Spring Flood Season and What It Does to Red River Valley Asphalt

Why the crest and the thaw arrive together

The Red River of the North flows north, toward Lake Winnipeg and Hudson Bay, across the flat bed of glacial Lake Agassiz, and through the valley it falls only about a foot per mile. Snowmelt starts in the warmer south while the river farther north is still frozen, so water moving downstream is slowed or dammed by ice. On ground that flat, overflow spreads across the old lakebed instead of staying in a channel.

The gauge record at Fargo shows the result. The National Weather Service lists the ten highest crests there, from 40.84 feet on March 28, 2009, down to 35.04 feet on April 8, 2019, and every one of them fell between March 21 and April 18. Seven of the ten have come since 1997. Minor flooding starts at 18 feet, moderate flooding at 25 and major flooding at 30.

Those are the same weeks when pavement is weakest. Hector airport averages 16.7 freeze-thaw days in March and 14.2 in April, and frost leaves the ground from the top down, trapping meltwater above a layer that is still frozen. NDDOT restricts heavy loads on state highways every spring because, in its words, the most significant pavement damage happens in the first four weeks after the thaw begins. A high river and a soft base belong to the same season here.

What flood season does to driveways and lots

Most driveways never see river water. What they see is everything around it. A winter's snow cover, which at Hector lies an inch deep or more on about 99 days a year, melts in a matter of weeks, the frozen soil cannot absorb it, and on land this flat it pools at the low edge of every drive and lot. The city runs 81 storm sewer lift stations because the terrain gives gravity so little to work with, and water that cannot get away sits against the asphalt.

Standing water does its damage through cracks and edges. It runs into any open crack, softens the base beneath and refreezes overnight, prying the crack wider. Along the edges, where snow banks have kept the soil saturated all spring, the asphalt loses side support and starts to crumble. Parking lots take the same punishment at their low corners, around catch basins and wherever the winter's plow piles melted.

Flood fighting adds weight. When the river at Fargo reaches 36 feet, the city starts building earth and sandbag levees, and afterward it removes clay levees and sandbag walls from public property. In neighborhoods near the river, trucks hauling clay and sand and stacks of sandbags can put heavy loads on streets and driveways at the moment their bases are softest. Ruts and depressions from that work may not show clearly until the ground firms up.

The clay that holds the water

The valley's soils explain why water lingers. The Fargo series, a soil named after the city, is a silty clay laid down in the old lake, rated poorly to very poorly drained, with a seasonal water table anywhere from the surface to 3 feet down and very low to low permeability. The Hegne series, another extensive clay across the valley, keeps its water table 6 to 18 inches below the surface from April through June.

For pavement, that means the base under a driveway can stay wet for weeks after the surface looks dry. Patching or sealing over a base in that state traps moisture under fresh material, and a patch set on saturated clay tends to settle once the clay dries and firms. After a wet, high-water spring, the ground under many drives is not ready for base repairs until late May or June.

A repair order for after the water drops

The sequence that works in this valley starts with patience and ends with sealer. The steps below assume a drive or lot that stayed dry but went through a wet, high-water spring. Property that actually flooded needs its base checked more thoroughly before any surface work, because silt and saturated gravel under the asphalt behave differently once they dry.

Drainage fixes belong in the same plan. If water stood against the garage or along one edge every spring, regrading, extending a downspout or moving next winter's snow pile will do more than any coat. The asphalt patching page explains saw-cut repairs, and the overview of Red River Valley sealcoating shows how the whole season fits together.

• Late March and April: watch and record where water stands, which cracks opened and where edges lifted
• Only for safety: cold patch any pothole that could catch a tire or a foot
• Late April and May: sweep off the grit, then fill cracks once highs sit between about 45 and 65 degrees
• Late May and June: saw-cut and patch ruts and soft spots with hot mix once the base has drained
• June through August: seal after the patches have cured, in warm, dry weeks
• October: a second crack pass, and a snow storage plan that sends next spring's melt away from the pavement

Protection that frames the season

Flood protection has grown a great deal since 1997. Since that year the City of Fargo has bought and removed more than 200 flood-prone properties and has made permanent levees, floodwalls and added pumping capacity its priorities. West Fargo and Horace are protected from the Sheyenne River by the Sheyenne Diversion, built between 1990 and 1992, which West Fargo reports held through the record floods of 1997 and 2009. The Fargo-Moorhead diversion, a 30-mile channel with a 22-mile southern embankment, is targeted to provide permanent protection by 2027.

None of that changes the physics on a single lot. The diversions keep river water out of neighborhoods; they do not dry the clay under a driveway or speed the thaw. In Moorhead, where the 2012 flood maps place a large portion of the city in the Special Flood Hazard Area, and in the low parts of Fargo, owners should expect a late, wet spring under their pavement whatever the river does. On the Minnesota side, any sealer laid afterward must also be free of coal tar under state law.

More from the blog

• How Cold Is Too Cold to Seal a Fargo Driveway?