Storms, runoff and fresh sealer in Fort Collins
Fort Collins is dry on paper, but its storms arrive in bursts, and whatever washes off a driveway heads for the Poudre.
A dry climate with wet months
The Fort Collins station on the CSU campus averages 16.10 inches of precipitation a year, which sounds like very little water to worry about. The catch is in the timing. Spring is the wettest season at 6.08 inches and summer brings 5.48, while May alone averages 2.72 inches on the monthly normals, more than any other month. On 8.7 days a year at least half an inch falls, and on 2.3 days an inch or more. For a driveway, the bursts matter more than the annual total, because a burst can overwhelm a drive's drainage and flood its cracks in an afternoon.
Hail belongs to the same months. A 2018 Denver7 report put Colorado's hail season at roughly mid-April to mid-August and counted seven to nine hail days a year across the region meteorologists call hail alley, which takes in Colorado, Nebraska and Wyoming. Those are the months in which sealing happens, so the storm record belongs in any sealing plan.
July 1997 and the network it started
The storm Fort Collins remembers came in late July 1997, when 10 to 14 inches of rain fell over about 31 hours and a flash flood ran through the city. It left a scientific legacy as well as a physical one. The Community Collaborative Rain, Hail and Snow Network began at the Colorado Climate Center at Colorado State University in 1998, thanks in part to that flood, and has since enlisted thousands of volunteers who measure rain, hail and snow where they live.
For a property owner, the flood's practical lesson is to look at where water goes. A driveway that sheds a summer downpour into the street or onto the lawn is doing its job. One that funnels water toward the garage, into a low spot or along an edge where the soil sits higher than the asphalt tends to show it within a few seasons as cracks and soft spots. Watching the drive during a real storm, rather than on a dry afternoon, shows which kind it is.
Where the water goes after it leaves the drive
Fort Collins has run a stormwater utility since 1980, and its own description is blunt: unlike sewage, which goes to a treatment plant, most stormwater flows through storm sewers directly to local creeks and the Cache la Poudre River without treatment. The city operates under a municipal separate storm sewer system permit and lists hazardous chemicals, automotive products, pesticides, fertilizers, pet waste and eroded soil among the pollutants that ride along.
The river at the end of that pipe is not an ordinary one. The Poudre runs 126 miles from Poudre Lake in Rocky Mountain National Park to the South Platte east of Greeley, and 76 miles of it were designated Wild and Scenic on October 30, 1986. Every property pays toward the stormwater system through a charge built from lot size and a runoff factor tied to how much of the lot is impervious, from 0.25 for very light coverage to 0.95 for very heavy, with 0.4 listed as typical for a house.
Why the sealer in the tank matters here
No Colorado state, county or city restriction on coal-tar pavement sealer appears on the national list of bans kept by Coal Tar Free America, so coal-tar products remain legal in Fort Collins. By the USGS count, coal-tar sealcoat holds about 1,000 times more PAHs than asphalt-based sealcoat, and it sees more use on the eastern side of the Continental Divide, Fort Collins' side. In USGS testing, runoff collected as long as 42 days after a coal-tar application killed day-old fathead minnows and water fleas.
An owner who wants to keep that out of the Poudre has a simple lever: ask for an asphalt-emulsion or acrylic sealer and have the product named in the bid. Both kinds do the same basic job on a sealed driveway, closing the surface against water and slowing the aging of the binder. They differ in how long their makers say they last and in what washes off during the first storms after application.
Scheduling a seal around the storms
SealMaster's guidance for application is 50 degrees and rising for 24 hours, no rain expected within 24 hours and a full day before traffic. On the campus normals, June through August is when nights mostly stay above 50, while May, the wettest month, still has 25.8 nights at or below 50. That leaves summer as the main window, and summer is also hail season. The practical answer is a morning start on a day with a clean forecast, so the coat has most of the day to set before any afternoon storm builds.
If rain reaches a coat that has not set, the damage is usually local: diluted sealer streaks, runs toward the gutter and leaves thin spots. The usual remedy is a touch-up coat on the affected area once it dries. The bigger mistake is starting a job with rain in the 24-hour forecast because the schedule is full, since the thin, streaked coat that results ends up being paid for twice.
After a storm: a five-minute check
The morning after a heavy storm is the most useful time to walk a driveway. Standing water shows the low spots. Dark lines that stay wet longest show the open cracks. Sand and fine stone washed to the bottom of the slope show a surface losing its binder. Soil washed out along an edge shows where the asphalt has lost its side support.
Most of what the check turns up is inexpensive to fix when caught early. Crack filling and a coat on schedule handle the surface, and drainage changes handle the rest. The overview of asphalt sealcoating in Fort Collins sets out prices and timing for each step.
- Water still standing a day later: a drainage or base problem to fix before sealing
- Cracks that stay dark and wet: fill them before winter
- Fine stone at the foot of the slope: surface aging, seal next summer
- Edge soil washed away: rebuild the shoulder before the edge breaks off
Want a sealing date planned around the storm season? Send a request through the quote form.
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Quick Answers
What happens if it rains right after a driveway is sealed?
It depends on how far the coat has set. Rain in the first hour or two can dilute the sealer, streak it and carry some toward the gutter, while rain after the surface has skinned over usually leaves little mark. SealMaster's guidance is to avoid application when rain is expected within 24 hours for that reason. Damaged areas are normally recoated once dry, which is why a clean forecast matters more than the date on the calendar.
Does sealer runoff really reach the Poudre?
Anything that leaves a Fort Collins driveway in a storm and enters a storm drain heads that way. The city's stormwater utility says most stormwater flows through storm sewers directly to local creeks and the Cache la Poudre River without treatment. That includes rinse water and any sealer that washes off before it cures, which is why the product choice and a dry forecast both matter on sealing day.
How can a resident see how much rain or hail fell at home?
The Community Collaborative Rain, Hail and Snow Network, which began at the Colorado Climate Center at CSU in 1998 partly because of the 1997 flood, relies on volunteers who take their own measurements of rain, hail and snow where they live. A volunteer's reading describes that one spot, which is more useful than a single citywide total when judging what a particular storm did to a particular driveway on a particular street.
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