Grading and erosion control
Construction activities expose soil to erosion caused by wind, rain, and snowmelt. Runoff from construction sites flows from streets and storm drains into creeks, lakes, and wetlands where it clouds the water with sediment and increases algal growth. Sediment deposits reduce the ability of our waterways to conduct and store water, increasing flooding. Topsoil loss and gully formation complicate landscaping and increase costs. Weather is difficult to predict; however, proper erosion control practices can reduce the loss of topsoil and deposition of sediment during grading and building.
Construction activity along I-494 north of Excelsior Boulevard
The city of Minnetonka requires that adequate erosion control be installed and inspected before a building or grading permit is issued and maintained until groundcover has been established on the site. City staff may outline necessary control measures before approving a permit and may require additional measures once a project has begun. Large projects may require submission of an erosion control plan for staff review and a letter of credit to insure compliance. Designing construction sites to avoid unnecessary disturbances is often the best method for preventing soil loss and preserving water quality.
Vegetation
Large-scale sod installation
Maintaining ground cover is the most effective tool for preventing erosion. Any existing vegetation should be preserved if possible. Bare soil should be seeded with quick-sprouting grasses such as annual rye (all year), oats (spring), and winter wheat (fall) as temporary cover. On large sites, construction should be staged so that one part of the site is stabilized before another section is disturbed. You may need to fertilize, water, and reseed to ensure the vegetative cover is maintained until permanent cover is installed.
The city of Minnetonka requires that property that has been disturbed by construction, grading, or other activity be seeded, sodded, or otherwise planted with a ground cover within 240 days. If the 240 days expires between November 1 and May 15, the ground cover must be established by the following July 15 unless the city approves a time extension (Minnetonka City Code Section 845.010 subdivision 7).
Seeding with straw mulch
Permanent groundcover should be established as soon as possible to avoid costly maintenance. Sodding and seeding may provide established groundcover in a few weeks. Sod may be used to protect problem spots in a seeded area and should be anchored with wire or biodegradable staples when installed on slopes. Erosion control blanket or mulch should be used when planting seed on slopes.
If the soil is not stabilized through landscaping when a certificate of occupancy is issued, the city may require financial security, not to exceed 150% of the estimated cost of erosion and sediment control on the site. The financial security will be returned when the landscaping is complete and established (Minnetonka City Code Section 300.028 subdivision 18b).
Hydroseeding with wood fiber mulch.
Hand-raked seed 4 weeks after application.
Erosion Control Blanket
Erosion control blankets are effective at stabilizing soils on slopes, channel bottoms, and adjacent to wetlands. They are a cost-effective alternative to sod, and provide substrate for grass, shrubs, and forbs. Blanket generally is composed of woven wood fiber, straw, or coconut attached to plastic netting. Blankets may be designed to last one season or several years depending on the manufacturer’s specifications.
Erosion control blanket is only effective if properly installed. Establish the leading edge in a 6 x 6-inch trench, staple, and pack with soil to prevent water from flowing under the blanket. Roll the blanket parallel to the direction of water flow and secure with staples every 1 to 3 feet, depending on application and width of blanket. Overlap the edges by several inches.
Permanent turf reinforcement mats provide greater stabilization than erosion control blanket, and may be necessary in areas of concentrated flow.
Mulch
Mulch may be effective where flows are less extreme or when vegetation can be established quickly. Mulch (wood, straw, or aggregate) may be spread in areas where it is not possible to establish vegetation (e.g.- heavy shade under tree canopies) or during the winter.
Six-inch layer of wood mulch to control erosion in winter.
Mulch should be spread thinly in seeded areas to allow germination. Under trees, spread mulch 4 inches deep as an alternative to vegetative groundcover or 6 inches deep to minimize root damage to trees during construction. Mulch may be anchored with tackifiers or netting on slopes. Inspect the site regularly and replace mulch if it washes away or decomposes.
Silt Fence
Silt fence is a temporary sediment barrier consisting of a geotextile attached to supporting posts trenched or sliced into the ground. Silt fence causes large flows of water and sediment to pool uphill. Sediment is filtered out as the water passes slowly through the fabric.
Heavy, wire-backed
silt fence
with metal stakes.
Silt fence should be trenched at least six inches or sliced at least 4 inches and soil compacted around the base. Heavy, wire-backed silt fence with metal posts should be used on slopes.
Silt fence may help slow sheet flows but is not recommended in channels or ditches. In addition, silt fence must be maintained to be effective. Tears in the fabric or broken support posts may direct water to form channels.
Silt fence maintenance
- Inspect silt fence each week or after each rainfall.
- Repair or replace if the fabric collapses, tears, or decomposes.
Machine-sliced silt fence
on metal stakesRemove silt deposits once they reach one-third the height of the fence to provide adequate storage volume for the next rain and to reduce pressure on the fence. Take care to avoid damaging the fence during cleanout.
Remove silt fence only after permanent groundcover has been established.
Silt fence on 2x2
wooden stakes
Double-row of heavy silt fence
(third row added for stockpile)
Ditch Checks
Biolog staked in channel
adjacent to silt fence
Gullies form quickly in ditches and swales once groundcover is compromised, and they can be difficult to repair. Check dams, installed in series, are effective for slowing water flow in channels and preventing gully formation. Check dams should be constructed using clean rock, triangular silt dikes, compost socks, or similar permeable materials.
Rock ditch checks
along Interstate-494
Dams should be spaced according to the chart below. Lower dams should crest at the same elevation as the base of the dam above it. Erosion control blanket may be used to form small ditch checks by trenching the leading edge, anchoring several feet of material, and then securing the remaining roll with stakes. Silt fence and hay bales are less effective than other methods for forming ditch checks because water will eventually flow beneath them.
| Ditch grade (%) | Spacing (feet) |
|---|---|
| 1 | 200 |
| 2 | 100 |
| 4 | 50 |
| 6 | 33 |
| 8 | 25 |
| 10 | 20 |
Biologs/Compost Socks
Biologs, compost socks, silt socks, wattles, and excelsior logs consist of a tubular membrane filled with straw, wood mulch, clean rock, or similar material. They are easy to install and available in any length and several thicknesses. Small diameter (~6 inch) biologs are sufficient to prevent runoff from paved driveways or into storm drains. Thick logs (9 to 12 inches or greater) should be used on slopes, along shorelines, in ditches, or areas with high water flow. Mulch biologs/socks can be seeded. Once groundcover is established, any non-degradable fabric should be removed and the mulch or rock may be spread over the site.
Biologs/socks should be staked to the ground to prevent rolling or shifting. Unlike silt fence, vehicles can drive over biologs on occasion; however, they lose their effectiveness after several passes. Biologs/socks should be replaced when they become torn, flattened, or filled with sediment.
Hay Bales
Hay bales may be used to slow water in channels or around storm water inlets, but they must be installed correctly and are usually less effective than other materials. Hay bales should be trenched 4 inches into the surrounding soil and secured with 2 metal or wood stakes. Bales need to be packed tightly against each other and straw should be forced into any cracks. Installing a second row, with the seams offset from the first may help prevent water from seeping between bales.
Once groundcover is fully established, hay bales should be removed or the hay should be spread over any exposed soil. Failure to maintain or remove decomposing hay bales often causes water to channel in the seams between the bales and produce the gullies that they should prevent. Loose straw from hay bales should not be allowed to flow into storm sewers or wetlands.
Storm Sewer Inlet Protection
Steel cage with geotextile
Proper inlet protection will help reduce sediment loads into wetlands and waterways and prevent costly storm sewer blockages that may cause flooding. Inlet protection should not be used in place of proper erosion control.
Silt sack with wood fiber log
Simple inlet protection formed from geotextile, biolog/compost sock, and similar materials, is effective for filtering fine particles. Geotextile may be installed under the storm grate and biolog/sock placed across the curb opening. It may be necessary to place a piece of lumber across the curb opening to prevent the biolog from falling into the drain. Biologs also may be placed in a horseshoe shape around the inlet to trap sediment, avoiding the need to lift the metal grate. If paving is not present, a box of silt fence may be trenched around the opening.
Silt fence and hay bale
around capped storm drain
Steel sediment trap (below grate)
Platic sediment trap (under grate) with rock log
Inlet protection should be checked after each rain. Sediment should be removed and material replaced as needed. Sand bags tend to block drainage and tear, releasing sand into the drain, and should be avoided. In areas of high volume flow, rock piles may be used to filter medium to large particles. Wire mesh should be placed over the grate to prevent rock from entering the drain. Prefabricated units are effective when cleaned regularly and some are designed with filters for fine particles.
Stockpiles
Silt fence placed
below stockpile on slope
Stockpiles should be located away from trees, wetlands, and other natural resources. Silt fence should be installed down slope of the pile location and material should be placed no closer than 8 feet from the fencing to prevent overtopping. Small piles should be covered with a tarp. Larger piles should be seeded unless they are to be removed from the site within 2 weeks.
Stockpile with
silt fence and mulch
Stockpile seeded
with vegetation
Street Cleaning
The city’s public nuisance ordinance regulates the deposition of sediment onto the street during and following construction. Any dirt and debris should be cleaned from the street by the end of the day. Contact a commercial street sweeping service to schedule a weekly sweeping for ongoing projects.
Temporary gravel
driveway leading
to shared
private drive. Parking off-site and avoiding unnecessary travel on work sites following rainstorms and snowmelt will help prevent depositing dirt and debris on city streets. Temporary rock driveways should be constructed in locations where entering the worksite is necessary. Biologs should be placed at the end of the rock entrance to prevent sediment flows onto the street.
Temporary rock entrance
to construction site
Dust Control
Large projects may create dust hazards for neighboring residents during dry weather. If construction dust is a problem, soil on the site should be sprayed with water regularly until groundcover is established. Non-hazardous chemical soil stabilizers may be appropriate for some sites.
For a site inspection or additional information about erosion control techniques and compliance contact the Minnetonka natural resources division at 952.988.8422.
References:
Diagrams and text were adapted from the following:
- “Sediment and Erosion Control for New Homeowners”(PDF, MPCA)
- “Protecting Water Quality In Urban Areas—Best Management Practices for Dealing with Storm Water Runoff from Urban, Suburban and Developing Areas of Minnesota” (PDF, MPCA)
