Many Front Range detention ponds were built decades ago to a simpler standard. They hold the big storm and drain afterward, and nobody has had a reason to look closer. That changes when the property redevelops, when the jurisdiction reviews older facilities, or when an inspection says the pond doesn’t meet current standards. At that point the owner has three choices: retrofit the pond, replace it, or build new storage somewhere else on the site. Retrofitting is often the best of the three, because the excavation, outfall, and land are already committed.
What Older Ponds Are Missing
Ponds built before the 2000s were usually designed to knock down flood peaks. Most did nothing about the small, frequent storms that carry most of the pollution and do much of the erosion damage downstream.
| Component | Typical older pond | Current Front Range practice |
|---|---|---|
| Outlet | A single pipe or orifice sized for the flood peak. Small storms drain straight through. | A multi-stage outlet structure: an orifice plate that releases the water quality volume over roughly 40 hours, a second stage for larger storms, and an overflow for the 100-year storm. |
| Forebay | None. Sediment spreads across the pond bottom. | A hardened forebay at each major inlet, where sediment can be cleaned out cheaply. |
| Micropool and trash rack | None. | A small permanent pool and trash rack that protect the orifice plate from clogging. |
| Trickle channel | Sometimes none. | A defined low-flow channel from the forebay to the micropool. |
| Emergency spillway | Sized to older standards. | Sized for the undetained 100-year peak inflow, with freeboard to the top of the embankment. |
| Design basis | Older rainfall data and methods. | Current rainfall data and current criteria. |
When a Retrofit Is Required
Redevelopment is the most common trigger. When a site with an older pond redevelops, the jurisdiction will often require the stormwater facility to meet current standards, and retrofitting the existing pond is frequently cheaper and uses less land than building new. See detention pond requirements for commercial sites for how redevelopment rules usually work.
The other triggers are a change upstream, where more area now drains to the pond than it was designed for, and repeated inspection findings, where the same problem keeps coming back because the design, not the maintenance, is the cause. Some jurisdictions also retrofit older public and private facilities through their own stormwater programs.
What a Detention Basin Retrofit Involves
A new outlet structure is the core of most retrofits. Replacing a single-pipe outlet with a multi-stage structure adds water quality treatment and controls the mid-size storms, and on many ponds it accomplishes most of the goal by itself. How the new structure should work is covered in detention pond outlet structure problems.
From there, the work gets more invasive. Adding a forebay at the main inlet makes the pond far cheaper to maintain for the rest of its life. A micropool and a trickle channel protect the orifice plate and keep the bottom dry. Regrading adds storage if current requirements need more than the pond holds. Bringing the emergency spillway and freeboard up to current standards can be the most expensive piece, especially if the embankment has to change.
What Limits a Retrofit
Volume is the first constraint. The water quality capture volume is counted within the pond’s total volume, at the bottom, with the larger storm volumes stacked above it. Adding water quality doesn’t usually take separate storage. It takes an outlet that releases that bottom volume over about 40 hours. The real volume question is whether the pond’s total storage still meets current requirements for the larger storms, and if it doesn’t, whether there is room to grow. Outfall depth is the second. The water quality orifice has to sit low enough to drain the bottom of the pond by gravity, and a shallow outfall limits the design. Colorado water law sets the third: the pond has to hold small storms long enough to treat them and still drain within the statutory limit. Raising water levels also means checking the embankment and spillway, and a larger embankment can bring in state dam safety review.
Every retrofit starts with what the pond actually is, not what the old plans say. A topographic survey of the pond and an inspection of the existing outlet are the first steps.
What a Retrofit Costs
An outlet-focused retrofit on a typical Front Range pond, with a new multi-stage structure and the grading around it, often runs in the range of $15,000 to $60,000 to construct. A fuller retrofit that adds a forebay, reshapes the pond, and upgrades the spillway more commonly runs $75,000 to $250,000. When the pond also has to get bigger, the number climbs fast. Colorado Springs’ 2017 upgrade of its King Street pond, which added water quality features and a new concrete outlet and nearly doubled the storage from 3.35 to 6.5 acre-feet, cost about $519,000, roughly twice the city’s early estimate. Design and review are on top of construction in every case. These are planning ranges; the real number depends on how far the pond is from current standards and how much room there is to work.
Common Questions
What is a detention basin retrofit?
Modifying an existing detention pond, usually its outlet and often its forebay, grading, and spillway, so it provides water quality treatment and meets current detention standards.
Do old detention ponds have to meet current standards?
Not automatically. Most older ponds can keep operating as approved until something triggers an upgrade, most often redevelopment of the site.
How much does a detention pond retrofit cost?
Roughly $15,000 to $60,000 for an outlet-focused retrofit and $75,000 to $250,000 for a fuller one, plus design and review. Retrofits that add significant storage can cost more.