If you're developing property along the Front Range, your civil engineer is going to propose some type of permanent stormwater facility as part of your site design. Depending on the project, that might be a grassy basin in the corner of your site, a series of landscaped planters along the parking lot, an underground vault nobody ever sees, or a sand-filled box tucked behind the building.
Each of these facilities serves a different purpose, carries a different price tag, and comes with a different set of long-term maintenance obligations. Understanding the tradeoffs before your engineer starts drawing helps you make better decisions about site layout, project budgeting, and what your property will look and function like for decades after the last contractor leaves.
Here are the four stormwater facility types you're most likely to encounter on a Front Range project, what each one actually does, and when each one makes the most sense.
Extended Detention Basins (EDBs)
If there's a default stormwater facility along the Front Range, it's the EDB. This is the facility type that most engineers reach for first, and for good reason: it's well-understood, widely accepted by reviewers, relatively simple to construct, and effective at what it does.
How It Works
An EDB captures stormwater runoff and releases it slowly through a small orifice in the outlet structure. The MHFD criteria call for a 40-hour drain time for the Water Quality Capture Volume (WQCV), which is the runoff generated by the 80th percentile storm (approximately 0.6 inches of precipitation within the MHFD boundary). During that 40-hour drain period, suspended sediment and particulate-bound pollutants settle to the bottom of the basin. The cleaned water discharges slowly through the outlet.
Between storm events, the basin is dry. The floor and side slopes are typically vegetated with native grasses, which stabilize the soil and provide some additional filtration benefit.
An EDB can also be designed to provide flood detention for larger storm events by incorporating additional storage above the WQCV pool. This dual-function approach, capturing the WQCV for water quality and the Excess Urban Runoff Volume (EURV) and larger storms for flood control, is common on Front Range projects where a single facility needs to serve multiple purposes.
When It Makes Sense
EDBs are the right choice for most residential subdivisions and commercial developments where there's adequate space for a surface facility. They're particularly well-suited when the site has a natural low point that can be graded into a functional basin without excessive earthwork, when the local jurisdiction is comfortable with EDB design (which along the Front Range, most are), and when long-term maintenance will be handled by an HOA or property owner without specialized equipment.
What to Watch For
The biggest design mistake I see with EDBs is undersizing the orifice or miscalculating the WQCV, both of which affect the drain time and treatment performance. MHFD published updated EDB recommendations in 2022 that refined the relationship between upstream impervious area and orifice sizing. Make sure your engineer is using current criteria.
The biggest operational mistake is neglecting sediment removal. Over time, sediment accumulates on the basin floor and reduces the available storage volume. A well-maintained EDB should have the accumulated sediment removed every few years, depending on the contributing watershed. If the sediment builds up enough to clog the orifice, the basin loses its water quality function entirely.
On the positive side, a well-designed EDB with gentle side slopes, native grasses, a walking trail around the perimeter, and a few benches can be one of the most attractive features on a development. Lots adjacent to a thoughtfully designed EDB consistently command premium pricing. It's a regulatory requirement that can pay for itself through lot values if you design it with that intent from the beginning.
Bioretention (Rain Gardens)
Bioretention has gained significant traction along the Front Range over the past decade, particularly on commercial, mixed-use, and urban infill projects where space for a traditional EDB is limited. The MHFD USDCM Volume 3 includes design criteria for bioretention, and many Front Range jurisdictions actively encourage it as part of a distributed stormwater management approach.
How It Works
A bioretention facility is essentially an engineered planting bed designed to capture, filter, and treat stormwater runoff. Water flows into a shallow depression filled with a specially blended media (typically a sand, compost, and soil mix), passes through the media where pollutants are filtered and absorbed, and is collected by an underdrain system at the bottom that conveys the treated water to the storm drain system.
On the Front Range, underdrains are almost always required because the native clay soils don't infiltrate fast enough to drain the facility between storms. Bioretention without an underdrain works in sandy soils, but those conditions are relatively uncommon along the urban corridor.
The vegetation on top of the media provides evapotranspiration, root-zone filtration, and aesthetic value. Native grasses, perennial flowers, and adapted shrubs are all common plant selections for Front Range bioretention facilities.
When It Makes Sense
Bioretention works well on sites where space is too tight for an EDB, where the stormwater management needs to be distributed across the site rather than concentrated at one point, or where the developer wants the facility to double as a landscape amenity. Common applications include parking lot islands, streetscape planters, building perimeter swales, and plaza edges.
It's also effective for volume reduction, since some portion of the captured runoff is absorbed by the media and plants rather than being discharged. For jurisdictions that offer volume reduction credits, bioretention can reduce the sizing requirements for downstream detention.
What to Watch For
Bioretention requires more specialized maintenance than an EDB. The media surface can clog with fine sediment over time, requiring periodic raking or thin-layer replacement. Vegetation needs management: dead plants need replacing, invasive species need removal, and the plant palette needs to be appropriate for Colorado's climate, which means drought-tolerant species that can survive both the July heat and the January freeze-thaw cycles.
Supplemental irrigation is typically needed during the establishment period (first one to two growing seasons) and may be needed ongoing in drier years. Factor that into the water budget and long-term maintenance plan.
The most common failure mode I see with bioretention on the Front Range is sediment loading from an upstream source that wasn't adequately controlled during construction. If the bioretention facility is installed before the contributing drainage area is fully stabilized, fine sediment washes in and clogs the media surface within the first year. Sequence your construction so that bioretention goes in last, after the surrounding area is paved or vegetated.
Underground Detention
Underground detention systems have become increasingly popular along the Front Range, particularly on commercial and higher-density residential projects where surface land is too valuable to dedicate to a stormwater basin. These systems store runoff below grade in vaults, chambers, or large-diameter pipe arrays, and release it through a controlled outlet just like a surface facility.
How It Works
The concept is the same as surface detention: capture stormwater, hold it temporarily, and release it at a controlled rate. The difference is that the storage is buried. Common configurations include precast concrete vaults, corrugated HDPE chamber systems (StormTech, Cultec, and similar products), and large-diameter pipe arrays.
Water typically enters through standard storm drain inlets, fills the underground storage, and discharges through an outlet control structure. Some systems include pretreatment components (sediment traps, hydrodynamic separators, or filter cartridges) to provide water quality treatment in addition to flow attenuation.
When It Makes Sense
Underground detention is the right choice when surface area is at a premium and the project budget can support the higher construction cost. Typical applications include commercial retail sites where every square foot of surface area translates to leasable space or parking, higher-density residential projects where lot yield drives the financial model, and urban infill sites with tight footprints and no room for a surface basin.
Underground systems can also make sense on sites with unusual topography or grading constraints where a surface basin would require excessive earthwork.
What to Watch For
Cost is the most obvious consideration. Underground detention systems are significantly more expensive to construct than surface EDBs. The storage volume per dollar is much lower, and the installation requires excavation, structural backfill, and often dewatering during construction. Depending on the system type and site conditions, underground detention can cost two to five times more per cubic foot of storage than a comparable surface facility.
Maintenance is the other critical issue. Underground systems are out of sight and therefore out of mind. Sediment accumulation in vaults and chambers reduces storage volume over time, just like in a surface basin, but you can't see it happening. Regular inspection (typically annually, sometimes more frequently depending on the jurisdiction) is essential. Cleaning requires vacuum trucks or confined-space entry, both of which cost more than mowing a grass basin.
Some Front Range jurisdictions are cautious about approving underground detention, particularly for water quality credit, because the long-term maintenance track record is mixed. If you're planning to use underground detention, confirm early in the design process that your jurisdiction will accept it and understand their specific requirements for access, inspection, and maintenance documentation.
Sand Filters
Sand filters are less common than EDBs or bioretention along the Front Range, but they fill a specific niche for sites that need water quality treatment in a compact footprint and don't have the space or conditions for other facility types.
How It Works
A sand filter captures stormwater in a pretreatment sedimentation chamber, then passes it through a bed of engineered sand media that filters out suspended sediment, particulate pollutants, and some dissolved contaminants. The filtered water is collected by an underdrain and discharged to the storm system. Sand filters can be designed as open surface facilities or enclosed below-grade structures.
The filtration process is effective at removing fine particulates that would pass through an EDB. Sand filters generally outperform EDBs on total suspended solids removal and can also address some dissolved metals and hydrocarbons, depending on the media composition.
When It Makes Sense
Sand filters work well on constrained sites where there isn't room for a basin with the side slopes and footprint an EDB requires. They're also useful in areas with high pollutant loads (commercial loading areas, fueling stations, heavy parking usage) where enhanced treatment is warranted. Because they can be built below grade, they offer similar space-saving benefits to underground detention while also providing water quality treatment.
What to Watch For
Maintenance is non-negotiable with sand filters. The sand media has a finite lifespan and will eventually clog with captured sediment and pollutants. When that happens, the top layer of sand needs to be removed and replaced. Depending on the facility design and pollutant loading, this might be needed every three to five years, or more frequently on high-loading sites.
If the pretreatment chamber isn't maintained (sediment removed regularly), the sand bed clogs much faster. I've seen sand filters that were designed for a five-year media life fail in under two years because the pretreatment was neglected and the full sediment load went straight to the sand.
Sand filters also don't provide flood control detention unless they're specifically designed with additional storage volume above the filter bed. In most cases, a sand filter addresses water quality only, and a separate detention facility is still needed for flood control.
Choosing the Right Facility for Your Project
There's no single best facility type. The right choice depends on your site size, budget, land use, soil conditions, local jurisdiction requirements, and a realistic assessment of who will maintain the facility and how well they'll do it.
Most Front Range projects end up with an EDB because it's the most versatile and cost-effective option for the majority of situations. Bioretention earns its place on commercial and urban projects where distributed treatment and landscape integration add value. Underground detention solves the space problem when the budget allows. Sand filters fill the gap where enhanced treatment is needed in a compact footprint.
The best projects often use a combination: distributed bioretention for water quality near the source, with a regional EDB or underground system for flood control at the downstream end. That layered approach aligns with the MHFD philosophy of managing runoff close to where it's generated, and it often produces a more efficient and more attractive site design than relying on a single large facility.
If you're early in the design process and want help evaluating which approach makes the most sense for your site, that's exactly the kind of conversation that saves money and avoids surprises later.