7.0 Water Quality Improvements
Comprehensive stormwater management includes a wide variety of rainfall scenarios from exceedingly rare peak flow events to statistically average events to brief afternoon rain showers. The best plans acknowledge all phases of stormwater management and provide benefits throughout the watershed and throughout the range of recurrence intervals.
Constraints to this approach are clearly financial, the major capital improvement projects are expensive and the minor water quality BMPs can become costly pieces of infrastructure to maintain in working order. However, there are localized water quality improvements that can be implemented in certain watersheds in specific communities. These smaller scale projects fit within smaller towns well because a reasonable implementation density (i.e. participation percentage) is within reach of public-private partnerships.
Other larger scale water quality projects are just as important to the overall stream health. Larger ponds integrated into public spaces are a typical method of capturing a large water quality volume. Ordinances that require water quality implementation for new and redevelopment can be equally important.
7.1 Small Scale Water Quality Improvements
7.1.1 Tree Wells
The downtown area could benefit from design of landscape and streetscape improvements that incorporate water quality treatment. One example of this multi-purpose streetscape is a tree-well water quality installation. Additional information is provided by the EPA’s “Stormwater to Street Trees” informational guide.
The tree wells have an advantage of water quality treatment in the highly impervious main street area, but also have application in side street and park areas. In any case, the tree wells treat for water quality, but from a volumetric standpoint are a small scale solution.
7.1.2 Rain Gardens
Another small scale option for targeted water quality is installation of rain gardens. Generally accepted as a means of treating roof top runoff from private property downspouts, there are applications for rain gardens in commercial and municipal applications. A commercial property or municipal building could route downspouts to a street side rain garden, with proper consideration for saturation of soils resulting impact on roadways, sidewalks, icing, and maintenance. A cul-de-sac or small parking area could be routed to an infiltration basin or rain garden for additional water quality treatment. Additional design and maintenance information is provided by UDFCD.
7.1.3 Rainwater Harvesting
Recent changes in state law (HB 1005) allow for private properties to harvest a small amount of rainwater from their rooftop for allowable uses (irrigation) on their property. Typically, a 55 gallon barrel or cistern is connected to roof downspouts to collect rainfall and snowmelt. Although rooftop runoff is not the largest source of stormwater pollution, it can still have a beneficial impact on urban water quality. The dust and wind blown litter that reaches rooftops can be captured in rain water harvesting systems. More importantly, the peak runoff events from the impervious surfaces are attenuated, or delayed, from entering the municipal stormwater system. This reduces the volume of stormwater treated by downstream systems.
7.2 Large Scale Water Quality Improvements
7.2.1 Confluence Area Water Quality Pond
At the time of this report, the ongoing confluence area and St. Vrain restoration projects are making improvements in and around the rivers. One area that could have an advantageous siting for a water quality pond is the east end of Prospect in the confluence area. If other project designs allow for a pond to be constructed in this area, the outlet could be configured to treat a water quality capture volume for the confluence area basin.
There are other water quality pond locations, but the confluence area was identified as the largest potential pond with urbanized runoff. The existing pond at Eagle Canyon could be studied for reconfiguration, but the impervious area is relatively low in that overall basin. Large water quality ponds along the northern side of Town near the river would treat the most urbanized stormwater runoff. However, a property and topographic review of the area could not identify areas large enough to capture a full water quality capture volume. Sandstone Park was the largest open space parcel, but would require significant grading and storm sewer installation that would significantly limit the park uses. Other upstream areas are available, but generally treat only the undeveloped portions of town.
7.2.2 Streambank Wetlands
There are several good locations for water quality treatment along the St. Vrain, particularly downstream of Lyons Valley stormwater outlet structures. The side channels north and south of McConnell Drive as well as the Bohn Park outfall east of 2nd Avenue can treat stormwater runoff upstream of the St. Vrain main channel. Wetland plantings in a specifically designed low velocity channel can absorb pollutants conveyed through nuisance and first flush stormwater events, while allowing larger runoff events to pass through the channel. The streambank wetlands are similar to the St. Vrain restoration work upstream, but designed and managed on a smaller scale to adapt to the side tributary flows.
7.2.3 Pervious Pavers
The reduction of impervious area is a direct means of increasing infiltration and interrupting the transmission of water borne pollutants in the urbanized watershed. For the Main Street and downtown areas, the addition of pervious pavers can be a good way to reduce impervious area. There are several candidate locations in parking areas, sidewalks, plaza areas that could infiltrate runoff before it reaches the curb and gutter system. However, there are also many lessons learned on maintenance of pervious pavement surfaces, specifically related to the winter environment and non-infiltrating soils of Colorado. The impact of a snow plow on pavers, expansion of clay soils creating ponding in pervious asphalt, or the freeze thaw action on pervious concrete surfaces are all documented issues. Ultimately, pervious pavement surfaces work well when combined with traditional pavement surfaces for high traffic, heavy load, and high maintenance pathways. The UDFCD has a good worksheet describing the opportunities and limitations of pervious surfaces.
7.3 Outfall Water Quality
There is a separate category of water quality treatment that is particularly suited to the Town of Lyons – outfall water quality. Given the limitations on property, soils, and age of the infrastructure in Lyons the outfall pipes discharge directly into the rivers. In many ways, the outfall is the last option for water quality treatment in a particular watershed. Other communities along Front Range have investigated these same challenges and found a few specific treatment options that have unique water quality advantages. It takes a very unique topographic condition with a small tributary basin for these elements to meet a full water quality capture volume. But, in all cases, the advantage of intercepting even the ‘urban drool’ nuisance flows in a small, maintainable, vegetated basin improves even the perception of a standard metal culvert dribbling directly into the river.
A list of outfall water quality options are detailed in the appendix. Each option is suited for different outfall locations and uses. The infiltration basin concepts are essentially miniaturized infiltration basins with a design suitable for curbside maintenance. Whereas the level spreaders are more complicated structures diverting low flows into hillside infiltration laterals that can irrigate and help sustain native vegetation on the stream banks. These concepts require additional design based on site specific conditions, but can be a starting point for high visual impact outfall improvement projects.