Monitoring the Success of Infiltration Requirements
Combining regulation with outreach and demonstration sites
Alternative stormwater techniques are becoming more widely used in jurisdictions around the country as local, state, and federal regulations increase. Infiltration, filtration, and bioretention practices are being considered more often as an option or a supplement to more traditional wet-pond treatments. These alternatives are not always easily implemented, however, due to resistance from the development community, skepticism from the planning and engineering community, lack of regulation, climate, soil conditions, and inexperience in construction. Implementation in suburban Minnesota has been slow to progress. Carver County, MN, a rapidly developing Twin Cities suburban area, has been working on this implementation since 2002 and has learned that a combination of uniform regulation, flexible techniques, demonstration, monitoring, and a willingness to evolve has proved successful.
This article covers the results of monitoring; the methods used in outreach to the stormwater management audience (developers, engineers, contractors, and landowners); and the types of practices installed, including the use of “pond shelves.” The approach of combining regulations with outreach, technical assistance, stakeholder input, and demonstration sites has led to successful implementation of innovative stormwater practices and has applicability to other jurisdictions.
Carver County Overview
Carver County is located approximately 20 miles southwest of Minneapolis, MN, and has been experiencing the type of rapid growth associated with outer-ring suburbs in most major US metropolitan areas. The five-year growth rate is approximately 21% (2000–2005) and ranks the county in the top 100 for growth in the US. Land prices in the 11 cities and 10 rural towns are rising rapidly, causing a “land rush” of developers and investors. Much of the development is residential, but commercial and industrial acreage is beginning to increase at a rapid pace. The area receives more than 30 inches of precipitation each year (nearly 50% of that in summer storm events), with frozen cold-climate conditions possible for five months.
Based on Minnesota state law, the metro-area county is the water management authority for more than 80% of the geographic area and has been implementing a water management plan since 2001. The plan covers many issues, including the protection of water resources through urban stormwater management.
The county is a municipal separate storm sewer system (MS4) community and must meet all Phase II requirements of the National Pollutant Discharge Elimination System. The county is also charged with the development of total maximum daily load plans to address 26 lakes and streams that are on the state’s list of impaired waters, mostly for excess nutrients.
The county’s goal is to ensure protection of the water resource and accommodate a variety of development patterns. In short, Carver County is an ideal testing ground for trying to meet regulations, curb stormwater impacts, and accommodate rapid growth.
Regulatory Standards
The county Water Management Rules, adopted in 2002, required, among other things, increased treatment in addition to traditional National Urban Runoff Program–style wet ponds. In developing these standards, the county determined that wet ponds alone were not sufficiently protecting water resources from the increased pollutants and runoff amounts associated with increases in impervious areas.
The standards require that the first 0.34 inch of storm runoff from new impervious areas be treated through infiltration, filtration, or bioretention. These standards are in addition to total phosphorus removal, total sediment removal, and rate control, as shown in Table 1.
Demonstration Sites
To correspond with the adoption and implementation of the Water Management Rules, the county developed two demonstration sites at county facilities: the Environmental Center and the Public Works Facility. The goal of the sites was to reduce impacts of runoff from those sites, provide a working model of alternative treatment methods, and educate stakeholders and citizens.
Environmental Center
The county’s Environmental Center consists of a public recycling facility on a 5-acre site with impervious area totaling approximately 1.5 acres. The site was selected due to the high citizen visibility (approximately 16,000 visitors per year) and its lack of pre-existing treatment. The building site is bordered by wetlands on two sides; has some wooded areas; and previously discharged most impervious runoff to stormwater inlets on the street, which then emptied into the adjacent wetlands. Approximately 70% of this stormwater runoff was redirected to flow to two infiltration rain gardens. The rain garden design incorporated a rock trench for storage and included native planting (Figures 1 and 2). Because one of the site goals was to create a nearly invisible treatment system, the native plants were installed as plugs (no seed) to ensure a quicker and more reliable vegetated area. Monitoring results for these rain gardens are discussed later in the article.
Public Works Facility
The second demonstration site was a filtration basin installed at the new County Public Works Facility.
The site consists of 45 acres with approximately 11 acres of impervious area. Future construction of county facilities will occur on the remainder of the site. The site was previously used for tilled row crop agriculture and has soils consisting of tight clays (mostly hydrogroup B or C). The main goal of the designed stormwater system was to treat stormwater runoff while incorporating the system into the surrounding landscape. The stormwater is directed through a grass waterway to a dry infiltration pond/basin, which is designed to provide treatment up to the 100-year/24-hour rain event on the site (Figure 3, left). Infiltration occurs through underdrains—a series of rock-filled trenches holding perforated drain tile. These trenches are covered with a sand and soil mixture, which acts as a surface sand filter, allowing infiltration yet sustaining plant material. Underdrains or tiles outlet the pond in the same location as emergency overflows (Figure 3, right). Native plants within the dry pond are incorporated with plantings on the remainder of the site, including the grass waterways and areas around the parking lot. The system is used in conjunction with other stormwater mitigation in and around the parking lot and building area, including sheet flow runoff to grass swales, filter strips, native plantings, and small-rain-event capture (no curb and gutter). Monitoring results for the system are discussed later in the article.
Outreach
Both of the demonstration sites provided county staff the ability to promote functional alternative designs through tours, images, site signage, workshops, and other educational materials. The county has held annual workshops with the development and engineering communities since 2002 to promote altenative designs and to collect feedback and input from these groups on the pros and cons of utilizing alternative designs. The input received at these venues has been instrumental in making revisions to the Water Management Rules, in particular the infiltration requirements. These workshops have also proved valuble in the installation of alternative treatments throughout the county, because site designs and construction practices are not being done as an afterthought or “on the fly.”
Two other elements have been used to intentionally create better implementation of alternative stormwater practices: the Development Review Team (DRT) and construction assistance from the Carver Soil & Water Conservation District (SWCD).
The DRT process involves a meeting of the county, local city staff, the developer, and all of the involved engineers. This meeting usually occurs prior to any plat approval and focuses on stormwater and natrual resources onsite. Many of the stormwater issues as well as potential alternative designs are discussed at this meeting, saving significant review time.
SWCD assistance consists of ensuring that staff are present onsite to assist contractors with field installation. Many times, the contractors are unfamliar with the design or have ideas to ensure a more functional design. Onsite SWCD assistance can ensure that the plans may be altered or tweaked without compromising the intent of the system.
Treatment Types
Since the inception of the county standards, approximately two dozen systems have been installed, with another three dozen planned for installation. The types of treatment systems installed can be categorized as follows: rain gardens, filtration basins and swales, infiltration basins, bioretention basins, and filtration pond shelves.
Rain gardens, for the purposes of this discussion, are described as small capture basins with a high aesthetic vegetated appearance and some infiltration. For this reason, the application of rain gardens has not been widespread around the county, although a few higher-density residential sites have incorporated a design similar to that used at the Environmental Center. Many sites have incorporated the rain garden concept but have added a filtration component, so for this discussion those installations are described as filtration basins.
True infiltration basins have been limited to areas of the county with sandy soils (hydrogroup A)—a relatively small area. Two basins of this type, which rely on natural drawdown only, have been installed.
Bioretention areas are described here as a combination of filtration, rain gardens, and constructed wetlands. The county categorizes these basins as highly vegetated with wetland-type plants. These systems are becoming more popular because of their ability to sustain wetter conditions.
Filtration basins (similar to that at the county’s Public Works Facility described above) have become widely used because of the nature of heavier clay soils in the area. The basin incorporates a trench containing a drain tile and surrounded by aggregate material and engineered soil. The basin is vegetated and is designed to draw down in three to four days (Figure 4 (Image courtesy of Minnesota Pollution Control Agency)). Filtration swales are an adaptation of the filtration basin but are used in areas with space restrictions or linear projects. They essentially take a grass swale and add a trench/drain tile component.
Filtration pond shelves have also been installed in a variety of locations due to their ability to fit within a typical wet-pond footprint (Figure 5). These pond shelves provide a water-quality benefit and some volume control benefit, and they have been successful in meeting the additional challenges of infiltration standards in the Carver County watershed, including tight clay soils and effects of a cold winter climate.
Monitoring
As part of the implementation program, the county has been monitoring installed sites since 2002. Due to resource constraints, the county’s approach was to visually monitor as many sites as possible for runoff control, function, aesthetics, and ease of construction. For a few select sites, water-quality data were collected. The overall goal was and is to determine the applicability and functionality of alternative practices in the county.
Table 2 provides a summary of the results for the sites on which data were available. Total suspended solids (TSS) removal was very high in two of the three sites, and total phosphorus removal was recorded as moderate at the one site where levels were measurable. The PW Facility data do not reflect several hundred feet of grass swale treatment prior to the inflow sampling point. The variations in data are assumed to be due to the “young” nature of the systems and the large differences in storm events when samples were taken.
Table 3 shows the observational data for the various sites monitored in 2004 and 2005. Many of the sites were installed in 2003 or 2004 and had ongoing function problems due to a constrcution error when the site was installed. (An example of this is the Office Townhomes site, which was connected to the municipal storm sewer at an incorrect elevation, leaving 6 inches of standing water at all times.) The variance in drawdown times can be attributed to two back-to-back events during the sampling years of more than 4 inches (i.e., the basins were still drawing down when the next storm event occurred). The drawdown time does not reflect the duration of tile flow (i.e., extension of hydrograph), which extends base flow times beyond typical wet-pond bouncing. The biggest problem with the sites was vegetation. Seeding has proved to be an inefficient way to establish native plants, particularly due to the short growing season. Seed needs to germinate, survive heavy rain conditions, and stabilize prior to frost conditions. In some cases, the water-quality benefits existed, but aesthetically the areas were lacking.
Table 4 shows that filtration methods (especially the pond shelf) have become the preference to meet the county standards. The number and type of practice installed by year is shown in the table.
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Conclusions
There are several points learned from Carver County’s experience:
- Regulations and uniform rules provide incentive. County requirements ensured that alternative practices would happen. Despite the good intentions of many developments, the construction of these practices would likely not have occurred without regulation.
- Flexibility is essential. Each development plan, associated party, and drainage pattern is unique. Staff need to be open to trying new ideas and working within the regulations. The standard sets the goal; all parties involved need to figure out how to get there. In short, one size does not fit all.
- Demonstration sites show intent. Construction of demonstration sites shows a level of seriousness when regualtions are being implemented. They are highly effective in persuading parties that sites can be built and will work. This being said, demonstration sites alone may not be an effective persuader (i.e., without required standards).
- Outreach efforts save time. The availablity of staff to conduct predevelopment DRT meetings and work with contractors in the field has proven to be worthwhile. The sites with the most problems were those where pre-meetings were not held and staff was not assisting in the field.
- Monitoring is key. Because the short- and long-term effectiveness of these practices is not known (both the general application and the site-specific installation), even low-cost monitoring is essential. If water-quality sampling is not available, regular observation is needed to adjust standards and guidelines.
- Regulations must evolve. Based on monitoring, adjustments to regulations need to occur. In Carver County’s case, rule revisions allowed more flexibility in meeting the treatment standard.
- Combination systems appear more effective. The most effective systems appear to be those that “multitreat”: filtration basin and wet pond, filtration swale and basin, and multicelled rain gardens. Filtration basins alone appear to function only as well as or slightly better than wet ponds alone.
- Plugs and/or mature shrubs work best. In most cases, the aesthetic issues for these basins are important. Using plugs or semimature shrubs and trees is much preferred over seed in a cold climate or tight soil setting.
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Author's Bio: Paul E. Moline, AICP, is the Carver County watershed administrator in Chaska, MN.
January-February 2007
Monitoring the Success of Infiltration Requirements
Combining regulation with outreach and demonstration sites
Alternative stormwater techniques are becoming more widely used in jurisdictions around the country as local, state, and federal regulations increase. Infiltration, filtration, and bioretention practices are being considered more often as an option or a supplement to more traditional wet-pond treatments. These alternatives are not always easily implemented, however, due to resistance from the development community, skepticism from the planning and engineering community, lack of regulation, climate, soil conditions, and inexperience in construction. Implementation in suburban Minnesota has been slow to progress. Carver County, MN, a rapidly developing Twin Cities suburban area, has been working on this implementation since 2002 and has learned that a combination of uniform regulation, flexible techniques, demonstration, monitoring, and a willingness to evolve has proved successful.
This article covers the results of monitoring; the methods used in outreach to the stormwater management audience (developers, engineers, contractors, and landowners); and the types of practices installed, including the use of “pond shelves.” The approach of combining regulations with outreach, technical assistance, stakeholder input, and demonstration sites has led to successful implementation of innovative stormwater practices and has applicability to other jurisdictions.
Carver County Overview
Carver County is located approximately 20 miles southwest of Minneapolis, MN, and has been experiencing the type of rapid growth associated with outer-ring suburbs in most major US metropolitan areas. The five-year growth rate is approximately 21% (2000–2005) and ranks the county in the top 100 for growth in the US. Land prices in the 11 cities and 10 rural towns are rising rapidly, causing a “land rush” of developers and investors. Much of the development is residential, but commercial and industrial acreage is beginning to increase at a rapid pace. The area receives more than 30 inches of precipitation each year (nearly 50% of that in summer storm events), with frozen cold-climate conditions possible for five months.
Based on Minnesota state law, the metro-area county is the water management authority for more than 80% of the geographic area and has been implementing a water management plan since 2001. The plan covers many issues, including the protection of water resources through urban stormwater management.
The county is a municipal separate storm sewer system (MS4) community and must meet all Phase II requirements of the National Pollutant Discharge Elimination System. The county is also charged with the development of total maximum daily load plans to address 26 lakes and streams that are on the state’s list of impaired waters, mostly for excess nutrients.
The county’s goal is to ensure protection of the water resource and accommodate a variety of development patterns. In short, Carver County is an ideal testing ground for trying to meet regulations, curb stormwater impacts, and accommodate rapid growth.
Regulatory Standards
The county Water Management Rules, adopted in 2002, required, among other things, increased treatment in addition to traditional National Urban Runoff Program–style wet ponds. In developing these standards, the county determined that wet ponds alone were not sufficiently protecting water resources from the increased pollutants and runoff amounts associated with increases in impervious areas.
The standards require that the first 0.34 inch of storm runoff from new impervious areas be treated through infiltration, filtration, or bioretention. These standards are in addition to total phosphorus removal, total sediment removal, and rate control, as shown in Table 1.
Demonstration Sites
To correspond with the adoption and implementation of the Water Management Rules, the county developed two demonstration sites at county facilities: the Environmental Center and the Public Works Facility. The goal of the sites was to reduce impacts of runoff from those sites, provide a working model of alternative treatment methods, and educate stakeholders and citizens.
Environmental Center
The county’s Environmental Center consists of a public recycling facility on a 5-acre site with impervious area totaling approximately 1.5 acres. The site was selected due to the high citizen visibility (approximately 16,000 visitors per year) and its lack of pre-existing treatment. The building site is bordered by wetlands on two sides; has some wooded areas; and previously discharged most impervious runoff to stormwater inlets on the street, which then emptied into the adjacent wetlands. Approximately 70% of this stormwater runoff was redirected to flow to two infiltration rain gardens. The rain garden design incorporated a rock trench for storage and included native planting (Figures 1 and 2). Because one of the site goals was to create a nearly invisible treatment system, the native plants were installed as plugs (no seed) to ensure a quicker and more reliable vegetated area. Monitoring results for these rain gardens are discussed later in the article.
Public Works Facility
The second demonstration site was a filtration basin installed at the new County Public Works Facility.
The site consists of 45 acres with approximately 11 acres of impervious area. Future construction of county facilities will occur on the remainder of the site. The site was previously used for tilled row crop agriculture and has soils consisting of tight clays (mostly hydrogroup B or C). The main goal of the designed stormwater system was to treat stormwater runoff while incorporating the system into the surrounding landscape. The stormwater is directed through a grass waterway to a dry infiltration pond/basin, which is designed to provide treatment up to the 100-year/24-hour rain event on the site (Figure 3, left). Infiltration occurs through underdrains—a series of rock-filled trenches holding perforated drain tile. These trenches are covered with a sand and soil mixture, which acts as a surface sand filter, allowing infiltration yet sustaining plant material. Underdrains or tiles outlet the pond in the same location as emergency overflows (Figure 3, right). Native plants within the dry pond are incorporated with plantings on the remainder of the site, including the grass waterways and areas around the parking lot. The system is used in conjunction with other stormwater mitigation in and around the parking lot and building area, including sheet flow runoff to grass swales, filter strips, native plantings, and small-rain-event capture (no curb and gutter). Monitoring results for the system are discussed later in the article.
Outreach
Both of the demonstration sites provided county staff the ability to promote functional alternative designs through tours, images, site signage, workshops, and other educational materials. The county has held annual workshops with the development and engineering communities since 2002 to promote altenative designs and to collect feedback and input from these groups on the pros and cons of utilizing alternative designs. The input received at these venues has been instrumental in making revisions to the Water Management Rules, in particular the infiltration requirements. These workshops have also proved valuble in the installation of alternative treatments throughout the county, because site designs and construction practices are not being done as an afterthought or “on the fly.”
Two other elements have been used to intentionally create better implementation of alternative stormwater practices: the Development Review Team (DRT) and construction assistance from the Carver Soil & Water Conservation District (SWCD).
The DRT process involves a meeting of the county, local city staff, the developer, and all of the involved engineers. This meeting usually occurs prior to any plat approval and focuses on stormwater and natrual resources onsite. Many of the stormwater issues as well as potential alternative designs are discussed at this meeting, saving significant review time.
SWCD assistance consists of ensuring that staff are present onsite to assist contractors with field installation. Many times, the contractors are unfamliar with the design or have ideas to ensure a more functional design. Onsite SWCD assistance can ensure that the plans may be altered or tweaked without compromising the intent of the system.
Treatment Types
Since the inception of the county standards, approximately two dozen systems have been installed, with another three dozen planned for installation. The types of treatment systems installed can be categorized as follows: rain gardens, filtration basins and swales, infiltration basins, bioretention basins, and filtration pond shelves.
Rain gardens, for the purposes of this discussion, are described as small capture basins with a high aesthetic vegetated appearance and some infiltration. For this reason, the application of rain gardens has not been widespread around the county, although a few higher-density residential sites have incorporated a design similar to that used at the Environmental Center. Many sites have incorporated the rain garden concept but have added a filtration component, so for this discussion those installations are described as filtration basins.
True infiltration basins have been limited to areas of the county with sandy soils (hydrogroup A)—a relatively small area. Two basins of this type, which rely on natural drawdown only, have been installed.
Bioretention areas are described here as a combination of filtration, rain gardens, and constructed wetlands. The county categorizes these basins as highly vegetated with wetland-type plants. These systems are becoming more popular because of their ability to sustain wetter conditions.
Filtration basins (similar to that at the county’s Public Works Facility described above) have become widely used because of the nature of heavier clay soils in the area. The basin incorporates a trench containing a drain tile and surrounded by aggregate material and engineered soil. The basin is vegetated and is designed to draw down in three to four days (Figure 4 (Image courtesy of Minnesota Pollution Control Agency)). Filtration swales are an adaptation of the filtration basin but are used in areas with space restrictions or linear projects. They essentially take a grass swale and add a trench/drain tile component.
Filtration pond shelves have also been installed in a variety of locations due to their ability to fit within a typical wet-pond footprint (Figure 5). These pond shelves provide a water-quality benefit and some volume control benefit, and they have been successful in meeting the additional challenges of infiltration standards in the Carver County watershed, including tight clay soils and effects of a cold winter climate.
Monitoring
As part of the implementation program, the county has been monitoring installed sites since 2002. Due to resource constraints, the county’s approach was to visually monitor as many sites as possible for runoff control, function, aesthetics, and ease of construction. For a few select sites, water-quality data were collected. The overall goal was and is to determine the applicability and functionality of alternative practices in the county.
Table 2 provides a summary of the results for the sites on which data were available. Total suspended solids (TSS) removal was very high in two of the three sites, and total phosphorus removal was recorded as moderate at the one site where levels were measurable. The PW Facility data do not reflect several hundred feet of grass swale treatment prior to the inflow sampling point. The variations in data are assumed to be due to the “young” nature of the systems and the large differences in storm events when samples were taken.
Table 3 shows the observational data for the various sites monitored in 2004 and 2005. Many of the sites were installed in 2003 or 2004 and had ongoing function problems due to a constrcution error when the site was installed. (An example of this is the Office Townhomes site, which was connected to the municipal storm sewer at an incorrect elevation, leaving 6 inches of standing water at all times.) The variance in drawdown times can be attributed to two back-to-back events during the sampling years of more than 4 inches (i.e., the basins were still drawing down when the next storm event occurred). The drawdown time does not reflect the duration of tile flow (i.e., extension of hydrograph), which extends base flow times beyond typical wet-pond bouncing. The biggest problem with the sites was vegetation. Seeding has proved to be an inefficient way to establish native plants, particularly due to the short growing season. Seed needs to germinate, survive heavy rain conditions, and stabilize prior to frost conditions. In some cases, the water-quality benefits existed, but aesthetically the areas were lacking.
Table 4 shows that filtration methods (especially the pond shelf) have become the preference to meet the county standards. The number and type of practice installed by year is shown in the table.
Conclusions
There are several points learned from Carver County’s experience:
- Regulations and uniform rules provide incentive. County requirements ensured that alternative practices would happen. Despite the good intentions of many developments, the construction of these practices would likely not have occurred without regulation.
- Flexibility is essential. Each development plan, associated party, and drainage pattern is unique. Staff need to be open to trying new ideas and working within the regulations. The standard sets the goal; all parties involved need to figure out how to get there. In short, one size does not fit all.
- Demonstration sites show intent. Construction of demonstration sites shows a level of seriousness when regualtions are being implemented. They are highly effective in persuading parties that sites can be built and will work. This being said, demonstration sites alone may not be an effective persuader (i.e., without required standards).
- Outreach efforts save time. The availablity of staff to conduct predevelopment DRT meetings and work with contractors in the field has proven to be worthwhile. The sites with the most problems were those where pre-meetings were not held and staff was not assisting in the field.
- Monitoring is key. Because the short- and long-term effectiveness of these practices is not known (both the general application and the site-specific installation), even low-cost monitoring is essential. If water-quality sampling is not available, regular observation is needed to adjust standards and guidelines.
- Regulations must evolve. Based on monitoring, adjustments to regulations need to occur. In Carver County’s case, rule revisions allowed more flexibility in meeting the treatment standard.
- Combination systems appear more effective. The most effective systems appear to be those that “multitreat”: filtration basin and wet pond, filtration swale and basin, and multicelled rain gardens. Filtration basins alone appear to function only as well as or slightly better than wet ponds alone.
- Plugs and/or mature shrubs work best. In most cases, the aesthetic issues for these basins are important. Using plugs or semimature shrubs and trees is much preferred over seed in a cold climate or tight soil setting.
-