Concrete Boardwalk in Charlotte at Lower McAlpine Creek GreenwayRecently, several hundred members of Charlotte’s local ASCE (American Society of Civil Engineers) and ASLA (American Society of Landscape Architects) heard about the successful completion of a new greenway connector to the Lower McAlpine Creek Greenway in South Charlotte.
Mr. Rod Fritz, Project Manager with Mecklenburg County Parks and Recreation, and Mr. Jeff Oden, Manager of Civil Engineering with Stewart Engineering, spoke at the Carolina Thread Trail Forum in Mooresville on December 6, 2012. They also presented again to a branch meeting for ASCE in Charlotte on January 15, 2013. (For more, click to see their McAlpine Creek Greenway Case Study presentation).
Upon completion of the popular Torringdon development (located on the northwest corner of I-485 and Johnston Road) in June 2008, the developer donated a 1400 lf greenway connector that consisted of a 60 ft truss bridge, 90 ft timber boardwalk, and an asphalt trail. This connector was properly designed and permitted for the floodplain, meeting the “no-rise” requirements. However, soon after Charlotte received several heavy storm events, stream debris became lodged against the structural framing of the truss bridge, thus reducing the effective flow area under the bridge. This debris became not only a maintenance challenge to Mecklenburg County Parks and Recreation, but also costly and unsafe. As the hydraulic flow under the bridge was restricted, flow was forced around the bridge, leading to scouring around the abutments and failing of the asphalt trail. For safety reasons, Mecklenburg County P&R had no choice but to remove the bridge from service (see photo album at bottom of this article).
Earthen Fill versus Elevated Boardwalk
After many evaluations of their options, Mecklenburg County P&R decided the best overall solution was to elevate the bridge 5 ft from its original location. This would allow for increased stream capacity under the bridge. Rather than build earthen berms as ramps on either side, an elevated boardwalk was constructed. Unlike earthen berms, the boardwalk would allow for additional water flow under the boardwalk instead of acting as a dam, limiting flow.
Boardwalk in Floodplains
One of the obvious challenges with boardwalks in floodplains is the fact that generally they spend much of their life either wet or underwater, leading to slippery or decaying conditions for some of the traditional boardwalk materials. Boardwalk ramps within the floodplain should not only be designed as ADA compliant, but should remain ADA compliant throughout the structure’s life span.
In addition to the wet/dry cycle, softer boardwalk materials (southern yellow pine and composites) are not well suited for commercial loading (greenways typically require 5000 – 10000 lbs as design loads). Normal life expectancies are greatly reduced for these timber or composite boardwalk materials in floodplains.
Recognizing the need for a durable boardwalk through the floodplain that could meet the 10,000 lb design load, Mecklenburg County chose to use a new local product, PermaTrak. The boardwalk system was installed on a driven H-pile foundation system and a textured surface was integrated into the boardwalk to improve its slip resistance. Finally, thanks to contractor J.D Goodrum, the elevated greenway connector was opened in November 2012. Greenway enthusiasts from skateboarders to dog walkers were quick to begin using their new elevated greenway connector.
For more information about Lower McAlpine Creek, McMullen Creek and Four Mile Creek Greenways, see Mecklenburg County Parks and Recreation. To learn more about PermaTrak's design through a floodplain or for cost information on this project, contact PermaTrak President Jason Philbin, PE. Enjoy the greenway!