In September 2004, the town of Newland was among the communities in western North Carolina that was severely affected by the flooding caused by the remnants of Tropical Storm Frances and Hurricane Ivan. Thirty-five businesses and six residential structures experienced major flooding, numerous roads in the downtown area of this small community overtopped, and several culverts washed out or were destroyed. With flooding so severe, the town of Newland was included in the planning report administered by the North Carolina Department of Environment and Natural Resources–Division of Water Resources (DWR) through Senate Bill 7, which identified a series of projects in the western part of North Carolina that, when completed, were expected to alleviate future flooding problems. The Newland By-Pass Channel project was one of those identified. As outlined in the original DWR report, the project would involve careful excavation of an overflow relief channel through a restrictive section of the floodplain located 1,500 feet downstream of downtown Newland.
“We were in crisis mode after Ivan,” says Keith Hoilman, public works director. “We had been dodging bullets for years until 2004, and we knew we couldn’t afford to take another hit from Mother Nature.”
Through evaluation of the original concept and a detailed hydraulic investigation, W. K. Dickson & Co. Inc. out of Raleigh recognized that, as proposed, the project would not help the town realize the flood mitigation benefits it had intended. W. K. Dickson proposed an alternative design and worked closely with local officials to develop a project that met the town’s flood reduction goals and created added value for the region.
Notching a Mountain
The Newland By-Pass Channel was a “chance of a lifetime” project for a water resources engineer involved with floodplain management. Excavating an entire side of a large mountain hillside to create a new emergency overflow is not your typical floodplain improvement project. To construct the 65-foot-wide grass-lined by-pass channel, it was necessary to excavate to a depth of approximately 20 feet to create the connection to the downstream floodplain.
One of the main contributing causes of flooding in Newland involved the collection of debris in the downstream restrictive floodplain. The mountainous watershed supplied an excessive volume of woody debris that created a 75-foot-wide, 5-foot-high dam that further exacerbated the September 2004 flooding. The by-pass channel was designed not only to provide increased floodplain capacity but also to reduce the future potential for woody debris to accumulate by providing an additional maintained floodplain free of foliage.
Within two weeks of construction, the town saw 6 inches of rain over a 24-hour period, which caused floodwaters to activate the by-pass channel to a depth of 18 inches. The unvegetated channel held up extremely well with no measurable loss of topsoil. “Having the channel hold up so close to construction gave us confidence that maintenance would not be an issue for the long-term sustainability of the channel,” says Hoilman.
A Nested Channel
One of the biggest challenges of the project involved constructing a new greenway trail and floodplain benches inside the narrow downtown stream corridor, which was flanked with numerous utilities, HVAC units, and building foundations. To make best use of the flood-carrying capacity of the channel and mitigate future flooding in Newland, it was necessary to maximize the size of the channel without jeopardizing the structural integrity of the nearby shopping center and other downtown businesses.
A unique nested channel was designed to activate in any two-year flood event yet quickly overtop and access the floodplain benches in the larger flood events. To construct this nested channel, the design team proposed a geogrid lift wall with a boulder façade. The geogrid lift wall was designed to withstand the lateral forces of the buildings, and the boulder front face was designed to protect the geogrid wall from the erosive force of large flood events.
Allowing the greenway trail to flood in the case of a two-year flood event helps mitigate future flooding, but not at the expense of creating a maintenance issue for the town or creating a channel that is not stable and sustainable. If the greenway was set too low, the town would be forced to clean the sand, gravel, and other debris off the trail too frequently following even relatively small flood events. Creating a design based on a two-year flood event allowed for flood mitigation goals to be achieved without creating a maintenance nuisance for the town.
“If we had implemented the original plan provided by DWR, we might have seen some improvement downstream, but it would not have provided the much-needed flood relief in the downtown business district,” adds Hoilman. “Instead, W. K. Dickson’s design greatly reduced our susceptibility to future flooding and added a great recreation feature at the same time.”
Added Value
W. K. Dickson served as the lead civil engineering consultant to DWR and was responsible for developing the overall concept plan, final engineering study, and construction documents, as well as performing construction administration and securing all required environmental permits. Recognizing that the primary objective of the design was to maximize flood reduction potential without significantly affecting project cost or adversely impacting the stability of the channel, nearby buildings, or the sensitive trout habitat, W. K. Dickson applied additional vision and saw an opportunity to achieve the primary goals and much more.
A low-impact, environmentally friendly project would also improve the functionality of the downtown greenway system, recover the river’s trout population, increase local and commercial property values, and help speed up the process of securing permits from the various regulatory agencies in the future. For this reason, W. K. Dickson focused its design efforts on creating a project that took advantage of the natural surroundings, connected two parks located on opposite sides of Highway 194 (Pineola Street), enhanced the trout habitat, and improved water quality. The team also focused on creating a partnership with town officials to assist with the local buy-in from property owners and the town itself.
To fulfill the expanded vision of the project, W. K. Dickson was faced with solving some complex challenges. A few of the more complex components included:
- Construct a greenway trail through one of the existing Pineola Street concrete box culverts without reducing flood reduction potential upstream and without creating a maintenance challenge for the town. This was accomplished by designing a wall that could be overtopped by small flood events but not overtopped every time it rained. This concrete wall also includes a series of drains with backflow preventers to allow floodwaters to be effectively drained following storms.
- Innovatively collect the stormwater runoff from the Pineola Street culvert to prevent standing water inside the culvert. To create positive drainage, it was necessary to first fill the fourth barrel of the quad 11-foot by 10-foot box culverts with 2 feet of concrete. The base of the culvert was further modified to include a “notched wedge” channel that provided positive drainage and did so without creating a safety hazard to users of the greenway.
- Nine environmental permits were required and obtained successfully from the local, state, and federal regulatory agencies, including a FEMA Conditional Letter of Map Revision and a NCDOT encroachment agreement. The original design was modified to eliminate the boulder and geogrid lift walls from inside the NCDOT right of way.
- One of the more complex challenges with modeling the FEMA floodplain involved determining the downstream impacts to insurable structures as a result of excavating the new by-pass channel. An EPA SWMM model was used to determine the peak flows that would be input into the HEC-RAS split flow analysis. The elevation of the by-pass channel base was iteratively changed until there was a balance to upstream flood reduction and increases to downstream peak flows.
- One of the primary focuses of the by-pass channel was to limit flood velocities to 4 feet per second in a 10-year flood event. This was achieved by iteratively adjusting the width, elevation, and slope of the channel. Also included with the project was a concrete level spreader designed to diffuse flow prior to entering the downstream floodplain.
Conclusions
The Newland By-Pass Channel project is a true example of creative engineering and the chance for a consulting firm to think beyond the initial goal of a project and explore added value benefits to the client and the community at large. Brenda Pittman, Newland’s town manager, says, “The innovation W. K. Dickson brought to the design of this project not only accomplished our flood reduction goals, it also improved safety and enhanced the recreational infrastructure of our community by connecting two downtown parks and improving the water quality of the North Toe River. We couldn’t be happier with the outcome.”
Since its completion, the project has been met with a great deal of enthusiasm from local residents and business owners as well. Cary Dominguez, owner of the Ugly Mugg Bistro, says, “We see a whole new group of people that now use the greenway at lunch, which very often results with someone stopping into our restaurant. Business has never been better.”
But even more important, the new by-pass channel has already been activated by three large flood events in recent months (each classified to be between a 10- and 50-year flood event) with no floodwater leaving the channel banks in the downtown business district. Following one of the wettest spring seasons on record, the town saw close to 8 inches of rain on May 2, 2013, over an eight-hour period. This equated to approximately a 50-year flood and exceeded the rainfall totals from Hurricane Ivan that subjected the town to some extreme flooding.
The excitement and enthusiasm of the project has been enhanced by the elements town leaders and W. K. Dickson added to the project: The green-way trail is heavily used by pedestrians, the trout that were missing have returned and are thriving, and the new riverwalk has become a sense of pride for the local community.