Broomfield's Park Is Also a Pipe, and That's the Point
On a dry August afternoon, Community Park looks like exactly what it is: grass, a playground, maybe a dog chasing a frisbee past a stand of cottonwoods. What it does not look like is infrastructure. But part of that park, along with stretches of Midway Park and Emerald Park, is doing double duty as a flood control channel, carrying storm water from US 287 down to Spader Way. The city council's consent agenda this month includes funding from the Mile High Flood District for the first phase of improvements to that channel, and the mundane bureaucratic language hides a genuinely interesting bit of engineering logic.
Start with the basic problem. Colorado's Front Range sits in a semi-arid climate that mostly behaves itself, then occasionally does not. Convective thunderstorms, the kind that build fast on summer afternoons and dump an inch of rain in twenty minutes, can send more water downhill than a normal creek bed can handle. The devastating Big Thompson Canyon flood of 1976 and the widespread 2013 Front Range floods are the extreme, headline-making versions of this pattern, but smaller versions happen most years somewhere along the urban corridor. Water that has nowhere orderly to go finds the disorderly option instead: streets, basements, parking lots.
This is where the term “floodplain” earns its keep. A floodplain is simply the low ground next to a waterway that periodically fills with water during high flows, whether people have built there or not. Engineers further split it into the floodway, the central channel that has to stay open to carry the fastest-moving water, and the floodway fringe, the surrounding buffer area that fills up more slowly and can tolerate some obstruction. Broomfield's park system, deliberately or through decades of fortunate zoning, sits mostly in that fringe. That is not an accident. It is much cheaper, and much smarter, to let a soccer field flood than a subdivision.
The statistic that usually gets attached to this kind of planning is the “100-year flood,” a phrase that confuses almost everyone who hears it. It does not mean a flood that happens once every hundred years, on a schedule. It means a flood so large it has roughly a 1 percent chance of occurring in any given year, based on historical rainfall and stream data. Run that math forward and a 1 percent annual chance adds up to something like a 26 percent chance of at least one such flood over a 30-year mortgage, which is one reason floodplain maps matter more to insurers and builders than the “100-year” label suggests at first glance.
Designing a channel to handle that kind of event without building a concrete-lined ditch through three parks takes some deliberate choices. Newer flood control work, including what the Mile High Flood District specializes in, increasingly leans on what is sometimes called green stormwater infrastructure: keep the channel wide, keep it vegetated, let the banks and adjacent low ground absorb and slow the water rather than just speeding it downstream toward the next town's problem. Slower water is calmer water, easier on erosion, easier on whatever is downstream, and it means the same acreage can be a floodway on a bad day and a perfectly normal park the other 360-some days of the year.
The Mile High Flood District itself is worth a mention, since water is famously indifferent to city limits. The district exists precisely because a channel that starts in Broomfield does not stop being Broomfield's problem once it crosses into the next jurisdiction, so a regional agency coordinates funding and design standards across boundaries that only show up on a map.
Phase 1 funding for the City Park Channel is not a dramatic announcement. No ribbon will be cut, and most residents will never notice the difference on an ordinary Tuesday. But the next time a summer storm rolls in fast and hard off the foothills, the reason a park absorbs the water instead of a living room is a century of hydrology working quietly under the swing sets, doing its job by looking like it is doing nothing at all.