About the author: Dan Line, P.E., is water quality extension specialist for the NCSU Water Quality Group. He can be reached at 919.515.8243 or by e-mail at [email protected].
Dan Line, P.E.
undefinedThe Sedgefield Lakes/King’s Mill project was initiated to document the effects of highway construction on the water quality in the Sedgefield Lakes and King’s Mill residential watersheds. Both of these watershed communities encompass small lakes that are highly valued for aesthetic and recreational purposes. The fact that the I-40 bypass around Greensboro, N.C., was being constructed within the watersheds was a concern to residents.
Their main concern was the potential increase in sediment and other pollutants to area lakes. Residents of the Sedgefield Lakes community, in conjunction with the North Carolina State University (NCSU) Biological and Agricultural Engineering Department, began a citizen monitoring program. They began observing tributaries to Sedgefield Lake at least one year prior to the start of construction in their watershed. This effort, combined with several subsequent meetings, led to an agreement with the North Carolina Department of Transportation (NCDOT) to continue and expand the monitoring conducted by NCSU during the period of highway construction.
As part of this agreement, the NCDOT also began meeting quarterly with residents on site to report on monitoring results and hear and address their concerns. Although the NCDOT regularly inspects the highway corridor for sediment control needs, the observations of residents identified potential pollutant sources that needed to be addressed. The close cooperation between residents and the NCDOT improved sediment control and now serves as a model for future projects.
Erosion and Sediment Control
Construction contractors, under the supervision of the NCDOT, installed many erosion and sediment control measures to reduce sediment export from the highway corridor. These included sediment basins with skimmer (floating pipe to drain the basin from near the top of the water column) outlets and coir baffles, sediment traps, rock dams, accelerated seeding and mulching, slope drains, silt fence and flocculant applications on selected areas.
In addition, floating silt or turbidity curtains were installed around the inlets of the two major tributaries to Sedgefield Lake. These two silt curtains were effective for relatively small storm events occurring shortly after the installation; however, visual observation indicated that the longer they remained in place and the larger the storm, the less effective they became at containing the sediment/turbidity plume.
Water Quality Monitoring Stations
The water quality monitoring program involved installing six continuous-monitoring stations: four in the Sedgefield Lakes watershed and two in the King’s Mill watershed. In Sedgefield Lakes, two stations, Ellery-up and Tilly-up, were located on tributary streams immediately downstream of the highway corridor; another two stations, Ellery-down and Tilly-down, were located on the same tributaries, just upstream of the lake.
For the upstream stations, the highway corridor comprised at least 26 percent of the total drainage, with the rest of the area being wooded or residential homes with established lawns and landscape. The intent was that sediment loading to the upstream stations would be almost exclusively from the highway corridor, as the remainder of the drainage area was stable. In the King’s Mill watershed, stations were located on a single tributary upstream and downstream of the highway corridor. The highway corridor comprised 24 percent of the area between the stations.
The Sedgefield Lakes stations were installed about six to eight months before highway construction began. In the King’s Mill watershed, the stations were installed five months before any major earth-disturbing activities took place. Ideally, at least 18 months of data would have been collected prior to major construction activities to adequately characterize background or preconstruction conditions.
For King’s Mill, the upstream station serves as a background site that also may be used for the Sedgefield Lakes sites. The stations were installed from February 2004 to June 2004 and are still in place, although the data reported in this article includes collection through May 2007. This period of monitoring encompasses the major construction grading and other preparation, which is likely the worst period in terms of sediment export.
Each monitoring station was equipped with an automated sampler and flowmeter programmed to continuously record discharge and collect flow-proportional samples. All samples collected at the sites were analyzed for total suspended solids, total solids and turbidity. Measurements of temperature, specific conductance, dissolved oxygen and pH were made occasionally. A recording rain gauge was also maintained near the Tilly-up monitoring sites in the Sedgefield Lakes watershed.
Calculating Sediment Loss
Despite an array of erosion and sediment control measures installed on the highway corridor, sediment loss at the Tilly-up site increased from 0.01 to 7.3 ton/acre per year when comparing the preconstruction and construction periods. The average turbidity of samples went from 25 nephelometric turbidity units (NTU) to 1,530 NTU.
About 40 percent of the total sediment export for the period occurred during two tropical storm systems that hit the Greensboro area in September 2004, with the majority of the export occurring during the second storm. Although these storms dumped more than 8.5 in. of rain on the area in three weeks, neither event was of a 10-year return period magnitude. This is significant, as most erosion and sediment control practices in North Carolina are designed for the 10-year storm event. However, runoff is computed as if the storms occurred individually and not in close time proximity. The close proximity created a very wet antecedent condition, which resulted in increased runoff and more than twice as much sediment export during the second event. Following these storms, additional sediment basins with skimmer outlets and coir fiber baffles, flocculation logs and sediment traps were installed in the drainage area.
Sediment loss rate in the much larger Tilly-down watershed increased from 0.07 ton/acre per year prior to construction to 3.50 ton/acre per year during construction. The much smaller increase was likely due to the fact that the highway corridor encompassed only 15.8 percent of the Tilly-down watershed and that a greater section of the highway was near grade, thereby requiring less fill and making the section less susceptible to erosion. The section of the highway corridor draining to Tilly-up was comprised mostly of a fill section where the roadbed was raised at least 25 ft, thereby creating steep fill slopes that were more vulnerable to erosion. Mean turbidity levels in samples increased from 54 NTU preconstruction to 1,197 NTU during construction, which resulted in a corresponding increase in the turbidity of the downstream lake.
Sediment loss rate at the Ellery-up site increased from 0.04 ton/acre per year before construction to 2.02 ton/acre per year during construction. This increase was less than half that of Tilly-up even though the highway corridor encompassed more than 25 percent of both drainage areas and the section of highway contained considerable fill slopes. The main difference was that construction in the Ellery-up area was at an earlier phase of construction with fewer fill slopes at the time of the tropical storms of September 2004. The highway corridor, as such, was less vulnerable to erosion at the time of the storms.
At Ellery-down, the sediment loss rate increased from 0.20 ton/acre per year before construction to 1.35 ton/acre per year during the construction period. Mean turbidity levels in samples increased from 140 NTU to 504 NTU. These increases can be attributed to a combination of the I-40 bypass construction, the widening of a city road just upstream from the station and residential construction in the watershed.
At King’s Mill, sediment loss upstream the highway corridor was 0.09 ton/acre per year, while downstream it was 1.63 ton/acre per year. The average turbidity of upstream samples was 41 NTU, while downstream it was 593 NTU.
This project highlights the difficulty in controlling sediment export resulting from large storm events that occur at times of high erosion vulnerability, as was the case for the Tilly-up drainage area. Furthermore, the Tilly-up drainage area included a large fill section of highway, which is particularly susceptible to erosion given the steep roadbanks. Continual cooperation and coordination with local residents helped project leaders identify and mitigate erosion and sediment control issues.
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