|Hydromulch at Hmong Alliance church, Maplewood|
While construction has slowed considerably in the District due to frigid winter temperatures, there remain a handful of active sites that continue to be checked on throughout the season. On active construction sites, the transport of sediment via rain or runoff (also known as erosion) can negatively affect nearby water bodies in which the sediment is deposited. The risk of surface water pollution diminishes significantly when the ground is frozen. During frozen conditions, there is little to no runoff so the sediment is forced to stay put. Nevertheless, sites that stay active during the winter are checked on a regular basis.
What about the sites that pause work for the winter? District and state rules require that certain practices be implemented for construction sites that do not stay active during the winter season. Prior to the first heavy snowfall, sites must install Best Management Practices (BMPs) to ensure the open soil stays covered and remains in place when the snow melts in the spring.
Here are a few recommended BMPs that stabilize soil over the winter and help prevent erosion. BMPs are determined based on a variety of site factors and intended uses:
|Straw coverage following a mass grading
at Buerkle Honda, Vadnais Heights
Mulch is used to cover large areas of open soil and has a variety of purposes. Mulch is a soil insulator that retains moisture and helps encourage the growth of native plants and grasses. It also helps prevent erosion by holding the soil in place. Mulch is generally applied in the form of straw or wood chips and can be a cost-effective option for the winter season. Proper application rates are required to ensure maximum benefit. The Minnesota Pollution Control Agency (MPCA) cites a 2 tons/acre or 90% coverage to achieve the proper erosion prevention threshold. The use of mulch is a temporary soil stabilizer. Mulch is required to be replaced at a frequency that depends on site slopes and rainfall amounts/durations until the site is permanently stabilized with vegetation or hard surfaces like buildings and pavement.
2. Erosion blanket
|Erosion blanket with grass starting to sprout on
Hwy 36 project, Maplewood.
Erosion blankets are mats with single or multiple layers that help provide soil stability. Erosion blankets can be synthetic or biodegradable (recommended) and are often used in combination with an approved seed mix. The blanket keeps the soil in place and promotes vegetative growth when used with seed. Erosion blanket is often used on sites with steep slopes. The blankets are anchored in the soil and are generally better capable of preventing erosion than mulch.
A combination of straw and erosion blanket on
the Lake Phalen waterfall project, St. Paul
Hydraulic soil stabilizers, also referred to as hydromulch, act as a glue for the soil that helps it stay in place. Hydromulch is sprayed from a large hose and can be applied with or without seed. Hydromulch applied with seed is a better, more permanent stabilization method. For sites that are simply pausing for the winter, hydromulch without seed can be applied more cheaply and will serve as a temporary solution. Without seed, hydromulch will wear down after a number of rain events and may need to be re-applied.
The appropriate method for stabilizing soil over the winter depends on a variety of site factors. Whatever the method, these practices help prevent pollution to nearby lakes and streams by holding the soil in place until work can continue in the spring.