Red River Rising: a Top-Ten Fargo Flood in 4 of the Past 5 Years

The Red River at Fargo, North Dakota surpassed major flood level on Sunday and continues to rise, with a peak expected Wednesday at the 9th highest flood level observed since 1897. On Friday, the President an emergency declaration for North Dakota because of the flooding, and millions of sandbags have been filled in anticipation of the huge flood. This year will be the fourth time in the past five years that Fargo has experienced a top-ten flood in recorded history. Flood stage is eighteen feet, and the Red River has now reached flood stage at Fargo for an astounding nineteen of the past twenty years, according to the U.S. Army Corps of Engineers. Prior to this remarkable stretch of flooding (which began in 1993), the river flooded in just 29 of 90 years. The Army Corps of Engineers calculates that in the last twenty years, the Red River has had ten 1-in-10 year floods--one every two years, on average. Two of these floods (1997 and 2011) were greater than 1-in-50 year floods, and one (2009) was a 1-in-100 year flood. That year, the Red River hit a record high-water mark of nearly 41 feet, or 23 feet above flood stage. Thousands of people had to leave home for higher ground, and about 100 homes were badly damaged or rendered unlivable. This year's flood will be somewhere between a 1-in-10 year to 1-in-50 year flood. Since a 1-in-10 year flood, historically, has a 10% chance of occurring in a given year, the incidence of flooding along the Red River over the past twenty years has clearly been extraordinarily abnormal.


Figure 1. View of the Red River of the North at the Fargo gauge taken on April 24, 2013 (top) and April 29, 2013 (bottom.) The river rose from 17' on the 24th (flood stage is 18') to 31' on the 29th. Image credit: USGS.

Reasons for this year's flood: unfavorable weather conditions
The USGS cites five weather factors that can act to increase flooding along the Red River. Four out of five of these factors occurred to a significant degree this year:

1) Above-normal amounts of precipitation in the fall of the year that produce high levels of soil moisture, particularly in flat surface areas, in the basin. North Dakota had its 9th wettest fall since 1895 during 2012.

2) Freezing of saturated ground in late fall or early winter, before significant snowfall occurs, that produces a hard, deep frost that limits infiltration of runoff during snowmelt. Fargo had temperatures that hit 50°F on December 2 - 3, 2012, followed by a sudden plunge to below-freezing temperatures that began on December 7. Temperatures remained below freezing the rest of December, and this froze the saturated ground to a great depth.

3) Above-normal winter snowfall in the basin. Fargo received 68.4" of snow during the winter, which is well above the city's average of 50".

4) Above-normal precipitation during snowmelt. Fargo has received 2.06" of precipitation so far this April, compared to the average of 1.23".

5) Above-normal temperatures during snowmelt. Fargo got lucky here. High temperatures in Fargo have been above average only two days during April, on the 26th and 27th.


Figure 2. Current and forecast flood stage for the Red River of the North at Fargo, ND. The river passed major flood stage on Sunday, and is headed for a crest near 35.5' (which is 17.5' above flood stage) on Wednesday. You can access images like these using our wundermap for Fargo with the "USGS River" layer turned on. Click on the icon for USGS station 05054000, then hit the "click for graph" link.

Reasons for flooding: increased urbanization
Urbanization has had a major impact on increasing flooding not only along the Red River, but in every river basin in the U.S. Many cities and developed areas are located in flood plains next to major rivers and their tributaries. Highways, streets, parking lots, sidewalks, and buildings now cover large areas of the ground that used to absorb excess rain water and slow the rate at which run-off from precipitation and melting snow reached rivers. By developing large portions of our flood plains, run-off now reaches rivers more quickly, generating higher floods.

Reasons for flooding: building more levees and flood defenses
Defending ourselves against floods has made floods worse. Every time a new levee is built, or an old flood wall raised in height to prevent overtopping, more and more water is forced into the river bed, which raises the height of the flood. Flood waters that used to be able to spread out over their natural flood plains are now forbidden from spilling out over newly developed land in flood plains. For example, a 2010 proposed improvement to the flood defense system in Fargo could cause a 4 - 10 inch rise in floods immediately downstream from the city, according to the Army Corps of Engineers.


Figure 3. Peak flow of the Red River at Fargo, North Dakota from 1901 - 2012. Three of the top five floods since 1901 have occurred since 2009. The projected crest for 2013 would be the seventh greatest flood since 1897. The U.S. Army Corps of Engineers lists the 10-year flood level for the Red River at Fargo to be 10,300 cubic feet per second (cfs), and a 50-year flood to be 22,300 cfs. A 10-year flood, historically, has a 10% chance of occurring in a given year. In the last twenty years, the Red River has had ten 10-year floods--one every two years, on average. Two of these floods (1997 and 2011) were greater than 1-in-50 year floods, and one (2009) was a 1-in-100 year flood. This year will be the fourth year out of the past five with a greater than 1-in-20 year flood. Image credit: U.S. Geological Survey.

Reasons for flooding: precipitation is increasing
Over the past century, precipitation over the Red River of the North drainage basin in Eastern North Dakota and Western Minnesota has increased by about 15%--more than any other region of the country. This fits the pattern expected by climate change models, which predict that winter and spring precipitation will increase by another 15% by the year 2100 over the Red River of the North drainage basin. As the climate warms, evaporation of moisture from the oceans increases, resulting in more water vapor in the air. According to the 2007 IPCC report, water vapor in the global atmosphere has increased by about 5% over the 20th century, and 4% since 1970. Satellite measurements (Trenberth et al., 2005) have shown a 1.3% per decade increase in water vapor over the global oceans since 1988. Santer et al. (2007) used a climate model to study the relative contribution of natural and human-caused effects on increasing water vapor, and concluded that this increase was "primarily due to human-caused increases in greenhouse gases". This was also the conclusion of Willet et al. (2007).


Figure 4. The colors on the map show annual total precipitation changes (percent) for 1991-2011 compared to the 1901-1960 average, and show wetter conditions in most areas (McRoberts and Nielsen-Gammon 2011). The bars on the graphs show average precipitation differences by decade for 1901-2011 (relative to the 1901-1960 average) for each region. The far right bar is for 2001-2011. (Figure source: NOAA NCDC/CICS-NC. Data from NOAA NCDC.) Note that precipitation over the Red River of the North drainage basin in Eastern North Dakota and Western Minnesota (outlined in red) has increased by about 15%--more than any other region of the country. Image credit: National Climate Assessment Draft, 2013.


Figure 5. Projected seasonal precipitation change for winter and spring (percent) for 2071-2099 (compared to1901-1960) as projected by the climate models used to formulate the 2013 IPCC climate change report, assuming we keep emitting heat-trapping gases like carbon dioxide to the atmosphere at current rates. Teal indicates precipitation increases, and brown, decreases. Hatched areas indicate confidence that the projected changes are large and are consistently wetter or drier. In general, areas that are wet are expected to get wetter, and areas that are dry will get drier. White areas indicate confidence that the changes are small. The Red River Valley is expected to see a precipitation increase of at least 20%, which would lead to bigger and more frequent spring floods. (Figure source: NOAA NCDC / CICS-NC. Data from CMIP5; analyzed by Michael Wehner, LBNL.) Image credit: Preliminary draft of the 2013 U.S. National Climate Assessment report.

A permanent fix for Fargo's flooding problems: a $2 billion diversion canal?
As the population continues to expand, development in flood plains and construction of new levees and flood protection systems will continue to push floods to higher heights. With global warming expected to continue and drive ever higher precipitation amounts--falling preferentially in heavy precipitation events--it is highly probable that flooding in the Red River Valley--and over most of the northern 1/3 of the U.S. where precipitation increases are likely (Figure 5)--will see higher and more frequent spring floods. With these higher and more frequent floods comes the increased risk of multi-billion dollar disasters, when a record flood event overwhelms flood defenses and inundates huge areas of developed flood plains. Obviously, we need to make smart decisions to limit development in flood plains to reduce the cost and suffering of these future flooding disasters.

A permanent fix for Fargo's flooding woes may lie in the construction of a 36-mile long canal that would steer flood waters around Fargo and neighboring Moorhead, Minnesota, according to an April 28, 2013 Associated Press article. The proposed canal could cost $2 billion and take ten years to complete, but has drawn strong opposition from farmers, homeowners and businesses who lie in the path of the proposed diversion channel. The http://www.redriverbasincommission.org/ has the latest long-term options on new flood control options for the Red River.

References
Kunkel, K. E., D. R. Easterling, K. Redmond, and K. Hubbard, 2003, "Temporal variations of extreme precipitation events in the United States: 1895.2000", Geophys. Res. Lett., 30(17), 1900, doi:10.1029/2003GL018052.

Groisman, P.Y., R.W. Knight, T.R. Karl, D.R. Easterling, B. Sun, and J.H. Lawrimore, 2004, "Contemporary Changes of the Hydrological Cycle over the Contiguous United States: Trends Derived from In Situ Observations," J. Hydrometeor., 5, 64.85.

McRoberts, D. Brent, John W. Nielsen-Gammon, 2011, "A New Homogenized Climate Division Precipitation Dataset for Analysis of Climate Variability and Climate Change," J. Appl. Meteor. Climatol., 50, 1187–1199.
doi: http://dx.doi.org/10.1175/2010JAMC2626.1

Milly, P.C.D., R.T. Wetherald, K.A. Dunne, and T.L.Delworth, Increasing risk of great floods in a changing climate", Nature 415, 514-517 (31 January 2002) | doi:10.1038/415514a.

Santer, B.D., C. Mears, F. J. Wentz, K. E. Taylor, P. J. Gleckler, T. M. L. Wigley, T. P. Barnett, J. S. Boyle, W. Brüggemann, N. P. Gillett, S. A. Klein, G. A. Meehl, T. Nozawa, D. W. Pierce, P. A. Stott, W. M. Washington, and M. F. Wehner, 2007, "Identification of human-induced changes in atmospheric moisture content", PNAS 2007 104: 15248-15253.

Trenberth, K.E., J. Fasullo, and L. Smith, 2005: "Trends and variability in column-integrated atmospheric water vapor", Climate Dynamics 24, 741-758.

Willett, K.M., N.P. Gillett, P.D. Jones, and P.W. Thorne, 2007, "Attribution of observed surface humidity changes to human influence", Nature 449, 710-712 (11 October 2007) | doi:10.1038/nature06207.

Links
A good way to track the flooding event is to use our wundermap for the Red River with the USGS River layer turned on.

The Fargo Flood webpage of North Dakota State University, Fargo, has some excellent links.

I'll have a new post on Wednesday at the latest.

Jeff Masters