Drainage plan proposed for northern Roanoke Island
Published 11:12 am Saturday, October 5, 2019
A phased plan for drainage improvement on northern Roanoke Island will help drain stormwater faster.
Drainage recommendations were presented by Ryan Smith with Moffatt & Nichol, the consulting firm hired by the North Carolina Department of Transportation to study the Brakewood subdivision and surrounding areas.
In July 2018, Roanoke Island received 15 inches of rain. The expectation is that infiltration will drain away stormwater. With a substantial rainfall, “the groundwater table rises to the surface and limits the potential for drainage by infiltration.” In one 16-hour period, the area received 5.17 inches of rain.
A model was created to simulate current flood conditions using topography, building footprints, land cover, pipes and open ditches and soil infiltration patterns. The study assumed that the water table was saturated. The existing conditions model was adjusted by resident observations.
The report states “much of the persistent flooding that occurs in the Brakewood subdivision and surrounding neighborhood areas is due to a lack of stormwater infrastructure and/or connection to existing outlets that allow for flood relief.”
The study recommends three phases for improving drainage in the area.
Phase 1 calls for upsizing existing stormwater pipes to expand the flow capacity of the downstream end of the study area. The plan calls for increasing the size of four culverts and installing a new connection between Airport Road and the surface ditch near Roanoke Trail. The estimated construction cost is $765,180 with a 25 percent contingency of $191,296 for a total conceptual level cost of $956,475. The estimate does not include costs for engineering or easement acquisition.
Phase 2 proposes to create a new surface ditch to reroute flow from the northern part of Airport Road to relieve the Airport Road system. The new ditch would run south along the woods and then tie into an existing ditch near Old County Road.
The estimated construction cost is $474,985 with a 25 percent contingency of $118,746.25 for a total conceptual level cost of $593,731.25. The estimate does not include costs for engineering or easement acquisition.
Phase 3 addresses flooding in neighborhoods with a series of suggestions:
– Install roadside ditches with culverts on Daphne Lane, Bradford Lane and Langley Lane to collect water and convey to Airport Road system. For Bradford and Langley Lane, stormwater would flow to existing basin area and then to Airport Road system via a new culvert.
– Grade two areas to channel stormwater to the U.S. 64/264 system.
The proposed grading work would be 20 feet wide tapered to two-foot depth.
The estimated construction cost is $ 776,500 with a 40 percent contingency of $310,600 for a total conceptual level cost of $1,087,100. The estimate does not include costs for engineering or easement acquisition.
The total construction cost for all three phases is $2,016,665. With contingencies, the cost is $2,637,306.
The almost 50 people at the meeting peppered Ryan Smith with questions.
Smith estimated that Phase I would take six to 12 months to construct and Phase II, three to six months. He did not venture an estimate for Phase 3, the largest project, which involves private property owners.
Another issue is maintenance. One speaker reported that the existing swales are private property, that the county cut a ditch in August and that NCDOT maintains Airport Road.
Smith said Holly Ridge Road is not addressed, but the road is “pretty close” to a drainage conveyance, namely a creek.
All seven commissioners attended the presentation.
At one point a questioner veered toward a policy question. County manager Robert L. Outten responded: “it’s illegal to have a policy discussion.” He indicated the board would take up the study at a future meeting. He also said the county will need to consult with the Dare County Airport Authority and NCDOT.
The meeting was held Sept. 23 in the Dare County commissioners meeting room in Manteo.
The document, made available at the meeting, has maps showing the effects of the various recommendations.
The entire plan is available by calling planning director Donna Creef at 252-475-5973.