Construction stormwater best management practice – Site stabilization

Site stabilization

NPDES/SDS Construction Stormwater permit requirements

Improperly treated and untreated runoff from exposed soils of a construction site can cause soil erosion and sedimentation problems resulting in the pollution of lakes, rivers, and other water bodies. This factsheet provides guidance on stabilizing a construction site to protect the quality of our water resources.

The erosion process

Subsoil erosion at a construction site

Erosion is the natural process in which soil and rock material is weathered and carried away by wind, rain, snow or ice. Factors such as rainfall, climate, location, and soil type that influence erosion.

Construction activities increase the ability of soil to erode once the vegetation and the top soil are removed. When the subsoil is exposed to the elements, the soil can erode very quickly. Wind and rain carry the soil off the construction site and sediment is deposited into our surface waters. This causes adverse effects on the quality of our water and to the health of aquatic life. Sediment runoff often carries additional pollutants such as nutrients, petroleum products, fecal matter and other contaminates from construction sites that further impact our lakes and rivers. It has been shown that it is far less costly to prevent erosion than to attempt to mitigate the effects of erosion after it has occurred. Through proper use of best management practices (BMPs), including temporary and permanent covers to stabilize soils along with other practices to prevent erosion; such as the use of construction phasing, horizontal slope grading, vegetative buffers and water diversions, these effects can be substantially minimized.

What is required by the NPDES/SDS Construction Stormwater permit?

The National Pollutant Discharge Elimination System (NPDES)/State Disposal System (SDS) Construction Stormwater permit identifies certain site requirements in regard to erosion prevention that, if utilized properly along with all other BMPs required by the permit, can effectively control erosion and sediment problems on a construction site.

Site stabilization is the process of implementing specific BMPs for the purpose of preventing soil from eroding. BMPs utilized for this purpose involve establishing a cover of some type over the exposed soils such as mulch, staked sod, riprap, erosion control blankets, or other material that prevents soil from eroding. Covering the soil with seed only, is not considered a stabilization practice. Mulch needs to be applied with the seed to stabilize the soil until the vegetation is established.

Sediment control BMPs, such as silt fence, rock checks, bio rolls, drainage swales, sediment traps or perimeter controls are utilized along with the soil stabilization BMPs. Perimeter controls are used to trap sediment prior to leaving the site, but alone are not considered soil stabilization practices.

At a minimum, the permit requires that the following practices are utilized to prevent soil from eroding at construction sites:

• Phase the construction when feasible to limit the amount of soil exposed at one time.
• Delineate areas not to be disturbed with flags stakes, silt fence or with signs. These areas also need to be noted on the plan sets.
• Avoid disturbance of steep slopes or utilize stabilization practices designed for steep slopes, such as terracing or slope draining to minimize erosion.
• Route stormwater around exposed soil areas through use of conveyance channels when feasible.
• Direct stormwater discharges from the site to vegetated areas in order to increase sediment removal and infiltrate stormwater on the site when feasible.
• Utilize vegetative buffers, horizontal slope grading, and maintenance to protect surface waters.
• Use check dams, sediment traps, riprap, or grouted rip rap at outlets and along conveyance channels to control velocity and minimize erosion along the channel and outlets.
• Install energy dissipation such as concrete aprons, splash pads, rip rap, and gabions at pipe outlets within 24 hours of connecting to a surface water.
• Initiate soil stabilization immediately on portions of the site, including soil stockpiles, where construction activity has temporarily or permanently ceased and will not resume for 14 days.
• Stabilize exposed soils within 24 hours on portions of the site that drain to a public water where the Department of Natural Resources has declared “work in water restrictions” during fish spawning time frames.
• Stabilize the last 200 feet of the normal wetted perimeter of any temporary or permanent drainage ditch or swale that drains water from the site within 24 hours of connecting to a property edge or a discharge point to a surface water. Use erosion blankets, rip rap or other cover designed for concentrated flow areas.

Note that the permit definition for surface water in the permit is not just lakes or streams, but also any natural or artificial water feature including drainage systems such as ditches, swales, or inlets. Therefore, ditches or swales that connect to another drainage system must follow these requirements.

• Stabilize the remaining portions of the ditch or swale within 14 days where construction of that portion of the ditch has temporarily or permanently ceased.

Special considerations

If the construction site is within one mile of a special or impaired water (defined in Part B.1. through B.9 and B.10 in Appendix A of the permit), as determined by the aerial radial distance from the site and not the flow distance; the following additional erosion prevention requirements are required in regard to preventing erosion:

• Initiate stabilization of all exposed areas immediately at the site and ensure the stabilization is complete no later than seven days after the construction activity in that area has temporarily or permanently ceased.
• Maintain an undisturbed buffer zone of not less than 100 feet from special waters at all times. If construction is such that the buffer must be encroached, use redundant BMPs sufficient to protect the special water. For more information on buffers see http://www.pca.state.mn.us/index.php/view-document.html?gid=7400 .

For full details of the permit requirements, a copy of the NPDES/SDS Construction Stormwater permit is available online.