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The major constraints for sizing the basin include the ponding area, the basin area, and the depth of the ponding area. The available space in the open area in the northern portion of the BMP Neighborhood limits the maximum ponding area of the basin. This is controlled by a required 10’ setback from the streets to avoid the infiltration basin being directly connected to pavement foundations to avoid seepage and frost heave concerns, per MPCA design criteria. This additionally provides pretreatment of any direct runoff not routed through the upstream BMPs, allowing for impervious surface disconnection over a grass strip between the street pavement and the infiltration basin. The basin itself will extend the entire width and length of the parcel, less the required 10’ offset in order to maximize stormwater capture from the site. The ponding area will include a shallower 10:1 side slope to the basin bottom to keep the space easily accessible to the public. The sizing parameters for the infiltration basin are described in the Infiltration Basin Sizing Input Parameters table below. Excess stormwater collected in the infiltration basin will be routed through the dry swale system to the remaining downstream treatment train BMPs. | The major constraints for sizing the basin include the ponding area, the basin area, and the depth of the ponding area. The available space in the open area in the northern portion of the BMP Neighborhood limits the maximum ponding area of the basin. This is controlled by a required 10’ setback from the streets to avoid the infiltration basin being directly connected to pavement foundations to avoid seepage and frost heave concerns, per MPCA design criteria. This additionally provides pretreatment of any direct runoff not routed through the upstream BMPs, allowing for impervious surface disconnection over a grass strip between the street pavement and the infiltration basin. The basin itself will extend the entire width and length of the parcel, less the required 10’ offset in order to maximize stormwater capture from the site. The ponding area will include a shallower 10:1 side slope to the basin bottom to keep the space easily accessible to the public. The sizing parameters for the infiltration basin are described in the Infiltration Basin Sizing Input Parameters table below. Excess stormwater collected in the infiltration basin will be routed through the dry swale system to the remaining downstream treatment train BMPs. | ||
− | + | {{:Infiltration Basin Sizing Input Parameters-TT}} | |
These parameters were entered into the MIDS Calculator to evaluate the performance of the BMP. The results are summarize in Table 3.5.5. | These parameters were entered into the MIDS Calculator to evaluate the performance of the BMP. The results are summarize in Table 3.5.5. |
This scenario is for a new residential development in the rural town in central Minnesota. The new development is to be 40 acres with four 10-acre sub-neighborhoods. The upstream neighborhoods have slopes of 2% draining to the downstream southwest neighborhood, hereby referred to as the “BMP Neighborhood.” The basic site conditions for the New Development setting are summarized in Table 3.5.1. For this scenario two treatment train options will be evaluated: a Varied BMP Approach and Stormwater Ponds in Series Approach. The existing site layout for the New Development scenario can be seen in the New Residential Development Scenario Base schematic.
At the southwest corner of the BMP Neighborhood is an outlet to Union Creek, which continues southwest to a small lake. This lake is known to be nutrient impaired with excess levels of phosphorous possibly due to fertilizer use from upstream lawns, gardens and agricultural areas. The use of fertilizers is not uncommon and often necessary as sandy soils are generally not productive for vegetation growth. Interception of nutrients through BMPs before entering the stream system is a preferable method of pollutant reduction for this project. The New Development Scenario Existing Site Condition table below describes the site conditions for this new development.
New Development Scenario Existing Site Conditions
The BMPs selected for the New Development site need to achieve the goals of decreased runoff volume or peak flow rate attenuation and pollutant removal, as well as fit within the site constraints. The New Development Scenario BMP Practice Selection table below discusses the applicability of BMP practices to this site.
New Development Scenario BMP Practice Selection
Infiltrator, filter, swales & strips, and constructed basin BMPs are determined to best address the goals of this project for the New Development scenario. The process of selecting and placing BMPs on a site is typically iterative, working between the site constraints, project goals, and available budget. The approach and considerations for this scenario are discussed in the following sections.
The New Development site constraints will influence successful BMP integration within this residential community. These constraints must be considered with BMP selection and design. Some specific site constraints for this scenario in the new residential development area include:
To meet space allotments, allow for maximum volume and pollutant removal via a centralized collection point, and to promote community recreation, the most viable locations for the BMPs are within the southwesterly quadrant of the neighborhood which is closest to Union Creek, as shown in the New Residential Development Scenario Base schematic. The BMPs should be designed to fit within the already pervious areas to prevent construction to the existing roads. Further considerations for placement of BMPs are the required minimum dimensions for rural residential roads, curbs, sidewalks and right-of-way zones as well as minimum setback requirements.
Step 2 introduced the practices that would best address the goals and site constraints for the New Development scenario as infiltrators, filters, swales & strips, and constructed basin BMPs. Various BMPs may be considered for a New Development setting where the goals of stormwater management are the reduction of runoff volumes and pollutant loads. However, for the New Development setting the efficacy of two treatment train options were evaluated. The first was a traditional Varied BMP Approach and the second was Stormwater Ponds in Series Approach. The selected BMPs are discussed in the following sections.
The following BMPs were evaluated as plausible treatment train options for the Varied BMP Approach in the New Development setting.
Infiltrator: Infiltration Basin
Filter: Dry Swale System
Filter: Iron-Enhanced Sand Filter (Minnesota Filter)
Constructed Basin: Stormwater Pond
The selected BMPs for the Varied BMP Approach include an infiltration basin, a dry swale conveyance system, an iron-enhanced sand filter, and a stormwater pond. The Development Scenario Varied BMP Approach BMP Layout schematic below displays the proposed BMP treatment train layout for the Varied BMP Approach of the New Development Scenario.
The Stormwater Ponds in Series Approach maintains the downstream stormwater pond, however an upstream larger stormwater pond replaces the other BMPs at the location of the former infiltration basin, as depicted in the New Development Scenario Stormwater Ponds in Series Approach BMP Layout schematic below. This stormwater pond is also designed to meet the Level 3 requirements for maximum TSS and TP removal. Although stormwater ponds do not contribute to reduction of runoff volume, they allow for water retention and controlled water release. Volume control prevents unwanted ponding by capturing water in the constructed ponds and releasing it at a rate such that the capacity of the natural soils, vegetation, and downstream waterbody is not overwhelmed in a storm event.
A cost-benefit evaluation of implementing two vs. three stormwater ponds in series was completed. It was determined that two ponds would require less initial capital and future operations and maintenance costs. The MIDS calculator also showed that two ponds would perform to a standard exceeding that of three ponds for TSS and TP removal
BMPs in the New Development scenario have both qualitative and quantitative goals. BMP sizing and placement must consider the quantitative performance goals, but also enhance the aesthetic quality of the community for residents. The MIDS calculator provides a performance goal requirement based on the New Development site conditions. Assumptions included an annual Phosphorus EMC of 0.3 mg/L, and an annual TSS EMC of 54.5 (mg/L). Table 3.5.3 summarizes the existing site runoff, volume and pollutant retention goals.
New Development Scenario Performance Goal
BMP Sizing for each component of a treatment train is an iterative process where available space, performance goals and regulatory requirements must be considered. The BMPs were sized to best meet the project performance goals within the parameters of the site.
Infiltration Basin
The major constraints for sizing the basin include the ponding area, the basin area, and the depth of the ponding area. The available space in the open area in the northern portion of the BMP Neighborhood limits the maximum ponding area of the basin. This is controlled by a required 10’ setback from the streets to avoid the infiltration basin being directly connected to pavement foundations to avoid seepage and frost heave concerns, per MPCA design criteria. This additionally provides pretreatment of any direct runoff not routed through the upstream BMPs, allowing for impervious surface disconnection over a grass strip between the street pavement and the infiltration basin. The basin itself will extend the entire width and length of the parcel, less the required 10’ offset in order to maximize stormwater capture from the site. The ponding area will include a shallower 10:1 side slope to the basin bottom to keep the space easily accessible to the public. The sizing parameters for the infiltration basin are described in the Infiltration Basin Sizing Input Parameters table below. Excess stormwater collected in the infiltration basin will be routed through the dry swale system to the remaining downstream treatment train BMPs.
Infiltration Basin Sizing Input Parameters-TT
These parameters were entered into the MIDS Calculator to evaluate the performance of the BMP. The results are summarize in Table 3.5.5.