Stormwater generally refers to water running off the landscape after a precipitation event. In an undeveloped state, stormwater runoff is a much smaller constituent of the water budget, but as land becomes increasingly urbanized, stormwater runoff increases as well.

This change increases runoff volumes, peak discharges, runoff velocities, flooding, and lower baseflows in our streams and rivers. This has an impact on erosion, water quality, and aquatic habitat. You can learn more about why stormwater matters by visiting the Minnesota Pollution Control Agencies Stormwater Program webpage.

In 1983, the Environmental Protection Agency published the Final Report of the Nationwide Urban Runoff Program and the Results of the Nationwide Urban Runoff Program. These findings recognized the importance of urban runoff regarding the quality of our streams and lakes.

As a result of this program, local government units began to construct stormwater detention ponds to treat stormwater and control the rates at which water leaves a site. As the science of stormwater management has grown, so have the practices used to manage and treat runoff.

As a homeowner and resident, you impact the health of Chanhassen water bodies. Chemicals, lawn clippings, fallen leaves, and sediment can be introduced directly into the pond or flow into the street and will travel to our lakes, streams, and wetlands through storm sewers. These components draining into our water bodies result in excess nutrients and pollutants that can cause ugly algae blooms and unsafe drinking and swimming water.

Stormwater ponds are one of many types of best management practices (BMPs). They remove pollutants from stormwater runoff before discharging the water downstream. This process is referred to as "water quality treatment."

The primary way in which a stormwater pond provides water quality treatment is through the settling of suspended materials such as sediment, grit, and organic material sediment. Water runoff from impervious surfaces, and everything collected along with it, enters a stormwater pond through a series of pipes and channels. As the water sits, the sediment, phosphorus, and other pollutants have time to settle to the bottom and be absorbed by aquatic plants. Clean water then moves out of the pond through a controlled outlet system and eventually into our lakes, rivers, and streams. The sediment builds up over time and decreases the pond's capacity to capture pollutants until, eventually, the pond needs to be cleaned out.

Infrastructure is regularly maintained to preserve the functionality of the BMP, but a complete cleanout becomes necessary at some point. The city must maintain the functionality of these ponds to protect the water quality of the water resources within Chanhassen.

Benefits of Stormwater Ponds:

• Trapping sediment
• Trapping pollutants collected from impervious surfaces
• Rate control
• Flood relief
• Erosion control 

The city maintains over 250 stormwater ponds. Per MS4 permit requirements, the city inspects 20% of the stormwater ponds annually and all of the stormwater ponds throughout the city within five-year rotations. The city uses two scoring systems to prioritize which ponds should be inspected. One online tool is the Stormwater Pond Asset Management Program (SWAMP). SWAMP analyzes the characteristics of each pond, including construction year, pond volume, drainage area, and original design. SWAMP then weighs these characteristics and generates a score that ranges from 0-100, where 100 would be a newly constructed pond, and 0 would be a completely dysfunctional pond.

The second assessment rating is an Overall Condition Index (OCI) which is also a score that ranges from 0-100. city field staff perform this assessment, retrieving a comprehensive survey of pond depth, infrastructure, and the status of the area surrounding the pond. Using both scores ensures an efficient and accurate evaluation.

Every year, the scores are updated and evaluated. The city then selects approximately 5-10 ponds for detailed studies to determine which ponds should be included in the maintenance projects. The city has $750,000 budgeted in 2023 for stormwater maintenance projects, including pond clean-out projects.  

The information gathered from the detailed studies includes:

• Permitting
• Access-work with property owners to obtain a right of entry agreement 
• Cost
• Water quality and other benefits
• Stormwater infrastructure to maintain
• Feasibility report 

An important note—the city does not own all stormwater ponds in Chanhassen. Other agencies, such as Carver County and MnDOT, are also MS4 permittees; as such, they have similar reporting and maintenance requirements as the city. Chanhassen does not perform maintenance on privately owned stormwater infrastructure. Instead, the city enters an Operation and Maintenance Agreement (O&M) to ensure the obligation of maintenance falls on the private owner. 

Project Plans and Specification Development

Depending on the cost of each pond cleanout, 4-6 ponds are selected, and the final design process can begin. Engineering plans, documents, and cost estimates are created during the final design process for the selected ponds. The project plans include the specifics of the sediment removal areas and the stormwater infrastructure to be maintained or replaced. The engineers calculate quantities and detailed cost estimates for the work.

The project plans also outline the final grading of the site and how the disturbed area will be re-established. In general, the engineers design the project to restore the area around the pond to the pre-project conditions. The city then reviews project plans and construction documents. Once approved, the construction plans and documents are compiled and sent to construction contractors for competitive bids. 

Timeline and Schedule

Stormwater pond cleanout projects typically begin construction in December and January. Frozen ground conditions are preferred because it is easier and cleaner to work with frozen material rather than wet muck. Below is a general timeline of the construction process:

• The city opens up the projects for contractors to bid on.
• The city selects a contractor to do the work, and the city approves.
• The contractor mobilizes and begins with tree removal and vegetation grubbing around the access route and grading limits.
• De-watering and grading take place. Excavated sediment is exported off-site. Topsoil is imported as needed for restoration purposes.
• New stormwater infrastructure may be added, and/or existing infrastructure is rehabilitated.
• A temporary restoration is implemented until weather permits final restoration and landscaping. Tree planting is also done at this time.

This process takes about 2-3 months and can vary depending on the weather. Punchlist items and restorations sometimes take place in the spring when weather permits. Pond cleanout schedules and timelines are updated regularly on the city website.

What You Need to Know

Whether a pond cleanout is happening (or going to happen) in your backyard or you pass by a construction site on your way to work, there are some important things you need to know:

• Be careful around construction—large equipment, such as excavators, dump trucks, and bobcats, will be actively moving around the site. Be sure to drive slowly around these projects.
• Let the city know about private utilities in the area—some common examples include invisible dog fences, satellite dish lines, sprinkler systems, and pool heating systems. The engineers will work with the information to avoid impacts if possible. 
• Let the city know about issues in the greater pond area (erosion concerns, invasive species, etc.)