(Editor’s Note: This is the first in a series of guest columns examining stormwater runoff and how the city can address it.)
The city of Colorado Springs describes its “stormwater” policy on its website as follows: “This definition describes the components of stormwater (rain and snow), it determines the generators of stormwater runoff (streets, parking areas, rooftops and other developed areas), and finally it describes the natural and man-made infrastructure used to manage stormwater (either flows directly into nearby streams or travels there through drainage systems, such as curbs and gutters, inlets, storm sewers, detention ponds and channels).”
Is stormwater just rain and snow? Or is there more to it?
When precipitation lands anywhere within our city limits, in any form, it can:
1. be released back into the atmosphere as water vapor through one of three processes: evaporation, sublimation or transpiration;
2. infiltrate into the ground — either by increasing the percentage of ground moisture or adding to a body of water underground;
3. become stormwater runoff.
When the city refers to its Stormwater Program, it only includes rain and snow in its definition. Hailstorms in the summer months can cause more damage than rain or snow. Hail produces stormwater runoff and must be managed. The other forms of winter precipitation typically produce less runoff when they transform into water than rain, snow and hail, but they should be included in the definition of stormwater.
The city’s definition of “stormwater” means that for rain or snow to become runoff, it must fall on impervious surfaces such as “streets, parking areas, rooftops and other developed land and either flows directly into nearby streams or travels there through drainage systems …”
Streets can generate as much as 50 percent of the runoff. But do the other impervious surfaces, such as parking areas, driveways and rooftops, and other developed land make up the remaining 50 percent or 30 percent or even 10 percent?
Significant amounts of stormwater runoff can come from vacant land that contains no impervious surfaces. In a 2007 Douglas County report, the county provided calculations of the impact precipitation had on vacant land.
They found the percentage of runoff from vacant land during storm events varies depending on whether it has been a dry year, a normal year or a wet year. During a dry year, the average runoff from most storm events would be negligible; during a normal year the average runoff would be about 2 percent; and during a wet year the average runoff would be about 12 percent.
For example, take 1 acre of vacant land and assume an inch of rain falls. The rain would provide 27,000 gallons of water over the entire acre of land or .62 gallons of water on each square foot of land. During a dry year, one would expect the water to be absorbed into the land during this storm event. During a normal year, about 540 gallons would become runoff. During a wet year, 3,240 gallons of water would have to be managed. The runoff has nothing to do with impervious surfaces.
In addition to the quantity of stormwater that lands on our roads, the stormwater draws out and mixes with the pollutants that lay on the road surfaces … pollutants that are deposited by vehicles. It can also be demonstrated that about 50 percent of the pollutants that enter stormwater systems comes directly from roads. The pollution does not include any of the additional runoff from other impervious surfaces.
The city also has significant amounts of groundwater that seep out of the ground. This groundwater seepage combines with stormwater runoff and either flows directly into nearby streams or into the city’s stormwater drainage system. For example, the Pine Creek Drainage Study says: “The Pine Creek Channel has a year-round base flow with peaks reaching as high as 500 G.P.M. at the western boundary of the study area. The primary source of this ‘live water’ is groundwater seepage along the creek banks in some areas.”
The city has more than 30 drainage basins that produce varying amounts of groundwater runoff from this “groundwater seepage,” and all the runoff from this seepage must be managed by the city’s stormwater program.
Vince Rusinak is owner of Rusinak Real Estate. He can be reached at email@example.com.