Estimating how much, how long, and how often rain will occur is a valuable tool for Alaska’s transportation engineers, as well as many other technical professions. Without this data, planning for culverts, bridges, dikes, diversions, roads and other structures—especially those built over rivers or floodplains—becomes vulnerable to major setbacks and costly infrastructure damage.
As June flooding resulted in closures along the Alaska Highway, state engineers were reminded of the crises that emerge when unexpected rainfall causes vital conduits for people and goods to halt. The costs of even isolated incidents of rainfall-related infrastructure failure are also significant. In 2003, unexpected storms in Colorado triggered a culvert failure causing $4.2 million in damages. On a statewide level, these costs are even more concerning. Ohio recently received $145 million in federal aid to cover road and bridge damages from unforeseen storms in 2011.
While more robust designs can mitigate these costs, Alaska’s engineers and planners until recently relied upon precipitation inventories published in 1963 and 1965. As Alaska DOT&PF begins the process of charting new transportation corridors, predicting and planning for the intensity, duration, and frequency of future rainfall events has become even more valuable.
The Hydrometeorological Design Studies Center (HDSC) within the National Oceanic and Atmospheric Administration’s (NOAA) National Weather Service (NWS), led by Dr. Sanja Perica, continually updates precipitation frequency estimates for various parts of theUnitedStates and affiliated territories, in NOAA Atlas 14 volumes. Alaska’s portion has not been updated since the mid-1960s due to the sparse station coverage, a relatively short period of record, and great variations in topography across the state. Dr. Doug Kane, a faculty emeritus with UAF’s Water and Environmental Research Center (WERC), was looking to update Alaska’s precipitation estimates for the state’s key transportation corridors. NOAA decided to collaborate with AUTC and WERC in expanding the effort statewide.
The partnership proved adept at overcoming numerous data collection challenges unique to Alaska. Rainfall measurement gauges often suffered from wind, snow, sleet, hail, and wildlife damage. Only some of the stations had observers present, whereas many stations were unattended. This led to data quality control procedures becoming critical for working with these types of records.
Data compilation was a key facet of this work, as the team sought to evaluate a large body of existing data. Researchers gathered data from a long list of agencies including the National Weather Service, National Resources Conservation Service, U.S. Geological Survey, the University of Alaska, and many others. The researchers aimed to make the data adequate for statistical analysis through merging records from nearby stations, performing various quality control tests, and detailed investigation of suspicious values. Using up-to-date modeling and computer techniques, they improved the quality of precipitation frequency estimates contained in the original publications. The project moved engineers from utilizing hand interpolations to determine location-based rainfall estimates to sophisticated computer-assisted techniques incorporating a more detailed and thorough collection of figures. This resulted in more accurate, efficient estimates.
The differences in estimates between these are attributed to a number of factors, including improved frequency analysis approaches and interpolation techniques. In addition, the increase in the number of stations—including high-elevation stations and more diverse topography—and periods of record across all durations used in frequency analysis were significant improvement factors.
The team determined which of the data sets from Alaska’s more than 1,600 gauges could be merged together to yield useful results. Their reliance upon longer rainfall records enables better estimates and improved statistics.
The precipitation frequency estimates with accompanying information are available at NOAA’s online interface. With a zoom-in map function, well-displayed graphs, and the ability to generate point-and-click location specific data on the fly, this functionality is a dramatic improvement from the original product. Before this project, engineers relied upon a series of printed isopluvial maps from the 1960s.
Online users and engineers move a curser anywhere in Alaska, and click on a location. Then, the system generates rainfall frequency estimates specific to that exact coordinate. The user can click to view these figures in tab or full-frequency color graph format. It also displays upper and lower bounds of 90% confidence intervals, helping engineers improve risk analysis by incorporating uncertainty into existing estimates. Users can also click to view a variety of supplemental data including cartographic maps, high resolution grids estimating a range of durations and frequencies, and accompanying documentation describing the whole process in detail. The format also gives temporal distribution, seasonality analysis, and data source and watershed information—all for a specific location.
The project has garnered significant interest from a number of stakeholders. In January, the Alaska Center for Climate Assessment and Policy (ACCAP) invited researchers led by Dr. Perica to present a live webinar for more than 80 people. In February, Dr. Kane gave a presentation at the Department of Interior’s North Slope Science Initiative (NSSI), and a graduate student gave a seminar on her role in the project to WERC colleagues at UAF. In March, team members were invited to present their work in Juneau to more than 100 attendants at the annual meeting of the Alaska Section of the American Water Resource Association (AWRA), and Dr. Sveta Stuefer headed this presentation.
Beyond these audiences, anyone can access the updated rainfall frequency estimates online at NOAA’s website, located at: hdsc.nws.noaa.gov/hdsc/pfds/pfds_map_ak.html. There visitors can explore the variety of functions and data accessible through this user-friendly format.
This story originally appeared in AUTC’s recent quarterly newsletter. Read this and other stories by visiting AUTC’s publications page online here.
 “The Economic Costs of Culvert Failures,” Perrin, Joseph and Chintan Jhaveri, Prepared for the Transportation Research Board, January 2004.
 “Ohio Receives $145 Million in Federal Aid to Deal with Storm Damage to Roads and Bridges,” Tom Breckenridge, The Plain Dealer, January 9, 2012.