Sediment control practices - Buffer zones

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Protected buffer zone around a water body. Silt fence perimeter controls mark the beginning of the buffer zone.

A buffer zone is natural band of vegetation that borders a body of water with the objective of protecting and enhancing water quality and habitat. Buffer zones may also be called riparian buffers.

There are two separate but complementary regulations regarding implementation and preservation of buffer zones in the state of Minnesota: the “Buffer Law” (Amended 2017) and the directives of the MPCA Construction Stormwater General Permit that require preservation of buffer zones both during and after site disturbance related to construction activities. The specific requirements of each regulation are dependent on how the adjacent waterbody is classified; this is discussed further below.

Minnesota’s Buffer Law was signed into law in 2015 and amended in 2016 and 2017. Under the Buffer Law requirements, contiguous perennially rooted vegetative buffers of 50-foot width must be provided along all public waterways including lakes, reservoirs, wetlands, rivers, and streams. Further, buffers of 16.5-foot width must be provided along public ditches. Per the regulations, these buffer zones were required on all applicable parcels adjacent to public waters by November 1, 2017 and adjacent to public ditches by November 1, 2018. Minnesota DNR has developed an interactive Buffer Map to help landowners determine if their property is identified for buffer protection. The most recent and complete version of the Buffer Law, as well as related resources, can be accessed through the Minnesota Board of Water and Soil Resources.

The guidance included in this section pertains specifically to buffer zone that is required to preserve during and after construction activities per the conditions of MPCA’s Construction Stormwater General Permit.

Purpose and function

Caution: Buffer zones should not be used as stand-alone stormwater practices for a construction site.

Buffer zones protect and enhance water quality and aquatic habitat by providing shade that moderates sunlight and water temperature, infiltrating and slowing runoff flows, trapping sediment and other pollutants in stormwater, providing habitat for fish and wildlife, and stabilizing shorelines and preventing erosion. Buffers also provide visually appealing shorelines, all of which can improve surrounding property values. Maintaining buffer zones during construction is important to preserving these benefits.

Buffer zones should not be used as stand-alone stormwater practices for a construction site and should be paired with other stormwater management and erosion and sedimentation control practices.

Applicability

Per the conditions of MPCA’s Construction Stormwater General Permit, buffer zones are required during and after construction activities depending on (1) the proximity of the project’s disturbed area to the adjacent surface water, and (2) the nature of the adjacent surface water. The specific conditions used to determine buffer zone applicability for each individual site are discussed below.

Site applicability

MPCA’s Construction Stormwater General Permit has two distinct buffer zone requirements, each with a different purpose.

1. Near any surface waters: a temporary undisturbed natural buffer zone 50-feet in width must be preserved during construction adjacent to surface waters when the surface water is located within 50-feet of the project’s disturbed area and when stormwater from the site flows to the surface water. This temporary buffer zone must be used in combination with other sediment and erosion control BMPs. Surface waters applicable to this requirement include lakes, wetlands, rivers, and streams. Buffer zones are not required during construction adjacent to ditches (including road, county, and judicial ditches), storm drain inlets, stormwater conveyance channels, or sediment basins.
2. Near special waters: a permanent undisturbed natural buffer zone 100-feet in width must be preserved at all times adjacent to any “special water” to protect the waterbody from stormwater runoff and preserve scenic quality. The 100-foot buffer zone must be maintained both during construction and as a permanent feature post construction.

Natural buffers must remain undisturbed and must not be used as sediment treatment areas or as storage areas for construction equipment or materials. As noted above, a 100-foot buffer is required surrounding waters classified by the state as “special waters.” Examples of special waters include the Mississippi River, Lake Superior, scenic or recreational river segments, wilderness areas, trout streams, and calcareous fens. A 50-foot buffer is required around all other surface waters. The 50-foot buffer does not apply if there is no stormwater discharging to the surface water, if all stormwater flows have been diverted from the site, or if the adjacent water body is classified as a ditch, storm drain inlet, stormwater conveyance channel, or sediment basin. MPCA has developed an online mapping tool to identify proximity to special waters.

Additional critical planning considerations for buffer zones are discussed below in Planning considerations, including the proper procedure for measuring buffer zone distance.

Permit applicability

Warning: The MPCA Construction Stormwater General Permit has several requirements regarding buffer zones.

Examples of permit language include the following:

• temporary natural buffers are required for all surface waters;
• permanent natural buffers are required for all surface waters identified by MPCA as special waters;
• both 50- and 100-foot buffers should be delineated in a SWPPP; and
• the 50-foot buffer does not apply if there is no stormwater discharging to the surface water or if all stormwater flows have been diverted from the site.

Specific permit requirements are discussed below.

Specific permit language-Location and sizing

• Regarding the location of other sediment control BMPs on site relative to buffer zones, the MPCA Construction Stormwater General Permit Section 9.2 states: “…Permittees must locate sediment control practices upgradient of any buffer zones.”
• Regarding placement of stockpiles related to construction activities, Section 9.10 states: “Permittees must locate stockpiles outside of natural buffers or surface waters, including stormwater conveyances such as curb and gutter systems unless there is a bypass in place for the stormwater.”
• Regarding site disturbance in proximity to surface waters, Section 9.17 states: “Permittees must preserve a 50 foot natural buffer or, if a buffer is infeasible on the site, provide redundant (double) perimeter sediment controls when a surface water is located within 50 feet of the project's earth disturbances and stormwater flows to the surface water. Permittees must install perimeter sediment controls at least 5 feet apart unless limited by lack of available space. Natural buffers are not required adjacent to road ditches, judicial ditches, county ditches, stormwater conveyance channels, storm drain inlets, and sediment basins. If preserving the buffer is infeasible, permittees must document the reasons in the SWPPP. Sheet piling is a redundant perimeter control if installed in a manner that retains all stormwater.”
• Regarding placement of other stormwater practices within buffer zones, Section 14.8 states: “Permittees must locate temporary basins outside of surface waters and any buffer zone required in item 23.11.” Additionally, Section 18.9 states: “Permittees must locate [wet sedimentation] basins outside of surface waters and any buffer zone required in item 23.11.”
• Regarding site disturbance in proximity to special waters, Section 23.11 states: “Permittees must include an undisturbed buffer zone of not less than 100 linear feet from a special water (not including tributaries) and must maintain this buffer zone at all times, both during construction and as a permanent feature post construction, except where a water crossing or other encroachment is necessary to complete the project. Permittees must fully document the circumstance and reasons the buffer encroachment is necessary in the SWPPP and include restoration activities. This permit allows replacement of existing impervious surface within the buffer. Permittees must minimize all potential water quality, scenic and other environmental impacts of these exceptions by the use of additional or redundant (double) BMPs and must document this in the SWPPP for the project.”

Specific permit language-SWPPP requirements

Regarding Stormwater Pollution Prevention Plan (SWPPP) requirements related to buffer zones, Section 5.12 states: “Permittees must identify locations of 50 foot buffer zones as required in item 9.17 and 100 foot permanent buffer zones as required in item 23.11, on plan sheets in the SWPPP.” Delineation of buffer areas in SWPPPs is highly recommended.

Effectiveness

Buffer vegetation intercepts overland flows, traps sediment, promotes infiltration, and retains nutrients. The preservation of a buffer zone is not considered a standalone best management practice for construction stormwater but a protective area around a water body to remain undisturbed during or after construction activities. Buffer zones are effective at reducing flow velocity, reducing shoreline erosion, and increasing sediment and nutrient removal. For more information on the effectiveness of buffers see the MS4 fact sheet on Vegetated Swales and Buffer Strips in the Minnesota Stormwater Manual.

Planning considerations

Planning considerations include

• measuring buffer zone distance,
• determining actions when meeting buffer requirements is infeasible,
• potential encroachment on the buffer zone, and
• replacement or relocation of impervious surfaces.

Measuring buffer zone distance

Schematic illustrating the ordinary high water level of a surface water. Source: Minnesota Department of Natural resources. Click on image to enlarge.

The buffer zone distance is measured horizontally from the water’s ordinary high water level (OHWL) mark. An OHWL is defined in Minnesota Statute 103G.005 Subd. 14 as the boundary of basins, watercourses, public waters wetlands and:

1. the ordinary high water level is an elevation delineating the highest water level that has been maintained for a sufficient period of time to leave evidence upon the landscape, commonly the point where the natural vegetation changes from predominantly aquatic to predominantly terrestrial;
2. for watercourses, the ordinary high water level is the elevation of the top of the bank of the channel; and
3. for reservoirs and flowages, the ordinary high water level is the operating elevation of the normal summer pool.

OHWL elevations can be found on the Minnesota Department of Natural Resource’s webpage. Local DNR hydrologists should be contacted prior to construction design and planning activities to determine if any special requirements exist.

When required buffer distance is infeasible

As discussed in Permit applicability, redundant sediment control BMPs are required in the event that the required buffer distance is infeasible; in other words, in addition to perimeter control BMP(s), an additional level of protection must be installed that can reasonably be expected to provide the same or better pollutant removal than the natural buffer zone.

Encroachment on the buffer zone

As discussed in Permit applicability, exceptions to the buffer zone rules may be allowed when a water crossing or other encroachment are necessary to complete projects, as well as other cases where construction within a buffer zone is unavoidable (e.g., for buffer restoration or maintenance). Per Section 23.11 of MPCA’s permit, “Permittees must fully document the circumstance and reasons the buffer encroachment is necessary in the SWPPP and include restoration activities.” If the installation of additional stormwater treatment BMPs for buffer restoration is not feasible due to space or other constraints, then construction activity encroaching on the buffer zone is not allowed.

Replacement or relocation of impervious surfaces

Existing impervious surfaces within a buffer zone can be replaced by new impervious surfaces as long as (1) the new impervious surfaces are the same size as or smaller than the existing impervious surfaces, and (2) the new impervious surfaces are not closer to the water body than the existing impervious surfaces. Any environmental or scenic impacts relative to existing conditions as a result of replacement or relocation of impervious surfaces must be mitigated and documented in the SWPPP. Permittees should contact MPCA during the design phase of the project to determine which additional BMP(s) may be necessary and to ensure appropriate restoration measures are implemented.

Design

The preservation of a buffer zone is not considered a standalone best management practice for construction stormwater but a protective area around a water body to remain undisturbed during or after construction activities. As such, there are no specific buffer zone design recommendations with respect to construction stormwater. Enhancement of the buffer zone, however, is both allowed and encouraged. More information on buffer enhancement including recommended plant species is provided in the MS4 fact sheet on Vegetated Swales and Buffer Strips in the Minnesota Stormwater Manual.

Construction recommendations and specifications

There are no specific construction recommendations or specifications for the protection of a buffer zone during construction. Information on optional natural buffer zone enhancement is provided in the Design section above.

Inspection and maintenance

The natural buffer zones that are required during and after construction activities (either temporary or permanent depending on the classification of the adjacent surface water) should require little maintenance once their required widths (50 feet for surface waters and 100 feet for special waters) are established. The following guidelines are recommended to ensure optimal function of buffer zones for protection of surface water quality.

• Inspect buffer delineations regularly to determine if the flags or other markers remain in the correct position and to ensure that construction activity remains outside of buffer delineation (buffer zone remains undisturbed)
• Do not place or store equipment or materials within the established buffer zone
• Do not remove or mow vegetation within buffer zones

Costs

There are no costs specific to the preservation of a buffer zone during construction activities. Depending on site conditions, overall costs of the construction may potentially be higher than if a buffer zone was not required.

Reference materials

Except where more stringent requirements are presented in this guidance, BMPs shall comply with MnDOT and other state requirements. Primary design references include:

• MnDOT Erosion Control Handbook II
• 2018 Minnesota NPDES/SDS Construction Stormwater General Permit
• MPCA Guidance: Temporary natural buffers for surface waters and permanent buffers for special waters (September 2018; wq-strm2-30)
• 2018 MnDOT Standard Specifications for Construction
• 2017 MPCA Stormwater Manual MS4 fact sheet - Vegetated Swales & Buffer Strips
• Minnesota Board of Water and Soil Resources: Minnesota Buffer Law
• 2003 MPCA Plants for Stormwater Design