Debris Flow Warning Systems:
an Opportunity for the National Weather Service

Arthur Zeizel
Policy Manager
U.S. Emergency Management Agency
Federal Center Plaza
Washington, DC


This paper seeks to alert the National Weather Service to the opportunity of improving and making operational debris flow warning systems. After a brief discussion of the nature of the phenomenon, its economic and social impacts, trends, and relationship to the emergency management sector, suggestions are made on how to improve in the San Francisco Bay Region.


Debris flows, often called mudslides, are rapidly moving flows of mixed rocks and mud that move downhill at speeds of 35 miles per hour or more, kill people and destroy homes, roads, bridges, and other property. They are caused chiefly by prolonged, heavy rainfall on saturated hillslopes, rapid snowmelt, earthquakes and the works of human kind. As we meet, four Western states are suffering major flood and landslide disasters, including debris flows.

The economic and social costs of storm-related debris flows and other landslide phenomena are estimated to be about $2.0 billion and 25 to 50 deaths annually, occurring throughout the nation. These figures are increasing as development moves into marginal lands and onto the alluvial fans with desirable views. In the San Francisco Bay region, past storms causing debris flows occurred in 1982, 1986, 1995 and 1996, thousands of such flows were caused by the severe storms in 1982 and caused $66 million in damages and the deaths of 25 people.

Emergency Management and Mitigation

The reduction of these unnecessary costs to the nation can be achieved by improved emergency management and mitigation. Emergency management includes the prediction of the hazard, the issuance of warnings, and the measures that are necessary to insure that the warning and response will be effective. Emergency response includes such measures as evacuation, search and rescue, and provision of emergency food, shelter and health care. Mitigation, or long term hazard reduction, is accomplished by such means as avoidance, design, building, and grading codes, control and stabilization, and insurance.

Successful mitigation of these losses can be achieved by informed land use planing and management, engineering works, and warning and evacuation programs. This is perhaps best illustrated in Japan where in 1938 130,000 homes were destroyed and more than 500 lives lost due to landslides in the Kobe area. After a strong Japanese mitigation program became law, losses declined to only 2000 homes and 125 lost lives in 1976, one of their worst years for landslides.

This paper focuses upon the opportunity that exists in improving the debris flow warning systems drawing upon the experience of the Real-Time Warning System, a pilot program, that was put into place in the San Francisco Bay region by the National Weather Service and the U.S. Geological Survey from 1985 to 1995. Many valuable insights into how similar debris flow warning systems should be established could be gained from a review of the history and demise of the Bay Region warning system.

Debris Flow Warning System in San Francisco Bay Region

The warning system was established in 1985, issued its first warnings during the severe storms of 1986 and gave its last warnings in January, 1995. Then it died. The warning system was developed by the U.S. Geological Survey and the National Weather Service and operated in cooperation with the State of California Office of Emergency Services and local governments agencies. The warnings issued by the system accurately predicted the times of actual debris flow events during storms and were used by several local governments as a basis for planning emergency response and for recommending temporary evacuation of hazardous areas.

The warning system was based upon empirical and analytical relations between rainfall and debris flow generation, real time regional monitoring of rainfall data from telemeter rain gauges, NWS precipitation forecasts, and delineation of debris flow hazard areas. It was developed by research geologists and weather forecasters and operated on a shoestring budget.

From most perspectives, the warning system was a successful demonstration of applied interdisciplinary science conducted on an interagency basis, and constituted a prototype system that could be adopted for use in other high-risk areas in the nation. But what went wrong? Why did such a promising warning system fail? Especially since similar warning systems in the Los Angeles area are working.

It would be presumptuous to say that anyone, particularly this author, could definitively figure out what went wrong and how to avoid making similar mistakes in the future. But certain insights were gained from long conversations with many of the scientists involved. These observations are offered in the hope that they may prove helpful in any future efforts to establish and maintain an effective debris flow, or any other hazard, warning system.


Enough science had been done to establish the relationship between actual and predicted rainfall, ground saturation, and geological parameters to set threshold levels for the occurrence of debris flows and to provide the basis for accurate warning. The San Francisco Bay Region Debris Flow Warning System moved through the research and demonstration phase during the ten years of its existence and was at a point where it could have become operational on a continuing basis. It was at this critical juncture that it was shut down, primarily because of the lack of support by the Federal agencies involved. Why weren't the minimal staff and dollar resources needed to continue a proven debris flow warning system provided by anyone? Why didn't the U.S. Geological Survey that identified the warning system as an exemplary project in their budget documents or the National Weather Service step forward with the necessary resources?

A scientist involved with the system cited the severe budgetary constraints in 1995 and the reorganization and reduction in force of the U.S. Geological Survey as contributing factors. The lack of a formal agreement between the two Federal agencies that resolved the issue of who would fund and staff continued operation was mentioned. Also, the absence of a "broker" to let the state and local governments know in time that new support had to be provided to continue the warning system was suggested as another contributing factor.

Important as these factors may have been, it seems that a critical element needed for success was missing. The users and beneficiaries of the warning system, the state and local governments, were not able or willing to step forward in time to assume a major responsibility for its continued operation. Even if their support was sought at the time when the system was being dismantled, by then it was too late. Experience has shown that the use of scientific and technical information can best, and perhaps only, be assured if the user community is involved early and continuously in the process. In the similar debris flow warning system in operation in Los Angeles County, the county fire department has the main responsibility for operation of much of the warning system and for emergency response, in cooperation with the National Weather Service.


The Debris Flow Warning System experience in the San Francisco Bay region shows that the scientific knowledge exists to put similar systems into place in other high risk areas in the nation. With the advent of NEXRAD and GIS capability and of models for predicting the probabilities of debris flows and processing spatial, temporal and socio-economic variables, the probabilities of debris flow occurrence and their impacts now can be predicted spatially in great detail for any given hour. Such a real-time model for predicting debris flow occurrence would provide local emergency management officials with the ability to prioritize evacuations and to take other emergency response and mitigation measures during severe storms.

An administrative mechanism is in place to conduct the necessary scientific and technical studies in the geologic, hydrologic, and atmospheric sciences to improve the capability for public warning of potential hazards from debris flows and other landslides that result from severe weather conditions. A formal Memorandum of Understanding agreeing to the joint conduct of such research was signed in 1995 by the U.S. Geological Survey and the National Weather Service. Progress has been slow and no joint research project has been started.

What remains to be done is for these agencies to begin to conduct the joint research and development called for in their formal agreement and to help formulate the necessary emergency management and mitigation tools required to deal with the debris flow hazard. If the cooperative effort moves forward as it should, the emergency management community at all levels of government and other key users of this information should be fully involved in the process, early and continuously. Their substantial involvement is particularly critical as projects under the cooperation move from research and demonstration to becoming operational.

Sources of Technical Information

Reducing Losses from Landslides in the United States, by the Committee on Ground Failure Hazards, National Research Council, 1985, 41 p.

Landslide Loss Reduction: A Guide for State and Local Governmental Planning, by Robert L. Wold, Jr., Candace L. Johnson, Federal Emergency Management Agency Publication 182, August, 1989.

Operation of a Real-time Warning System for Debris Flows in the San Francisco Bay Area, California, by R.C. Wilson, R.K. Mark, and G. Barbato; Hydraulic Engineering 1993, Proceedings of the 1993 Conference, ASCE, p. 1908-1913.

Real-time landslide warning during heavy rainfall, by D.K. Keefer and others: Science, vol 238, November 1987, p. 921-925.

Memorandum of Understanding Between the U.S. Geological Survey, and the National Weather Service, 1995.

Societal Aspects of Weather

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