Improving Mixed Populations Flood Frequency Analysis

By: Mike Bartles, P.E. (HEC), Avital Breverman, P.E. (HEC), and Dave Rosa, Ph.D. (FEMA)

Within the United States, federal guidelines such as Bulletin 17C outline the methods used to estimate flood frequency (England, Jr. et al., 2019). These guidelines recognize that floods can result from various causes, including rainfall-only, rain-on-snow, snowmelt, and tropical cyclones, amongst others. At a single location, floods may be triggered by one or more of these processes, each contributing to the overall likelihood of significant flood events. Bulletin 17C also clearly notes the need for additional research to more effectively account for these multiple flood-generating processes, often referred to as mixed populations, in flood frequency analyses.

The Federal Emergency Management Agency (FEMA) has partnered with the U.S. Army Corps of Engineers (USACE) and U.S. Geological Survey (USGS) in order to enhance state-of-the-practice tools and techniques that are used to quantify flood risks at locations that are subjected to floods from multiple mechanisms. Six pilot regions were selected to develop and demonstrate flood frequency analysis and flood type classification methodologies throughout the United States. These pilot regions included the Puget Sound, Upper Colorado River, Red River of the North, Iowa River, Delaware River, and Trinity River, as shown in the following figure.

Using a combination of historical narrative sources, and meteorological and land surface data, manual flood and storm type classifications were developed for three locations within each of the six pilot watersheds. This was done to provide a "truth" data set that could be used to verify automated flood typing approaches in the future. Floods were assigned primary and, when appropriate, secondary causal mechanisms, distinguishing amongst rain-only, mixed, and snowmelt-only events, with further subdivision by storm type (e.g., atmospheric rivers, frontal systems, convective storms, tropical cyclones/tropical storm remnants, monsoon, long-duration rainfall) and modifiers such as frozen or saturated ground, ice jams, and wildfire. A histogram detailing the distribution of flood types for a location within the Delaware River watershed is shown below.

Afterwards, automated flood and storm typing algorithms were developed for each region in order to more efficiently and uniformly assign flood types to an time series of peak flows. The goal of the automated procedures was to reproduce the results of the manual flood typing using zonal statistic time series from gridded data sources and tropical storm track information. An example algorithm from the Puget Sound watershed is shown below, starting in the upper left with precipitation accumulation.

Using the automated flood typing results, flood generating mechanisms were found to vary geographically within the pilot regions.  For instance, within the Delaware River watershed, floods driven by mixed events are more common in the headwaters (northern latitudes) of the watershed than further south, as a result of colder temperatures and increased precipitation falling as snow or remaining on the ground as snow. Conversely, a greater percentage of flood events are driven by rain-only floods in the southern portion of the watershed. In addition, tropical cyclone/tropical storm remnants (TC/TSR) driven flood events are more prevalent in the southern portion of the basin, which is closer to the Atlantic Ocean, as shown in the following figure.

Once the flood and storm typed data was available, multiple approaches were then utilized to estimate at-site flood frequency. First, a "baseline" approach to estimating flood frequency was employed which used Bulletin 17C procedures (England et al., 2019). This baseline approach disregarded all flood mechanism information and treated the entire annual maximum series (AMS) as a single homogeneous sample. Then, numerous mixed population analysis techniques, such as those contained within Engineer Manual 1110-2-1415 (EM 1415; USACE, 1993), were used to estimate flood frequency. A figure comparing the baseline and EM 1415 flood frequency results is shown below.

For locations that exhibited large disparities in magnitudes between the various flood mechanism-specific AMS, extrapolations beyond the range of observed data were notably different between the EM 1415 and baseline methods. These extrapolations to rare annual exceedance probabilities were afforded by the use of flood mechanism-specific AMS, which were disregarded when using the baseline method. Notice that the combined curve computed using the EM 1415 method has a noticeable inflection upwards (as compared to the baseline results) and follows the upward trend of the empirical plotting positions for the largest observed annual maxima. This plot simultaneously represents the challenge of correctly analyzing mixed population hydrology data and demonstrates the value of this research in producing more accurate flow frequency results.

FEMA Perspective

Historically, FEMA has focused on providing flood data and maps specific to National Flood Insurance Program (NFIP) mandatory purchase and floodplain management requirements. These flood maps are widely used for risk communication but fall short of offering a complete view of flood risk due to several limitations including a simplistic in/out presentation of flood hazards, incomplete coverage, absence of velocities and pluvial (rainfall induced) flood data, minimal accounting for future conditions, as well as assumptions about spatial and temporal distributions of rainfall and homogenous flood generating mechanisms, among other factors. To leverage and advance modern scientific and technological methods to identify flood hazards and evaluate flood risks, FEMA launched the Future of Flood Risk Data (FFRD) initiative.  The FFRD initiative is intended to help FEMA achieve their goals of developing a risk informed NFIP by addressing the need and statutory requirement to deliver trusted, authoritative data that can enable consistent flood risk-informed decision making across the nation.  Advancing the science of mixed population analyses supports FFRD and the evolution of FEMA's flood modeling to a graduated, probabilistic approach that provides comprehensive flood hazard and risk information over a wide range of magnitudes and frequencies as well as a quantification of the uncertainty of those estimates. Improved understanding of mixed populations and flood-type classification is expected to result in improved flood frequency methods that can be leveraged through FFRD, leading to reductions in both flood damage and excessive infrastructure expenses. Additional benefits include improved flood hazard and risk products, including regulatory maps, improved estimates of low-probability, high consequence flood events, and improved understanding of how floods are generated under existing conditions, which can then be leveraged to estimate future conditions. 

Value to the Field

The FEMA, USACE, and USGS team has delivered numerous webinars and conference presentations and published several journal articles related to this effort. In the fourth and final year of the work unit, the teams are developing technology transfer and training materials and software features to support operationalization of methodologies developed in the work unit. Additionally, HEC has developed and delivered updated mixed population related material in the Flood Frequency Analysis PROSPECT course in April 2026. Links to these tech transfer materials in addition to the flood- and storm-typed data that was generated as part of this work unit can be found here: FEMA-USGS-USACE Mixed Populations Work Unit Overview. Enhancements to both HEC's Statistical Software Package (HEC-SSP) and its planned successor application, HEC-Neptune, will be released in the near future that make the aforementioned 1) automated flood and storm typing algorithms and 2) enhanced mixed population flood frequency techniques available to all users. Look for an HEC-SSP Version 2.4 and HEC-Neptune Version 1.0-alpha release within Calendar Year 2026. Finally, formal updates to USACE guidance (e.g., EM 1110-2-1415) pertaining to the generation and use of mixed population flood frequency data are planned for the near future.

References

England, J.F., Jr., Cohn, T.A., Faber, B.A., Stedinger, J.R., Thomas, W.O., Jr., Veilleux, A.G., Kiang, J.E., and Mason, R.R., Jr., (2018). Guidelines for Determining Flood Flow Frequency - Bulletin 17C (ver. 1.1, May 2019): U.S. Geological Survey Techniques and Methods, book 4, chap. B5. Reston, VA: U.S. Geological Survey.

U.S. Army Corps of Engineers. (1993). Hydrologic Frequency Analysis. Engineer Manual 1110-2-1415. Washington, D.C.: Department of the Army.