Scenario for developing a stormwater treatment train for constructed ponds in new development

Treatment Train Scenarios

New Development & Stormwater Ponds in Series Scenarios

Step 1: Review Project Goals & Site Conditions

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.

New Residential Development Scenario Base

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

Step 2: Review Pollutant Removal Processes & Identify Potential Practices

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.

Step 3: Determine Site Constraints & BMP Placement

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:

• Available Space –The existing road configuration should remain to avoid additional unnecessary costs of redesigning and reconstructing roads. Selected BMPs must fit within the already existing pervious areas.
• Public Integration – New development settings are usually characterized by residential neighborhoods with high interaction between the public and their environment. Selected BMPs should add to the aesthetic value of the neighborhood and be safe for interaction with all ages.
• Utilities –Utilities will have to be constructed or adjusted to meet the requirements of a new residential area. BMP sizing and location should try to avoid major conflict with existing or planned utilities.
• Regulatory Requirements - All local, state, and federal regulatory requirements must be met. The design criteria and recommendations from the Minnesota Stormwater Manual will be followed.

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 4 Select Individual BMPs & Evaluate Range of Performance

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.

Step 4.1: Varied BMP Approach BMP Selection

The following BMPs were evaluated as plausible treatment train options for the Varied BMP Approach in the New Development setting.

Infiltrator: Infiltration Basin

• An infiltration basin allows for a pervious open area that could provide both infiltration and attenuation of stormwater runoff. The basin can incorporate grass vegetation to provide an open activity space for the community during dry weather periods. This will be placed in the northeast corner of the BMP neighborhood allowing for capture of stormwater from the