Why does my subdivision have ponds?

Technically they are not ponds, they are basins. They are part of the Engineered Stormwater Management System (ESMS) that was designed when the subdivision was built. All development (commercial, industrial, residential…)  includes stormwater management as part of the development plan.

What is an Engineered Stormwater Management System?

The ESMS is designed to manage the stormwater that falls on buildings, roads, parking lots, and the land. The purpose is to release the water at a similar rate to the land before development. It also acts as a filter for the pollutants and eroded soil that the stormwater picks up from the developed areas.  

 

Why is developed land different from undeveloped land when it comes to stormwater?

Undeveloped land retains much of its natural soil structure. As such its acts like a big sponge and can absorb water. When land is developed the topsoil and subsoil are manipulated to make it suitable for development of building, roads, landscapes, and parking. When soil is manipulated, it loses its structure. Good soil structure has lots of air spaces that allow for water to be absorbed and transported down towards the water table. Manipulated soils get compacted and loose the airspace as well as the capacity to hold and transport water. Buildings, roads, and parking are impervious to water. Therefore, developed land loses a lot of its capacity as a sponge thus the need for a system to mitigate that difference. The ESMS is a system of open spaces such as naturalized areas, bioswales, lawns, parks, wetlands, detention basins and retention basins that guide and collect the water. All along its journey the stormwater is slowed down to allow some absorption by the soil and for pollutants to be degraded. There may be points along the route that hold the water back such as berms, pipes, or weirs (dams). Ultimately the water is guided to a point where it will leave the property. The rate that it leaves the property is the same rate as before development.

 

What is the difference between a retention basin and a detention basin?

The difference can be thought of in two colors, green and blue. A retention basin hold water for a short period of time and then it dries up. The bottom of the basin is planted and therefore it is green with plants. A detention basin hold water all year and rises with a storm. The water gets released back to the normal water level. The basin is designed to hold water all year long thus it is blue. Retention basins are green bottom and detention basins are blue bottom.

  

What kind of maintenance is needed for the ESMS?

First it is essential that any pipes that handle stormwater be free from obstructions. Inspect them after every storm. Keep the grates in the streets and parking lots free from debris. Keep dense woody plants and invasive species from growing in the stormwater management areas and along the shorelines. Brush and trees create debris that can clog the pipes. Next inspect he pipes to make sure there is no erosion around them from waves or animals like muskrats. Erosion can cause the pipes to pull a part. Fixing broken stormwater pipes is expensive. Promptly remove any trash as it frequently causes the pipes to be blocked. If the shoreline is covered in rip rap (rock) it is important to add more rock where is has eroded. This is expensive but necessary if further erosion is to be prevented. Rip rap is a static system that cannot adapt to the season.  If the shoreline is lawn, then there is probably already erosion. Lawn grasses do not have deep roots necessary, 4” at best, to hold the soil against the waves and the freeze thaw cycle in the winter. The options are to place rip rap along the shore. This is very expensive. The other option is to use native plants with their deep roots (24”-15’) to hold the soil in place. This is a more permanent solution as the native plants are dynamic and can adapt to the changing seasons. Native plantings are cost effective compared to rip rap. If the shoreline is already native plantings, then it is important to keep the invasive species out. Controlled burns help stimulate the native plants.

 

What about wildlife and my ESMS?

Most wildlife is a benefit of having natural spaces in our developments. Some species thought can cause damage. Beaver will damage trees and build dams that block the flow of water through the system. Muskrat burrow into the shorelines and cause erosion. If Muskrat burrow around the concrete pipes they can collapse causing expensive repairs. Muskrat holes are also dangerous to people too if you fall into one. Muskrat and Beaver are best controlled with trapping. The Canada Goose is a frequent resident of these areas. Geese are aggressive when defending their nests. Geese produce copious amounts of feces that foul the water. Geese are best controlled by planting naïve plants where there are large areas of open lawn. Geese leave areas such as native plantings that could hide predators. Carp and Goldfish frequently are found in these ponds. Both are called rough fish. They feed in the bottom sediments and stir them up. The sediments when stirred make the water murky. The resuspended sediments release the nutrients back into the water which feeds algae. Controlling or eradicating the rough fish will help water quality.