A buffer zone is natural undisturbed area that borders a body of water with the objective of protecting and enhancing water quality and habitat by restricting construction activities and maintaining existing vegetation. The buffer includes the vegetation, exposed rock, or barren ground that exists prior to commencement of earth-disturbing activities. Buffer zones are sometimes 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 a buffer zone required to be preserve during and after construction activities per the conditions of MPCA’s Construction Stormwater General Permit.
The buffer zone in the MPCA Construction General Permit is intended to restrict earth disturbing activities and maintain existing vegetation within the sensitive area adjacent to surface waters. 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.
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.
MPCA’s Construction Stormwater General Permit defines a natural buffer as “an area of undisturbed cover surrounding surface waters within which construction activities are restricted.” The permit further states that the “natural buffer includes the vegetation, exposed rock, or barren ground that exists prior to commencement of earth-disturbing activities.” The permit has two distinct buffer zone requirements, each with a different purpose.
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.
In most cases, the buffer zone prior to construction activities will consist of stable soils, natural vegetation, and other erosion-resistant features such as rock, woody debris, etc. In cases where the soils in the buffer are actively eroding prior to construction, and discharging sediment-laden runoff to the adjacent waterbody, the permittee should consider stabilizing the buffer area prior to construction. Addressing this situation in the SWPPP and discussing possible stabilization approaches with the permitting authority will help to avoid any misunderstandings regarding responsibility for unstable conditions in the buffer. Additional critical planning considerations for buffer zones are discussed below in Planning considerations, including the proper procedure for measuring buffer zone distance.
Examples of permit language include the following:
Specific permit requirements are discussed below.
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.
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 include
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:
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.
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.
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.
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.
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 sections of a buffer zone that are actively eroding and contributing sediment or other pollutants to the adjacent surface water is both allowed and encouraged. Such enhancements – which must be addressed in the SWPPP and approved by the permitting authority – can include targeted grading if necessary, seeding and mulching, application of rolled erosion control products, and other measures intended solely to address the actively eroding areas (i.e., naturally vegetated and stable areas should remain undisturbed). 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.
At a minimum, activities within the buffer should be restricted through the use of construction fencing, flagging, signage, or other means. Disturbed areas upgradient from the buffer must be managed via appropriate perimeter controls, such as silt fencing, stormwater diversion berms/ditches, sediment traps, fiber logs, or other BMPs that prevent sediment-laden runoff from entering the buffer. As noted above, where areas within the buffer are actively eroding they should be stabilized via carefully targeted management practices prior to construction. Where encroachment within the buffer is permitted in accordance with permit conditions, the double perimeter controls (e.g., silt fencing) must be sited, installed, and maintained properly. Use of wire-reinforced silt fence or rock berms is recommended where heavy stormwater volumes are expected to flow toward the buffer. Placing the required double line of perimeter controls more than five feet apart will help in vegetation establishment and maintenance. Addressing upgradient flow volume, erosion, and sediment movement conditions will greatly reduce the stress on BMPs protecting the buffer. Information on optional natural buffer zone enhancement is provided in the Design section above.
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.
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.
Except where more stringent requirements are presented in this guidance, BMPs shall comply with MnDOT and other state requirements. Primary design references include: