, the USGS is the Nation's leading earth science information agency. As part of its mission, the USGS provides practical information about the Nation's rivers and streams that is useful for mitigation of hazards associated with floods and droughts and defines the hydrologic and hydraulic characteristics needed for the design and operation of engineering projects, such as dams and levees. The primary source of this information is the USGS streamflow-gaging station network.
The USGS operates and maintains more than 85 percent of the Nation's stream-gaging stations, which includes 98 percent of those that are used for real-time river forecasting. Currently, this network comprises 7,292 stations dispersed throughout the Nation, 4,200 of which are equipped with earth satellite radios that provide real-time communications. The NWS uses data from 3,971 of these stations to forecast river depth and flow conditions at 4,017 forecast-service locations on major rivers and small streams in urban areas (fig. 1).
Figure 1. Locations of U.S. Geological Survey stream-gaging stations that
are used by the National Weather Service to develop river
forecasts.
By using automated equipment in the gaging station, river stage can be continuously monitored and reported to an accuracy of 1/8 of an inch. Linking battery-powered stage recorders with satellite radios enables transmission of stage data to computers in USGS and NWS facilities even when extreme high waters and strong winds disrupt normal telephone and power services. In this way, USGS and NWS hydrologists know the river stage at remote sites and how fast the water is rising or falling.
It is much more difficult to measure river discharge accurately and continuously. As a matter of practicality, discharge is usually estimated from pre-established stage/discharge relations, or rating curves. The rating curves are constructed by USGS field personnel who periodically visit the gaging station to measure river discharge (fig.3). For more information about measurement of river discharge see Wahl and others (1995).
Changes in river cross sections that result from the scour or deposition of sediment or changes in streambed and bank roughness alter the stage/discharge relation. Such changes are particularly prevalent during floods. Occasionally, changes are so severe as to require development of a new stage/discharge rating; this occurred at the North River at Stokesville, Va., as a result of a major flood in 1985 (fig. 4). Thus, even after a stage/discharge rating is well established, additional discharge measurements are required periodically to detect and track changes and to update the rating. Updated rating curves are provided to the NWS. Because documentation of flood discharges is so important, USGS field personnel are routinely deployed to stream-gaging stations during periods of high flow to measure river discharge during inclement weather, day or night.
National Weather Service
State College, PaPer your letter request dated June 5, enclosed are updated rating tables for the following USGS streamflow gaging stations in Virginia: James River at Lick Run, Jackson River below Gathright Dam, Jackson River at Covington, Maury River near Buena Vista, Rappahannock River near Fredericksburg, and Craing Creek at Parr. Ratings at the other six sites are unchanged....The low-end portion of the Gathright Dam rating is currently undergoing reassessment but the high flow portion of the rating is not expected to change.
U.S. Geological Survey
Richmond, Virginia
River Flood Warning
National Weather Service,
Washington, D.C.
4:15 pm EDT Tuesday, June 27, 1995Heavy rain across the Rappahannock River basin in northern Virginia will cause significant flooding. At 4:10 pm the Rappahannock River at Remington was 12.4 feet and rising sharply. The river should reach its 15 foot flood stage tonight and crest between 18 and 20 feet early Wednesday morning.
As forecasts are prepared, water that flows into large rivers from upstream points and tributary streams must be considered; in fact, gaging important tributary streams is often needed even at locations where forecast services are not provided. These points are used in the forecast models as model control points. Because none of the models can predict exactly what will happen on a river, the use of river stages and the associated rating curve to reassess continuously how much water is in every stream is a vital part of the forecast process.
River Flood Statement
National Weather Service,
Washington, D.C.
11:00 pm. EDT Tuesday June 27, 1995At 10:00 pm the Rappahannock River at Remington was 15.02 feet and still rising. The river has reached its 15 foot flood stage and will crest between 16 and 18 feet overnight.
These rises will continue to move downstream toward Fredericksburg. At 10:00 pm the Fredericksburg gage was reading 6.48. A sharp rise and crest of around 20 feet is expected Wednesday afternoon.
The 1993 Midwest flood presented both agencies with an unprecedented challenge. During the floods, USGS hydrographers made more than 2,000 visits to stream-gaging stations in the flood- affected areas to verify that instruments were working and communicating properly, to make repairs as needed, and to measure river discharge. The NWS issued more than 135 river flood warnings and 2,562 river flood statements from June 1 to August 15, 1993. Both agencies supplied the U.S. Army Corps of Engineers (COE), the Federal Emergency Management Agency (FEMA), and many State and local agencies with a continuous stream of water-related information for their use in flood management and disaster mitigation.
Although the economic damages caused by the 1993 Midwest flood were a financial disaster for the Nation, the loss of human lives was relatively small; a smaller flood in 1903 claimed 100 lives. Most of the savings in human lives can be attributed directly to the early and accurate river forecasts that were made possible by recent advances in remote stream-gaging telemetry and data-intensive river-flow modeling, as well as to flood-control dams, locks, and levees.
River Flood Statement
National Weather Service,
Washington, D.C.
3:00 pm EDT Wednesday, June 28, 1995At 1:00 am the Rappahannock River at Remington river stage was 17.6 feet and rising.
Stages are at hazardous levels throughout the Rappahannock basin.
Of all USGS stream-gaging stations, 90 percent are operated by the USGS in cooperation with other Federal, State, and local agencies. About 50 percent of the stations are funded through cost-sharing arrangements whereby the USGS provides one-half of the funds for the stations and the cooperating agencies provide the other half. Another 40 percent of the stations are funded entirely by the cooperating agency. However, the resulting streamflow data are available to all potential users through USGS data bases, on the Internet, and through USGS publications.
River Flood Statement
National Weather Service,
Washington, D.C.
4:26 am EDT Thursday, June 29, 1995River stages are falling across the Rapidan and Rappahannock Rivers. At 3:45 am the level on the Rappahannock River at Remington was 13.46 feet, well below its 15 foot flood stage. The Rappahannock River at Fredericksburg crested at around 25.1 feet at 2:30 am this morning.
This will be the last statement of this flood event.
Demand for NWS river-forecast services continues to grow owing to an expanding population, urbanization, and economic growth-NWS now provides forecast services at about 4,000 locations. Although new radar technologies and computer visualization techniques hold significant promise for improving the timeliness and accuracy of river forecasts and flood warnings, ground-based verification will still be needed even after such technologies are in place. The need for real-time verification of river discharge and subsequent model adjustment is more than a scientific quest for accuracy; it is critically important to maintain model accuracy to minimize economic damage and human suffering. The detail and timeliness of the required data can be furnished only by on-site stream-gaging stations
--Robert R. Mason, Jr., U.S. Geological Survey, and Benjamin A. Weiger, National Weather Service
from U.S. Department of the Interior, U.S. Geological Survey, Fact Sheet FS-209-95