Maintenance

 The City of Woodbury has almost 1,700 structural stormwater best management practices (BMPs) within its borders. These BMPs are integral to the overall stormwater system and are designed to control and treat stormwater runoff. They provide storage and capture pollutants from the runoff to decrease nutrient loading in downstream receiving waters such as lakes, rivers or streams. They serve a very critical role in the overall management of our essential resource, water.

Why are BMP’s Important?

  • Water Quality
  • Flood Control
  • Erosion Control
  • Groundwater Protection
  • Habitat & Aesthetic Value

 

The City created a holistic stormwater BMP maintenance plan, finalized in 2021.

The Goals of the plan were to address:

  1. Compliance with Regulatory standards
    1. As a Municipal Separate Storm Sewer System (MS4) permittee, the City must meet requirements as determined by the Minnesota Pollution Control Agency
  2. Annual inspection program
    1. The city inspects 25% of all stormwater pipes, catch basins and structures each year to inform maintenance and budgetary needs
  3. Maintenance of BMP’s
    1. Including but not limited to vegetation maintenance, pond dredging, addressing inspection findings and responding to complaints
  4. Maintaining a BMP Inventory
    1. New BMP’s are added to the asset management software, inspections and maintenance activities are tracked by each BMP.

The results of the BMP plan:

  1. Inspection and Maintenance Framework
    1. While the City has actively inspected and maintained BMP’s through their existence, the framework includes industry standards and BMP types that are slowly becoming more popular (such as iron enhanced sand filters)
  2. Inspection and Maintenance Guides
    1. Similar to the framework, this process formalized what staff inspects and how the inspections translate to determining maintenance
  3. Cost Estimation Model
    1. While Woodbury continues to grow and add BMP’s, this model is a tool to help inform budgetary decisions

What is stormwater runoff?stormwater2

Urban areas contain roads, sidewalks, roofs, and other hard surfaces that are impermeable to water. When rain falls or snow melts on these surfaces, the water drains to storm sewers and eventually to lakes, rivers, and wetlands. Water that drains off impervious surfaces is called stormwater runoff.

Large storm events contribute large volumes of runoff moving at an increased rate, which raises the potential for erosion and flooding downstream. Stormwater runoff also contains pollutants that collect from the impervious surfaces, such as sediment, oil, salt, nutrients associated with fertilizer and plant material like grass and leaves. These pollutants can contaminate lakes, rivers, and wetlands. To reduce these negative impacts, stormwater professionals can implement several practices, including rain gardens, permeable pavement, and green roofs.

Whastormwater1t is a stormwater pond?

Retaining stormwater runoff in a constructed basin or pond is a crucial practice. Water is held in the pond for a period, allowing sediment to settle and release into the water over days instead of hours, which relieves flooding downstream. The most common type of pond is a wet pond, which has a permanent pool of water throughout the year. This pool of water allows for sediment and pollutants associated with that sediment to remain in the wet pond, where they can later be removed and properly disposed.

How does a stormwater pond work?

Water from impermeable surfaces drains to a wet pond. The water level is controlled by an outlet structure. If the water level exceeds the outlet elevation, such as after a heavy rainfall, the water is slowly released through the outlet structure. This diminishes the likelihood of downstream flooding since the water is slowly released downstream rather than accumulating all at once during the rainfall. In the case of extreme rain events, water may flow over the highest elevation, called the emergency overflow. 

What is stormwater maintenance?

Maintenance of the stormwater system is necessary based on specific site characteristics such as watershed size, age of infrastructure and upstream land uses. Pond depths and sediment contamination levels are analyzed prior to maintenance.

Pond maintenance specifically is best completed during the winter months when water and ground is frozen. This limits disturbance to vegetation and wildlife. However, project timelines can still be impacted by weather (i.e., winter rain or sub-zero temperatures).

Stormwater maintenance projects are funded through the Stormwater Utility Fund and not assessed to property owners.