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Incorporating wetland and water quality enhancements into greenway design

A Case Study: Lower McAlpine Greenway, Mecklenburg, NC -- by Gary Wirth

Project Environs:

The project is located in southeastern Mecklenburg County and is at the confluence of McAlpine Creek and McMullen Creek. The overall drainage basin for these creeks drains a large portion of the whole County and, thus, is subject to frequent flooding. The frequency of this flooding has been steadily increasing as the upstream watershed has become urbanized.

This section of Lower McAlpine Greenway is a greenway corridor approximately 4* miles in length located in southern Mecklenburg County, NC. The area of this study is quite residential in character and is completely built-out. Carmel Road is situated on the ridge line and divides the project area into distinct watersheds.

Project Goals and Objective: Planning of this greenway trail encountered many significant wetland and environmentally sensitive areas. Wirth & Associates, Inc., Planners and Landscape Architects, in association with A Fine Line Design, was retained by the Mecklenburg County to analyze the existing conditions, provide recommendations for wetlands mitigation and enhancements, and research the opportunities for water quality improvement techniques that could become standard improvements for all future County greenways.

The goals of the study were to:

1) provide effective management of the storm water runoff while preserving stream ecosystems by implementing similar techniques throughout the watershed as the County constructs its Greenway system; and, 2) Creating public awareness of the value of the floodplain and the role wetlands play in the environment.

The objective was to develop a Demonstration Project Area that will establish the various techniques to be utilized that would:

1) Preserve and Enhance the existing Wetlands; 2) Increase the holding capacity of the wetlands for flood control; 3) Provide opportunities for Pollutant removal and Infiltration of flood waters; and, most importantly, 4) Provide interpretive stations and signage to enhance public perception of wetlands and of their critical role in the natural environment.

Site Inventory The first step in the design process was to understand the existing riparian characteristics of McAlpine Creek, its floodplain, and of the surrounding upland which contributes stormwater runoff to the site. Since many wetland areas existed within the floodplain it was necessary to locate and evaluate the existing functions and values of these areas so that they could be preserved and enhanced where possible.

Analyzing the watershed and the floodplain's components provided key information on the characteristics of the Greenway. Stormwater inflow points from adjacent residential and commercial areas carry increased volumes of stormwater and pollutants. Dedicated easements carry stormwater straight through the floodplain via pipes or channels discharging low volume runoff directly into the creek. This floodplain and its ecosystem are the environment's last defense to the post development runoff and the non-point source pollutants it carries.

The site inventory mapped floodways, contributing watershed areas, drainage easements and utility corridors, inflow and outflow points.

Site Assessment

Since a primary objective of the design was to enhance the functions and values of wetlands, the site was divided into three zones based upon existing wetland characteristics. The first zone consisted of predominantly wetland. These areas generally occurred within the depressions of the floodplain. Some of the wetland areas were heavily forested while others were primarily open water with little or no woody vegetation. Existing wetlands were generally evaluated for their functions and values using a Wetland Rating System developed by the NCDENR Division of Water Quality. Functions and values of wetlands include: flood storage, pollutant removal, shoreline stabilization, benefits to aquatic life, wildlife habitat, recreation, and education.

The second zone within the floodplain was identified as braided area that serves as a transition between existing wetland and surrounding upland. These areas are typically a mosaic of wetland and upland and exhibit characteristics of each. Most of the braided areas consist of a series of small islands and drainageways. In most areas, the understory was very open between stands of mature trees. The surface flow through these areas was indirect and in many areas the velocity of flood waters had scoured channels which reduced the effectiveness of surrounding wetlands for water storage and pollutant removal.

The third zone within the floodplain is comprised of predominantly upland. This zone occurs in areas where the soil is deep and well drained such as along stream levees and along the outer edge of the floodplain. Several broad islands were identified within the floodplain, which were at a slightly higher elevation than surrounding depressional wetlands or braided areas and as a result were more well drained. Most of the upland areas within the floodplain were generally open with widely spaced mature trees and a herbaceous understory. These areas provided the best opportunity for enhancement strategies that would take advantage of the relatively open character and deep well-drained soils.

Eight distinct systems became apparent based upon the inflow and discharge points. The ecosystem's components were quantified and evaluated. Each system was evaluated on its size, drainage basin, amounts of upland, braided and wetland areas, its average width, its Envelope to Basin Ratio and its Upland to Wetland/Braided Ratio.

Conceptual Plan

The conceptual plan employed a series of sixteen enhancement strategies which were divided into three categories: methods which would enhance water quality, methods which would enhance the flood control aspect of the floodplain and methods which would enhance wetland functions and values. One of the most effective strategies employed is the diversion of unfiltered flow from stormwater channels which discharge directly into the creek so that the flow can be retained and infiltrated into floodplain soils thereby removing pollutants and reducing overall flood volume. All of the strategies encouraged the use of natural materials and methods such as bioengineering to minimize impact and to reduce the cost of long term maintenance.

The plan proposes to create enhancements to wetlands, flood storage capacity and water quality. By repairing discharge points and diverting water flows into the wetland and braided areas, residence time of stormwater is increased. This will encourage increased infiltration of stormwater and pollutants as well as increase evapotranspiration of retained water resulting in better water quality. Wetland enhancements, based upon the NC Division of Water Quality rating system, include improvements to wildlife habitat and aquatic life as well as providing recreational and educational opportunities for the general public.

Demonstration Area Plan

System 6 was selected for the initial demonstration project. The decision is based upon the evaluation performed in the Site Assessment, its topographical characteristics and its existing wetland to braided to upland area ratios. This area offers public access and opportunities to implement recreational and educational amenities. Boardwalks will span all jurisdictional areas ensuring minimal impacts to existing wetland systems. Observation structures will be provided with interpretive signage to educate the public to the environmental systems they are witnessing.

Recommendations to enhance existing wetland areas included providing vegetative plantings, wildlife food sources and nesting sites, and in limited circumstances, redirecting surface flow into wetland areas or expanding the size of the area. The benefits of additional wetland vegetation, especially in open water areas enhances nearly all aspects of the functions and values typically associated with wetlands. Recommendations for enhancing transitional braided areas include slowing the velocity of water through them by redirecting flow and by vegetating scoured channels. Increasing the density of understory plantings increases the filtering and pollutant absorption of the area and will provide food sources, cover, and nest sites for wildlife.

Upland areas provide the best opportunity for enhancements to water quality and flood control. Since these areas provide fewer wetland functions and values than the depressional areas and braided transitional areas, there is the greatest room for enhancement. The strategies employed in upland areas include lowering the grade where the soils are open and well drained to provide sites for infiltration of runoff. Planting these areas with vegetation improves the filtering of surface flow and stabilizes the shoreline to prevent erosion. Additional plantings provide for wildlife and increase the evapotranspiration of flood waters so that the volume can be reduced more quickly.

The alignment of the trail, boardwalk and interpretive facilities in the detailed plan are designed to take advantage of the educational and recreational opportunities that the enhancement strategies provide. New plantings and other enhancements provide for better wildlife observation. Enhancements to wetlands and water quality are described with signage and interpretive displays to benefit public education.

Project Summary:

The greenway trail system in Mecklenburg County is intended to support multiple uses in addition to pedestrian access and transportation. Potential additional uses of the trail system include providing enhancements to wetlands, stormwater runoff control, pollutant removal, opportunities for educational interpretation and improving wildlife habitat and biodiversity. In many ways, this Riparian Enhancement effort supports the multiple use aspect of the County's Greenway System.

However, just this one demonstration project area, or even several similar enhancement projects will not have a major impact on either the water quality or flood storage capacity of the floodplain. The major impacts will be achieved as the elements identified in this study are implemented throughout the County Greenway System. Also, as water sheds and drainage basins do not recognize political boundaries, an opportunity arises for a regional commitment to protect stream ecosystems, increase flood storage capacity and improve water quality.

The inventory and analysis process implemented in this study can be repeated throughout the County's greenway system to identify wetland and water quality enhancement opportunities. Implementation of the recommended enhancement strategies throughout the watershed will have the greatest impact on enhancing this and other riparian corridors.

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