NOAA-USGS Debris-Flow Warning System—Final Report

By NOAA-USGS Debris Flow Task Force

This report was prepared in response to the Debris Flow Team charter

and NOAA/USGS Memorandum of Understanding 052-49

Circular 1283

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

P. Patrick Leahy, Acting Director

U.S. Geological Survey, Reston, Virginia: 2005

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NOAA-USGS Debris Flow Task Force, 2005, NOAA-USGS debris-flow warning system—

Final report: U.S. Geological Survey Circular 1283, 47 p.

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Foreword

Landslides threaten lives and property in every State in the United States. The autumn and 

winter of 2004 and 2005 were particularly active landslide seasons, with numerous landslides 

caused by hurricanes on the East Coast and heavy rainfall on the West Coast. Debris flows 

are an especially destructive form of landslides and cause loss of life and millions of dollars in 

damages annually in the United States. Debris flows following wildfires also pose significant 

hazards. 

This document presents the findings and recommendations of a joint NOAA-USGS Task 

Force that assessed the current state-of-the-art in precipitation forecasting and debris-flow 

hazard-assessment techniques. This report includes an assessment of the science and resources 

needed to establish a demonstration debris-flow warning project in recently burned areas of 

southern California and the necessary scientific advancements and resources associated with 

expanding such a warning system to unburned areas and, possibly, to a national scope.

The development of this document has been a truly collaborative process between NOAA and 

USGS and highlights the potential for strong continuing scientific and operational partnerships 

between the agencies. Implementation of the prototype warning system will support the NOAAUSGS goal of issuing timely debris-flow Warnings to public officials and affected communities.

P. Patrick Leahy D.L. Johnson

Acting Director, USGS Director, NOAA’s National Weather Service

iii
Executive Summary

Landslides and debris flows cause loss of life and millions of dollars in property damage annually in the United States (National Research Council, 2004). In an effort to reduce loss of life by 

debris flows, the National Oceanic and Atmospheric Administration’s (NOAA) National Weather 

Service (NWS) and the U.S. Geological Survey (USGS) operated an experimental debris-flow 

prediction and warning system in the San Francisco Bay area from 1986 to 1995 that relied on 

forecasts and measurements of precipitation linked to empirical precipitation thresholds to 

predict the onset of rainfall-triggered debris flows. Since 1995, there have been substantial 

improvements in quantifying precipitation estimates and forecasts, development of better 

models for delineating landslide hazards, and advancements in geographic information technology that allow stronger spatial and temporal linkage between precipitation forecasts and hazard 

models. Unfortunately, there have also been several debris flows that have caused loss of life 

and property across the United States. Establishment of debris-flow warning systems in areas 

where linkages between rainfall amounts and debris-flow occurrence have been identified can 

help mitigate the hazards posed by these types of landslides. Development of a national warning system can help support the NOAA-USGS goal of issuing timely Warnings of potential debris 

flows to the affected populace and civil authorities on a broader scale.

This document presents the findings and recommendations of a joint NOAA-USGS Task 

Force that assessed the current state-of-the-art in precipitation forecasting and debris-flow 

hazard-assessment techniques. This report includes an assessment of the science and resources 

needed to establish a demonstration debris-flow warning project in recently burned areas of 

southern California and the necessary scientific advancements and resources associated with 

expanding such a warning system to unburned areas and, possibly, to a national scope.

The principal findings of the NOAA-USGS Task Force are:

Interviews with potential users of the demonstration warning system revealed that (1) some 

users saw the benefit from warning system for burned areas, whereas others thought that 

they already have sufficient knowledge to deal with such a situation. However, users clearly 

stated the need for an enhanced effort. Further, users clearly expressed the importance of 

executing the research plan outlined in this document and incorporating its results into better 

operational models, as appropriate, and as added resources warrant. (2) In addition to advisory 

Outlooks, Watches, and Warnings, map products that provide information about areas that 

could be impacted by debris flows are considered useful. (3) Lead times of 24 to 48 hours for 

Outlooks and Watches and 24 hours for Warnings are desired. (4) Extension of a system to cover 

unburned areas would be a valuable contribution.

It is possible to institute a demonstration debris-flow forecasting and warning project for 

recently burned areas in southern California. Debris flows are common following wildfires, and 

rainfall intensity duration thresholds for debris-flow occurrence have been developed for parts 

of southern California using analyses of rainfall and response data from recently burned areas. 

These quantitative thresholds provide an improvement over the present method of identification 

of dangerous rainfall conditions based on professional opinion and experience. The demonstration project will cover the service area of the National Weather Service Weather Forecasting 

Offices (WFO) at Oxnard and San Diego, Calif., which includes the counties of San Luis Obispo, 

iv
Santa Barbara, Ventura, Los Angeles, San Bernardino, Orange, Riverside and San Diego. Areas 

within those counties are prone to wildfires in close proximity to developed areas, and heavy 

rainfall over those areas has resulted in debris flows that caused considerable loss of life and 

property damage. 

Given presently available resources, the Task Force determined that the only practical way to 

start a demonstration system is to use the National Weather Service Flash Flood Monitoring 

and Prediction (FFMP) system, which is currently operationally available at the WFOs. FFMP 

is used to identify when flash floods are likely to occur based on comparisons between radar 

precipitation estimates and rainfall intensity-duration threshold values. Advisory Outlooks, 

Watches, and Warnings are disseminated to emergency-management personnel through the 

Advanced Weather Information Processing System (AWIPS). Given that basic levels of debrisflow forecasting can also utilize precipitation thresholds, the Task Force concluded that the 

FFMP provides the most cost-effective and expedient approach to implement a warning system 

on a 24 hour x 7 day (24x7) basis. The approach relies on USGS providing NWS with precipitation thresholds developed for post-wildfire flash floods and debris flows, and NWS determining 

actual gridded precipitation accumulations and forecasts and then issuing the appropriate advisory Outlook, Watch, or Warning. The USGS also provides necessary training to WFO staffers for 

interpreting debris-flow thresholds. 

The Task Force recommends that, as a first step in the advancement of the science, and with 

appropriate funding, a smaller area within the larger demonstration area be dedicated to intense 

instrumentation and research to enhance and develop new geological, hydrological, and hydrometeorological methods to improve precipitation forecasts and measurement techniques and 

debris-flow-forecasting models. Funding for hydrometeorological and geological instrumentation 

and research should be about $1.0 million for a 5-month deployment and 1 year of analysis. 

Given appropriate resources and scientific focus, considerable potential exists for enhancing 

and expanding the warning system to provide spatially and temporally explicit information 

specific to debris flows. Expansions include incorporating improved forecasts and measurements of precipitation as well as methods for delineating where debris-flows might occur, how 

big the events might be, and what areas might be impacted. The team defined the scientific and 

operational requirements necessary to enhance the system in the near and long terms. Issues, 

research needs, and potential warning-system products that can be developed over different 

time scales and in different areas are identified in tables 3, 4, 5, 6, 7, 8, and 9. Personnel and 

expertise needs for the implementation, operation, and maintenance of any expanded system 

are included in tables 10, 11, 12, 13, 14, and 15.

In the near term (2–5 years), the demonstration warning project can be refined and expanded 

to other burned areas within southern California. Expansion of the warning system beyond the 

demonstration project but within the FFMP framework would consist of refinement of existing 

rainfall thresholds and development of new regionally specific thresholds. Implementation of 

models to provide near-real-time mapping of basin-scale debris-flow probability, magnitude, 

and areas of inundation is possible but requires the development of an operational framework 

beyond that of the FFMP to address issues specific to debris flows.

Near-term (2–5 years) expansion of the demonstration warning system to burned areas beyond 

southern California is possible within the FFMP framework but will require the refinement of 

existing rainfall thresholds and development of new thresholds for additional areas. Developv
ment of models that provide near-real-time mapping of basin-scale debris-flow probability, 

magnitude, and areas of inundation is possible but requires the development of an operational 

framework beyond that of the FFMP. Physically based models that characterize the hydrological 

response of burned areas could be incorporated into a warning system. 

Near-term (2–5 years) development and expansion to a nation-wide debris-flow warning system 

in areas other than those burned by wildfire requires development of an operational framework 

separate from the FFMP and requires, at a minimum, development of regionally specific rainfall 

intensity-duration thresholds. An expanded warning system could potentially provide map products that identify areas of instability in the event of heavy rainfall and areas of inundation for a 

range of possible debris-flow volumes.

More substantive development and expansion of a debris-flow warning system over the longer 

term (5–10 years) within burned areas in the United States includes the development and calibration of physically based models for post-fire runoff and erosion and improvement of inundation-area mapping. 

Longer term (5–10 years) development and expansion to a national debris-flow warning system 

in areas other than those burned by wildfire requires development and implementation of physically based models for slope failure that, when linked with spatially distributed precipitation 

forecasts and measurements, can provide near-real-time information on where and when within 

a storm debris flows are likely to occur. Methods for predicting possible debris-flow volumes can 

potentially be linked with inundation-area mapping to provide map products showing probable 

impacts. We expect this effort to be considerably leveraged by current research at the NWS’ 

Office of Hydrologic Development on high-resolution distributed models. The implementation of 

those models will allow the computation of soil-moisture forecasts that could be coupled with 

high-resolution slope-stability models.

Although this report describes a likely fruitful collaboration between the NWS and the USGS 

and potential capabilities of a debris-flow early warning system, the Task Force wishes to 

emphasize that both the human capital and financial resources required to successfully implement, operate, and advance such a system are beyond those available to either agency at this 

time. A long-term commitment of such resources from both agencies is needed prior to the 

implementation of any such warning system.

vi
Contents

Foreword ........................................................................................................................................................iii

Executive Summary......................................................................................................................................iv

Introduction.....................................................................................................................................................1

Motivations, History, and a New Opportunity ..................................................................................1

Hazards Posed by Debris Flows.........................................................................................................2

Terminology............................................................................................................................................3

Objectives of the Task Force...............................................................................................................3

Acknowledgments.........................................................................................................................................3

Leadership Team...................................................................................................................................3

Management Team...............................................................................................................................4

Task Force ..............................................................................................................................................4

External Cooperators............................................................................................................................5

Debris Flows ...................................................................................................................................................5

Triggers...................................................................................................................................................5

Transformation from Landslides................................................................................................5

Mobilization of Channel Deposits .............................................................................................5

Hazard Assessments............................................................................................................................6

When? (Rainfall Intensity-Duration Thresholds) ....................................................................6

Where? (Hazard Mapping).........................................................................................................6

How Big? (Volume or Peak Discharge)....................................................................................7

How Far? (Runout and Inundation-Area Mapping)................................................................8

Precipitation....................................................................................................................................................9

Measuring Precipitation......................................................................................................................9

Rain Gages....................................................................................................................................9

Radar ...........................................................................................................................................9

Multisensor Techniques ...........................................................................................................10

Forecasting ..........................................................................................................................................10

Debris-Flow Warning Systems ..................................................................................................................11

Review of Existing Systems ..............................................................................................................11

Current NOAA-USGS Capabilities and Limitations .......................................................................12

Elements of a Debris-Flow Warning System..................................................................................12

Products ......................................................................................................................................12

Data Forms and Formats..................................................................................................12

Procedures and Protocols .......................................................................................................13

Training ........................................................................................................................................13

Outreach and Information ........................................................................................................13

Validation Methodology............................................................................................................13

A Prototype Debris-Flow Warning System.....................................................................................14

Southern California Setting......................................................................................................14

History of Debris-Flow Activity.......................................................................................14

Geology...............................................................................................................................15

Vegetation ..........................................................................................................................15

vii
Climatology and Meteorology.........................................................................................15

Existing Meteorological Operational Systems.............................................................16

Identification of Customers/Collaborators and Their Requirements ................................19

Elements of the Prototype Warning System..........................................................................20

Products .............................................................................................................................20

Implementation .................................................................................................................21

Procedures and Protocols ..............................................................................................21

Training ...............................................................................................................................21

Outreach and Information ...............................................................................................21

Validation Methodology...................................................................................................22

Research ............................................................................................................................22

Implementation Plan of the Prototype Warning System.....................................................22

Implementation and Operational Costs of the Prototype Warning System .....................22

Operational Considerations .....................................................................................................23

Entrance/Exit Strategies..................................................................................................23

Public Affairs Factors.......................................................................................................23

Intensive Research Area..........................................................................................................23

Validation Methodology...................................................................................................23

Implementation and Operation Costs.....................................................................................24

Future Development ....................................................................................................................................24

Precipitation Measurement ..............................................................................................................24

Refined Precipitation Forecasts and Measurements..........................................................24

Operational Costs and Impacts ...............................................................................................26

Distributed Hydrologic Modeling Issues ........................................................................................26

Hydrology and Geology Modeling Issues for Burned Areas.......................................................27

Developments Possible Within 2 to 5 Years Time Frame to Expand Capability of the 

Existing Prototype System for Burned Areas in Southern California ..................27

Refining Existing Rainfall Thresholds ............................................................................27

Generation of Basin-Scale Debris-Flow Probability and Volume Maps..................27

Generation of Debris-Flow-Inundation-Area Maps....................................................27

Developments Possible Within 5 to 10 Years Time Frame to Expand Capability of the 

Existing Prototype System for Burned Areas in Southern California ..................28

Develop Magnitude/Frequency Relations for Debris Flows......................................28

Develop Automated Warning System ...........................................................................28

Developments Possible Within 2 to 5 Years Time Frame to Expand Prototype 

System to Burned Areas beyond Southern California ...........................................28

Definition of Rainfall Thresholds ....................................................................................28

Generation of Basin-Scale Debris-Flow Probability and Volume Maps..................28

Developments Possible Within 5 to 10 Years to Expand Prototype System 

to Burned Areas beyond Southern California .........................................................28

Generation of Debris-Flow-Inundation-Area Maps....................................................28

Development and Calibration of Physically Based Models.......................................28

Hydrology and Geology Issues for Unburned Areas ....................................................................28

Developments Possible Within 2 to 5 Years for Unburned Areas......................................29

Development of Rainfall Thresholds..............................................................................29

Statistical and Physically Based Methods...................................................................29

viii
Debris-Flow-Inundation Modeling.................................................................................29

Developments Possible Within 5 to 10 Years for Unburned Areas....................................29

Physically Based Models for Spatially and Temporally Specific Projections........29

Methods for Predicting Debris-Flow Volumes.............................................................29

Refine Inundation-Area Mapping Using Ground-Based or 

Airborne-Based LiDAR .......................................................................................29

Magnitude/Frequency Relations for Debris Flows......................................................29

Develop Automated Warning System ...........................................................................30

Snowmelt-Triggered Debris Flows.................................................................................30

Developments in 10+-Year Time Frame for Unburned Areas..............................................30

Issues and Products Tables .....................................................................................................30

Resources, Expertise, and Impacts .................................................................................................32

Conclusions and Recommendations ........................................................................................................35

Conclusions..........................................................................................................................................35

Recommendations..............................................................................................................................36

References....................................................................................................................................................36

Appendix A—Functions of NWS Organizations Involved in the Preparation of 

Precipitation Forecasts .................................................................................................................42

Meteorological Development Laboratory.......................................................................................42

Hydrometeorological Prediction Center.........................................................................................42

Weather Forecast Offices and River Forecast Centers................................................................43

Appendix B—Snowmelt and Rain-on-Snow Triggering of Debris Flows...........................................43

Appendix C—List of Potential Customers and Collaborators ..............................................................45

Appendix D—Customer Requirements....................................................................................................46

Customers in the WFO Oxnard area ................................................................................................46

San Luis Obispo..........................................................................................................................46

Santa Barbara ............................................................................................................................46

Ventura ........................................................................................................................................46

Los Angeles.................................................................................................................................46

Customers in the WFO San Diego area...........................................................................................46

Riverside......................................................................................................................................46

San Diego....................................................................................................................................46

San Bernardino ..........................................................................................................................46

Figures

1. Proposed prototype region for issuing debris-flow Forecasts and Warnings .................2

2. Rainfall thresholds for La Honda study site in northern California.....................................7

3. Probability of debris-flow occurrence after the 2002 Missionary Ridge fire 

in Colorado in response to 25-year recurrence, 1-hour duration rainstorm.....................8

4. Topography of southern California.........................................................................................16

5. Map of mean annual precipitation for southern California................................................17

6. Topography map of southern California and the location of ALERT rain gages, 

NWS operational radars, upper-air radiosonde sites ........................................................17

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7. Southern California ALERT rain gage locations and WSR-88D radar 

low-level coverage ..................................................................................................................18

8. Example of storm-total maps of precipitation accumulation available 

in real time on the Internet from every NEXRAD radar site...............................................19

9. Vertical section of radar coverage across the Los Angeles Basin area.........................20

10. Base map of an enhanced precipitation measurement network 

that could be deployed in and near a recently burned mountainous 

area susceptible to debris flows ............................................................................................25

Tables

1. Prototype implementation plan...............................................................................................22

2. Summary of estimated meteorologic and geologic implementation 

costs for a 5-month deployment of the Intensive Research Area....................................24

3. Issues and products by expanding capability of existing prototype—2–5 years..........30

4. Issues and products by expanding capability of existing prototype—5–10 years........30

5. Issues and products by expanding prototype to burned areas beyond 

southern California—2–5 years..............................................................................................31

6. Issues and products by expanding prototype to burned areas beyond 

southern California—5–10 years............................................................................................31

7. Issues and products to establish warning system in areas other than 

recent fires—2–5 years ...........................................................................................................31

8. Issues and products to establish warning system to areas other than 

recent fires—5–10 years .........................................................................................................31

9. Issues and products to establish warning system in areas other than 

recent fires—10+ years ...........................................................................................................31

10. Personnel and expertise necessary to expand capability of a prototype 

system for burned areas in southern California (and to move beyond 

reliance on FFMP system).......................................................................................................32

11. Operation and maintenance items to expand capability of a prototype 

system for burned areas in southern California (and to move beyond 

reliance on FFMP system).......................................................................................................33

12. Personnel and expertise necessary to expand prototype system to a region

with burned areas beyond southern California ...................................................................33

13. Operation and maintenance items to expand prototype system to a region

with burned areas beyond southern California ...................................................................33

14. Near- and long-term implementation items to establish warning system 

in areas other than recent burns per year per region........................................................34

15. Operation and maintenance items to establish warning system 

in areas other than recent burns per year per region........................................................35

C-1. List of potential customers and collaborators .....................................................................45

x
List of Selected Acronyms Used in this Report

 Acronym Meaning 

 AL NOAA’s Aeronomy Laboratory 

 ALERT Automated Local Evaluation in Real Time 

 ASCII American Standard Code for Information Interchange 

 AWIPS Advanced Weather Interactive Processing System 

 CNRFC California-Nevada River Forecasting Center 

 CWA County Warning Areas 

 DFWS Debris Flow Warning System 

 DMIP Distributed Model Intercomparison Project 

 DSD Drop size distribution 

 EAS Emergency Alert System 

 EM Emergency Management 

 ETL Environmental Technology Laboratory 

 FAA Federal Aviation Administration 

 FAR False Alarm Ratio 

 FFMP Flash Flood Monitoring and Prediction 

 FTP File Transfer Protocol 

 GeoMAC Geospatial Multi-Agency Coordination 

 GOES Geostationary Operational Environmental Satellite 

 GRIB2 Gridded Binary Data Edition 2 

 HPC NOAA’s Hydrometeorological Prediction Center 

 IPMA Intensive Precipitation Measurement Array 

 IT Information Technology 

 LAMP Localized Aviation MOS Program 

 MIC Meteorologist In Charge 

 MND Mass News Disseminator 

 MOS Model Output Statistics 

 NCEP NOAA’s National Centers for Environmental Prediction 

 NDFD National Digital Forecast Database 

 NEXRAD Next Generation Radar 

 NMQ National Mosaic and Multisensor QPE 

 NOAA National Oceanic and Atmospheric Administration 

 NOHRSC National Operational Hydrologic Remote Sensing Center 

 NRC National Research Council 

 NSSL NOAA’s National Severe Storms Laboratory 

 NWP Numerical Weather Prediction 

 NWS NOAA’s National Weather Service 

 NWSH National Weather Services Headquarter 

 NWSTG NWS Telecommunications Gateway 

 OAR NOAA’s Oceanic and Atmospheric Research 

 OHD NOAA’s Office of Hydrologic Development 

 PACJET Pacific Jets Experiment 

 PNS Public Information Statement Product 

xi
 Acronym Meaning 

 POD Probability of Detection 

 PWS Prototype Debris-Flow Warning System 

 QPE Quantitative Precipitation Estimation 

 QPE-SUMMS Quantitative Precipitation Estimation and Segregation 

 Using Multiple Sensors 

 QPF Quantitative Precipitation Forecasting 

 RDBMS Relational Database Management System 

 RFC River Forecasting Centers 

 SMART-R C-Band Doppler Radar 

 S-PROF S-Band Precipitation Profiler Radar 

 SSH Senior Service Hydrologist 

 UGC Universal Geographic Code 

 USGS U.S. Geological Survey 

 VTEC Valid Time Event Coding 

 WCM Warning Coordination Meteorologist 

 WFO Weather Forecast Office 

 WR/MSD Western Region, Meteorological Services Division 

 WRH NWS/Western Region Headquarters 

 WSR-88D Weather Service Radar, 1988- Digital 

 X-POL X-Band Polarimetric Radar 

xii
Introduction

Motivations, History, and a New Opportunity

Landslides result in an estimated 25 to 50 deaths and 

damages exceeding $2 billion annually (National Research 

Council (NRC), 2004). Debris flow is a type of landslide most 

commonly initiated when heavy rainfall or rapid snowmelt 

mobilizes soil on steep slopes, sending a slurry of rocks, soil, 

and mud downhill with tremendous force. These are often 

called mudslides by the public. Because of their close link 

with precipitation, debris flows are somewhat more predictable than most other types of landslides. The weather conditions that provoke them are the same as those monitored by 

meteorologists for flood predictions, although the critical 

precipitation-warning thresholds and areas vulnerable to flood 

and debris-flow damage may differ significantly. Geological 

research has established rainfall intensity and accumulation 

thresholds above which debris flows are likely to occur in 

some mountainous or hilly locations. Meanwhile, the ability 

to monitor and forecast precipitation and issue timely weatherhazard Warnings is a well-established and ever-improving 

operational meteorological capability.

Thus, prospects are good for devising an effective operational system to predict debris flows and issue Warnings of 

their imminent threat. A joint cooperative program involving 

the U.S. Geological Survey (USGS) and the National Oceanic 

and Atmospheric Administration (NOAA) is proposed here 

to develop and implement such a system, beginning with a 

limited-scale prototype demonstration. The complementary 

expertise and capabilities of the two agencies provides an 

ideal partnership for addressing this problem. This problem is 

directly related to the missions of both agencies—to provide 

services to protect life and property from natural disasters. 

An exploratory program for predicting debris flows and 

issuing Warnings was operated in the San Francisco Bay area 

from 1986–95 through an informal cooperation between the 

USGS and NOAA’s National Weather Service (NWS). This 

program demonstrated a notable degree of success and established rudiments for future efforts. 

In the ensuing decade, significant advances have occurred 

in precipitation forecasting and monitoring. These improvements, now operational at the NWS, include higher resolution 

numerical weather-prediction models that use more realistic 

precipitation-formation physics, the deployment of a national 

network of WSR-88D (NEXRAD) storm-surveillance Doppler radars, and an explosion of real-time information sharing 

through the Internet. Recent NOAA studies of West Coast 

winter storms offer new understanding of these storms and 

have employed technologically advanced instruments that may 

become operational forecast-aid tools in the future. Simultaneously, the geology and physics of debris flow has been refined 

though continued USGS research involving case studies and 

model development. 

As of 2005, a very few NWS Weather Forecasts Offices 

(WFO) are issuing weather Warnings that experimentally 

include comments about debris-flow threats in recently 

burned areas, based on very simplistic and untested rainfall 

thresholds provided by a panel of geologists. In January 2005, 

the USGS conducted an ad hoc warning exercise as a recordbreaking storm deluge approached and crossed southern 

California, producing numerous debris flows and the deadly 

landslide at La Conchita, Calif. (cover photo). These temporary activities have further clarified the opportunity and 

the need for a more formal, operational debris-flow forecast/

warning system.

Important recent science and technology advances suggest that the time is right to develop an operational warning 

system. It could build on and extend the earlier San Francisco 

area exploratory program using the new tools and knowledge 

now available. Mountainous areas recently burned by wildfires are the best candidates for testing the prototype of such a 

system because the likelihood of debris flows occurring is high 

and, unlike unburned areas, largely independent of antecedent 

precipitation and soil-moisture conditions. As such, burned 

areas also offer a comparatively simple study “laboratory.” The 

mountainous areas of southern California have been identified 

as an excellent location for the warning program’s prototype 

demonstration. Recently burned slopes in these areas are 

prevalent, debris flows are likely to occur frequently, a large 

nearby population is at risk, and data from earlier events in 

some of these mountain ranges are available to establish sitespecific rainfall/flow thresholds. The area (fig. 1) is served by 

two NWS WFOs and has relatively good upper-level coverage 

from nearby NEXRAD radars and surface monitoring by realtime rain-gage networks. In figure 1, the two NWS Weather 

Forecast Office regions are shown with different shading. 

Also, a smaller (movable) region for intensive research is 

shown in red.

NOAA-USGS Debris-Flow Warning System—Final Report

By NOAA-USGS Debris Flow Task Force
Initially, the prototype system will rely primarily on 

operational tools (such as existing rain gages and NEXRAD) 

to monitor storm conditions. However, although the current 

operational weather tools and the present geological knowledge of debris flows far exceed what was available a decade 

earlier, they are known to have shortcomings that render them 

less than ideal for developing Forecasts and Warnings. Thus, 

the warning development program and its prototype demonstration must also include a strong research component (both 

geologic and meteorological) that addresses weaknesses if 

the system is to be more broadly relevant. The research to 

be focused in a smaller subregion (fig. 1) would include, for 

example, testing supplemental instrumentation for improved 

measurements of rainfall and the development of geologic and 

hydrologic models that more accurately predict areas at risk 

from debris flows. 

A successful prototype demonstration in southern California will serve as a model for implementing similar systems 

in other areas of the Nation vulnerable to debris-flow hazards. 

However, the geology and meteorology associated with debris 

flow in other areas may differ greatly from those in southern 

California. Widespread winter storms are the primary catalyst 

for triggering debris flows in southern California. However, 

in other parts of the country, the main threat arises from other 

sources, such as localized convective thunderstorms in the 

Rocky Mountains, summer monsoon rains in Arizona, and 

hurricane rainfall in the Appalachian Mountains. The regional 

and local geology affecting debris flow varies greatly. Hence, 

another important topic for the program’s research is to assess 

transferability of results from one area to another and the 

specific data and observational requirements for implementing 

Forecasts in new areas.

Hazards Posed by Debris Flows

Landslides are among the most widespread geological 

hazards on Earth and threaten lives and property in every State 

in the Nation. Landslides result in an estimated 25 to 50 deaths 

and damages exceeding an average of $2 billion annually 

(National Research Council, 2004). Despite advances in science and technology, these events continue to result in human 

suffering, property losses, and environmental degradation. As 

our population increases and our society becomes ever more 

complex, the economic and societal costs of landslides will 

continue to rise unless there is significant intervention.

Figure 1. Proposed prototype region for issuing debris flow Forecasts and Warnings.

 NOAA-USGS Debris-Flow Warning System—Final Report
Debris flows are among the most hazardous types of 

landslides. They pose a hazard distinct from other landslide 

processes because of their unique destructive power. They 

can occur with little warning and can exert great impulsive 

loads on objects in their paths. Even small debris flows can 

strip vegetation, block drainage ways, damage structures, and 

endanger human life (Iverson, 1997). The deaths of 19 people 

during the winter storms of 2004–05 that impacted much of 

southern California highlight the most drastic consequences 

of debris flows. In addition to the lives lost, damage to public 

property by debris flows and floods from this event has been 

estimated to approach $0.5 billion (Los Angeles Times, 2005). 

Hillslopes burned by wildfire are particularly susceptible to 

debris-flow activity. Sixteen people were killed during the 

Christmas Day 2003 storm that impacted recently burned 

hillslopes in southern California (Los Angeles Times, 2004). 

In response to this one event, $26.5 million was spent to repair 

roads and to remove the 4.1 million m3 of material deposited 

in debris retention basins (Pat Mead, San Bernardino Flood 

Control District, oral commun.).

Terminology

Debris flows are gravity-driven mixtures of sediment 

and water that are intermediate between landslides and water 

floods. They have mechanical characteristics that are distinct 

from either of these processes (Johnson, 1970; Iverson, 1997; 

Iverson and Vallance, 2001) and are commonly described as 

resembling flowing, wet concrete. A debris flow is commonly 

defined as a flowing mixture of approximately equal parts sediment and water in which a broad distribution of grain sizes, 

commonly including gravel, is vertically well mixed. Debris 

flows exhibit behavior that is strongly affected by interactions 

between the solid and fluid components. Debris flows can 

travel through steep channels, over open hillslopes, and across 

gently sloping surfaces, where they are known to build their 

own channels (Costa, 1984). Flow properties vary with water 

and clay content, and sediment size and sorting (Costa, 1984). 

They commonly travel along channels in a series of waves or 

surges (Costa, 1984). Debris flows can have apparent viscosities that are five to six orders of magnitude greater than water, 

and fluid densities almost twice as great. As a consequence 

of the high fluid densities of debris flows, large rocks can be 

carried along and cause significant damage by impact (Wieczorek and others, 2002). In this report, debris flows broadly 

encompass mudflows (slurries containing mostly fine grained 

material) and debris avalanches (a variety of very rapid debris 

flow (Varnes, 1978)). Debris flows and mudflows are commonly referred to in the vernacular as mudslides.

Hyperconcentrated flow is another form of sedimentwater mixture found in mountainous terrain (e.g., Pierson, 

2005a). Hyperconcentrated flow is defined as a phase of flow 

that is transitional between debris flow and sediment-laden 

streamflow in which stresses exerted by the fluid are responsible for the transport of sediment. A hyperconcentrated 

flow contains volumetrically more water than sediment, but 

it is very sediment rich compared to normal streamflow. As 

a result, the coarsest particles settle rapidly and the flowing 

sediment-water mixture usually contains a narrower distribution of grain sizes than is found in a debris flow. The sediment in many hyperconcentrated flows is predominantly sand. 

Hyperconcentrated flow can form when a debris flow moves 

downslope and mixes with streamflow. It can also form when 

water flow erodes and ingests large amounts of sediment.

The debris-flow warning system will use the same terminology used by the NWS in the delivery of hazardous weather 

messages (Outlook, Watch, and Warning). It is, therefore, 

important to describe these terms and their application. An 

Outlook is used by the NWS to indicate that a hazardous 

weather or hydrologic event may develop. It is intended to 

provide information to those who need considerable lead time 

to prepare for the event. A Watch is issued when the risk of a 

hazardous weather or hydrologic event has increased significantly, but its occurrence, location, and (or) timing is still 

uncertain. It is intended to provide enough lead time so that 

those who need to set their plans in motion can do so. Lead 

times are, at most, less than 3 days and can be as short as a 

few hours. A Warning is issued when a hazardous weather or 

hydrologic event is occurring, is imminent, or has a very high 

probability of occurring. A Warning is used for conditions 

that pose a threat to life or property. Desired lead times for 

Warnings would be within 1 day, but developing conditions 

might cause them to be issued with lead times as short as 30 

minutes.

Objectives of the Task Force

The objective of the NOAA-USGS Task Force is to 

develop a plan for the implementation and operation of a 

NOAA-USGS system to issue joint Outlooks, Watches, and 

Warnings for areas deemed to be at risk from debris flows as a 

result of current or forecast precipitation. As part of this effort, 

a prototype system will be developed, implemented, and tested 

in recently burned areas in southern California using technologies currently available to NOAA and the USGS. The potential 

and resources required for expansion of a warning system to 

additional geographic settings and other debris-flow-triggering 

conditions using more advanced techniques in precipitation 

forecasting and measurement and debris-flow hazard delineation are identified.

Acknowledgments

Leadership Team

The leadership team is formed by the Directors of NOAA 

line offices and the Associate Directors of the U.S. Geological 

Survey for Water and Geology.

Acknowledgments 
Management Team

The management team has as its responsibility the overall 

direction of the project and is formed by senior personnel of 

the four offices involved in the project.

Leadership Team composition. 

[AD, Associate Director] 

 Name Title Affiliation

 Robert Hirsch AD for Water U.S. Geological Survey 

 D.L. Johnson Director NOAA’s National Weather Service 

 Linda Gundersen AD for Geology, Acting U.S. Geological Survey 

 Rick Rosen Director NOAA Oceanic and Atmospheric Research 

Management Team composition. 

 Name Affiliation

 Steve Blanchard USGS, Water Resources Discipline 

 Gary Carter NOAA’s Office of Hydrologic Development 

 Peter Lyttle USGS, Geologic Resources Discipline 

 Marty Ralph NOAA, Environmental Technology Laboratory 

Task Force

The Task Force was formed to carry out the research 

and draft the overall implementation plan. The product of this 

assignment is this report.

Task Force composition. 

 Name Affiliation

 Jim Bowers USGS, Water Resources Discipline, Calif. 

 Sue Cannon USGS, Geologic Resources Discipline 

 Tom Carroll NOAA, National Operational 

 Hydrologic Remote Sensing Center 

 John Costa USGS, Water Resources Discipline 

 Jonathan Godt USGS, Geologic Resources Discipline 

 J.J. Gourley NOAA’s National Severe Storms Laboratory 

 David Jorgensen NOAA’s National Severe Storms Laboratory 

 David Kingsmill NOAA, Environmental Technology Laboratory 

 Jon Major USGS, Water Resources Discipline 

 Brooks Martner NOAA, Environmental Technology Laboratory 

 Pedro Restrepo NOAA’s Office of Hydrologic Development 

 David Savage NOAA, National Operational

 Hydrologic Remote Sensing Center 

 Michelle Schmidt NOAA’s National Weather Service,

 Western Region Headquarters 

 Robert Tibi NOAA’s National Weather Service, 

 Western Region Headquarters 

 Kevin Werner NOAA’s National Weather Service, 

 Western Region Headquarters 

 Gerry Wieczorek USGS, Geologic Resources Discipline 

 NOAA-USGS Debris-Flow Warning System—Final Report
External Cooperators 

The project benefited immensely from a number of 

individuals and organizations that were not part of the Task 

Force. They contributed either by commenting on sections of 

the report or by being interviewed and expressing their opinion 

with regard to the value of this debris-flow-warning system.

Transformation from Landslides

Many debris flows in undisturbed landscapes begin as 

discrete landslides on steep (greater than about 15°) hillslopes 

as a result of a relatively rapid influx of large amounts of water 

(e.g., Iverson and others, 1997). Slope failures usually originate at the head of swales (small zero-order drainages), but 

they can also occur on flat or convex side slopes. Landslide 

failures typically occur when rainfall infiltrates through a relatively competent block of soil. As the block of soil gradually 

saturates, pore-water pressures increase, and shear strengths 

decrease. It is not necessary for the entire thickness of soil to 

be saturated for failure to occur. The initial landslide failure 

then mobilizes into the muddy slurry that is the debris flow. 

Landslide-generated debris flows can move rapidly downslope 

and frequently incorporate significant volumes of channel 

sediment, thus increasing in size with distance.

In landscapes disturbed by wildfire, forest practices, 

volcanic eruptions, etc., infiltration-triggered landslide activity 

is frequently attributed to increased soil moisture caused by 

reduced transpiration rates as a result of loss of vegetation 

and to root decay associated with decreases in soil cohesion 

(e.g., Schmidt and others, 2001; Swanson and Major, 2005). 

In areas burned by wildfire, Cannon and Gartner (2005) found 

that debris flows caused by landslide mobilization could 

occur during the first rainy season immediately after the fire, 

and as much as about 10 years after the fire. Such landslides 

generally occurred in response to prolonged, infrequent 

rainfall events. The most extensive landslide events occurred 

in response to week-long, or multiweek storms, or prolonged 

rainfall in combination with rapid snowmelt.

Mobilization of Channel Deposits

Large-scale sediment entrainment has proven to be a 

process capable of generating debris flows. Material stored 

in channels can be mobilized by rainfall runoff or flash-flood 

surges and act as the source sediment for debris flows. In 

several areas, volume increases in flow caused by sediment 

entrainment have been recognized as an important or even 

predominant factor in creating large debris flows. Such an 

initiation process, known as bulking in the sedimentologic literature, in reference to the associated increase of flow volume 

that accompanies sediment entrainment, has been recognized 

to be an important process around volcanoes (Gallino and 

Pierson, 1985; Scott, 1988; Scott and others, 2005) as well 

as in nonvolcanic terrain. Scott (1971) noted that channel 

deposits are a major source of debris-flow sediment in the San 

Gabriel Mountains of southern California. 

Mobilization and entrainment of channel sediment is 

particularly important in areas recently burned by wildfires. 

Meyer and Wells (1997) and Cannon and others (2001, 

2003) describe a process whereby storm runoff progressively 

entrains sediment eroded from hillslopes and channels and 

transforms into a debris flow. Convergence and concentration 

External cooperators. 

[See “List of selected acronyms used in this report” at end of table of contents 

for acronym definitions] 

 Eric Boldt, WCM, NOAA/NWS/WFO, Los Angeles/Oxnard 

 Ed Clark, WCM, NOAA/NWS/WFO, San Diego 

 Dan Keeton, MIC, NOAA/NWS/WFO, Oxnard 

 Jayme Laber, SSH, NOAA/NWS/WFO, Oxnard 

 Jim Purpura, MIC, NOAA/NWS/WFO, San Diego 

 Greg Romano, NOAA/NWS Public Affairs 

 Craig Schmidt, WR/MSD 

 Mike Smith, Research Hydrologist, NOAA/NWS/OHD 

 San Bernardino County Office of Emergency Services 

 San Diego County Office of Emergency Services 

 San Luis Obispo, Office of Emergency Services 

 Riverside County Office of Emergency Services 

Debris Flows

Triggers

Debris flows originate when poorly sorted rock and 

soil debris are mobilized from hillslopes and channels by the 

addition of moisture (Costa, 1984). Prerequisite conditions 

for most debris flows include an abundant source of unconsolidated regolith, steep slopes, and a source of moisture. The 

most common moisture sources are intense or prolonged rainfall and rapid snowmelt (or some combination of the two). 

Debris flows can be triggered by a variety of mechanisms. Commonly, debris flows occur when landslides transform into rapidly flowing masses. However, they may also 

occur when rapid hillslope runoff or flood surges erode and 

entrain channel sediment. At volcanoes, they can be triggered 

by rapid melting and mixing of snowpack with volcanic debris 

during eruptions (Waitt and others, 1983; Major and others, 

2005) and by heavy rainfalls (Gallino and Pierson, 1985; 

and Scott and others, 2005). Earthquakes and dam failures 

can also result in debris flows (Scott, 1988; O’Connor and 

others, 2001; Evans and Bent, 2004). Initiation mechanisms 

can greatly influence debris-flow volumes, compositions, 

and hydrographs. Any conditions that foster slope instability, 

enhanced and rapid runoff, or flash flooding can favor formation of debris flows. This report is focused on rainfall-induced 

debris flows.

Debris Flows 
of surface runoff within small, first-order drainages initiates 

erosion, often to bedrock, and the transport of material downchannel. Field observations indicate that at some distance 

down the channel network, a sufficient amount of eroded 

material is incorporated, relative to the volume of surface runoff, to impart debris-flow characteristics to the flow. In a study 

of the response of 410 burned basins throughout the Western 

United States, Cannon and Gartner (2005) found that about 75 

percent of debris flows were produced through this process. 

This mechanism of debris-flow generation usually occurred in 

response to short-recurrence (<2- to 10-year) rainfall events. 

In addition to enhanced and rapid runoff, landslides can 

influence mobilization of channel sediment in several ways. 

First, a landslide may override and mobilize saturated channel sediment (e.g., Hungr and Evans, 2004). If a sufficient 

amount of channel sediment is mobilized, it can augment 

the travel distance of a debris flow. Second, rapid release of 

ground water from a landslide rich in coarse debris can trigger 

a flood that subsequently entrains sediment and transforms 

into a debris flow (e.g., Scott and others, 2005). Third, a 

landslide can dam a stream channel and temporarily impound 

a lake. Breaching of the impoundment can trigger a flash flood 

capable of entraining enough channel sediment to transform 

into a debris flow (e.g., Scott, 1988).

Hazard Assessments

Reliable and accurate debris-flow Watches and Warnings 

must be based on sound identification of areas susceptible to 

debris flows and recognition of the conditions that will result 

in their occurrence. Hence, a comprehensive debris-flowhazard assessment must address the following questions:

When? (Rainfall Intensity-Duration Thresholds)

To address when debris flows are likely to occur, both 

within a storm or within a storm season, two complementary 

triggering thresholds that relate to different time scales must 

be considered: (1) an antecedent rainfall threshold, requiring 

an accumulation of a certain amount of rainfall during the 

season, and (2) a storm intensity-duration threshold, requiring 

that a critical combination of rainfall intensity and duration 

be exceeded during the course of the storm (Wilson, 1993). 

The antecedent rainfall threshold exists because hillside 

soils become dehydrated during the dry season, and a certain 

amount of rainfall is necessary to replenish soil moisture. 

Until soil moisture is restored, the pore pressures necessary 

for slope failure cannot form and debris flows are unlikely, 

even in heavy rainfall (Wilson, 1993). Note that in recently 

burned areas, where debris flows are most likely to be initiated 

through runoff-dominated processes rather than from infiltration-triggered landslides, the largest and most extensive debris 

flows generally occur in response to the first significant storm 

to impact an area (Cannon and Gartner, 2005). Thus, definition of antecedent rainfall conditions is not as important for 

debris-flow initiation in burned landscapes as it is in undisturbed landscapes. In burned areas, rainfall intensity-duration 

thresholds can provide sufficient information for the initial 

prediction of debris flows.

For landslide-triggered debris flows, once an antecedent rainfall threshold has been exceeded, approaching 

storms are evaluated to see if the intensity and duration of 

the expected rainfall are sufficient to initiate movement. 

Storm thresholds are usually defined by identifying those 

rainfall intensities and durations that are unique to debrisflow-producing storms. A threshold line delineates a range 

of rainfall combinations—from short-duration, high-intensity 

to long-duration, low-intensity—any of which can result in 

debris-flow activity within a storm. Thresholds commonly 

take the form of a power-law function, and because of 

variations in the rainfall conditions that trigger debris flows 

in different regions, such thresholds typically are region 

specific. For example, distinct landslide-triggering rainfall 

thresholds have been reported for Puerto Rico (Larsen and 

Simon, 1993), Hong Kong (Au, 1993; Finlay and others, 

1997), Taiwan (Taiwanese Soil and Water Conservation 

Bureau), central and southern California (Campbell, 1975; 

Cannon and Ellen, 1985, Wieczorek 1987; Cannon, 1988; 

Wilson and Wieczorek, 1995), Seattle, Wash. (Baum and others, 2005), the Blue Ridge in Virginia (Wieczorek and others, 

2000), New Zealand (Crozier, 1986, 1999; Glade and others, 

2000), and the Piedmont Region in Italy (Aleotti, 2004). To 

account for local variability in precipitation amounts due 

to local orographic effects and the concurrent variability in 

local debris-flow-triggering rainfall conditions, some workers have proposed normalizing rainfall thresholds by mean 

annual precipitation (Cannon, 1988), or by rainy-day normals 

(Wilson, 1993).

Rainfall intensity-duration thresholds can also be used 

to indicate different levels of potential hazards, as shown in 

figure 2. For example, in the operation of a debris-flow warning system in the San Francisco Bay region of California, 

a lower “safety” threshold identified a rainfall level below 

which significant debris-flow hazards are considered unlikely, 

and above which debris flows are likely. An upper “danger” 

threshold represented a rainfall level above which abundant 

debris flows large enough to destroy structures are likely to 

occur across broad areas (Wilson, 1993).

Where? (Hazard Mapping)

During the past 3 decades, geoscientists have developed 

several approaches to debris-flow-hazard analyses, which can 

be broadly classified as inferential, statistical, and processbased (Hanson, 1984; Varnes, 1984). All three approaches 

are currently applied to produce a variety of map products 

described below; there is no standard approach used in the 

United States. These approaches vary considerably in their 

spatial and temporal resolution and physical basis. 

 NOAA-USGS Debris-Flow Warning System—Final Report
An inferential approach is very common and relies on 

information from a variety of sources to create interpretive 

maps that show the potential extent of landslide activity. 

An example of an inferential product would be a landslide 

susceptibility map. Such a map ranks slope stability of an area 

into categories that range from stable to unstable to indicate a 

sense of relative stability. The controls on landslide occurrence 

and the relative effects of each of the controls are generally 

inferred based on expert opinion, field observations, and data 

from landslide inventories. For example, in a series of maps 

for the southern California area, Morton and others (2003) 

present the susceptibility to shallow landsliding as a function 

of the product of geologic unit, slope, and aspect. Such a product provides a static representation of landslide susceptibility 

in the event of some unspecified rainfall accumulation.

A statistical approach consists of mapping large numbers 

of parameters considered to potentially affect debris-flow 

occurrence and subsequent (statistical) analyses of those 

parameters to identify controlling factors and their relative effects. For example, multivariate statistical models for 

landslide-hazard zonation have been developed in Italy, mainly 

by Carrara (1983) and colleagues (Carrara and others, 1991, 

1992). Suspected relevant factors are evaluated spatially within 

grid cells or by morphometric units to develop a predictive 

model. Cannon and others (2004) developed a multivariate 

statistical model that can be used to determine the probability of debris flows from recently burned drainage basins as a 

function of measures of basin gradient, material properties, 

burn severity, and storm rainfall. One example is shown in figure 3 (Cannon and others, 2003). Such a product can provide 

a dynamic representation of debris-flow probability during a 

developing storm. 

A process-based approach uses deterministic analyses 

to delineate landslide potential. Quantitative theory for slope 

instability forms the basis of a model, which is then applied 

using digital elevation data and measures of soil properties. 

Slope-instability theory is commonly coupled to ground-water 

flow models to calculate grid-cell-scale factors of safety (e.g., 

Montgomery and Dietrich, 1994; Pack and others, 1998). This 

approach emerged in the last 10 years and is undergoing rapid 

evolution, driven in part by new observational technology. For 

example, in recent years, methods have advanced from the 

necessity of evaluating saturated soil thicknesses to models 

that evaluate time-dependent rainfall infiltration into unsaturated soils (Savage and others, 2003; Morrisey and others, 

2004). These more advanced models can potentially be used 

to provide spatially and temporally explicit information on 

hillslope instability.

How Big? (Volume or Peak Discharge)

Debris-flow magnitudes are usually characterized either 

as a peak discharge that can issue from a basin outlet, the 

planimetric area that can be inundated, or the potential volume 

of material that can be mobilized from hillslopes and channels 

and thus deposited in the inundation zone (Rickenmann, 1999; 

Pierson, 2005b). Rickenmann (1999) recommends geomorphic 

Figure . Rainfall thresholds for La Honda study site in northern California.

Debris Flows 7
assessments of potential sediment volumes stored in channels as the most reliable method of characterizing debris-flow 

volume. D’Agostino and Marchi (2003) describe such an 

approach for basins in the Dolomites in Italy. In a different 

approach, Cannon and others (2004) used data from recently 

burned basins throughout the Western United States to develop 

a multivariate statistical model that can be used to estimate the 

potential peak discharges of a debris flow issuing from a basin 

mouth as a function of the extent of the burn, basin gradient, 

material properties, and the triggering rainfall. 

Note that, although peak discharge is the standard indicator for flood magnitude, it is not always a good representation 

for debris-flow magnitude. Because many debris flows occur 

in ungaged basins, and it is rare for a direct gaging system to 

survive a debris-flow event, it is necessary to rely on indirect 

methods to reconstruct peak discharges. Indirect methods for 

determining peak discharges are of limited use for debris-flow 

reconstructions because debris flows are non-Newtonian flows 

and orders-of-magnitude changes in peak discharge can occur 

over very short distances (hundreds of meters) downstream. 

Estimates of measurements of total planimetric area inundated 

or total volume delivered beyond a canyon mouth are considered to be more useful data for some hazard assessments 

(Pierson, 2005b).

How Far? (Runout and 

Inundation-Area Mapping)

Traditionally, assessments of debris-flow hazards and 

runout are based on detailed mapping of the extents of debrisflow deposits and extrapolation of estimated inundation limits 

among drainage basins. Such methods are, however, highly 

subjective. Simulation models provide a somewhat more 

objective way of evaluating hazards, and a variety of empirical 

and numerical models have been proposed to simulate runout 

distances and inundation limits of debris flows (e.g., Hungr, 

1995; O’Brien and others, 1993). 

Although theoretical and numerical analyses of debrisflow mechanics are advancing, there is yet no universally 

accepted physically based model for routing debris flows 

across three-dimensional terrain. At present, the most practical methods for identifying debris-flow runout and limits of 

inundation rely on empirical analyses that are based on historical inundation patterns. However, reason suggests that the 

extent and distance of runout of a debris flow will be related to 

the flow volume (contributed from both the initial failure and 

channel bulking). One of the more useful empirical methods for identifying debris-flow runout and inundation relies 

on relationships between flow volume and inundation area 

Figure 3. Probability of debris-flow occurrence after the 2002 Missionary Ridge fire in Colorado in response to 25-year recurrence, 1-hour 

duration rainstorm.

 NOAA-USGS Debris-Flow Warning System—Final Report
(Iverson and others, 1998). This method relies on a physically 

based, statistically constrained simulation model calibrated 

with data from historical debris flows from a variety of settings. It relies on scaling and statistical relationships among 

expected debris-flow volume, cross-sectional area of channel 

inundation, and planimetric area of inundation. Estimation 

of initial failure volume before an event is difficult, but by 

using a range of prospective debris-flow volumes, a range of 

inundation areas can be plotted for debris flows of increasing 

volume and decreasing probability. Detailed descriptions of a 

geographic information system (GIS) computer program for 

implementation of this approach, and discussion of limitations 

of the approach, are found in Iverson and others (1998), Schilling (1998), and Griswold (2004).

Precipitation

Measuring Precipitation

This section reviews the primary current methods for 

quantitative precipitation estimation (QPE) and quantitative 

precipitation forecasting (QPF).

Rain Gages

Rain gages are the oldest tool for measuring precipitation. Reports of daily precipitation are made by a network of 

approximately 10,000 cooperative observers across the United 

States. However, the vast majority of these reports are only of 

daily total accumulations, and they do not provide the kind of 

information on rainfall intensity, nor report it quickly enough, 

to be useful in flood- or debris-flow-forecasting applications. 

More useful for forecasting, but much less prevalent, are 

tipping bucket precipitation gages that provide time-resolved 

measurements of rainfall and snowfall (with heated gages). 

The reporting period of most tipping gages is usually 1 to 10 

minutes, with a basic resolution amount of 0.01 to 0.04 inches 

per tip. 

Although rain gages are often considered to be the 

standard of truth for QPE, their data are vulnerable to errors, 

including under-catch during windy conditions (especially for 

snowfall) and the inability of the tipping bucket mechanism 

to keep pace with intense rainfall. The tipping mechanism can 

also easily get out of proper balance, resulting in erroneous 

data for all rain rates, unless the gage is calibrated in the field 

periodically. More important, however, a gage provides only a 

point measurement, which may not be representative of other 

nearby locations and areas, particularly in mountainous terrain 

and for convective rain. Networks of gages are required to 

observe the area patterns of precipitation and to estimate the 

basin-average rainfall that contributes to debris-flow conditions. Denser networks of gages are preferable, but more 

expensive to install and maintain. Data from gage networks are 

only useful for warning of precipitation-related hazards if they 

are communicated to users in real time. In spite of these shortcomings, a well planned and maintained network of precipitation gages is essential for a debris-flow forecasting system. 

Fortunately these kinds of networks are already operational for 

flood-forecasting purposes in the debris-flow prototype region.

Radar

Radar has the ability to observe the intensity and spatial patterns of precipitation with resolutions that could only 

be duplicated by extremely dense networks of rain gages. 

Unlike gages, radar can also observe the precipitation aloft 

and offshore and can determine the trajectory of precipitation 

cells. Radar is able to detect approaching heavy rain cells, and 

its data are used to extrapolate rain amounts that are likely to 

fall within the next hour or two. This “nowcasting” capability provides precious forecast lead time for Warnings that 

cannot be obtained with a rain-gage network alone. Stormsurveillance radar is the cornerstone for short-term forecast 

and warning procedures for various weather hazards. The 

national network of operational storm-surveillance radars was 

tremendously improved with the arrival in the 1990s (at about 

the time the San Francisco Bay area debris-flow Warnings 

were terminated) of the WSR-88D (NEXRAD) radars. There 

are more than 100 of these Doppler radars across the United 

States operating around-the-clock with highly sophisticated 

displays and algorithms for detecting various weather hazards, including heavy rainfall (Crum and Alberty, 1993). The 

NEXRAD configuration and capabilities in southern California are described in more detail in the section on “Existing 

Meteorological Systems.” Spatially detailed instantaneous 

maps of rainfall intensity and accumulation from these radars 

can provide vital input for Warnings of water-related hazards. 

Thus, the operational radars are essential for a new debris-flow 

forecast/warning system. But they too have limitations that 

must be addressed.

Radar transmits pulses of microwave energy that are 

reflected back to the radar antenna from raindrops, snowflakes, and other particles in the atmosphere. For rainfall, the 

strength of the reflected signal (reflectivity, Z) is related to 

the rainfall intensity, R. However, the physical relationship is 

imprecise, because Z and R are fundamentally related to different moments of the drop size distribution (DSD). The DSD 

can vary from one storm to another and within regions of the 

same storm, and it is almost never known a priori. Therefore, 

meteorologists usually resort to using empirical Z-R relations 

from past storms that may be inaccurate when applied to new 

storms, depending on the DSD actually present.

These reflectivity-based QPE may be further degraded 

by other circumstantial factors, including inaccurate hardware 

calibrations; partial beam filling; partial beam blockage by terrain; attenuation; and contaminations by the presence of hail, 

snow, and melting snow at the freezing level (which causes a 

signal enhancement called the “bright band”), ground clutter, 

Precipitation 
and echoes from airborne nonhydrometeor targets. Many of 

these problems are accentuated with increasing distance from 

the radar because a horizontally directed radar beam increasingly departs from the ground and broadens with range. In 

carefully controlled analyses, where most problematic factors 

are ruled out, comparisons of radar-reflectivity-derived rainfall 

accumulations agree with coincident gage measurements to 

within about 20 percent. However, under more general conditions, comparisons frequently show differences of as much 

as a factor of 2. The situation is worse for radar estimates of 

snowfall. 

Many of the problems associated with reflectivitybased radar QPE can be avoided or mitigated if the radar has 

polarimetric capability. Comparing the amplitude and (or) 

phase of the reflected signals in two orthogonal polarizations 

provides information about the shape of the scattering particles. This allows polarimetric radar to distinguish raindrops, 

snowflakes, hail, and various other hydrometeors as the 

dominant particles within different portions of a storm cloud. 

The differential phase method uses the polarimetric information about raindrop shapes to obtain estimates of rainfall rate 

that avoid many of the problems related to reflectivity-based 

estimates (NRC, 2002). Only a few meteorological research 

radars worldwide now have these polarimetric capabilities. 

The national network of operational NEXRAD radars is 

expected to begin upgrades to include polarimetry starting in 

about 2008–10, however.

Unfortunately, many NEXRAD radars, especially in 

mountainous States, have large scan coverage gaps that 

occur because the beam is blocked at low levels by terrain 

features. Some NEXRAD radar antennas are mounted on 

peaks to minimize terrain blockage. However, this tactic also 

produces problems because the antenna does not point below 

the horizon, and, thus, key low altitudes of the atmosphere 

beneath the peak are not observed. This data-void region 

expands vertically with distance from the antenna. Although 

coverage at higher altitudes is good, precipitation generally 

exhibits sharp vertical gradients that render higher altitude 

observations poor indicators of the more important nearsurface conditions. 

The cost of relocating a NEXRAD radar for better scan 

coverage in a debris-flow warning system is prohibitive. However, the data voids can be effectively filled using additional, 

much smaller radars that are mobile or transportable (NRC, 

2002). Strategically positioned near a crucial basin or mountain range, these “gap-filling” radars can augment the WSR88D coverage with high-resolution (~150 m) observations of 

the storms in and beyond the gaps. Upward-pointing profiling 

radars can also provide continuous high-resolution measurements of the boundary-layer winds that govern the upslope 

forcing of precipitation production and reveal microphysical 

features aloft of the precipitation itself, such as the height of 

the melting layer. 

Transportable and mobile gap-filling radars and profiling radars are available from the NOAA research laboratories 

and other agencies. Prime examples include an X-band transportable polarimetric radar (X-Pol) from NOAA’s Environmental Technology Laboratory (ETL), mobile Doppler radars 

(SMART-R) from NOAA’s National Severe Storms Laboratory (NSSL), and 915-MHz wind-profiling and S-band precipitation-profiling (S-PROF) radars from NOAA/ETL and 

NOAA’s Aeronomy Laboratory (AL). These are advanced 

remote-sensing instruments, currently used for research, have 

great potential as operational monitoring tools. The section 

“Precipitation Measurement” describes how they can immediately become important parts of an intensive research facet 

of the proposed debris-flow warning program in southern 

California to address anticipated weaknesses of the prototype 

system that will initially rely on operational, but limited, 

tools.

Multisensor Techniques

In addition to using better rain gages and radars, QPE 

inaccuracies can also be reduced by making intelligent use 

of the complementary nature of these and other sensors. 

One approach, simple in concept, is to use real-time pointspecific data from gages beneath the radar beam as continuously updating “calibrators” of the radar data and then applying these adjusted or “blended” estimates to the entire scan 

region, including the vast area between gages (Brandes, 1975). 

More sophisticated multisensor (blended or hybrid) 

methods for improving area estimates of precipitation are now 

experiencing rapid development at NOAA’s Office of Hydrologic Development (OHD) and NOAA/NSSL. Eventually a 

single system is expected to evolve from these beginnings to 

become an operational tool for the NWS. The method developed at NSSL, for example, called Quantitative Precipitation 

Estimation and Segregation Using Multiple Sensors (QPESUMS) integrates and quality-controls data from rain gages, 

NEXRAD, and Federal Aviation Administration (FAA) 

Doppler radars, Geostationary Operational Environmental 

Satellites (GOES), lightning-detection networks, surface and 

upper-air meteorological data, and numerical-forecast models to produce an ensemble of precipitation estimates. Each 

ensemble product has strengths and weaknesses that vary 

with season, geography, and precipitation type. For instance, 

research from a limited-area deployment (Gourley and others, 

2002) showed that calibrated satellite-based estimates outperform those from radar-only algorithms during the cool season 

in complex terrain (where NEXRAD data voids are a serious 

problem). An experimental national-scale implementation of 

this system begins in 2005. Blended precipitation estimates 

from systems, such as this, are another new key element for a 

debris-flow warning program.

Forecasting

Note: The information in points 1 through 9 in this section is adapted from Carter and others (1999). 

10 NOAA-USGS Debris-Flow Warning System—Final Report
The current NWS forecast process for quantitative precipitation forecasting (QPF) involves numerical weather prediction (NWP) and statistical guidance, the Hydrometeorological 

Prediction Center (HPC), Weather Forecasting Offices (WFO), 

and River Forecasting Centers (RFC). The final step, (10 

below) “preparation of the National Digital Forecast Database 

(NDFD)” is available only in experimental fashion as of the 

writing of this report. Because of its importance to the debrisflow forecasting system, the separate steps involved in its 

preparation are also shown. Additional information is included 

in Appendix A “Functions of NWS Organizations Involved in 

the Preparation of Precipitation Forecasts.”

The fundamental steps for the preparation of precipitation 

forecasts at the NWS are:

1. The real-time collection of observations, which include 

all in situ and remotely sensed data;

2. The assimilation of data into operational NWP models 

in real time via the National Centers for Environmental 

Prediction (NCEP) model-based data assimilation systems;

3. The application of the NCEP global, regional, and 

mesoscale atmospheric models and ensemble prediction systems;

4. The automated generation, dissemination, and use of 

national statistical guidance products;

5. The manual generation and dissemination of national 

QPF guidance products from the HPC of NCEP;

6. The manual production of local QPF products at WFOs 

and their dissemination to servicing RFCs and other users;

7. The assimilation of WFO-prepared QPFs by RFCs for 

input to the hydrologic forecast system;

8. The production of hydrologic and flood guidance at 

RFCs and its provision to the WFOs within the RFC 

forecast domain;

9. The preparation of all public hydrologic Forecast, 

Watch, and Warning products at WFOs, and the coordination of this information with emergency managers, 

the media, and other end users; and

10. Preparation of the National Digital Forecast Database 

(NDFD) (adapted from Boyer, 2003)

· WFOs send new forecast grids as they are updated;

· gridded forecast data received from WFOs around 

the clock;

· WFO grids decoded at NDFD Central server every 

5 minutes and stored to the relational database 

management system (RDBMS);

· NDFD mosaics are generated hourly, starting at 10 

minutes prior to the top of the hour;

· mosaics are encoded in Gridded Binary Data Edition 2 (GRIB2) format and transferred to NWS 

Telecommunications Gateway (NWSTG); and

· latest hour’s mosaics available for File Transfer 

Protocol (FTP) download at NWSTG by 15–20 

minutes past the top of the hour.

Debris-Flow Warning Systems

This section starts with a review of existing debris-flow 

warning systems, followed by an evaluation of the current 

capabilities and limitations within NOAA and the USGS. 

“Elements of an Operational Debris-Flow Warning System,” and the following section, “A Prototype Debris-Flow 

Warning System,” cover the prototype system that the Task 

Force recommends for implementation given the operational 

capabilities and limitations at the USGS. The same section 

includes a subheading with feedback received from potential 

users of the system. This feedback is compared to the characteristics of the recommended prototype debris-flow warning 

system.

Review of Existing Systems

Intensity-duration thresholds, in combination with 

rainfall forecasts and real-time rainfall measurements, have 

been the basis for operational landslide warning systems 

in several areas. These systems are typically operated over 

broad regions where people and infrastructure are at risk from 

shallow landslides. The Hong Kong Geotechnical Engineering Office established a warning system in 1977 (Chan and 

others, 2003); continuous data collection and periodic review 

has resulted in significant improvement of the criteria for 

issuing and canceling Warnings of impending landslides. The 

USGS, in cooperation with the National Weather Service, 

operated a debris-flow warning system in the San Francisco 

Bay region from 1986 to 1995 (Keefer and others, 1987; 

Wilson and others, 1993; Wilson, 1997). In Rio de Janeiro, 

the Alerta Rio System consists of a network of 30 telemetered 

rain gages and weather radar and has been used by operational forecasters to issue Warnings for landslides and (or) 

flash flooding to government agencies and the public during 

severe rainstorms (d’Orsi and others, 2004). The Soil and 

Water Conservation Bureau of the Council of Agriculture 

in Taiwan has developed a debris-flow warning system that 

consists of a linked network of real-time-reporting rain gages, 

a series of flow-detection devices, and cameras (Taiwan Soil 

and Water Conservation Bureau). The State of Oregon operates a landslide warning system in western Oregon based 

on comparison of rainfall intensity-duration thresholds with 

measured rainfall accumulations (Mills, 2002). In the United 

Kingdom, data from an extensive instrument network that 

detects movement in a large landslide complex are combined 

with rainfall information to warn residents of periods when 

landslide activity can be expected (Cole and Davis, 2002). 

Baum and others (2005) proposed an integrated system 

consisting of field measurement of precipitation, soil wetness, 

and pore pressures coupled with time-dependent infiltration 

models for unsaturated soils, rainfall forecasts, and intensityduration thresholds as a comprehensive early-warning system 

near Seattle, Wash.

Debris-Flow Warning Systems 11
Current NOAA-USGS Capabilities 

and Limitations

The NOAA-USGS Task Force determined early in the 

study that the initial concept—that called for the NWS to supply the USGS with precipitation estimation and forecasts grids 

and for the USGS to operate the debris-flow warning system—would not be feasible under current funding constraints. 

The reason was lack of resources to establish a 24 hour × 7 

day (24×7) operation. As a feasible, implementable, and workable solution at the prototype level, the Task Force agreed to 

use the NWS Flash Flood Monitoring and Prediction Program, 

to be followed by a system operating under the initial concept, 

once resources for its operation could be procured.

Elements of a Debris-Flow Warning System

A Debris-Flow Warning System (DFWS) should ideally consist of separate products for Outlooks, Watches, and 

Warnings that are specific in both space and time to allow for 

useful lead and planning times for system users—emergency 

managers, planners, and responders. Training of USGS and 

NWS personnel to understand pertinent issues in debris-flow 

processes and hazard-assessment techniques as well as in 

precipitation forecasting and measurement is critical. Outreach 

and information products designed to inform both system 

users and the public of the uses and limitations of the system 

are necessary. And last, methods to verify, and thus improve, 

the effectiveness of a system are required. These elements are 

described below.

Products

Products issued by a DFWS should include Outlooks, 

Watches, and Warnings for debris-flow activity. (See definitions in “Terminology” section.)

In the event of a forecast for precipitation in excess of 

a 10-year recurrence storm for a given area, the USGS will 

develop an Outlook statement. The Outlook statement will 

identify those counties that have some potential to experience 

debris flows and will include information specific to the threat 

of debris flows. Outlook statements will be updated as more 

precise precipitation forecasts become available. 

A Watch will be issued by the USGS when forecast 

precipitation input to detailed hydrological and geological 

models for debris-flow susceptibility, hillslope hydrology, 

and slope stability indicate that debris flows are probable. 

The Watch statement will identify those areas that are most 

susceptible to debris flows, given the precipitation forecast, 

and will reflect the spatial and temporal uncertainty associated with the forecast. The statement will include information specific to the threat of debris flow. Watch statements 

will be updated as more precise precipitation forecasts 

become available.

Warnings will be issued by the USGS when radarderived precipitation estimates, observed rainfall data from 

ALERT (Automated Local Evaluation in Real Time) networks, spotter information, and other gage data input into 

detailed hydrological and geological models for debris-flow 

susceptibility, hillslope hydrology, and slope stability indicate 

that debris flows are imminent or have a high probability of 

occurring. A Warning statement may identify the probability 

of debris-flow activity for specific areas, and will reflect any 

uncertainty with the precipitation measures. The statement 

will include information specific to the threat of debris flow. 

Warning statements will be updated as real-time precipitation 

data and reports of debris-flow occurrences warrant. 

Any Outlook, Watch, or Warning statement will indicate that the product is a joint NOAA-NWS/USGS product 

and is based on USGS guidance. The description of the 

potential hazard will be written in a way that is understandable 

by emergency managers and public officials. Any Outlook, 

Watch, or Warning statement will include the following 

information: 

· Issuance time: Debris-flow products are nonscheduled, 

event-driven products;

· Valid time: Debris-flow products will be valid until a 

time/date specified in the product or until cancelled or 

updated by another debris-flow product as advised by 

USGS;

· Universal geographic code (UGC) type: County codes 

will be utilized for debris-flow products;

· Mass news disseminator (MND) product line: The 

product line will indicate “Debris-Flow Outlook,” 

“Debris-Flow Watch,” or “Debris-Flow Warning” 

as appropriate and as indicated by the USGS for the 

threat;

· Headline defining the type of event expected and additional vernacular names (e.g., debris flow, mudslide, 

mudflow);

· Identification of the area covered by the statement;

· Potential time period for which the event can be 

expected;

· Relevant factors (e.g., quantitative precipitation forecast, soil conditions, heavy rainfall, relation to threshold conditions, etc.);

· Likelihood of potential event;

· Call to action statements as appropriate for the 

expected event; and

· A closing statement indicating when additional information will be provided by the USGS and NOAANWS.

Data Forms and Formats

To assess the potential for debris flows and to determine 

the necessity of issuing Outlooks, Watches, and Warnings, the 

NWS will provide the USGS with data in three primary forms: 

(1) quantitative precipitation forecasts (QPF), (2) radar-derived 

12  NOAA-USGS Debris-Flow Warning System—Final Report
precipitation estimates, and (3) observed rainfall data from 

the ALERT networks as well as spotter information and other 

gage data available for the County Warning Areas (CWAs) of 

the NWS Weather Forecast Offices (WFOs) participating in 

the Debris-Flow Warning Program. These data can potentially 

be used as near-real-time input into detailed hydrological and 

geological models for debris-flow susceptibility, hillslope 

hydrology, and slope stability by the USGS. 

NWS forecast (QPF) data and radar observations will 

be transmitted by the NWS to the USGS in a format that can 

be readily incorporated into a geographic information system 

(GIS). Such a format might be, for example, ASCII grids in 

a defined coordinate system at the highest resolution possible. The NWS will post the data to a site that can be readily 

accessed by USGS systems and (or) push the data directly 

from NWS systems to USGS systems. NWS and USGS Information Technology (IT) personnel will determine the most 

efficient and effective methodology for these data transfers 

based on the systems capabilities of both agencies. The participating WFOs will provide USGS with information on QPF 

storm totals and durations. These forecasts would be made 

available on a routine basis at time frequencies ranging from 

twice daily (every 12 hours during routine hydrometeorological periods) to four times per day (every 6 hours during events) 

with more frequent updates as required by the hydrometeorological and (or) geological conditions. Quantitative estimates 

of rainfall accumulations from radar will be made available 

by NWS to USGS in near-real-time. Radar derived precipitation will be available for each WFO area every 5 minutes and 

will include 5-minute, 1-hour, 3-hour, and 6-hour accumulated 

precipitation as well as storm-total precipitation. Rainfall 

accumulations from the ALERT rain gage network and (or) 

other local mesonets will be made available to the USGS 

in near-real-time via transfer from NWS WFOs or through 

direct collection by USGS via an ALERT base station. NWS 

and USGS IT personnel will determine the most efficient and 

effective methodology for each participating WFO area. Other 

rain gage data, spotter reports of rainfall, and (or) reports of 

debris flows collected at WFOs within the area(s) of interest will be transmitted to USGS as received using established 

data-exchange methods. 

Procedures and Protocols

USGS personnel will contact the appropriate WFO to 

advise they are providing Outlook/Watch/Warning statements 

via the Advanced Weather Interactive Processing System 

(AWIPS). The message will be formatted to alert the forecaster of its arrival via an “Alarm” message at the forecaster’s 

workstation and will be a text product. NWS will assist the 

USGS with the technical and IT security details of delivering 

messages to the NWS through AWIPS. 

Upon receipt at the WFO(s) of a USGS message indicating the need for a debris-flow Outlook, Watch, or Warning, 

the impacted WFO forecaster will format and disseminate the 

recommended product via a Public Information Statement 

Product (PNS) and via NOAA All Hazards Radio (also known 

as a NOAA Weather Radio (NWR)). The NWS electronic 

product will be a text product with a unique “Debris Flow” 

product header that the NWS/Western Region Headquarters 

(WRH) will obtain from National Weather Services Headquarters (NWSH). In addition, the product will contain Valid Time 

Event Coding (VTEC) and a request for Emergency Alert 

System (EAS) activation as appropriate for the level of alert 

product being issued.

Any Outlook, Watch, or Warning statements will reference the USGS Landslide Hazards Program Web page, where 

the statements will be posted along with maps that show 

potential locations of debris-flow activity and impact zones, 

as are appropriate to the level of alert. Any statements will be 

provided to the USGS Senior Advisor for Science Applications, the USGS Associate Directors for Geology and Water, 

the Director of the USGS National Landslide Information 

Center, the appropriate USGS Water Science Center, and 

the appropriate USGS Regional and Headquarters Offices of 

Communication for their information. If considered newsworthy by the USGS, a press release that includes the Outlook, 

Watch, or Warning statement will be issued through the USGS 

Communications Office, but only after the PNS has been disseminated.

Training

Training developed by the USGS will be provided to 

WFO and River Forecast Center (RFC) staff, interested spotters, and other NWS volunteers. Training will address how and 

where debris flows are generated, what physical and meteorologic conditions are most likely to produce debris flows, 

and what products the USGS will be using as the basis for 

issuing Outlooks, Watches, and Warnings. Training will occur 

as 1-day sessions at each WFO and RFC and (or) as digital 

products that can be accessed as needed.

Outreach and Information

The USGS Landslide Information Officer will coordinate 

with his/her counterpart in the NWS region to develop an Outreach Plan for each region for which a DFWS is implemented. 

Possible outreach products include press releases that describe 

the DFWS, and Fact Sheets that describe controls on debrisflow generation. The USGS Landslide Information Center 

toll-free number will be available for inquiries. All products 

should be in both English and Spanish.

Validation Methodology

Although the exploratory debris-flow-forecast project 

in the San Francisco area from 1986–95 listed a number of 

Debris-Flow Warning Systems 13
successful Forecasts and early Warnings of debris flows (e.g., 

Wilson and others, 1993), the evidence cited was often anecdotal and subjective rather than systematic and objective. Any 

warning system should be evaluated more rigorously, while 

not discarding the less quantitative, but informative, testimonies. 

The NWS conducts ongoing assessments of its skill for 

predicting various weather features, including Forecasts of 

heavy precipitation, flash-flood Warnings (was a flood forecast and did it occur or not?), and flash-flood Warning lead 

time (how far in advance of the flood was a Warning issued?). 

Predictions of binary (yes/no) events, such as the forecast of a 

flood or whether heavy precipitation (1 inch in 24 hours) will 

occur, are often evaluated by their the false alarm rate (FAR) 

and probability of detection (POD) scores. The threat score, or 

critical success index (Glickman, 2000), combines aspects of 

both FAR and POD and is used by the NWS to assess the skill 

of its heavy-precipitation and flash-flood forecasting methods 

and to chart year-to-year improvements. These same metrics 

can also be applied to debris-flow forecasting, assuming that 

all significant flows will be reported. However, agreement 

must be reached before the forecasting tests begin as to exactly 

what constitutes a validating debris flow in the prototype 

forecast area. Must the flow (or combined flows) exceed some 

specified volume to qualify? Is a single flow in the area during the forecast period sufficient to justify a forecast success? 

Following storms, field reconnaissance by USGS will be 

required to survey the Forecast/Warning region for evidence of 

flows and to document them with video photography. NOAA’s 

Storm Data reports from a variety of sources, including spotters, newspaper reports, and damage surveys will all be useful 

in the debris-flow-occurrence assessments. Additional, more 

sophisticated monitoring methods will be available within the 

smaller Intensive Research Area.

The success or failure of any warning system, however, 

goes beyond simple forecasting skill measures. Another 

important facet is assessing the effectiveness of the Warnings. 

This includes determining whether the Watches and Warnings 

reach the at-risk population in a timely manner and if actions, 

such as evacuation, are actually taken to escape the imminent 

danger. Victims killed by a debris flow that was correctly 

forecast may not have heard the Warning, or may have heard 

but ignored it. Their deaths will be well publicized, but it may 

never be known how many other people escaped peril because 

of the forecasts. Evacuations in Warning areas where debris 

flows fail to materialize (false alarms) are disruptive, costly, 

and may cause future Warnings to go unheeded. These matters 

are much more difficult to evaluate, but speak more directly to 

the overall effectiveness of the system than an assessment of 

forecasting skill alone. Therefore, the prototype system must 

interact closely with emergency management (EM) agencies to 

ensure the Forecasts and Warnings achieve maximum usefulness and that subjective evaluations by emergency managers 

will constitute part of the prototype’s assessment. The EM 

evaluations can be obtained from a scheduled program of 

direct interviews before, during, and after each winter season 

and by Web-page-based questionnaires. The latter tactic was 

successfully employed in NOAA’s Pacific Jets Experiment 

(PACJET) storm research project to evaluate the usefulness of 

new profiling radar observations to NWS forecasters. Evaluations by EM agencies must be a planned part of the program, 

not an afterthought.

A Prototype Debris-Flow Warning System

Southern California Setting

History of Debris-Flow Activity

Although limited numbers of debris flows are likely to 

occur somewhere in southern California during any given year, 

some winter storms have resulted in particularly abundant 

debris flows over broad areas. Summer thunderstorms have 

produced debris flows over limited areas.

· Storms during the winter of 1937–38 produced the 

floods of record for most gaging sites throughout 

southern California. At the time, a clear distinction was 

not made between floods and debris flows, but these 

events were reported to be responsible for 90 deaths, 

and 5,600 homes were destroyed (Los Angeles Times, 

2004, 2005). Approximately $65 million in damage 

was caused by these events. 

· Three months after the Bel-Air Brentwood fires of 

1962, storms triggered debris flows that traveled down 

Hollywood Boulevard. Twenty-one deaths were attributed to the storms (Los Angeles Times, 2005).

· The winter of 1968–69 brought back-to-back storms in 

January and February. These storms resulted in flooding 

and debris flows in the San Gabriel and San Bernardino 

mountains that caused 11 deaths (California Department 

of Water Resources, 2005). Debris flows and flash floods 

from hillslopes burned the previous summer above the 

town of Glendora transported more than a million m3 of 

material from the burned basins (Scott, 1971). 

· Heavy winter rainfall during February and March 1978 

resulted in widespread landslide and debris-flow activity in Ventura, Los Angeles, Orange, Riverside, and 

San Bernardino Counties (Weber and others, 1979).

· During the winter of 1979–80 1,450 homes were 

destroyed and $7 million damage was wrought by 

flooding and debris flows in Los Angeles, Orange, Riverside, and Ventura Counties (Weber and others, 1980). 

Some of this damage was caused by debris flows generated from hillslopes burned the previous summer 

in the San Gabriel Mountains (Shuriman and others, 

1985)

· Storms during the winter of 1982–83 resulted in $106 

million in damages from flooding and debris flows.

· The winter of 1992–93 brought flash floods and debris 

flows which resulted in eight deaths in southern California (Los Angeles Times, 2005).

14  NOAA-USGS Debris-Flow Warning System—Final Report
· The winter of 1997–98 brought debris flows and floods 

to the entire southern California area. Activity was particularly severe from recently burned basins (Cannon, 

2000). One death occurred in Laguna Canyon. 

· In late October and early November 2003, wildfires 

burned more than 700,000 acres in Los Angeles, Riverside, San Bernardino, San Diego, and Ventura Counties. A storm on Christmas Day 2003, which caused 16 

deaths, resulted in debris flows and flash floods from 

nearly all of the recently burned basins. In response, 

$26.5 million was spent to repair roads and to remove 

the 4.1 million m3 of material deposited in debrisretention basins (Pat Mead, San Bernardino Flood 

Control District, oral commun.). 

· A series of storms from October 2004 through March 

2005 impacted southern California, and many areas 

received record amounts of precipitation. These storms 

triggered widespread floods, landslides, and debris 

flows; losses are expected to reach $500 million.

Geology

Southern California consists of a series of prominent 

mountain ranges with intervening valleys, coastal plains, and 

river basins (fig. 4). The Peninsular Range, which includes the 

Santa Ana Mountains north of Los Angeles, is located south 

of the Los Angeles Basin and trends northwest to southeast 

following the San Andreas fault. The San Jacinto and San 

Bernardino mountains east of the Upper Santa Ana River basin 

follow a similar trend. Immediately north of the Los Angeles 

Basin, the mountains trend east-west, giving rise to the Transverse Range (the San Gabriel Mountains, the Santa Monica 

Mountains, the Topatopa Mountains, and the Santa Ynez 

Mountains). North of the Transverse Range, the Sierra Madre 

and Santa Lucia Ranges again trend northwest-southeast. Most 

of these mountains are bounded by faults and have broad 

alluvial plains extending to the south along which major highway corridors have been built. Active uplift, combined with 

fractured, weak rocks and actively downcutting streams, create 

remarkably steep hillslopes that are susceptible to debris flows 

following heavy rainstorms. 

The Transverse Ranges and San Bernardino Mountains 

are particularly well known for producing large flash floods 

and debris flows following wildfires (e.g., Anderson and 

others, 1959; Doehring, 1968; Campbell, 1986; Scott and 

Williams, 1978; Wells, 1981, 1982, 1987; Rice, 1974; Wohlgemuth, 1986; Cannon, 2000). Watersheds are steep and 

rugged, rising abruptly from valley floors to general elevations of 2,000 to 2,500 m, and to extreme elevations of more 

than 3,000 m. Drainage networks are deeply incised with 

steep side slopes. The mountains are composed of a complex 

assembly of easily weathered, extensively faulted, coarsely 

crystalline igneous and metamorphic rocks in the south, 

and sedimentary sequences in the north (State of California, 

1967, 1969; Scott and Williams, 1978). Soils generally are 

composed of shallow, rocky, sandy loams, less than 1 m 

thick, and they show little evidence of profile development 

(Wells, 1981).

The Peninsular Ranges, which include the Santa Ana 

Mountains, are generally less steep and less deeply incised 

than the Transverse Ranges and are composed primarily of 

weathered granite batholiths that intruded into marine sediments and volcanic and metavolcanic rocks (State of California, 1965; Hart, 1991). The Sierra Madre and Santa Lucia 

mountains are composed principally of marine and nonmarine 

sedimentary rocks and some metamorphic rocks (State of 

California, 1958).

Vegetation

Mountain-front slopes are vegetated most commonly 

with combinations of annual grasses, coastal sage scrub, 

and chaparral—a vegetation complex that is composed 

predominantly of highly flammable, woody, shrub-like 

plants. Typical coastal sage scrub species include Salvia 

mellifera, Salvia apiana, Encelia farinosa, and Eriogonun 

fasciculatum (Morton, 1989). Typical chaparral species 

include chamise (Adenostoma fasiculatum), California 

lilac (Ceanothus spp.), manzanita (Arctostaphylos spp.), 

and scrub oak (Quercus Dumosa) (Campbell, 1986). In 

general, grasses and coastal sage scrub species occupy 

lower elevations and transition into chaparral at slightly 

higher elevations, owing to generally increasing precipitation and cooling temperatures with elevation (Minnich, 

1989). Chaparral can be replaced by oak woodland and 

conifer forests at higher elevations (Mooney and Parsons, 

1973; Minnich, 1989). Riparian woodland occupies stream 

courses (Mooney and Parsons, 1973).

Climatology and Meteorology

The Mediterranean climate of southern California is 

characterized by hot, dry summers and cool, wet winters. The 

driest months are July and August, and the period from August 

to November is one of hot, dry desiccating winds called 

Santa Anas. These winds blow across California in a southsouthwesterly direction and can create conditions of extreme 

fire danger in a matter of hours (Wells, 1981). The rainy season begins in December and lasts until mid-April; January and 

February are the wettest months. 

Figure 5 shows a map of average annual precipitation for 

southern California. The orographic influence on the distribution of precipitation is apparent. Mountain ranges in the 

coastal region receive annual averages of approximately 40–45 

inches per year, whereas adjacent basins receive less than 

half that amount. The seasonal distribution of precipitation is 

also very uneven. More than 90 percent of the annual average precipitation at Los Angeles and San Diego falls in the 

November to April period. The spatial and temporal concentrations of precipitation make the southern California coastal 

region highly susceptible to flash floods and debris flows in 

the winter months.

Debris-Flow Warning Systems 15
The coastal mountains represent a significant barrier to 

lower tropospheric onshore flow. These mountains can provide lift of moisture-laden air even under the typically stably 

stratified conditions found in Mediterranean-type climates. 

Indeed, substantial rainfall often occurs when strong winds 

associated with extratropical cyclones force high-humidity 

air up the mountain slopes. Not infrequently, a concentrated 

plume of moisture originating in the subtropics near Hawaii 

(the “pineapple express,” or atmospheric river (e.g., Ralph and 

others, 2004)) produces persistently heavy precipitation along 

California’s coast. The intense storm events usually involve 

relatively shallow bands of rain and showers. It is not uncommon to see tops of radar echoes reaching only 3 km or less 

above sea level. Despite a low-lying freezing level (around 2 

km) during the winter months, the coastal convergence and 

orographic uplift may still result in significant concentrations 

of liquid water within the shallow warm cloud layer. In addition to the broad-scale airflow that produces the orographically 

enhanced precipitation, embedded thunderstorms contribute 

even more intense localized downpours for short periods, 

thereby increasing flood and debris-flow threats. Thus, winter 

storms can produce more than 200 mm (8 in.) of rain in 24 

hours and strong surface winds in excess of 50 mph. Interannual variations in the mean storm track due to “El Niño” conditions often exacerbate the southern California flood problem 

by setting up a pattern of repeated heavy storms over a period 

of weeks. The recent 8–10 January 2005 event had recorded 

weekly precipitation amounts in excess of 28 inches at some 

mountain rain-gage locations

Existing Meteorological Operational Systems

Successful debris-flow Warnings will depend strongly 

upon accurate real-time observations and short-term forecasts 

of rainfall. The rainfall observation infrastructure in southern 

California has been augmented through the years to provide 

for a relatively high density distribution of rain gages and a 

few NWS radar sites (fig. 6).

The ALERT gage network, maintained by local floodand water-control districts, is the backbone of this system. 

The ALERT system was developed by the National Weather 

Service (NWS) California-Nevada River Forecast Center 

Figure 4. Topography of southern California.

SAN LUIS

OBISPO

KERN

SAN BERNARDINO

SANTA BARBARA 

VENTURA

LOS ANGELES

ORANGE

RIVERSIDE

SAN DIEGO

Los Angeles

Basin

SAN BERNARDINO

MTS

SAN JACINTO

MTS

SAN GABRIE

MT

L

S

SANTA MONICA

MTS

SANTA YNEZ

MTS

SANTA LUCIA

RANGE

SANTA ANA

MTS

TOPATOPA

MTS

SIERRA MADRE 

CALIFORNIA

STUDY AREA

0 50 miles

0 50 kilometers

County boundaries

EXPLANATION

16  NOAA-USGS Debris-Flow Warning System—Final Report
Figure 5. Map of mean annual precipitation for southern California.

Figure 6. Topography map of southern California and the location of ALERT rain gages, NWS operational radars, upper-air radiosonde sites.

Debris-Flow Warning Systems 17
(CNRFC) in the late 1970s with the intention of providing local communities with a low-cost means of acquiring 

real-time rainfall and streamflow data to better evaluate flood 

potential. Components of the ALERT system particularly 

relevant for debris-flow warning include precipitation and 

stream gages. Most ALERT precipitation gages are a standard tipping bucket design. When the buckets tip (0.04 inches 

accumulation), a signal is sent to a central ALERT command 

center, which in turn transmits the information to a floodcontrol district office. Based on the history of precipitation at 

each ALERT station, an alarm will sound when the amount of 

precipitation reaches a threshold value that is known to result 

in flooding. Similarly, when river levels reach a predetermined 

stage, flood-stage sensors send information to the flood-control district offices and the local office of the NWS. Real-time 

data also go directly to the county’s Emergency Operations 

Center and county road-crew supervisors to prepare them for 

the potential of flash flooding. Reports also are received from 

trained spotters and observers, emergency managers, lawenforcement officials, and the public to indicate where flooding has begun or is imminent. A number of other networks 

of rain gages are operated by various other agencies at more 

limited locations. This includes USGS stream gages in some 

mountain ranges in southern California.

Figure 7 shows the distribution of ALERT rain gages and 

operational radar coverage in southern California from the 

network of NEXRAD radars. The coverage indicates where 

the radar beam can observe conditions less than 1 km above 

ground level. Significant gaps are evident in the low-level 

coverage due to beam blockage and other factors. Green lines 

are county boundaries. There are more than 300 ALERT rain 

gages in the region. Four NWS NEXRAD radars cover the 

region: San Diego (KNKX), Los Angeles–Santa Ana (KSOX), 

Sulphur Mountain–Ventura (KVTX), and Barstow (KEYX). 

NEXRAD radars operate continuously, scanning the threedimensional volume surrounding the radar with prescribed 

coverage patterns. To produce rainfall maps, an empirical 

relationship between reflectivity and rainfall is used and the 

data converted to Cartesian (i.e., x, y, z) grids. The grids can be 

updated every volume scan (i.e., 5–6 minutes) and made available on the Internet. (See fig. 8 and http://weather.noaa.gov/

radar/mosaic/DS.p19r0/ar.us.conus.shtml.)

NEXRAD’s data-void problems were previously 

described in general terms. These scan-coverage gaps are parFigure 7. Southern California ALERT rain gage locations (red dots) and WSR-88D radar low-level (below 1 km above 

ground level) coverage (blue regions).

18  NOAA-USGS Debris-Flow Warning System—Final Report
ticularly problematic in southern California. As figure 7 indicates, the low-level (less than 1 km above ground) NEXRAD 

scan coverage in southern California is quite limited. Beam 

blockage by terrain, the high site elevation (2–3 thousand feet 

above sea level) of some NEXRAD radars located on peaks, 

and the minimum scanning elevation angle (0.5°) used by 

the radar combine to produce these gaps. Often, shallow but 

intense regions of storms over the Los Angeles Basin are, 

therefore, poorly observed or missed entirely because of these 

factors. Other effects, such as beam ducting due to strong lowlevel temperature inversions, also reduce, from time-to-time, 

the radars’ ability to detect low-level precipitation. The effects 

of inadequate scan coverage on precipitation measurements 

over the Los Angeles Basin are illustrated in figure 9, where it 

can be seen that large regions of the important lower atmosphere entirely escape NEXRAD monitoring. These gaps are 

problematic because the most potent region of many winter 

storms and the winds that force orographic lifting and condensation of water vapor are at low levels. Vertical gradients 

of reflectivity increase the uncertainty of the radar estimate 

of surface rainfall. A recent National Academies study (NRC, 

2005) examined these problems in detail, specifically for the 

Los Angeles area, and recommended the use of gap-filling 

radars as part of a solution.

Identification of Customers/Collaborators 

and Their Requirements

A list of identified potential customers and collaborators 

is included in Appendix C.

The customers interviewed by Eric Boldt, WFO, Oxnard, 

Calif., and Ed Clark, WCM, WFO, San Diego, included the 

Department of Public Works, Watershed Department, County 

Flood Control, and County Offices of Emergency Services.

Interviews with potential users of the demonstration 

warning system revealed that (1) some users saw the benefit 

from warning of debris flows from burned areas, whereas 

others thought they already had sufficient knowledge to deal 

with such a situation. (2) In addition to advisory Watches and 

Figure 8. Example of storm-total maps of precipitation accumulation available in real time on the 

Internet from every NEXRAD radar site.

Debris-Flow Warning Systems 19
Warnings, map products that provide information on areas 

of impact are desirable. (3) Lead times of 24 to 48 hours for 

Outlooks and Watches and 24 hours for Warnings are desired. 

(4) Extension of a warning system to cover unburned areas 

would be a valuable contribution. For details, see Appendix D, 

“Customer Requirements.”

Elements of the Prototype Warning System

A Prototype Debris-Flow Warning System (PWS) should 

ideally consist of separate products for Outlooks, Watches, and 

Warnings that are specific in both space and time to allow for 

useful lead and planning times for emergency managers, planners, and responders. Training of USGS and NWS personnel 

to understand important issues in debris-flow processes and 

hazard-assessment techniques as well as in precipitation forecasting and measurement is critical. Outreach and information 

products designed to inform both the system users and the 

public of the uses and limitations of the system are necessary. 

And last, methods to verify, and thus improve, the effectiveness of a system are required. These elements are described 

below.

Products

Products issued by the PWS will include Outlooks, 

Watches, and Warnings for flash floods and debris flows from 

recently burned basins. 

An Outlook is used by the NWS to indicate that a hazardous weather or hydrologic event may develop. It is intended 

to provide information to those who need considerable lead 

time to prepare for the event. Within 10 days of the occurrence of any wildfires that consume more than about 200 acres 

within the San Diego and Oxnard Weather Forecast Office 

(WFO) areas, the USGS will develop an Outlook statement. 

The statement will identify the location of the fire by county, 

nearby city or town, geographic landmarks, and bounding latitude and longitude coordinates and will include information 

specific to the threat of debris flows and flash floods following 

fires. The description of the potential hazard will be written 

in a way that is understandable by emergency managers and 

public officials. Outlook statements can be updated to include 

site-specific observations of hillslope, fire, and drainage conditions, should they become available. 

USGS Landslide Hazards Program personnel will contact 

the appropriate WFO to advise they are providing an Outlook

Figure 9. Vertical section of radar coverage across the Los Angeles Basin area.

20 NOAA-USGS Debris-Flow Warning System—Final Report
via the Advanced Weather Interactive Processing System 

(AWIPS). The message will be formatted to alert the forecaster of its arrival via an “Alarm” message at the forecaster’s 

workstation and will be a text product. (NWS will assist the 

USGS with the technical and IT security details of delivering 

messages to the NWS through AWIPS.) Forecasters on duty 

will, in turn, disseminate the Outlook statement to emergency 

managers, responders and the general public through established dissemination procedures as a PNS (Public Information 

Statement product). The Outlook statements will reference the 

USGS Landslide Hazards Program Web page, where statements will be posted along with maps that show the locations 

of each fire. The Outlook statements will be provided to the 

USGS Senior Advisor for Science Applications, the USGS 

Associate Directors for Geology and Water, the Director of the 

National Landslide Information Center, the USGS California 

Water Science Center, and the USGS Western Region and 

Headquarters Offices of Communication for their information. If considered newsworthy by the USGS, a press release 

that includes the Outlook statement will be issued through the 

USGS Communications Office, but only after the PNS has 

been disseminated. 

A Watch is issued by the NWS when the risk of a 

hazardous weather or hydrologic event has increased significantly, but its occurrence, location, and (or) timing are still 

uncertain. It is intended to provide enough lead time so that 

those who need to set their plans in motion can do so. Lead 

times are at most, less than 3 days, and can be as short as a 

few hours. 

A Warning is issued when a hazardous weather or 

hydrologic event is occurring, is imminent, or has a very 

high probability of occurring. A Warning is used for conditions that pose a threat to life or property. Desired lead times 

would be within 1 day, but developing conditions might 

cause them to issued with lead times as short as 30 minutes. 

Within the PWS, Watches and Warnings will be issued by 

the NWS using the Flash Flood Monitoring and Prediction 

(FFMP) system described in the next paragraph. The system 

will use the debris-flow threshold curves developed by the 

USGS (see, for instance, fig. 2). Depending on whether the 

actual or forecast conditions fall within the orange or red 

zones, the NWS will issue a corresponding Watch or Warning, respectively.

Implementation

To best utilize available personnel and operational 

resources, the PWS will use as its foundation the existing 

NWS procedures in place to monitor and forecast the potential 

for flash floods, including the Flash Flood Monitoring and 

Prediction (FFMP) system. With the tools available to the 

WFO forecaster, precipitation forecasts (QPF), quantitative 

estimates of rainfall accumulations and intensities from radar 

observations, and actual precipitation accumulations (AP) are 

compared in real time to user-defined guidance, or thresholds, 

for event occurrences. Information is evaluated relative to 

counties and FFMP basins. Flash-flood thresholds, or flashflood guidance, is provided to the WFOs by the River Forecast 

Centers (RFC) to assist the forecaster in determining if conditions warrant the issuance of a flash-flood Watch or Warning. 

The USGS Landslide Hazards Program will provide geologic guidance for the onset of flash floods and debris flows 

from recently burned basins in the form of rainfall thresholds 

for each of the FFMP basins within the San Diego and Oxnard 

WFO areas in a form compatible with current NWS data 

structure. These data will be provided to the WFOs. NWS personnel will use their meteorologic expertise in predicting and 

monitoring precipitation to determine when and if threshold 

conditions will be met and to determine if conditions warrant 

the issuance of a flash-flood Watch or Warning. USGS Landslide Hazards Program personnel will be available through the 

AWIPS system for consultation during working hours, or on a 

24×7 basis if conditions warrant, in the event that precipitation 

forecasts appear to exceed the provided thresholds. 

Procedures and Protocols

Watches will be disseminated to emergency-response 

personnel and the public through AWIPS, with activation of 

the Emergency Broadcast System (EMS) when appropriate. 

Watch and Warning statements will include the elements presently included in such statements (Issuance Time, Valid Time, 

etc.) in addition to language specific to recently burned areas 

provided by the USGS. USGS Landslide Hazards Program 

personnel will be informed when a Watch or Warning is 

issued through the AWIPS system. Landslide Hazards Program personnel will, in turn, provide information to the USGS 

Senior Advisor for Science Applications, the USGS Associate 

Directors for Geology and Water, the Director of the National 

Landslide Information Center, the USGS California Water Science Center, and the USGS Western Region and Headquarters 

Offices of Communication for their information. 

Training

Training developed by the USGS will be provided to 

WFO and RFC staff, interested spotters, and other NWS 

volunteers. Training will address how surface hydrology is 

changed by wildfire, what processes result in the generation 

of fire-related debris flows (and how these processes differ 

from those that generate debris flows in unburned settings), 

what conditions are most likely to result in debris flows 

(short-recurrence-interval storms, material properties, etc.), 

the derivation of the rainfall thresholds, and our confidence in 

those thresholds. Training will occur as 1-day sessions at each 

WFO and at the RFC and (or) as digital products that can be 

accessed at one’s convenience.

Outreach and Information

The outreach and information plan for the prototype system is identical to the one for the full operational system.

Debris-Flow Warning Systems  21
Validation Methodology

The validation methodology for the prototype system is 

identical to the one for the full operational system.

Research

The initial prototype will be based on currently available 

operational capabilities and represents the lowest cost system 

that can provide a minimum of service. However, the prototype system should also involve a strong research component 

to address various anticipated weaknesses of these capabilities 

for the sake of making near-term and future improvements. 

These research activities, to be conducted primarily within 

the Intensive Research Area, are described in the “Intensive 

Research Area” section and described more extensively in the 

“Future Development” section. Some of the research activities 

could begin as part of the initial deployment of the prototype 

system as early as the winter of 2005–06; others involve 

longer lead times. All will require new funding beyond the 

requirements of the minimum system.

Implementation Plan of the 

Prototype Warning System

The time line and activities to be completed before October 2005 are shown in table 1.

Implementation and Operational Costs 

of the Prototype Warning System

The activities required to operate the prototype warning 

system are part of ongoing project and (or) operations costs 

both at the USGS and at the NWS. Therefore, no additional 

Table 1. Prototype implementation plan. 

 Accomplishment date Activity 

 June 15, 2005 USGS will establish agreements and procedures with Geospatial MultiAgency Coordination (GeoMAC) and fire suppression agencies to 

provide burn perimeter and burn severity data to USGS in a timely 

manner.

 June 15, 2005 An outreach plan will be developed by the USGS Landslide Information 

Officer and his/her counterpart in the NWS region. 

 June 15, 2005 FFMP basin coverage for the San Diego and Ventura/Oxnard WFO areas 

will be provided by the NWS to the USGS. 

 July 15, 2005 USGS will develop rainfall thresholds for debris flows and flash floods 

from recently burned areas within the San Diego and Ventura/Oxnard 

WFO areas. 

 August 1, 2005 FFMP basin coverage of the rainfall thresholds will be developed by the 

USGS and provided to the NWS. 

 August 1, 2005 NWS will provide USGS access to AWIPS system as the designated 

communication system. This access will be tested both by the WFOs 

and the USGS to ensure that two-way communication is established. 

 August 15, 2005 Base maps of the areas covered by the San Diego and Ventura/Oxnard 

WFOs showing cities, road networks, topography, and primary river 

systems will be assembled by USGS. 

 August 15, 2005 Text specific to flash-flood and debris-flow hazards from recently burned 

areas for inclusion in the Outlook, Watch, and Warning statements 

will be developed jointly by the WFOs and the USGS and approved 

by the appropriate officials. 

 September 1, 2005 The outreach plan developed by the USGS Landslide Information Officer 

and his/her counterpart in the NWS region will be implemented. 

 September 1, 2005 A training program for NWS personnel that includes digital products and 

1-day hands-on training will be provided by the USGS. 

 October 1, 2005 Updated rainfall thresholds to account for seasonal-scale vegetation 

recovery and sediment source depletion will be developed by the 

USGS and provided to the NWS. 

22  NOAA-USGS Debris-Flow Warning System—Final Report
operational costs are expected at either agency. Funding for 

the recommended research components of the system represents additional costs.

Operational Considerations

Entrance/Exit Strategies

The proposed debris-flow warning system for recently 

burned areas in southern California is designed to be a 

prototype system and is being assembled with NOAA and 

USGS resources that are currently available. The organization and hardware are designed to be an experiment whose 

input and output can be adjusted and modified to optimize 

results. Once in place, if preliminary results are promising, a 

debris-flow warning system for recently burned areas in parts 

of southern California would be hardened; an infrastructure 

created to support data collection, warning systems, reliable 

Forecast criteria, and Warning dissemination; and additional 

capabilities added. We anticipate that once this activity is 

begun, we would continue to try to provide debris-flow 

Warnings for recently burned areas in southern California in 

perpetuity. 

There are some circumstances under which this prototype debris-flow warning system could be dismantled. Among 

these would be a lack of success in defining thresholds for 

debris-flow production in recently burned areas, or the fiscal 

inability of the USGS or NOAA to continue support for the 

program. In these cases, any instruments or measurement 

devices deployed in the affected areas will be removed and 

either returned to service in another project or activity in 

NOAA or the USGS, or turned over to local officials who may 

wish to continue monitoring recently burned areas for debris 

flows with local support and expertise. Any information, data, 

protocols, or procedures generated during this experience 

will be made readily available to any agency that may wish to 

continue operation of the system. The agreement between the 

NOAA and the USGS to operate a prototype system will be 

considered null and void. 

Public Affairs Factors

Upon successful completion of preliminary testing, 

NOAA/NWS and USGS public affairs will work jointly to 

announce implementation (and expansion, if warranted) of the 

program. Ideally, timing of this announcement would be just 

prior to the southern California rainy season. Opportunities 

would likely exist to “show-off” the system to media, legislators, and other key constituents.

Exiting the program brings with it a unique set of challenges. Messages to be conveyed will depend largely on the 

rationale for exiting the program. It is fair to say that many 

constituents will be concerned about the loss of the program. 

In any event, announcements will be made that are simple and 

forthright. Briefings should be held with all major constituents 

prior to implementation of an exit strategy to:

· Determine whether other alternatives (to exiting) exist,

· Explain and discuss rationale for exiting, and

· Gather potential information for use in statement to 

media and other constituents impacted by exiting.

Intensive Research Area

The prototype system also involves a research component 

to address the anticipated shortcomings of the initial warning 

system, which will be based entirely on operational, but less 

than ideal, observations and models. Most of this meteorological and geologic research will be conducted in an Intensive 

Research Area within the much broader Prototype Forecast 

and Warning Region (fig. 1). Recent USGS studies show that 

debris-flow-initiating downpours are sometimes highly localized deluges from intense thunderstorms embedded within 

widespread precipitation. The Intensive Research Area will 

be densely instrumented to address this and to facilitate other 

research activities. Specially deployed tools in this area will 

provide streamflow measurements and video documentation 

of flows as they occur, including detailed information on the 

type of runoff—water flood, hyperconcentrated flow, or debris 

flow. Dense arrays of nested soil tensiometers and runoff plots 

will be used to characterize infiltration, runoff, and erosive 

processes. Installing these instruments will aid in the understanding of the debris-flow-triggering mechanisms, and hence, 

in the development of the more advanced physical-sciencebased models applicable to all areas, burned and unburned, 

across the United States. As described in the “Precipitation 

Measurement” section, the Intensive Precipitation Measurement Array (IPMA), composed of transportable gap-filling 

radars and other meteorological instruments, will augment the 

coarser documentation of atmospheric conditions available by 

existing operational networks. Post-storm ground crews will 

survey the flows and damage in the research area.

Validation Methodology

An important component of the Intensive Research Area 

should be an assessment of whether research fostered by 

the program has produced, or is likely to produce, improvements to the forecasting skill and Warning effectiveness. 

This research will also address the future transferability of 

techniques used in the prototype region to other parts of the 

country. For example, debris flows induced by rapid melting 

of mountain snow pack are not a factor in southern California, 

but they are a significant cause of debris flows in mountain 

ranges farther north. NOAA’s National Operational Hydrologic Remote Sensing Center (NOHRSC) uses aerial remote 

sensing to regularly measure snow-pack water content across 

the United States. In addition to monitoring antecedent snow 

conditions, these snow survey flights can use GPS-coordinated 

aerial photography to verify and document the occurrence 

and extent of debris flows with the aid of GIS-based imageanalysis techniques. The routine flights can be expanded to 

Debris-Flow Warning Systems  23
cover potential debris-flow areas in detail, and NOHRSC’s 

sophisticated snow model will help identify the snow-water 

equivalent and temperature thresholds that produce snowinduced debris flows.

Implementation and Operation Costs

The implementation costs of the hydrometeorological equipment for the intensive research area are outlined in 

table 2. For a description of the acronyms on this table, please 

consult the “List of Selected Acronyms Used in this Report” in 

the table of contents. The role of the advanced instrumentation 

listed in the table is described in the “Precipitation Measurement” section.

research activities may be required. Nevertheless, the desirable 

research, including meteorological, hydrologic and geologic 

aspects of the problem, is outlined in this section. Most of it 

would be conducted in the localized Intensive Research Area 

within the much broader prototype Forecast/Warning region.

Precipitation Measurement

The limitations of precipitation measurement and nowcasting with the current operational observing system were 

previously outlined, with specific examples of limitations in 

southern California. This section outlines the composition, 

design, logistical considerations, and costs of an enhanced 

observing network for precipitation measurement that could 

be used as improved input to a debris-flow warning system. 

The proposed Intensive Precipitation Measurement Array (fig. 

10) has been conceived as a research effort to supplement 

operational precipitation-measurement tools that will initially 

be the inputs to the prototype debris-flow warning system for 

southern California. In this prototype warning system, recently 

burned mountainous areas will be emphasized, given their 

much greater susceptibility to debris flows. The IPMA will 

also focus on these areas. Therefore, depiction of the IPMA 

in figure 1 should not be viewed as fixed, but rather as an 

example of how the array might be organized had the recently 

burned area been in the eastern San Gabriel and western San 

Bernardino Mountains, such as occurred in late 2003.

Refined Precipitation Forecasts 

and Measurements

The emphasis of the IPMA will be on filling gaps in 

operational radar coverage for the purpose of improving quantitative precipitation estimates (QPE)—the most critical input 

to the debris-flow warning system. Gaps in southern California operational radar coverage were highlighted in the section 

“Existing Meteorological Operational Systems,” where the 

poor coverage by NEXRAD of the key low-altitude regions 

of the atmosphere was illustrated. In the IPMA, scanning 

radars from the NOAA research laboratories will be deployed 

to fill these gaps. Two different varieties of these radars are 

envisioned to be part of the array. One is a transportable 

3-cm-wavelength (X-band) Doppler and polarimetric radar 

(X-POL) from the Environmental Technology Laboratory 

(ETL) and the other is a mobile 5-cm-wavelength (C-band) 

Doppler radar (SMART-R) from the National Severe Storms 

Laboratory (NSSL). X-POL, which has been used in dozens of 

field projects, has a 0.9° beam width and can collect 256 radial 

bins (gates) of data. At a radial resolution of 150 m, data can 

be collected out to a range of about 38 km. SMART-R has a 

1.5° beam width and can collect 2,048 gates of data. At a pulse 

repetition frequency of 1,000 Hz, data can be collected out to 

a range of 150 km for radial resolutions greater than or equal 

to 75 m.

Table 2. Summary of estimated meteorologic and geologic 

implementation costs for a 5-month deployment of the Intensive 

Research Area. 

Description Cost ($K)

 LIDAR characterization of topography of the site 

and documentation of changes...................................... 40 

 Site logistics ......................................................................... 40 

 Integration of IPMA data into QPE algorithms 

(one-time cost) (0.5 FTE) ........................................... 100

 SMART-R.......................................................................... 100 

 X-POL/sfc. met. site .......................................................... 100

 S-PROF/ disdrometer/ sfc. met. site.................................... 35 

 Rain-gage network (6 sites)................................................. 10 

 Wind profiler /sfc. met. /GPS IWV/ 

GPS balloon sounding................................................. 115 

 Soil moisture and pore-pressure-gaging network................ 50 

 Laboratory testing of materials............................................ 10 

 Post-deployment analysis (2 FTE) .................................... 400 

 Total ................................................................................ 1,000 

Future Development

The proposed prototype program will initially rely on 

the use of existing operational tools as a low-cost way to 

begin testing debris-flow forecasting and warning methods. 

These tools and the knowledge of debris-flow physics have 

advanced significantly since the end of the San Francisco Bay 

area exploratory program a decade ago. However, even with 

these advances, the current resources are less than ideal for the 

job. Therefore, it is essential that the program also include a 

strong research component to address anticipated shortcomings. This will require additional new funding, which is likely 

to take time to secure in the USGS and NOAA budgeting 

processes. Hence, a delayed or phased-in implementation of 

24  NOAA-USGS Debris-Flow Warning System—Final Report
The strength of X-POL is its polarimetric capability, 

which allows derivation of fields such as differential reflectivity and specific differential propagation phase (Bringi and 

Chandrasekar, 2001). It has been shown that these parameters 

can (1) be used to make more accurate estimates of precipitation than is possible from reflectivity alone (Zrnic and 

Ryzhkov, 1996; Matrosov and others, 2002) and (2) allow the 

types of hydrometeors (raindrops, snowflakes, graupel, etc.) 

present in the beam to be inferred. Polarimetry is also used for 

correcting attenuation, which diminishes X-band radar reflectivity values in moderate to intense precipitation. The strength 

of SMART-R is its mobility. As opposed to X-POL, which 

is transportable but takes a few days to set up and dismantle, 

SMART-R is truck mounted and thus, highly mobile, with the 

ability to deploy or stow in 10–20 minutes at each new location. The SMART-R is also C-band and thus not as susceptible to attenuation of its signal by intervening precipitation 

as X-band. In addition, SMART-R is due to be upgraded to 

dual-polarization capability by 2006. From a practical standpoint, X-POL will be sited at a fixed location near the primary 

mountainous burn area for an entire winter season, whereas 

SMART-R could be moved to different locations throughout 

the much broader prototype Forecast/Warning region on a 

storm-by-storm basis over the course of a winter season.

The strategy for siting these two scanning radar systems 

in the IPMA takes advantage of their unique capabilities. This 

is illustrated in the IPMA base map that, for the purpose of 

example, assumes its location is focused on a recent major 

burn area in the eastern San Gabriel and western San Bernardino Mountains (fig. 10). In this scenario, X-POL is sited 

Figure 10. Base map of an enhanced precipitation measurement network that could be deployed in and near a recently burned 

mountainous area susceptible to debris flows.

Future Development  25
within 30 km of mountainous areas with the highest likelihood 

of debris flows in an effort to take fullest advantage of its 

polarimetric capabilities. SMART-R could scan this region as 

well, but its mobility would also allow it to sample other burn 

areas in southern California. For example, figure 10 depicts a 

scenario where SMART-R scans a secondary burn area farther 

west in the central San Gabriel Mountains. Note that both 

X-POL and SMART-R are much closer to the target burn areas 

than the nearest operational NEXRAD radar (KSOX). Both 

are also located at much lower altitudes than KSOX. These 

sites allow the research radars to fill the crucial low-altitude 

NEXRAD gap.

Placement of X-POL and SMART-R in operational radar 

coverage gaps is not a solution to the QPE problem in and 

of itself. QPE algorithms need to be validated and their logic 

refined with better knowledge of the processes that lead to precipitation development. The additional instrumentation shown 

in figure 10 is intended to help address these important issues. 

Several ETL rain gages will be deployed in the primary mountainous burn area to augment the existing operational ALERT 

network for the purpose of validating QPE algorithm output 

from KSOX, X-POL, and SMART-R. A vertically profiling 

10-cm-wavelength (S-band) Doppler radar from ETL will be 

sited at a central location in the primary mountainous burn 

area. This instrument (S-PROF) provides the detailed vertical 

structure of reflectivity and particle fall speeds from just above 

the ground surface to the top of precipitating clouds. Its data 

allow precipitation rates aloft, at levels observed by the scanning radars, to be more accurately extrapolated to the surface. 

It also provides a continuous determination of the height of 

the melting level. A raindrop-counting and sizing instrument 

called a disdrometer will be collocated with S-PROF. The 

disdrometer is particularly helpful in validating output from 

polarimetric radar QPE algorithms. In addition, the disdrometer and S-PROF are both excellent tools for improving physical knowledge of precipitation development processes. 

Surface meteorology stations collocated with X-POL and 

S-PROF will have rain gages as well as sensors for measuring temperature, humidity, pressure, wind speed, and wind 

direction. These standard meteorological parameters provide 

further context for understanding precipitation-development 

processes. One such station will be sited southwest of KSOX, 

well upwind of the primary target burn area, along with a 915-

MHz wind-profiling radar, a GPS balloon-sounding system, 

and a GPS integrated water vapor receiver. The wind profiling 

radar provides continuous monitoring of winds aloft through 

the depth of the atmospheric boundary layer. Its measurements have revealed critical altitudes at which the winds are 

highly correlated with downstream orographic rainfall rates 

in California’s coastal mountain ranges (Neiman and others, 

2002). The instruments at this upwind site provide meteorological context over a depth of the atmosphere where precipitation-development processes are most active, especially 

those influenced by orography. Its upwind location near the 

coastline will allow sampling of air parcels that have not been 

altered by the inland topography. 

Data from the various standard and advanced meteorological instruments of the IPMA will be individually displayed 

in real time for NWS forecasters to view. The rainfall observations will also be ingested by prototype next-generation QPE 

algorithms based on the National Mosaic and Multisensor 

QPE (NMQ, Seo and others, 2005). The NMQ is a data and 

algorithm test platform that merges in near-real-time ALERT, 

NEXRAD, satellite, and other observations to form a multisensor QPE that mitigates radar problems with beam blockage 

and nonmeteorological artifacts. Thus, the IPMA instrumentation will serve both the operational forecasting and research 

aspects of the program.

Operational Costs and Impacts

Capabilities exist to implement the IPMA and begin 

its associated research as early as the winter of 2005–06, if 

sufficient funding is allocated for the task. However, a large 

part of the implementation effort involves preseason planning 

and preparations for instrument site logistics and communications, data ingest, and display. Scheduling conflicts for some 

instruments could also pose problems for the next few winter 

seasons, particularly the winter of 2005–06. Furthermore, the 

budgetary processes at USGS and NOAA generally require 

substantial lead time for new programs. Thus, a phased-in 

implementation of the IPMA plan and its research over the 

next few years may be more realistic. Approximate costs associated with the components of the IPMA are outlined in table 

2. Scaled-back versions of this plan would delay progress but 

may still be worthwhile, representing less expensive or more 

slowly implemented possibilities. However, the full IPMA 

described here would maximize benefits and is the recommended ultimate goal for advancing the meteorological side of 

debris-flow research.

Distributed Hydrologic Modeling Issues

Distributed hydrologic models have been available for 

a number of years for research purposes due to a number of 

factors such as the inadequate resolution of spatially distributed 

watershed data and QPE, and the still-excellent performance of 

properly calibrated lumped-parameter models (Reed and others, 2004). Because of improvements in the availability of both 

distributed watershed data and high-resolution gridded QPE 

and QPF, it is now foreseeable that distributed hydrologic models will have an ever-expanding role in the day-to-day operations of the National Weather Service RFCs and WFOs. In 

fact, their use in an operational setting in the National Weather 

Service is an area of active research and development, which 

is summarized in a special issue of the Journal of Hydrology 

dedicated to the recent Distributed Model Intercomparison 

Project (DMIP) (Smith, Georgakakos, and Liang, 2004; Smith, 

Seo, and others, 2004; Smith, Koren, and others, 2004; Reed 

and others, 2004; Carpenter and Georgakakos, 2004a, 2004b; 

26  NOAA-USGS Debris-Flow Warning System—Final Report
Ivanov and others, 2004; Ajami and others, 2004; Di Luzio 

and Arnold, 2004; Vieux and others, 2004; Bandaragoda and 

others, 2004; Georgakakos and others, 2004; Butts and others, 

2004; Liang and others, 2004; Guo and others, 2004).

In addition to the modeling of rainfall-runoff processes, 

the National Weather Service Office of Hydrologic Development is researching a new distributed-statistical approach to 

flash-flood modeling at ungaged sites that could prove useful 

in the development of high-resolution probabilistic assessment 

of debris-flow risk. In this approach, historical flow-frequency 

relationships for each cell in a high-resolution computational 

grid are developed using archived, gridded, radar rainfall 

estimates (Reed and others, 2005). In operations, real-time and 

forecast precipitation are input to the distributed model. The 

resulting flows are then evaluated against the historic flowfrequency curves to provide the forecaster a measure of severity.

One of the major payoffs of distributed hydrologic models is their ability to model hydrologic processes at a resolution limited only by the availability of observations. Using a 

combination of physically based and conceptual hydrologic 

models, the NWS Office of Hydrologic Development currently 

has the capacity to produce high-resolution forecasts of soil 

moisture (Koren and others, 2005) that include an estimation 

of the soil-moisture profile as a function of depth in each of 

the elements of the distributed model. 

High-resolution observations of soil moisture and their 

assimilation into distributed hydrologic models is now an 

active research area. It is expected that adding remotely 

sensed observations and forecasts of soil moisture will not 

only improve distributed hydrologic models but also future 

physically based debris-flow initiation models. Expanding 

this research into physically based debris-flow forecasts is a 

natural progression. (See section “Developments in 10+-Year 

Time Frame for Unburned Areas.”)

Hydrology and Geology Modeling Issues 

for Burned Areas

Developments Possible Within 2 to 5 Years 

Time Frame to Expand Capability of the 

Existing Prototype System for 

Burned Areas in Southern California

Refining Existing Rainfall Thresholds

The establishment of the Prototype Debris-Flow and 

Flash-Flood Warning System for areas recently burned by 

wildfires in the two WFO areas in southern California is based 

on the assumption that the potential for flash floods and debris 

flows can be reasonably characterized by regional rainfall 

intensity-duration thresholds. The capability of the system 

within southern California can be expanded by refining existing rainfall thresholds with additional storm/event data, by 

defining thresholds that are more geographically specific, and 

defining separate thresholds that are specific to flash floods 

and debris flows (and thus warranting an operational system 

other than the FFMP).

Generation of Basin-Scale Debris-Flow Probability 

and Volume Maps

The potential also exists to expand the capability of 

the prototype system from simply identifying those storm 

conditions likely to result in flash floods and debris flows 

to identifying those basins that are most prone to post-fire 

debris-flow activity and characterizing the potential magnitude 

of the event. Cannon and others (2004) developed a statistical method for calculating the probability that an individual 

basin will produce debris flows as a function of basin gradient, 

burned extent, material properties, and storm rainfall. With 

additional development, testing, and IT infrastructure expansion, this model could be implemented in near-real-time using 

measured precipitation to identify debris-flow-susceptible 

basins as a storm develops and to produce Web-based map 

products. The model could be made regionally specific by better characterizing the effects of basin shape and soil properties 

and by incorporating the elapsed time since the last fire (and 

erosive event) within burned basins. Cannon and others (2004) 

also developed a model for predicting the potential debrisflow peak discharge that can issue from a basin outlet as a 

function of basin gradient, burned extent, and storm rainfall. 

Again, with additional development, this model could be 

implemented in near-real-time using measured precipitation to 

identify those basins that can produce large events as a storm 

develops; the model could produce Web-based map products. 

The application of this approach could be broadened by developing predictive relations for debris-flow volumes rather than 

peak discharges. 

Generation of Debris-Flow-Inundation-Area Maps

Methods to accurately predict areas inundated by debris 

flows should be part of an expanded warning system. Although 

theoretical and numerical analyses of debris-flow mechanics are 

advancing, there is yet no universally accepted physically based 

model for routing debris flows across three-dimensional terrain. 

However, empirical methods for identifying two-dimensional 

debris-flow runout distance have been developed for unburned 

settings (e.g., Hungr, 1995; O’Brien and others, 1993). Further, 

a method that relies on a statistically constrained simulation 

model calibrated with data from debris flows throughout the 

world can be used to map potential inundation areas (Iverson 

and others, 1998; Griswold, 2004). The utility of these methods 

could be refined for burned areas by collecting data and developing calibration coefficients specifically for these settings. The 

methods further require estimates of the volume of material 

involved in a flow. In burned areas, the majority of the sediment 

in a debris flow originates from progressive bulking of storm 

runoff with material entrained from hillslopes and channels, in 

contrast to unburned settings where an initial landslide failure 

typically contributes material. By developing methods to conFuture Development  27
strain estimates of debris-flow volumes from recently burned 

areas, this statistically based inundation model holds promise 

for near-real-time implementation into a warning system. Maps 

showing potential inundation areas could be released as Webbased products. 

Developments Possible Within 5 to 10 Years 

Time Frame to Expand Capability of the 

Existing Prototype System for 

Burned Areas in Southern California

Develop Magnitude/Frequency Relations for 

Debris Flows

The development of magnitude/frequency relations for 

post-wildfire debris flows would provide a necessary tool for 

characterizing recurrence intervals of flows of particular sizes. 

These relations could be used to quantify the probability of 

debris flow of a particular size to be expected with the issued 

Watch or Warning. 

Develop Automated Warning System

Even though the human factor can never be removed 

from a warning system, it would be useful to develop an 

automated warning system that, in areas prone to repetitive 

debris flows, could warn in the event of a flow in progress. 

For example, an automated system could be used to close off 

roads cut by debris-flow channels if a flow is in progress. This 

would require implementation in coordination with regulatory 

agencies that have jurisdiction in these areas.

Developments Possible Within 2 to 5 Years 

Time Frame to Expand Prototype System to 

Burned Areas beyond Southern California

Definition of Rainfall Thresholds

Expansion of the prototype warning system in its present 

form to burned areas beyond southern California will require 

definition of region-specific rainfall intensity-duration thresholds. At present, such a threshold exists for recently burned 

areas in southern Colorado (Cannon and others, 2003), and 

data that might be used for preliminary definitions for western 

Montana and north-central New Mexico have been compiled, 

but not analyzed (Gartner and others, in press).

Generation of Basin-Scale Debris-Flow Probability 

and Volume Maps

Models developed by Cannon and others (2004) for 

estimating basin-scale debris-flow probability and magnitude 

as a function of basin morphology, burned extent, and storm 

rainfall could be implemented in near-real-time to generate Web-based map products with additional programming 

effort. The model could be improved by better characterizing 

the effects of basin shape and soil properties. As above, the 

application of this approach could be broadened by developing 

region-specific predictive relations for debris-flow volumes 

rather than peak discharges. 

Developments Possible Within 5 to 10 Years to 

Expand Prototype System to 

Burned Areas beyond Southern California

Generation of Debris-Flow-Inundation-Area Maps

Region-specific threshold models or statistical models for 

predicting debris flows in burned areas can also be coupled to 

empirical models for predicting debris-flow-inundation boundaries, but research is needed to develop techniques for predicting potential debris-flow volumes in burned areas (other than 

those in southern California) before such a model could be 

implemented in near-real-time.

Development and Calibration of 

Physically Based Models

The development and calibration of physically based 

models for post-fire erosional processes will provide means 

to advance a warning system from reliance on empirical and 

statistical models to one founded on more physically based 

analyses and will provide spatially and temporally specific 

hazard assessments. This effort will be leveraged by the development of high-resolution physically based hydrologic models 

that the NWS Office of Hydrologic Development is currently 

developing. These models will be able to produce soil-moisture forecasts at resolutions compatible with those required by 

physically based debris-flow models.

Hydrology and Geology Issues 

for Unburned Areas

The capability to predict debris flows in areas of the 

United States beyond burned areas is nontrivial. There are 

a variety of hydrological and geological mechanisms that 

can result in debris flows, including snowmelt, gravity, 

earthquakes, rapid drainage of natural and human-made 

dams, and volcanic eruptive processes. To understand and 

be able to issue reliable Watches and Warnings for debris 

flows beyond burned areas, the meteorological and geological processes that can lead to debris flows in a variety 

of different settings must be appropriately characterized. 

Examples of meteorological processes that can trigger 

debris flows include localized convective thunderstorms in 

the Rocky Mountains, summer monsoon rains in Arizona, 

and hurricane rainfall in the Appalachian Mountains of the 

Eastern United States. 

28  NOAA-USGS Debris-Flow Warning System—Final Report
Developments Possible Within 2 to 5 Years 

for Unburned Areas

Development of Rainfall Thresholds

The development of rainfall thresholds necessary for 

triggering debris flows under different meteorological conditions and in different parts of the United States is one of 

the most critical research needs for expanding the warning 

system. Well-defined thresholds exist in only a few specific 

areas, for example in Seattle (Baum and others, 2005); the San 

Francisco Bay Region (Cannon and Ellen, 1985; Wieczorek, 

1987), the Blue Ridge in Virginia (Wieczorek and others, 

2000), and Puerto Rico (Larsen and Simon, 1993). Some of 

these thresholds could be improved by better defining antecedent rainfall conditions and incorporating data from additional 

events. Because rainfall conditions that trigger debris flows 

are regionally specific, any expansion of the warning system 

to additional regions will require further data collection and 

analysis to define these thresholds. For areas for which data 

are not available, it might be possible to develop methods for 

extending thresholds beyond a local area where they were 

established. 

Statistical and Physically Based Methods

Some statistical and physically based approaches exist for 

defining where within a landscape debris flows are most likely 

to initiate (e.g., Carrara, 1983; Carrara and others, 1991, 1992; 

Montgomery and Dietrich, 1994; Pack and others, 1998). 

These preliminary approaches could be used to identify areas 

of potential instability in the event of significant rainfall. They 

do not provide a means to evaluate the response to specific 

storm rainfall conditions. 

Debris-Flow-Inundation Modeling

Methods to accurately predict areas inundated by 

debris flows should be part of an expanded warning system. 

Although theoretical and numerical analyses of debris-flow 

mechanics are advancing, there is yet no universally accepted 

physically based model for routing debris flows across threedimensional terrain. However, empirical methods for identifying two-dimensional debris-flow-runout distance have been 

developed for unburned settings (e.g., Hungr, 1995; O’Brien 

and others, 1993). Further, a method that relies on a statistically constrained simulation model calibrated with data 

from debris flows throughout the world can be used to map 

potential inundation areas (Iverson and others, 1998; Griswold, 2004). The utility of these methods could be refined for 

different regions by collecting data and developing calibration 

coefficients specifically for these settings. The methods further 

require estimates of the volume of material involved in a flow. 

By developing methods to constrain estimates of debris-flow 

volumes, these methods hold promise for near-real-time implementation into a warning system. Using a range of prospective debris-flow volumes, a range of inundation areas can be 

plotted for debris flows of increasing volume and decreasing 

probability. Maps showing potential inundation areas could be 

released as Web-based products. 

Developments Possible Within 5 to 10 Years 

for Unburned Areas

Physically Based Models for Spatially 

and Temporally Specific Projections

Ideally, it is desirable to advance any warning system 

from reliance on empirically defined rainfall intensity-duration 

thresholds to one founded on more physically based analyses 

and to be able to provide spatially and temporally specific hazard assessments. Coupled models for time-dependent rainfall 

infiltration and slope stability can provide spatially and temporally detailed projections of debris-flow activity (e.g., Savage 

and others, 2003; Morrissey and others, 2004). With additional 

development, testing, and IT infrastructure expansion, these 

models could be implemented in near-real-time using measured precipitation to identify debris-flow-susceptible hillslopes as a storm develops and to produce Web-based map 

products. Implementation of such models requires extensive 

data collection, including precipitation, soil moisture, and pore 

pressure. Additional research is needed to identify the necessary scales of data collection and to understand the details of 

storm-driven subsurface water movement over time. Information on soil-thickness distributions and other boundary conditions are also necessary to obtain reliable results.

Methods for Predicting Debris-Flow Volumes

Development of methods for predicting debris-flow volumes 

relative to storm characteristics would allow empirical methods for 

delineation of areas of inundation to be implemented in near-realtime and could produce Web-based map products. Methods for 

estimating potential volumes will require thorough understanding 

of channel-bed erosion and deposition relationships.

Refine Inundation-Area Mapping Using 

Ground-Based or Airborne-Based LiDAR

The detailed topographic information developed using 

LiDAR could provide vastly improved estimates of inundation 

areas. 

Magnitude/Frequency Relations for Debris Flows

The development of magnitude/frequency relations of 

landslide-initiated debris flows would provide a necessary tool 

for characterizing recurrence intervals of flows of particular 

sizes. These relations could be used to quantify the probability 

of a debris flow of a particular size to be expected with the 

issued Watch or Warning. These probabilistic definitions of 

Future Development  29
debris-flow risk could potentially be linked with precipitation 

forecast probabilities.

Develop Automated Warning System

Even though the human factor can never be removed 

from a warning system, it would be useful to develop an 

automated warning system that, in areas prone to repetitive 

debris flows, could warn in the event of a flow in progress. 

As examples, an automated system could be used to remotely 

close off roads cut by debris-flow channels if a flow is in progress. This would require coordination with regulatory agencies 

that have jurisdiction in these areas.

Snowmelt-Triggered Debris Flows

To expand a warning system to alpine areas to address 

issues for snowmelt-triggered debris flows, it is necessary to 

have in-depth data on the current state of the snow pack in areas 

of concern. There are various models in existence that predict 

snow-water equivalence, including NOAA’s Operational Hydrologic Remote Sensing Center (NOHRSC) snow model and the 

SNOW-17 models. The NOHRSC model provides estimates of 

snowmelt at the base of the snow pack and water release from 

rain-on-snow events, both of which can trigger debris flows. 

To document conditions under which snowmelt-induced debris 

flows occur and to develop predictive algorithms, remote-sensing tools such as aircraft, high-resolution MODIS satellite data, 

ground-based precipitation radars, and GIS can be used. 

Developments in 10+-Year Time Frame 

for Unburned Areas

The operational use of remotely sensed soilmoisture and soil-thickness data can be used to determine boundary and initial conditions for use in deterministic slope-stability modeling, but this will require 

great advances. The Office of Hydrologic Development 

of the National Weather Service will have soil-moisture 

data-assimilation systems in place in the 10+-year time 

frame. In coordination with high-resolution physically 

based models currently under development, the assimilation of remotely sensed information will assist in the 

improvement of debris-flow models.

Issues and Products Tables

The tables in this section summarize the issues, research 

needs, and potential products that can be developed with an 

expanded-capability warning system at several time scales for 

burned and unburned applications in southern California and 

beyond. The tables include issues for the near-term (2–5 years, 

table 3) and longer term (5–10 years, table 4); extension of 

the system beyond southern California in the near-term (2–5 

years, table 5) and longer term (5–10 years, table 6); extension 

of the system in areas other than burns in the near-term (table 

7), longer term (5–10 years, table 8), and more than 10 years 

(table 9).

Table 3. Issues and products by expanding capability of existing prototype—2–5 years. 

Issue Products 

 Refine existing rainfall thresholds More accurate Outlooks, Watches, Warnings 

 Define geographically specific rainfall thresholds More precise Outlooks, Watches, Warnings 

 Refine existing model for debris-flow probability Region-specific model for debris-flow probability 

 Implement model for basin-scale debris-flow probability Near-real time Web-based map 

 Develop predictive model for debris-flow volume Region-specific model for debris-flow volume 

 Implement models for basin-scale debris-flow volume Near-real-time Web-based map 

 Implement technique for debris-flow-inundation mapping Near-real-time Web-based map 

 Install additional rain gages, gap-filling radar, etc. More accurate and precise precipitation measurements for model input 

Table 4. Issues and products by expanding capability of existing prototype—5–10 years. 

Issue Products 

 Develop debris-flow magnitude/frequency relationships Quantified probability of debris flow in Outlooks, Watches, and 

 Warnings 

 Automated warning system Ability to remotely restrict access to dangerous areas in cooperation with 

 egulatory agencies r

30 NOAA-USGS Debris-Flow Warning System—Final Report
Table 5. Issues and products by expanding prototype to burned areas beyond southern California—2–5 years. 

Issue Products 

 Definition of region-specific rainfall thresholds Ability to issue Outlooks, Watches, and Warnings 

 Implement existing model for debris-flow probability Near-real-time Web-based map 

 Develop predictive model for debris-flow volume Region-specific model for debris-flow volume 

 Implement models for basin-scale debris-flow volume Near-real-time Web-based map 

 Install additional rain gages, gap-filling radar, etc. More accurate and precise precipitation measurements for model input 

Table 6. Issues and products by expanding prototype to burned areas beyond southern California—5–10 years. 

Issue Products 

 Develop additional predictive models for debris-flow volume Region-specific models for debris flow volume 

 Implement technique for debris-flow-inundation mapping Near-real-time Web-based map product of areas of impact 

 Physically based models of post-fire erosion Spatially and temporally specific predictive models 

Table 7. Issues and products to establish warning system in areas other than recent fires—2–5 years. 

Issue Product 

 Refine existing thresholds with better information on antecedent Ability to issue Outlooks, Watches, and Warnings in some areas

 conditions and with additional data 

 Definition of additional region-specific rainfall thresholds Ability to issue Outlooks, Watches, and Warnings in additional areas

 Link existing statistical and physically based stability models Near-real-time Web-based map product 

 with real-time precipitation measurements 

 Implement technique for debris-flow-inundation mapping Near-real-time probabilistic representation of area of impact as Web- 

 ased map product b

Table 8. Issues and products to establish warning system to areas other than recent fires—5–10 years. 

Issue Product 

 Implement coupled models for time-dependent rainfall Spatially and temporally specific predictive models that provide 

 infiltration and slope stability in near-real-time near-real-time Web-based map product 

 Develop predictive models for debris-flow volume Region-specific models for debris-flow volume 

 Implement technique for debris-flow-inundation mapping Near-real-time Web-based map product of areas of impact 

 Develop debris-flow magnitude/frequency relations Ability to characterize recurrence intervals of events of different sizes 

 Development of automated warning system Ability to remotely restrict access to dangerous areas in cooperation with 

 regulatory agencies 

 Develop methods to address snowmelt-induced debris flows Ability to characterize effects of rain-on-snow and snowmelt in 

 lpine areas a

Table 9. Issues and products to establish warning system in areas other than recent fires—10+ years. 

Issue Product 

 Use of remotely sensed soil-moisture information Initial condition input for deterministic stability models 

Future Development  31
Resources, Expertise, and Impacts

The tables in this section include estimated resources 

necessary for the implementation, operation, and maintenance 

for an expanded system. These tables were developed based 

on information from operation of river forecast centers and 

experience within the USGS Volcano Hazards Program.

Personnel and expertise needs to expand capability of a 

prototype system for burned areas in southern California (and 

to move beyond reliance on FFMP system) are shown in tables 

10 and 11, respectively.

Table 12 shows the personnel and expertise needed to 

expand prototype system to burned areas within southern California. Table 13 includes the personnel and expertise needed 

to expand a prototype system to burned areas beyond southern California. Table 14 displays the personnel and expertise 

needs per region to establish warning system in areas other 

than recent burns. Finally, table 15 shows the personnel and 

expertise needs per year per region incurred in extending the 

warning system to areas other than those burned by wildfire. 

Table 10. Personnel and expertise necessary to expand capability of a prototype system for 

burned areas in southern California (and to move beyond reliance on FFMP system). 

 Implementation item Personnel 

 1a. Refine existing rainfall thresholds 1 GS-12 scientist for 3 years 

 Event-response documentation, 

 data compilation and analysis 

 1b. Programming requirements 1 GS-12 to GS-14 programmer for 2 years 

 Automate comparison of forecast data 

 and measured precipitation with 

 rainfall thresholds 

 2a. Develop models for basin-scale debris-flow 1 GS-14 scientist for 3 years 

 probability and volume 1 GS-11 scientist for 3 years 

 Event-response documentation, 

 data collection and analysis 

 2b. Programming needs 1 GS-12 to GS-14 programmer for 2 years 

 Automate incorporation of forecast data 

 and measured precipitation into models 

 to generate near-real-time

 Web-based map products 

 3a. Methods to generate debris-flow- 1 GS-14 scientist for 2 years 

 inundation-area maps Data collection and development of 

 calibration coefficients for 

 burned settings 

 3b. Programming needs 1 GS-12 to GS-14 programmer for 1 year 

 Incorporation of volume estimates

 into debris-flow-routing program for 

 generation of near-real-time 

 Web-based map products 

 4. Develop magnitude/frequency relations for 1 GS-12 to GS-14 scientist for 2 years 

 debris flows Data collection and analysis 

 5. Develop automated warning system 1 GS-12 technician for 2 years 

 Development of apparatus 

32  NOAA-USGS Debris-Flow Warning System—Final Report
Table 11. Operation and maintenance items to expand capability of a prototype system for burned 

areas in southern California (and to move beyond reliance on FFMP system). 

 Operation and maintenance item Personnel 

 Personnel 4 GS-12 to GS-15 supervisors 

 (2 each NWS and USGS) 

 6 GS-12 to GS-14 warning operators 

 (USGS) 

 Rent & utilities (suggest sharing facility to facilitate 

 data and information transfer and decision-making) 

 Telemetry needs 

 Supplies and materials 

 Equipment (stream and rain gages, real-time cameras) 

 Equipment & network maintenance 

Table 12. Personnel and expertise necessary to expand prototype system to a region with burned 

areas beyond southern California. 

 Implementation item Personnel 

 1. Refine existing thresholds (utilize 1 GS-12 scientist for 3 years 

 programming developed above) Event response documentation, data

 compilation and analysis 

 2. Generate basin-scale debris-flow probability and 1 GS-14 scientist for 2 years 

 volume maps (using previously developed 1 GS-11 scientist for 2 years 

 regional models, programming developed above) Event-response documentation, data

 collection, and analysis 

 3. Methods to generate debris-flow-inundation-area 1 GS-14 scientist for 3 years 

 maps (utilize programming developed above) Data collection and development of 

 calibration coefficients for burned 

 settings 

 4. Develop and calibrate physically based models 1 GS-13 scientist for 5 years 

 for burned settings 1 GS-11 scientist for 5 years 

 Field experiments and modeling 

Table 13. Operation and maintenance items to expand prototype system to a region with burned 

areas beyond southern California. 

 Operation and maintenance item Personnel 

 Personnel 4 GS-12 to GS-15 supervisors 

 (2 each NWS and USGS) 

 6 GS-12 to GS-14 warning operators 

 (USGS) (including overtime) 

 Rent, communication, and utilities (suggest sharing 

 facility to facilitate data and information transfer 

 and decision-making) 

 Telemetry needs 

 Supplies and materials 

 Equipment (stream and rain gages, real-time cameras) 

 Equipment and network maintenance 

Future Development  33
Table 14. Near- and long-term implementation items to establish warning system in areas other 

than recent burns per year per region. 

 Implementation item Personnel 

 1a. Develop and refine rainfall thresholds 2 GS-12 scientists per year 

 Event-response documentation, data 

 compilation, and analysis 

 1b. Programming requirements 1 GS-12 to GS-14 programmer 

 Automate comparison of forecast data and 

 measured precipitation with rainfall 

 thresholds 

 2. Implement existing statistical and physically 1 GS-11 to GS-14 scientist per year 

 based models 2 GS-12 GIS specialists per year 

 3a. Generation of debris-flow-inundation-area maps 1 GS-14 scientist 

 Data collection and development of 

 calibration coefficients for different 

 regional settings 

 3b. Programming needs 1 GS-12 to GS-14 programmer 

 Incorporation of volume estimates into

 debris-flow routing program for 

 generation of near-real-time 

 Web-based map products 

 4a. Physically based models for spatially and 2 GS-12 to GS-14 scientists per year 

 temporally specific projections 2 GS-11 scientists per year 

 Model development and calibration 

 4b. Programming needs 1 GS-12 to GS-14 programmer 

 Automate incorporation of forecast data 

 and measured precipitation into models 

 to generate near-real-time

 Web-based map products 

 5. Methods for predicting debris-flow volumes 1 GS-12 to GS-14 scientist per year 

 1 GS-9-11 scientist per year 

 Field and air photo data collection and 

 analysis 

 6. Refine inundation-area mapping using ground- 1 GS-12 to GS-14 scientist per year 

 or airborne-based LiDAR 1 GS-9 to GS-11 scientist per year 

 7. Develop magnitude/frequency relations for 1 GS-12 to GS-14 scientist per year 

 debris flows 1 GS-9 to GS-11 scientist per year 

 Data collection and analysis 

 8. Develop automated warning system 1 GS-12 technician per year 

 9. Snowmelt-triggered debris flows 1 GS-12-14 scientist per year 

 10. Incorporation of remotely sensed soil 1 GS-12 to GS-14 scientist per year 

 moisture and thickness data 

34  NOAA-USGS Debris-Flow Warning System—Final Report
Conclusions and Recommendations

Conclusions

The research carried out by the Task Force indicated that:

1. Support from potential customers ranged from people 

expressing that even though they felt the current level 

of service from their local systems was adequate, an 

enhanced NOAA-USGS system would be welcome, to 

individuals that would fully support and benefit from a 

joint NOAA-USGS debris- flow-warning system, even 

at the prototype level.

2. It is feasible to establish a joint NOAA-USGS debrisflow warning system for recently burned areas using 

rainfall intensity-duration thresholds developed 

by the USGS and applying those thresholds to the 

Flash-Flood Monitoring Program of the NWS, at no 

additional annual operating costs. Rainfall intensityduration thresholds for debris-flow occurrence have 

been developed for parts of southern California using 

detailed analyses of rainfall and response data from 

recently burned areas. These quantitative thresholds 

provide an improvement over the present method of 

identification of dangerous rainfall conditions based on 

professional opinion and experience.

3. Given appropriate resources and scientific focus, considerable potential exists for enhancing and expanding 

the warning system to provide spatially and temporally 

explicit information specific to debris flows. Expansions include incorporating improved forecasts and 

measurements of precipitation as well as methods for 

delineating where debris flows might occur, how big 

the events might be, and what areas might be impacted. 

The Task Force defined the scientific and operational 

requirements necessary to enhance the system in the 

near and long terms.

4. In the near term (2–5 years) the demonstration warning 

project can be refined and expanded to other burned 

areas within southern California. Expansion of the 

warning system beyond the demonstration project but 

within the FFMP framework would consist of refinement of existing rainfall thresholds and development 

of new regionally specific thresholds. Implementation 

of models to provide near-real-time mapping of basinscale debris-flow probability, magnitude, and areas of 

inundation is possible but requires the development of 

an operational framework beyond that of the FFMP to 

address issues specific to debris flows.

5. Near term (2–5 years) expansion of the demonstration warning system to burned areas beyond southern 

California is possible within the FFMP framework but 

will require the refinement of existing rainfall thresholds and development of new thresholds for additional 

areas. Development of models that provide near-realtime mapping of basin-scale debris-flow probability, 

magnitude, and areas of inundation is possible, but this 

requires the development of an operational framework 

beyond that of the FFMP. Physically based models that 

characterize the hydrological response of burned areas 

could be incorporated into a warning system.

6. Near-term (2–5 years) expansion of a warning system 

to areas other than burned areas requires development 

of an operational framework separate from the FFMP 

and requires, at a minimum, development of regionally specific rainfall intensity-duration thresholds for 

unburned areas. An expanded warning system could 

provide map products that identify areas of potential 

instability in the event of heavy rainfall and areas of 

inundation for a range of possible debris-flow volumes.

Table 15. Operation and maintenance items to establish warning system in areas other than 

recent burns per year per region. 

 Operation and maintenance item Personnel 

 Personnel 4 GS-12 to GS-15 supervisors 

 (2 each NWS and USGS) 

 6 GS-12 to GS-14 warning operators 

 (USGS) (including overtime) 

 2 GS-12 technicians to operate and 

 maintain field-monitoring arrays 

 1 GS-14 scientist to download and process 

 remotely sensed data 

 Rent, communication, and utilities 

 (suggest sharing facility to facilitate data 

 and information transfer and decision-making) 

 Telemetry needs 

 Equipment and network maintenance 

Conclusions and Recommendations  35
7. More substantive development and expansion of a 

debris-flow warning system over the longer term 

(5–10 years) within burned areas in the United States 

includes the development and calibration of physically based models for post-fire runoff and erosion and 

improvement of inundation-area mapping.

8. Longer term (5–10 years) development (table 12) and 

expansion to a national debris-flow warning system 

in areas other than those burned by wildfire requires 

development and implementation of physically based 

models for slope failure that, when linked with spatially distributed precipitation forecasts and measurements, can provide near-real-time information on 

where and when within a storm debris flows are likely 

to occur. Methods for predicting potential debris-flow 

volumes can be linked with inundation-area mapping 

to provide map products showing potential impacts. 

There are exciting research possibilities for advancing 

debris-flow and hydrometeorological instrumentation 

and science.

9. Although this report describes a likely fruitful collaboration between the NWS and the USGS, and potential 

capabilities of a debris-flow early warning system, the 

Task Force wishes to emphasize that both the human 

capital and financial resources required to successfully 

implement, operate, and advance such a system are 

beyond those available to either agency at this time. 

A long-term commitment of such resources from both 

agencies is needed prior to the implementation of a 

warning system. 

Recommendations

The Task Force recommends:

1. Resources be made available for the implementation of 

the prototype debris-flow warning system in southern 

California before the end of FY 2005.

2. Resources be made available for the deployment of 

the Intensive Research Area monitoring program as 

quickly as possible

3. Once the system proves its value, resources be made 

available for expansion of the system to burned and 

unburned areas within southern California and other 

parts of the country.

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on debris flows in central Santa Cruz Mountains, California, 

in Costa, J.E., and Wieczorek, G.F., eds., Debris flows/

avalanches—Process, recognition, and mitigation: Reviews 

in Engineering Geology, Geological Society of America. 

Wieczorek, G.F., Morgan, B.A., and Campbell, R.H., 2000, 

Debris-flow hazards in the Blue Ridge of central Virginia: 

Environmental and Engineering Geosciences, v. 6, p. 3–23. 

Wilson, R.C., 1997, Operation of a landslide warning system 

during the California storm sequence of January and February 1993, in Larson, R.A., and Slosson, J.E., eds., Reviews 

in engineering geology: Geological Society of America. 

Wilson, R.C., and Wieczorek, G.F., 1995, Rainfall thresholds 

for the initiation of debris flows at La Honda, California: 

Environmental and Engineering Geoscience, v. 1, no. 1, 

p. 11–27. 

Wilson, R.C., Mark, R.K., and Barbato, G., 1993, Operation 

of a real-time warning system for debris flows in the San 

Francisco Bay area, California, in Shen, H.W., Su, S.T., and 

Wen, F., eds., Hydraulic engineering ‘93: Proceedings of the 

1993 Conference, San Francisco, Calif., July 25–30, 1993:

Hydraulics Division, American Society of Civil Engineers, 

v. 2, p. 1908–1913.

Wohlgemuth, P.M., 1986, Surface sediment transport—a 

review of current knowledge and a field study of its spatial 

and temporal distributions in the San Dimas Experimental 

Forest, California: University of California, Northridge, 

Northridge, California, M.S. thesis, 282 p.

Zrnic, D.S., and Ryzhkov, A., 1996, Advantages of rain 

measurement using specific differential phase: Journal of 

Atmospheric and Oceanic Technology, v. 13, p. 454–464.

References  41
This Appendix is adapted and updated from Carter and 

others (1999).

Meteorological Development Laboratory

The Meteorological Development Laboratory has produced statistical quantitative precipitation forecasts based on 

operational synoptic-scale NWP models for nearly 3 decades. 

Although much of this statistical guidance is disseminated routinely to the field in categorical form, probabilistic forecasts 

(PQPF) have long been an integral part of this system. 

Current operational synoptic-scale Model Output Statistics (MOS) QPF guidance is available in three distinct packages from the Global Forecast System (GFS), the Eta model, 

and the Nested Grid Model (NGM). The GFS-based MOS 

guidance is available for 6- and 12-h periods for projections 

up to 72 hours after 0000, 0600, 1200, and 1800 UTC. The 

Eta-based MOS guidance is available for the same projections, 

but only during the 0000 and the 1200 UTC forecast cycles. 

An extended-range GFS-based MOS system for QPF provides 

guidance for 12- and 24-h periods out to 156 hours after 0000 

UTC only. Both the GFS- and Eta-based MOS guidance packages are available for more than 1,500 stations in the contiguous United States, Alaska, Hawaii, and Puerto Rico. The 

oldest MOS QPF system, based on the NGM, is available for 

almost 700 stations throughout the contiguous United States 

and 60 stations in Alaska. For all of these MOS systems, categorical forecasts are available in alphanumeric form, whereas 

corresponding probabilistic forecasts are transmitted in binary 

format to all Weather Forecast Offices. Probabilistic and categorical forecasts are provided in graphical form to the NCEP 

Hydrometeorological Prediction Center. 

The synoptic-scale package of MDL QPF guidance is 

supplemented by short-range guidance from the Localized 

Aviation MOS Program (LAMP) QPF model. The LAMP 

system produces probabilistic, categorical, and expectedvalue forecasts on a 20-km grid for 1-, 3-, and 6-h periods 

in the 1- to 22-hour range. As input, the LAMP model uses 

the NGM-based MOS QPF probabilities together with a few 

direct model output fields from the NGM. Sub-synoptic-scale 

predictor input consists of objectively analyzed conventional 

surface observations and output from simple numerical 

models. Mesoscale predictor input includes fine-scale highresolution precipitation climatology, hourly and 3-hourly 

antecedent precipitation analyses, and topography.

MDL plans to produce in late 2005 an extended-range 

MOS QPF system from the 1200 UTC cycle of the GFS. 

There are no plans at this time to develop a LAMP system 

based on either the GFS or Eta MOS guidance.

Hydrometeorological Prediction Center

Manual QPFs, representing spatially averaged precipitation and covering the continental United States, have been 

routinely prepared and issued since 1960 by forecasters in the 

National Precipitation Prediction Unit (NPPU) of the Forecast 

Operations Branch (FOB) of the HPC. Twenty-four-hour Day 

1 QPFs are issued by FOB forecasters early each morning. 

These forecasts are valid from 1200 to 1200 UTC the following day, and are based on the 12- to 36-hour forecast from the 

0000 UTC NWP model run. In addition, a Day 2 QPF and a 

Day 2 Update are issued, each valid for the 24-hour forecast 

period beginning 1200 UTC the following day. The Day 2 

QPF and Day 2 Update products are issued in the early morning and early afternoon, respectively.

Forecast guidance to support flash- and river-flood 

operations throughout the contiguous United States include 

the Excessive Rainfall Outlooks and short-range 6-h QPF 

product. The Excessive Rainfall Outlooks consist of a graphical Rainfall Potential Exceeding RFC Flash Flood Guidance 

Product and an accompanying Excessive Rainfall Discussion. 

Six-hour forecasts are issued every 12 hours covering the 24-

hour period from 1200–1200 UTC or 0000–0000 UTC. The 

forecasts for the first 18 hours of each of these two periods 

are updated once daily. The Excessive Rainfall Outlooks are 

produced three times per day with special issuances as needed. 

HPC guidance is not issued for Alaska, Hawaii, Guam, and 

Puerto Rico.

FOB forecasters reconcile differences between the observational data, operational NCEP NWP models, and statistical guidance during the preparation of the aforementioned 

manual, spatially averaged QPFs and associated guidance 

products. Verification of these manual products over the past 

35 years has shown a steady improvement in forecast skill, 

with consistent improvements over the best NWP models. The 

HPC provides QPF products and services to a wide variety 

of partners and customers. A recent survey by the University 

Corporation of Atmospheric Research, as part of their review 

of the HPC, provided useful information as to which of the 

various HPC products are of most utility, and it also indicated 

the general level of user satisfaction with the products. The 

survey revealed that HPC QPF products are widely used and 

that the quality of these products appears to be acceptable. The 

private-sector meteorological community, in particular, uses 

both the graphic products and the text version of the isohyet 

locations to provide rainfall estimates to their customers in a 

repackaged format. The national media requires QPF to define 

areas of moderate to heavy rainfall across the contiguous 

United States. Other Government agencies including the Corps 

of Engineers, the Bureau of Reclamation, and various State 

Appendix A—Functions of NWS Organizations Involved 

in the Preparation of Precipitation Forecasts

42  NOAA-USGS Debris-Flow Warning System—Final Report
and local water managers use the HPC products for waterresources management and flood management.

Weather Forecast Offices and 

River Forecast Centers

WFOs utilize NWP, MOS, and HPC guidance to produce 

QPF. In response, forecasters edit or “re-draw” QPF through a 

Graphical Forecast Editor. The QPF (for each 6-hour period out 

to 72 hours) is then sent to the National Digital Forecast Database (NDFD). Gridded QPF received from WFOs is mosaicked 

and available for access via the NDFD for the coterminous 

United States (CONUS), Alaska, and Puerto Rico. 

RFCs create QPF for input to the NWSRFS in six 

hourly time steps. For RFCs, QPF is the prediction of basin 

average precipitation amounts accumulated over a river 

basin. RFCs utilize either the NMAP (Network Mapper) or 

Mountain Mapper software applications for QPF generation and dissemination. RFCs who use Mountain Mapper 

in their operations convert point QPFs to forecast MAPs 

for inclusion in the river models. Western Region offices 

have traditionally produced point QPFs. This is largely due 

to the complexity in topography, precipitation climatology, 

and lack of high-resolution forecast guidance across the 

West. 

WFOs and RFCs coordinate QPFs by way of telephone 

or 12Planet Chat software

Appendix B—Snowmelt and Rain-on-Snow 

Triggering of Debris Flows

The literature shows that snow-pack conditions commonly cause or contribute to debris flows in mountainous 

areas throughout the world. Rain and snow events in 1983 

resulted in widespread debris-flow activity in central and 

northern Utah, and throughout the Pacific Northwest in 

December 1996 through January 1997. In the Central European Alps, the alpine areas are relatively densely populaated 

when compared with other alpine regions of the world. The 

large potential for the loss of life and property, combined with 

recent warming trends in the Alps, have lead to increased 

interest in studying, predicting, and mitigating debris flows 

there. Snow-triggered debris flows have also been studied in 

the Himalayas and other regions.

Numerous debris flows occurred in Switzerland during 

the summer of 1987. The initiation of several debris flows on 

the talus slopes of the upper part of alpine catchment basins 

was attributed to a blockage of water underneath a perennial 

snow patch at the toe of the rock wall (Rickenmann and Zimmerman, 1993). Snowmelt was considered a triggering factor 

for the initiation of the first pulse of these debris flows. The 

melting of old snow patches during the spring and summer 

months enhanced local water saturation of underlying unconsolidated sediments. This was followed by a large second 

pulse of rain precipitation. The combination of the two pulses 

lead to more than 600 recorded debris flows in the Swiss Alps 

during the summer of 1987. There were two major rain storm 

events in July and August in the region. Air temperatures 

were high and the snow line was 3,000 m above mean sea 

level during most of the debris-flow events. Precipitation fell 

as rain-on-snow in the higher periglacial regions, resulting in 

pronounced debris-flow activity in the higher elevations. In 

July, there was still significant snow cover, which resulted in 

the long-term saturation of loose debris by melting snow. This 

saturation led to rain-triggered debris-flow events in both July 

and August.

Another widely examined debris-flow hazard in the 

Swiss Alps involves the breaching of periglacial lakes. Snow 

plays an important role in the formation of these lakes. Four 

potentially hazardous periglacial lakes were studied near 

Valais in the Swiss Alps (Haeberli and others, 2001). Three of 

the lakes were naturally dammed by ice and snowdrifts. The 

lakes were formed in topographic depressions at their associated glacier margins. During the 1960s and 1970s, the glaciers 

grew and the related depressions were filled year-round by 

accumulating snow and ice. From the late 1980s to present, 

during a general warming trend, the lakes have been enlarged 

by successive snow and ice melt toward the glaciers. Haeberli 

and others (2001) warn that the formation of slush avalanches 

in snow covering the outflow of these lakes can cause a sudden increased discharge toward two of the lakes during spring 

and early summer. The recommendation the authors made for 

debris-flow-hazard mitigation was to establish a comprehensive observation system, combined with additional installation 

of protective barriers such as flood-release overflow structures.

Single-flow and multiple-flow GIS models were created for the assessment of hazards from glacier lake outbursts 

(Huggel and others, 2003). The models are topography based 

and use primarily ASTER satellite-derived digital elevation 

models. The model was applied to the Tasch Lake area of 

Switzerland, which experienced a devastating debris flow 

on June 25, 2001, during a period without any significant 

precipitation. Considerable parts of the village of Tasch were 

Appendix B—Snowmelt and Rain-on-Snow Triggering of Debris Flows  43
destroyed or damaged by the event. Elevated air temperatures 

during the period prior to the event led to high snowmelt-water 

input to the lake. Tasch Lake had previously been dammed by 

pieces of lake ice and snow deposits. The elevated water level 

caused larger hydraulic gradients and piping in the moraine 

dam body. The snow and ice blockage ruptured, and the resulting water initiated the debris flow that devastated the village. 

The single-flow GIS model was able to more accurately recreate the event than the multiple-flow version and was suggested 

as a possible predictive tool for the regional assessment of 

debris-flow hazards. 

The winter of 1999 brought unprecedented amounts of 

snow to the European Alps. Switzerland was confronted with 

floods partly due to snowmelt (Bardou and Niggli, 2003). 

During the following two summers, large amounts of snow 

were still found in gullies down to 1,000 m above mean sea 

level. On average, more than 3 m of snow could be found in 

these gullies. The snow was found under a few centimeters 

of rock fragments that insulated the underlying snow. During 

the summers of 1999 and 2000, small debris flows occurred in 

many of the upper watersheds throughout the European Alps. 

Some of these debris flows occurred during clear weather 

and were not triggered by rain precipitation. The debris flows 

were observed to contain large amounts of snow. The authors 

performed statistical analyses over selected watersheds to 

correlate winter snowfall with the number of floods occurring during the following summers. They concluded that snow 

precipitation is often the underlying cause of debris flows in 

mountainous torrents during the first thunderstorms of the 

summer and that temperature and antecedent climatic history 

of the watershed must be examined in addition to rainfall to 

give a complete picture of debris-flow triggers.

Snow-avalanche deposits were examined as possible 

contributors to debris flows (Bardou and Delaloye, 2004). 

Snow avalanche deposits in gullies were shown to be both 

potential amplifying factors and potential reducing factors 

of debris flows in the Valais Alps of southwest Switzerland. 

Snow deposits increased the base flow under the snow pack 

and created a sliding plane for sediments, primarily during 

summer rain storms. Conversely, snow-avalanche deposits 

were shown to reduce the impact energy of raindrops, mainly 

during the time of winter storms. Bardou and Delaloye (2004) 

also contend that it is not currently possible to establish rainfall threshold values for debris-flow triggering. The authors 

also state that it is difficult to attribute debris-flow triggering 

in alpine environments to rainfall alone due to hydrogeological variability in these regions. In another study in the 

Himalayas (Wei and Gao, 1992), debris flows were triggered 

without simultaneous rainfall. An intense snowmelt in the 

early summer enhanced both superficial and subsurface runoff 

to provide the trigger for debris-flow initiation. The Swiss 

study discusses two contributions of snow-avalanche deposits 

to debris-flow triggering. The first is the increase in base flow 

of melt at the base of the snow pack. The second contribution 

occurs when the snow deposits are covered by sediments. 

Increased temperatures during the summer cause the upper 

zone of the snow pack to soften, forming a very efficient 

sliding plane for the sediments covering it. Subsequent small 

slips in these sediments fill gullies with loose, highly mobile 

material. Consequent debris flows can result simply from 

the alternation of snowmelt events with these sediment slips. 

Bardou and Delaloye (2004) concluded through debris-flowand snow-avalanche-event distributions that the number of 

debris flows is typically more significant during the two 

summers following a winter of large snow accumulation and 

subsequent avalanche activity. The severity of debris flows is 

generally the greatest during the second summer following 

significant snow avalanche activity. During the first summer, 

the extra deposits of very dense avalanche snow insulate the 

gully bed to reduce base flow.

Limited work has been done on the effects of snow pack 

on debris flows in the Pacific Northwest. The eruption of Mt. 

St. Helens in March of 1982 caused hot eruption products to 

interact with a thick snow pack (Waitt and others, 1983). The 

authors state that the eruption of Mt. St. Helens would have 

been confined to the crater and upper flanks of the volcano 

had it not been for the thick snow that mantled the steep crater 

wall. A lateral blast during the eruption generated a large snow 

avalanche that flowed 8 km down the volcano. The heat from 

the eruption also caused rapid snowmelt, creating a transient 

lake whose rapid discharge triggered a lahar (volcanic debris 

flow). There were two outlets to the transient lake. The snow 

melted faster than water could escape from the outlets, so the 

water discharged rapidly from the lake, creating a flood that 

swept down the crater breach. The flood picked up enough 

debris to emerge at the bottom as a lahar with a peak discharge 

of 13,800 m3/s. The lahar flowed into Spirit Lake and the 

Toutle River, severely eroding a debris retention dam 35 km 

downstream from the crater. 

Debris flows resulting from the breach of neoglacialage moraine dams in the Three Sisters and Mount Jefferson 

Wilderness Areas of Oregon have also been documented 

(O’Connor, Hardison, and Costa, 2001). This area has the 

highest concentration in the United States of lakes dammed by 

glacial moraines. There have been 11 moraine-breach debris 

flows described in this area since 1930. O’Connor, Hardison, 

and Costa (2001) state that many breaches were probably 

the result of erosion of steep lake outlet channels, triggered 

by unusually large discharges caused by avalanche waves in 

the lakes or by precipitation and melting snow and ice. The 

potential currently exists for at least five more neoglacial lake 

moraine-dam breaches in this area.

A debris flow created by a 1996 rain-on-snow event in 

the Blue Mountains of Washington was described at a Geological Society of America meeting (Carson, 2002). In February 

of 1996 there was a warm rain event on deep snow covering 

frozen ground in this area. Thousands of small slumps of loess 

and colluvium liquefied to become long, thin debris flows. 

Many of these flows were initiated at breaks in slope like road 

fills and farm berms. One particular flow originated from a 

logging road fill and formed a debris dam below the road. 

Continued runoff from this rain-on-snow event caused the dam 

44  NOAA-USGS Debris-Flow Warning System—Final Report
to breach. The discharge, estimated at 62 m3/s, inundated a 

home with four residents.

Another debris flows in Washington was reported at the 

H.J. Andrews Experimental Forest in the central Cascades 

during a 1996 rain-on-snow storm (Harris and others, 1997). 

Runoff from snowmelt and rain generated 24 debris flows in 

the Lookout Creek basin.

Debris flows of noneruptive origin have also been studied 

on Mt. Shasta in northern California (Blodgett and others, 

1996). The primary snow-related cause of debris flows in this 

area has been the release of meltwater into and over the ground. 

The area of historic debris-flow deposition below Mt. Shasta, 

the second largest volcano on the West Coast, is now being 

developed and populated. New noneruptive debris flows comparable to those that have occurred in the past could be devastating to this community. Debris flows on Mt. Shasta generally 

originate near the termini of glaciers above 2,740 m a.s.l. Some 

debris flows also are caused by snowmelt streams originating in snow fields that are regions of permanent snow cover, 

such as the upper areas of Diller Canyon. Debris flows on Mt. 

Shasta have only been observed during the summer. Warm air 

temperatures and ablation of glaciers and snow pack are the 

primary triggers of debris flows on the mountain. The specific 

cause of these debris flows is generally a combination of water 

released from natural impoundments, snow and glacial melt, 

and precipitation on the stream basins during the summer snow 

and glacial melt season. Ground water recharged by snowmelt 

or rain precipitation may also cause slump failure and debris 

flows into stream channels. One debris flow resulting from the 

release of water from channels blocked by snow and ice was 

observed in 1934. Warm summer weather caused rapid snow 

melt on top of Konwakiton Glacier. This melt caused waterfalls 

that flowed into fissures in the glacier and reached the base. 

Large blocks of snow and ice were carried away from the glacier in the resulting torrent. Temporary dams were formed and 

then breached, resulting in a large debris flow.

Appendix C—List of Potential Customers and Collaborators

Appendix C—List of Potential Customers and Collaborators  45

Table C-1. List of potential customers and collaborators. 

Agency 

 California Board of Mines and Geology 

 California Department of Water Resources 

 California Department of Transportation (CalTrans) 

 California Geological Survey 

 California Office of Emergency Services 

 County Departments of Public Works 

 County Flood Control Districts 

 County Offices of Emergency Services 

 Federal Emergency Management Agency (FEMA) 

 Natural Resources Conservation Service 

 NORAD/USNORTHCOM 

 U.S. Department of Agriculture Forest Service 
Customers in the WFO Oxnard Area

The following statement were compiled by Eric Boldt, 

WCM, WFO, Los Angeles/Oxnard, Calif., from interviews 

with the Department of Public Works, Watershed Department, 

County Flood Control, County Operations of Emergency 

Services (OES).

San Luis Obispo

Would like to see a 3- to 4-column product from USGS 

that uses different threat levels/categories similar to Advisory, 

Watch, Warning, or use a percent chance very similar to POPs.

Current USGS landslide Advisories have no concrete 

information included and would prefer advance notice before 

the Advisory is issued. SLO County would like to review and 

comment on any products proposed for development.

Santa Barbara

USGS currently shows a burn map with their Advisories, but would prefer a larger scale, county-specific 

map giving more detail. Do not think a flood Warning is 

appropriate for slides because floods have likely already 

occurred before Warning goes out. Maybe NWS needs an 

additional product to cover landslides? Like the idea of 

displaying landslide potential dangers on a Web site to get 

public attention.

Ventura

Would like better coordination between Federal, State, 

and county agencies on this type of project. Do not feel that 

the USGS is doing a good job of identifying their needs. Do 

not want redundant products and think there is an overload 

of Watch/Warning statements during big events. They have a 

good understanding of rainfall rates needed to create runoff 

problems near severely burned areas. Long lead time is preferred. 

Los Angeles

Department of Public Works issues a “Debris and 

Mudflow Potential Forecast” for severely burned areas of LA 

County during rain events, based on NWS QPS predictions at 

0400 and 1600 hours. They do not foresee any benefit from 

a USGS statement for their purposes but thought it would be 

useful if issued for unburned areas. Lead time would not help 

them in their decision-making

Customers in the WFO San Diego Area

The following statements were compiled by Ed Clark, 

WCM, WFO, San Diego, Calif., and from interviews with the 

Department of Public Works, Watershed Department, County 

Flood Control, County Operations of Emergency Services 

(OES).

Riverside

Riverside County does not seem concerned about their 

burn areas at the present time. Despite being offered, they did 

not ask for any special Flash Flood Criteria and have not had 

any significant mudslide or landslide problems. 

San Diego

The primary agencies responding to flooding issues 

would be the Public Works and Roads Division, Sheriff’s 

Communication, EOC, and Flood Control. Outlook products 

for a significant event of 2 to 3 days would be ideal. As the 

event approaches at least 24 hours notice would be needed. 

The initial criteria for the beginning of flooding and 

debris flows for the first two seasons has been 0.25 inch/hr for 

the beginning of flooding, and 0.25 inch/15 minutes for the 

beginning of moderate to severe flooding. That served them 

pretty well in the 2003–04 season. They still had some problems in Oct. 2004, but not as bad as last season, even though 

the rain rates were pretty high. For the remainder of this current rainy season, they had several periods of heavy rain, but 

because they also had an explosion of vegetative growth with 

the abundant rains that held the soil in place, they saw almost 

no debris flows and encountered very few flooding problems. 

As a result, they are seriously considering relaxing their Warning criteria (unless they get more fires) to maybe 0.35–0.50 

inch/hr for the start of flooding/debris flows and 0.50 inch/15 

minutes for the start of moderate flooding/debris flows. They 

had several periods this year where those new criteria were 

reached but encountered little problem.

San Bernardino

They want as much lead time as possible. The timing of 

Advisories, Watches, or Warnings is critical when preparing 

Appendix D—Customer Requirements

46  NOAA-USGS Debris-Flow Warning System—Final Report
for evacuations. They would hope to be alerted by an Advisory 

or Watch 48 to 24 hours prior to a possible event and have a 

Warning 24 hours prior to an event.

Currently all Watches and Warnings pertaining to flooding are transmitted to their Flood Area Safety Task Force 

(FAST). They would expect this to continue for any debrisflow products. The FAST group includes many potential 

users including representatives from Public Works (Trans and 

Flood), County Fire, OES, Sheriff, CHP, Red Cross, County 

Schools, and CALTRANS. 

They also wanted more details on (1) What types of products will be available? (2) Will this be similar to Advisories/

Watches/Warnings issued for possible flooding or something 

similar to the Flash-Flood-Potential Index? and (3) It was 

stated that, hopefully, mapping that identifies areas at risk will 

be provided.

Published in the Central Region, Denver, Colorado

Manuscript approved for publication August 8, 2005

Editing, page layout, photocomposition—Richard W. Scott, Jr.

Cover design—Carol A. Quesenberry

Graphics by the authors

Appendix D—Customer Requirements  47