Site assessment, preparation, design considerations and recommendations for vegetation in stormwater management

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Photo of a site with vegetation prior to implementing stormwater controls. Source: Jim Nelson, Restoring the Landscape.

Conducting a site assessment and inventory of existing conditions on a proposed BMP site is a essential step in a successful stormwater management project. The process and results of conducting an assessment and inventory will ultimately impact the design and function of the stormwater project. Taking the time to complete a review of existing conditions can also bring to light critical challenges or obstacles early in a project that can increase the success and decrease the risk in the BMP. When conducting a site assessment specifically for vegetation, it is critical to start with two areas of consideration:

• Conducting an assessment of existing vegetation
• Assessing soil health and function (effect of vegetation on soil health)

Conducting assessment of existing vegetation

Assessing existing vegetation can provide vital clues in understanding vegetation growth patterns, provide keys to underlying hydraulic conditions, provide insight into historic vegetation cover, and many more useful tools in the development of BMP design. Assessments, as described above, can be completed after an inventory of existing vegetation has taken place. Often inventory and analysis can provide helpful information that may influence critical decisions and impact the trajectory and increase the likelihood of a successful project.

The process of completing assessment of existing vegetation will depend upon the size and conditions of the site you are assessing. To aid in the design of a vegetated stormwater feature, you will want to understand the following basic information about your site:

• What vegetation was historically found on site?
• What vegetation is currently found on the site?

These two questions will guide you in future decision-making processes and help reduce risk and increase the likelihood of long-term success.

What vegetation was historically found on site?

In the process of designing a BMP, depending upon the size, condition, and location, it is often helpful to have a reference, or index, plant community to guide plant selection using a vegetative baseline. Reference plant communities from on-site or local similar conditions as the project site can provide a baseline indication of what vegetation or seed could be suitable for use. These baseline indices may also provide insight into site conditions that could impact the BMP function or design. To learn more about vegetation design, visit the plant and seed vegetation design section (link to other section of stormwater manual when developed - Plant and seed selection based on project goals and site conditions).

Below are several tools that designers can use in the inventory and analysis stages of a project to help determine reference points for determining what vegetation might have occurred naturally on a given project site:

• Marschner Map of Original Vegetation
• Minnesota Historical Aerial Photographs Online (MHAPO)
• Mississippi River Commission Maps
• Other Historic Vegetation Resources

Marschner Map of Original Vegetation

In order to restore a native vegetated state or plant community, it is helpful to start with a reference point or vegetation baseline that can be used as a guide in a restoration process. The presence of native vegetation on your site can provide both a reference plant community as well as provides indication of higher functioning biological and ecological systems. A common vegetative reference point for high-functioning ecosystems in the continental United States is that time prior to European settlement. The Marschner Map of Original Vegetation was developed from notes on pre-settlement vegetation in a Minnesota land survey completed between 1846–1848. These maps depict ecological communities encountered by surveyors and can be a useful tool in creating a reference vegetation community for a BMP. To review Minnesota’s Marschner Maps or to learn more about them visit the link to Minnesota Land Use and Cover: Historic

Historic Aerial Photography

Another resource to help determine historic vegetation is online aerial images. Even if specific vegetative species cannot be determined from an aerial photo, it may be helpful to understand what type of plant communities were present before current day conditions. Historic imagery can give you context clues, for example if the project site was historically a woodland, prairie, or oak savanna. Aerial imagery can also hint at hydraulic conditions, as wetlands are often easily identifiable and can provide a baseline understanding of the potential for wet conditions on a given site. It can also inform the vegetation design of your BMP, helping you better select species that will have higher rates of success given specific site conditions. Below are a few links to historical aerial imagery.

• MHAPO (Minnesota Historical Aerial Photographs Online)
• Historical Airphoto Information, Minnesota DNR

Mississippi River Commission Maps

The Mississippi River Commission Maps show the channel of the Mississippi River and adjacent area, including 10-foot contours and river cross-sectional depth increments, as derived from a special survey completed by the US Army Corps of Engineers in the late 1800’s. These historic hand-drawn maps offer both the channel and floodplain characteristics as well as vegetative cover, land ownership, and built features such as road, railroads, and buildings. Depending upon the location of the proposed BMPs, these maps can be useful tools to see how landscape has changed over time, what land uses may have occurred on the site during that period, and where remnant vegetation may be present from that time period.

• USGS Georeferenced Mississippi River Commission Maps

Other Historic Vegetation Resources

MCBS Native Plant Communities. The Minnesota Biological Survey Native Plant Communities and Rare Species dataset provides mapped areas of natural vegetation and general locations of existing rare and endangered species. Maps are available by county, and completed for most of the state in PDF and GIS shapefile formats. These provide a spatial resource for assessing what high-quality natural areas may be present on your site as well what adjacent areas may help inform your restoration goals.

• Minnesota Biological Survey Native Plant Community and Rare Species Maps

PLS Bearing Trees. The original Public Land Survey (PLS) for Minnesota was roughly completed between 1847 and 1908, organizing the land for sale and use. During this survey, the bearing trees were blazed and marked to be able to locate the survey corners if the wood or stone corners were lost or moved. These tree species were only captured at the corners of sections and quarter-sections, so broad translation of these as indicators of pre-settlement ecological communities should be performed with caution. Alternately, these can be used in combination with other vegetative maps such as the MCBS Native Plant Communities or Marschner Map to understand what types of trees were present and may be suitable for use.

• Original Land Survey Bearing Trees

What vegetation is currently on site?

Vegetation existing on a project site can play a significant role in the design and function of a BMP. High quality vegetation, specific vegetation communities, and key individual species may have local or national protections and potentially shift the location or design of the project. In the same manner, invasive species found within a project site may impact the function or vegetation design within a site. Additional management or restoration measures may need to be taken to increase the likely hood of vegetation establishment and success. Vegetation on site can be broken into three categories in the site assessment process:

• Native Vegetation
• Key Plant Species
• Undesirable Plants

Native Vegetation

Native Plant Communities
The Minnesota DNR defines native plant communities as, “a group of plants that interact with each other and with their environment in ways not greatly altered by modern human activity or by introduced organisms…classified and described by considering vegetation, hydrology, landforms, soils, and natural disturbance regimes.” (MnDNR, https://www.dnr.state.mn.us/npc/index.html). These groups of plants form recognizable communities classified by characteristic plant species or characteristic environmental features. These plant communities developed in response to natural environmental conditions so are often best adapted to the environmental and hydrologic conditions in site-specific BMP’s.

Understanding and classifying the vegetation on your site can be a useful tool in the design of your BMP. It can provide you with a list of common plant species found within a given plant community to serve as a reference in BMP vegetation selection. To learn more about native plant communities review the DNRs classifications below.

• Minnesota's Native Plant Communities, Minnesota DNR
• Native Plant Community Classification, Minnesota DNR

Wetlands
Minnesota supports a variety of wetland types, each with greatly varying characteristics. Wetland types include shallow open water, bogs, shrub swamplands, wet meadows, floodplain forests, and many more. Wetland types and wetland delineation can vary depending on the regulatory jurisdiction. However, generally speaking wetlands are identified by soils that develop in wet conditions, the presence of water (hydrology), and the presence of water loving vegetation.

Wetlands are an important part of our ecosystem and provide a variety of benefits:

• Water Quality: Wetlands filter and absorb pollutants from surface runoff thereby protecting the water quality of downstream lakes, streams, and rivers.
• Flood Control: Wetlands slow down and retain runoff from precipitation events. Wetlands serve as holding areas and can reduce flooding and soil erosion downstream.
• Groundwater Recharge/Discharge: Some wetlands recharge groundwater by allowing surface water slowly filter into groundwater storage reserves. Other wetlands discharge groundwater to the surface and can help maintain flows in nearby rivers and streams.
• Provide Habitat: Wetland vegetation provides habitat and forage for many insects and animals
• Education, Recreation, and Commercial Benefits: Some wetlands offer opportunities for education, recreation, and can even support the production of commercial products.

Regulation is in place to protect and prevent the loss of existing wetlands so that they can continue to be a public value. Wetlands are protected by federal, state, and local regulations. Because of this regulation, it is important to have a trained wetland delineator review your site thoroughly to determine if a wetland exists on your site as early in the design process as possible.

Additional information regarding the wetland delineation process, permitting, and regulatory requirements can be found here:

• Wetlands, Minnesota DNR
• DNR Regulations
• Delineation & Jurisdictional Determination Guidance
• NWI Wetland Finder: Minnesota Department of Natural Resources

Key Species

Tree Survey
Tree surveys provide critical information on what mature trees are on the site and are often required to be mitigated as part of site development. Check your local codes for the required information to be included in a tree survey. Typical information includes the tree’s trunk diameter (DBH), species, health, location, and any comments on condition or quality based on field observations. If repeat surveys are to be completed, a permanent metal tag with unique ID is often affixed to the tree to provide consistent numbering and identification. A tree survey should be completed by an arborist, ecologist, landscape architect, land surveyor, or other qualified person able to key the tree to the species level in the field.

Heritage Trees
Heritage trees are very large, unique, or old trees that have cultural or ecological value. Trees may be considered for heritage status based on age, rarity, size, aesthetic, ecological, or historical value. Depending upon your location, governing bodies may have ordinances governing what can be permitted and built around a heritage tree.

To learn more about Minnesota’s heritage or landmark tree programs follow the links below:

• Heritage Tree Program - Minneapolis Park & Recreation Board
• Minnesota's native big tree registry | Minnesota DNR
• Landmark Tree Program | Saint Paul Minnesota

Threatened and Endangered Species
State and federal laws exist to protect endangered or threatened species throughout the state. Before beginning a design process, it is important to engage local and state natural resource managers and review the site for any potential impacts a construction project may have on threatened or endangered animals. A review of the DNRs Natural Heritage Information System (NHIS) will provide information related to rare plants, animals, and native plant communities that have been identified within a particular area.

Additional information about threatened or endangered species regulation can be found at the following online resources: Minnesota's endangered, threatened, and special concern species, Minnesota DNR

Natural Heritage Information System Desirable Plants
Plants native to Minnesota are adapted to our soil and climate and are often easier to grow than non-native species. Native plants, if placed withing the correct habitat, produce a significant number of ecological benefits while reducing the amount of maintenance needed on a project. Below is a few links to plants that you may want to keep if found within a project site.

• Plants of Minnesota, Minnesota DNR
• Native plants, UMN Extension
• A guide to aquatic plants, Minnesota DNR

Undesirable Plants

Tree Removal & Mitigation
Each jurisdiction has different regulations regarding tree protection and mitigation of removed trees during a construction project. It is important to review local ordinances to identify if tree preservation plan or mitigation requirements exist for your project area. A detailed tree preservation plan can help minimize compliance cost, protect valuable trees, and ensure that your project conforms to local tree mitigation ordinances. A tree preservation and mitigation plan typically involve the following elements.

• Tree Inventory: Species identification, location, health, and size of existing trees on site. This is typically a plan figure with an associated table.
• Tree Preservation and Removals Plan: This is typically a figure showing tree protection measures (construction fence, trunk guards, etc.) for existing trees to be preserved. It should also identify trees to be removed as a result of the proposed construction or restoration.
• Mitigation Plan: Tree mitigation is the act of compensating for the removal of existing healthy trees by planting a proportional number of replacement trees. Many jurisdictions have mitigation requirements based on the species and number of trees to be removed as a result of a construction project. Siting design elements and grading in ways that limits the removal of trees, especially hardwood species, can often reduce the need for replacement tree stock.

It is important to review your project location’s municipal code documents early in the design process to ensure that you are able to meet all permit requirements.

Invasive Species
In the process of completing a vegetation site assessment, it is important to understand the amount and type of invasive species pressure that may occur on or around the site. Early identification of invasive species can save you time, money, and increase the likely hood of project success. Invasive species spread rapidly, especially when a site is disturbed due to construction and can out compete native species and reduce potential quality of the BMP.

To learn more about invasive species in Minnesota visit the links below:

• Invasive terrestrial plants - Minnesota Department of Natural Resources
• State prohibited noxious weeds - Minnesota Department of Health
• Identify invasive and non-native species - University of Minnesota
• Invasive plants - Board of Water and Soil Resources
• Minnesota Invasive Species Advisory Council
• Invasive Plant Species in Minnesota – Minnesota Wildflowers