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Two swales (Mango Creek and I-40) treating highway runoff were retrofitted with simple check dams typically used for construction site sediment and erosion control to determine if these devices might improve post-construction stormwater management. The Mange Creek swale was retrofitted with a pair of standard rock check dams. The I-40 swale was retrofitted with a system of excelsior fiber wattle check dams and bags with proprietary phosphorus-adsorptive media (ViroPhosTM) to investigate if additional treatment for dissolved phosphorus was provided. Each swale was instrumented for approximately six months to collect data prior to the installation of check dams (pre-retrofit period) in March and April of 2014. Monitoring continued for an additional twelve months post-retrofit. Hydrologic data were collected at Mango Creek to examine peak flow mitigation and volume reduction imparted by the check dams due to additional storage and infiltration. Flow-proportional water quality samples were obtained to investigate removal of nitrogen and phosphorus species and total suspended solids (TSS) during the pre- and post-retrofit phases of the research. The inclusion of rock check dams in the Mango Creek swale did not significantly improve the volume reduction or peak flow mitigation of the SCM; however, these statistics were impacted substantially by the small pre-retrofit data set. Improvements were observed for volume reduction, peak flow mitigation, and hydraulic retention time, especially for small (<0.75 inch) and moderate (0.75-1.5 inch) rainfall events. Rock check dams did not significantly improve the removal rates of nutrients or TSS. However, these statistics were again impacted by the small pre-retrofit data set. Total nitrogen (TN) and total phosphorus (TP) concentrations from the highway were low compared to the literature, limiting the potential for significant reduction. Even so, TN and TP concentrations exiting the swale (0.81 mg/L and 0.13 mg/L, respectively) r

This report constitutes the second phase to document the water quality benefits of a stream restoration in the coastal plain of North Carolina. This phase compares the pre-restoration and post-restoration states of the water quality and hydrochemical signature of ‘the canal’ at the Claridge nursery in Goldsboro, NC. We made the hypothesis that a robust way to quantify the water quality benefits of this stream restoration was the use of cumulative nutrient load indicators. However, these indicators required the use of state-of-the-art instruments capable of capturing flow and water quality on a near continuous basis over the long term. After meticulous correction and all the necessary verifications, we were able to show that the restoration of the Claridge nursery canal, effectively creating a flowing wetland, was able to lower the nitrate loads by about 30% over three post-restoration consecutive years. This was accompanied by an overall carbon sequestration. The seasonality of the nitrate retention suggests that much of the nitrate unaccounted for was associated with the growth of vegetation, either through plant uptake and/or through denitrification associated with the release of organic matter from dead vegetation and the exchange conditions that they provided in the channel and with the floodplain. Increased residence time associated with the aquatic vegetation working as a filter time likely increased the capacity of the stream to retain nitrate. The seasonality of the nitrate retention may suggest that shorter monitoring periods could be chosen to represent a ‘summer’ vs. a ‘winter’ effect. However, averaging these effects would not necessarily represent the effective benefits over the long term. Instead, we suggest that monitoring should focus on cumulative indicators used over the long term for projects for which there is a high potential for nutrient retention. Fewer projects may have to be monitored, but using the necessary investment.

Sediment is the most common pollutant affecting North Carolina’s waterways, impacting a range of aquatic organisms, reducing, reservoir capacity, and hurting their aesthetic value. Construction activities, including roadway projects, are a significant, contributor of state-wide sediment loading. The NC DOT program of widening and paving rural roads in the mountain region, provided an opportunity to evaluate new types of roadside erosion control BMPs with the goal of protecting the particularly, sensitive trout streams commonly found there., Two roadway paving projects were each divided into experimental sections installed with either 1) the standard DOT BMPs, consisting of narrow sediment traps and rock checks, 2) the new fiber wattle check dams (consisting of a mix of straw wattles and, coir logs) with 100 grams of granulated polyacrylamide (anionic PAM 705) added to each, or 3) the new wattle check dams alone, (no PAM added)., The results suggest a significant advantage to the use of the new BMPs. At the first site, from June 2006 to March 2007, the, average turbidity values (in NTUs) for the stormwater runoff for were 4,198 for the Standard BMPs, 30 for the Experimental, BMPs with PAM, and 187 for the Exp. BMPs alone. The second site showed similar results with average turbidity values of 64, for the Exp. BMPs with PAM, as compared to 852 for the Standard BMPs. Sediment loading at both sites was similarly skewed, with dramatic decreases in sediment discharged off site from the new BMPs. At the first site, the Standard BMPs lost an average, of 944 lbs (428 kg) of sediment per storm event as compared to just 1.93 lbs (0.88 kg) for the BMPs with PAM and 6.53 lbs (2.96, kg) for the Exp. BMPs alone. At the second site, the Standard BMPs lost an average of 8.84 lbs (3.63 kg) per storm event, compared with 1.67 lbs (0.76 kg) for the Exp. BMPs with PAM.

Stormwater runoff from roadways is a source of surface water pollution in North Carolina. The North Carolina Department of Transportation (NCDOT) is required to implement stormwater control measures (SCMs) in the linear environment. NCDOT has specific interest in runoff from bridge decks, which is often discharged directly to a stream below. The research presented herein focuses on retrofit stormwater SCMs for bridge deck runoff management. Two bioretention basins and a swale were constructed in the easement of a bridge deck on I-540 at Mango Creek in Knightdale, North Carolina. One bioretention basin was sized to capture (without overtopping) runoff from the 0.7 inch event. The second basin was undersized by one-half (as compared to the large cell) and captured runoff from the 0.4 inch event. Undersized bioretention basins might often be used in retrofit situations when space is limited; therefore, it is important to understand how an undersized bioretention basin performs with respect to hydrology and water quality. Both bioretention basins employed 20 in (51 cm) of fill media, and had an internal water storage layer (IWS) of 2 ft (0.6 m) including the gravel drainage layer. The swale was designed to convey the ten-year storm event without overtopping. Runoff was piped from the northbound and southbound lanes to the bioretention basins and swale, respectively., Data collection began in October 2009 and continued through April 2010. Weirs and stage recorders were used to monitor inflow to and outflow from each SCM. Flow-proportional, composite water quality samples were obtained at the inlet and outlet of each SCM. Monitored water quality parameters included TKN, NO2,3-N, NH4-N, TN, TP, TSS, Cu, Zn, and Pb. TN was calculated by summing TKN and NO2,3-N. For small storms [those with less than 1 in (25 mm) rainfall depth], flow volume reductions for the large and small bioretention basins were 69% and 47%, respectively. This shows the hydrologic importance of properly sizing bioretention basins when space is available. However, some benefit is also associated with undersized systems, if space is too limited to allow for “full sizing.” There was a 23% volume reduction benefit associated with the swale., Average concentrations of TN (0.74 mg/L), TP (0.12 mg/L), and TSS (32 mg/L) from the bridge decks were relatively low when compared to other highways and paved surfaces previously monitored in North Carolina. Median effluent concentrations for the large bioretention basin were lower than those for the small bioretention basin for all nutrient forms and sediment.

This report summarizes the results of a two-year water quality monitoring project to document the effects of, the construction of the Highway 1 bypass on the water quality of Crane (Crains) Creek and the Little River., Automated monitoring equipment were installed upstream and downstream of the highway corridor on both Crane, Creek and the Little River. For Crane Creek, discharge was monitored and samples of creek water were collected on, a flow-proportional basis throughout the project, while for the Little River samples were collected every 6-8 hours, during the project. A recording raingage was also maintained for most of the study at Crane Creek and in-situ, measurements of temperature, dissolved oxygen, conductivity, and pH were made at least monthly., The mean suspended sediment concentration and turbidity for Crane Creek was 48 mg/L and 40 ntu upstream, and 38 mg/L and 26 ntu downstream of the highway corridor. Statistical analysis of the bi-weekly sediment load, data from both sites showed that the loads at the upstream site were not significantly different than the downstream, site indicating that the construction had no effect on sediment loads of Crane Creek. The mean turbidity of samples, was greater upstream compared to downstream, which also indicates no negative effect of highway construction., Means of temperature, dissolved oxygen, conductivity, and pH were nearly the same upstream and downstream, indicating no effect on these water quality parameters., Monitoring results for the Little River were similar to Crane Creek in that there were no significant, differences between upstream and downstream sites according to paired t-tests conducted on the bi-weekly data. The, mean suspended sediment

A three year research project was conducted to develop and test a method to predict the distance borrow pits should, be set back from adjacent wetlands to avoid detrimental impacts on wetland hydrology. The method is based on the, Boussinesq equation. Knowledge of soil hydraulic properties and the time, T25, required for characteristic water table, drawdown is necessary to calculate the setback. DRAINMOD simulations were conducted to determine the T25, values for all 100 North Carolina counties. A survey of 27 borrow pits indicated seepage from the wetland to the pit, in 42% of the pits, pit to the wetland in 31% and sometimes in one direction and vice versa in 27% of the pits. Eight, borrow pits were instrumented to determine the response of the water level in the pit, and the water table in adjacent, wetlands, to rainfall, evapotranspiration and seepage. The time required after closure for the water level in pits to rise, to equilibrium varied from 10 to 23 months with an average of 17 months. The equilibrium water level is a critical, factor affecting the impact of the pit on hydrology of adjacent wetlands. This depth depends primarily on the, elevation of surface water outlets, the depths of adjacent drainage ditches or other subsurface drainage sinks in close, proximity to the pit, and the natural depth of the water table prior to the construction of the pit. Equilibrium water, levels in 5 of the 8 instrumented pits were lower than the surface of adjacent wetlands, with seepage from the wetland, to the pit. The average water levels in the 5 pits ranged from 1.0 to 4.1 ft. below the surface of adjacent wetlands., Water level elevations in the other 3 pits were equal to or above the elevation of adjacent wetlands. The method, developed herein was used to predict setback requirements for the 5 instrumented borrow pits having seepage from, wetlands to the pit. Results were compared to the potential lateral impacts determined from measured water table, data. The setback distances predicted using the measured pit water levels were about 60% greater than the measured, impacts on average. The method is based on conservative assumptions so the overestimation was not unexpected., Use of a constant 2 ft. depth of water in the pit predicted setback requirements within 16% of measured potential, impacts for 3 of the 5 cases; the setback was overestimated by more than a factor of 2 for the other two pits., However, the projected lateral impacts were less than 50 ft. in both cases. It was concluded that the proposed method, will provide conservative but reasonable estimates of the setback necessary to avoid impacts on the hydrology of, wetlands adjacent to borrow pits.

Stormwater runoff from roadways is a source of surface water pollution in North Carolina. The North Carolina Department of Transportation (NCDOT) is required to implement stormwater control measures (SCMs) in the linear environment. NCDOT has specific interest in evaluating pollutant loads from interstate highways and potential stormwater treatment measures. The research presented herein focuses on monitoring of highway runoff at four sites along Interstate 40 (I-40) in Johnston, Sampson, and Duplin counties. This entire stretch of I-40 had a permeable overlay [known as a permeable friction course (PFC)] applied in November, 1998. The overlay is porous, and allows water to pass through the surface of the pavement, reducing splash during rainfall and allowing for improved vehicle traction (Barrett et al. 2006). Drainage from the PFC was monitored at all four sites to determine highway pollutant concentrations and loads. Roadside filter strips are nearly ubiquitous on highways, as they are constructed to make grade and to hydraulically connect the roadway to the roadside swale. Two roadside filter strips (21.5 ft in width) were evaluated in this study. Finally, four linear roadside swales were monitored to determine their hydrologic and water quality benefits. Two of these swales were dry swales, meaning that they drained inter-event. The other two swales had wetland characteristics, including hydrophytic vegetation, hydric soils, and wetland hydrology.

FHWA/NC/20YY-NN, Appendices included as Supplemental File, Stormwater conveyance networks contribute to stream degradation and instability either through direct discharges of sediment or by eroding gullies downslope of pipe outlets. To limit erosion downslope of pipe outlets, current regulations require the 10-yr, 24-hr peak velocity discharged to a vegetated area to be less than the downslope soils’ permissible velocity. Otherwise, the conveyance system must be redesigned. The effectiveness of this design standard has not been evaluated. North Carolina State University assessed 60 pipe outlets in the Piedmont and Mountain physiographic regions of North Carolina to identify which watershed and downslope characteristics influence the severity of erosion caused by pipe outlets. The effectiveness of the current standard was also assessed. Six assessed sites in Raleigh were additionally monitored for hydrologic, hydraulic, and water quality impacts. These data were also used to evaluate the effectiveness of a swale with (1) rip-rap, (2) well-established vegetation, (3), check dams with mowed turf grass, and (4) check dams with well-established vegetation to limit downslope erosion. The results indicate the current standard does not limit downslope erosion, and designers should consider limiting the 1-yr, 24-hr peak velocity to the permissible velocity. Designers should also consider the downslope soil and vegetative conditions to reduce the potential for gully erosion. Designers can use the predictive tools developed from the data to help determine the optimal discharge point for pipe outlets. Preliminary results suggest turf grass swales with check dams may mitigate the potential for erosion downslope of pipe outlets. However, this design should be validated with field-scale studies before standards for pipe outlets are revised.