Soil erosion and sediment runoff to waterways are significant problems in Minnesota. According to the Minnesota Pollution Control Agency, approximately 30 percent of the state’s rivers and streams are impaired by sediment. Poorly managed construction sites can be substantial sediment sources to these surface waters. Pine County seeks to address these impacts by requiring:
- carefully crafted sediment and erosion control plans designed to reduce polluted runoff;
- implementation and updating of the plan;
- operation and maintenance of runoff reduction measures until the site is stabilized; and
- post-construction management plan
Development must be planned and conducted in a manner that will minimize the extent of disturbed areas, run-off velocities, erosion potential, and reduce and delay run-off volumes. Disturbed areas must be stabilized and protected as soon as possible, and facilities or methods used to retain sediment on site such as properly installed silt fence or straw bales.
When possible, leaving existing natural drainage way, wetlands, and vegetated soil surface will store, filter, and retain stormwater runoff effectively before being discharged into water ways. Native plants have an extensive root system stabilize soils to prevent erosion and absorb nutrients and stormwater effectively.
When natural features are not enough to adequately handle stormwater runoff, various of other types of constructed facilities can be used without the use of buried pipes. Examples include: shoreline buffers, rain gardens, vegetated swales, or an infiltration basin.
This section provides general information on best management practices (BMPs) during and post construction. Understanding this basic information and applying these principles in the field will reduce negative environmental impacts associated with erosion and sediment loss.
During Construction - Sediment Control and Perimeter Protection
Sediment controls should be planned as a system, taking the entire site into consideration, and installed prior to any land disturbing activity. The design of a sediment control should anticipate ponding that will occur up gradient of the controls and provide sufficient storage and deposition areas and stabilized outlets to prevent flows from over topping the controls. Flows should be strategically directed to specified deposition areas through appropriate positioning of the sediment controls and site grading.
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SILT FENCE
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HAY BALES/FILTER LOGS
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WHAT IT DOES: Intercept runoff, reduce flow velocity, and remove sediment from runoff. Generally used as perimeter control to keep sediment on site.
WHERE IT’S USED: Level areas, down slope of disturbed soil, around stockpiles, along streams and channels, and the perimeter of a project.
HOW TO INSTALL: The bottom of the silt fence must be buried at least 6 inches and support posts should be less than 6 feet apart.
MAINTENANCE ALERT: Must be maintained and replaced since the fabric has a limited lifespan.
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WHAT IT DOES: Intercept runoff, reduce flow velocity, and remove sediment from runoff.
WHERE IT’S USED: Up slope and/or down slope of disturbed soil, and along the perimeter of a project.
HOW TO INSTALL: Logs/Bales should be laid parallel to the slope contour, trenched at least 4 inches into the ground, and held in place by 2 – 2” x 2” wooden stakes
MAINTENANCE ALERT: Must be maintained and replaced as they become degraded.
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EXISTING TURF
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OTHER
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A strip of existing turf, at least four feet wide, shall be maintained around the perimeter of the site throughout all phases of construction. The turf will be inspected and maintained in a manner that keeps sediment from moving offsite. Vehicle traffic will be restricted to access driveways to protect the turf.
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- sediment traps or berms (rock, soil, compost, etc.);
- perimeter control ditches; and
- other (sand bags, rock logs, etc.).
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During Construction - Erosion Prevention
Erosion prevention practices are designed to prevent or minimize erosion. Typical erosion prevention practices involve one or more of the following:
- Planning approaches that minimize the size of the bare soil area and the length of time disturbed areas are exposed to the elements – especially for long, steep slopes and easily erodible soils
- Diverting or otherwise controlling the location and volume of run-on flows to the site from adjacent areas
- Keeping concentrated flows in ditches stabilized with vegetation, rock, or other material
- Covering bare soil with vegetation, mulch, erosion control blankets, turf reinforcement mats, gravel, rock, plastic sheeting, soil binder chemicals, etc.
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MULCHING |
Grasses are planted to help control erosion during construction activities because they sprout and provide protection quickly. Temporary grasses are meant to be replaced at the end of the project with permanent vegetative cover. Permanent vegetative cover consists of grasses or other plants that are intended to control erosion on the site as well as be part of the site’s permanent landscaping. If appropriate species are selected, grasses used for temporary vegetation can also be part of the permanent vegetation for the site.
Purpose
To minimize erosion and sedimentation by stabilizing exposed soils with vegetation and mulching
Conditions Where Practice Applies
- Ground surfaces likely to be exposed during the wet season (October 1 through April 30) or surfaces likely to be exposed for more than 3 weeks during the dry season.
- Areas that will not be subjected to heavy wear or are not working soils piles used by ongoing construction traffic.
- Exposed ground surfaces at the end of the construction period. (Permanent cover shall be established before removal of any erosion control BMPs.)
- Temporary or permanent stabilization of new or disturbed ditches, ponds, trenches, dikes, or swales.
Shall Not Be Used
- On active stockpiles
- By itself (without matting) after October 1
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Mulch is the name given to various organic or inorganic natural or synthetic materials that are spread or blown on the soil surface to prevent movement of soil by wind and rain. Mulches protect exposed soil surfaces from the force of falling rain, slow downslope flows, increase heat and moisture content for seeding and other vegetation, discourage weed growth, and, when adequately anchored, can provide slope stabilization. Mulches can be loose materials (compost), rock, or organic fibers in a water-based matrix that dry as mats on the soil surface (hydraulic mulch, bonded fiber matrix).
Purpose
Mulch minimizes erosion by providing a protective cover over disturbed, bare, or reseeded soils and will help ensure the success of seeding or revegetation. Minimal thickness protects soils from splash erosion, while thicker layers are effective for additional sediment control.
Conditions Where Practice Applies
- As a cover on ground surfaces and stockpiles exposed during the wet season (October 1 through April 30).
- As a mulch to enhance vegetation establishment in areas that have been seeded.
Shall Not Be Used
- Mulch other than gravel shall not be used on slopes greater than 3:1 without erosion blankets or netting on top of the mulch or without a tackifier.
- No organic mulches shall be used on streams or water quality/quantity facility banks below the high water line.
- Straw shall not be used by itself as a permanent erosion control practice.
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| EROSION CONTROL BLANKETS |
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OTHER |
Erosion control blankets are mats composed of organic fibers or inorganic materials held by synthetic or biodegradable netting. Most blankets are rolled products, but some may be sprayed onto exposed soils. Jute netting is not by itself an erosion control blanket.
Purpose
To provide immediate protection and physical stabilization of disturbed soils. Erosion control blankets are typically used when vegetative cover cannot be achieved because of soils or time of year or where slopes are too steep for mulch or other erosion prevention BMPs. They can be used to enhance the success of seeding, planting, and/or sod application. The blankets are usually left in place as a permanent BMP.
Conditions Where Practice Applies
- As channel stabilization against concentrated runoff flows (with adequate approval and permits for active waterways).
- On areas of steep slopes and areas that are prone to erosion.
- As a cover on ground surfaces exposed during the wet season (October 1 through April 30).
- As a supplemental aid to seed and/or mulch treatment on slopes or in ditches or swales.
- Erosion control blankets shall be installed only in accordance with the manufacturer’s specifications and limits on use.
Shall Not Be Used
- On slopes steeper than the manufacturer's recommendation.
- On rough surfaces (unless using spray-on mats) where there is not good contact with the ground.
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- Construction site phasing
- Lot benching
- Surface roughening
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Post Construction - Final Stabilization and Long-Term Stormwater Management
Final stabilization is achieved when all soil disturbing activity is completed and the exposed soils have been stabilized with a vegetative cover with a uniform density of at least 70 percent over the entire site or by equivalent means to prevent soil failure. Simply seeding and mulching is not considered acceptable cover for final stabilization. Final stabilization must consist of an established permanent cover, such as a perennial vegetative cover.
Long-Term Stormwater Management Plan
A post-construction permanent stormwater management plan should be designed so that all stormwater discharged from the project during and after construction activities does not cause a violation of state water quality standards, including nuisance conditions, erosion in receiving channels or on downslope properties, or a significant adverse impact to wetlands caused by inundation or decrease of flow.
RAIN GARDEN: Intentionally placed gardens planted with native shrubs, flowers, grasses, and perennials in a shallow depression designed to hold water during a rain event. Note: a Rain Garden should be located in a place that can collect as much runoff as possible. The best areas are where water naturally drains but doesn't hold water. It should also be located at least 5—15 feet away from your home.
PERVIOUS PAVEMENT: A specific type of pavement or blocks engineered with a high porosity that allows rainwater to pass through into the ground below
VEGETATED SWALE: Used to convey runoff from impervious surfaces to treatment areas such as rain gardens or basins. Note: These broad and shallow channels are vegetated along the bottom and sides of the channel. Swales can be an important tool for retention and detention of stormwater runoff.
SHORELINE BUFFER: A 10, 15, or 20 foot area of natural no-mow vegetation that spans the shoreline outside of the allowed recreational use area and behind any beach or rip rap.
