CONSTRUCTION & DEVELOPMENT: Soil erosion – Control by design
Soil erosion control during construction has become a high profile issue in northern Michigan. News articles detailing severe erosion problems along sensitive environmental areas, such as streams and lakes, cast an unfavorable light on projects.
Often the success of these projects depends on the natural beauty of the area. Even more important than the public relations considerations are the overall impacts on the natural resources. Remediating erosion problems delays schedules, increases costs and increases the focus of regulators on your project.
The challenge of designing to prevent soil erosion requires an understanding of why soils erode, and what factors influence the potential for erosion. Although erosion can occur from wind or ice, falling and flowing water is the issue of most concern to projects in our area.
Soil erosion results from the sequence of raindrops falling directly on ground and dislodging soil particles, subsequent sheet flow of water over a wide area transporting these particles, and the run-off concentrating in a gully or channel with velocity sufficient to erode the sides or bottom of the channel. Whether a slope erodes or not depends on the intensity of the rainfall and run-off, slope length and gradient, soil characteristics, and vegetative cover.
Factors which could change during a construction project include slope length and gradient, vegetative cover, and occasionally, soil characteristics.
How is erosion controlled?
Controlling erosion requires the engineer, developer and contractor to be aware of the modifications the construction has on the land, and adhere to the following five basic principals:
1. Keep disturbed areas small
2. Stabilize and protect disturbed areas as soon as possible
3. Keep storm water velocities low
4. Protect disturbed areas from storm water runoff
5. Retain sediment within the site area
The most direct way to control the potential for erosion is to minimize the area under construction and to stabilize the disturbed areas as soon as possible. Grass is the least expensive and most effective material for permanent control of eroding soils. This approach should be used where possible, but many projects require significant earthwork, and take place over a long period of time.
Larger and longer-term projects necessitate using a combination of techniques to reduce water velocities, protect areas from excessive run-off, and retain sediment within the site area. These techniques consist of constructed check dams, diversion swales, sediment ponds, berms and drains. Materials such as rip-rap, filter fabric, hay bales, and stone filters are used, and the designs must be coordinated so that all of the erosion control measures work together.
Applicable to most project sites are such practices as access roads and buffer strips. Access roads keep the roadway free of soil–a potential hazard–and also reduce the amount of sediment leaving the site. Buffer strips consisting of natural or planted vegetation along the perimeter of the project provide sediment filters and nutrient uptake. These measures must be inspected after rainfall events, and maintained on a regular basis.
The development of a soil erosion control plan requires a coordinated effort by the developer, engineer, contractor, and local soil erosion control agency. The construction of larger projects is typically staged over a full construction season or more, and it may take a variety of techniques and stages to provide the necessary control. The contractor may require areas for storing topsoil, materials, and equipment to work efficiently. Knowing where and how the contractor is planning to construct the project allows the engineer to design the erosion control measures for efficiency during the construction, while providing the necessary protection. A well-prepared plan with proper installation and maintenance can minimize adverse impacts to adjacent properties.
Many developers and owners of property under construction know the requirement for having a soil erosion control plan. Depending on the size of the project, and how the stormwater discharges from the site, they may not be aware that additional permit coverage may be required.
Michigan has developed rules under the Federal Storm Water Regulations that require the owner of the property with a project construction area greater than five acres and a point source discharge, such as a culvert or ditch, to apply to MDEQ for coverage under the permit-by-rule requirements.
The owner must have a state-certified storm water operator inspect the construction site once per week and within 24 hours after a precipitation event that causes a discharge from the site. A log must be kept of the certified stormwater operator’s inspections, and the operator must report to the owner any soil erosion control measures that need maintenance or repair.
Can a properly-designed soil erosion system fail? Unfortunately, it is possible. The amount of rainfall the system is designed to accommodate could be exceeded, materials could be defective, unknown subsurface conditions–such as an intermittent spring–could all create situations where the capacity of the system is exceeded.
With proper design and foresight, though, even the impacts of the unexpected situations can be minimized.
Soil erosion is a situation that must be addressed for the preservation of the integrity of our natural resources. With a thorough understanding of the factors that cause erosion, a design can be developed and implemented to minimize or eliminate unnecessary erosion brought about by construction.
Garth Greenan, PE, is a civil engineer with EC&S, a firm specializing in land transaction services, land development and environmental management. He has over 22 years of civil engineering experience, specializing in site design and engineering for commercial, residential and industrial development projects. BIZNEWS