Browsing by Author "William F. Hunt, Committee Co-Chair"

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    Effect of Urban Stormwater BMPs on Runoff Temperature in Trout Sensitive Regions
    (2008-11-17) Jones, Matthew Paul; William F. Hunt, Committee Co-Chair; Aziz Amoozegar, Committee Member; Garry L. Grabow, Committee Member; Daniel H. Willits, Committee Co-Chair
    While the negative impact of warm urban stormwater runoff on coldwater stream environments has been studied, little is known about the effect of urban stormwater best management practices (BMPs) on runoff temperature. A monitoring study was conducted from May through October of 2005, 2006, and 2007 in western North Carolina, along the southeastern extent of United States trout populations, to examine the effect of urban stormwater BMPs on runoff temperature. The monitoring sites consisted of a stormwater wetland, wet pond, and four bioretention areas. Runoff temperatures at all monitoring locations significantly (p<0.05) exceeded the 21°C trout temperature threshold from June through September. Monitored runoff temperatures at a parking lot surrounded by a mature tree canopy and a parking lot covered with a light-colored chip seal were cooler than nearby un-shaded and standard asphalt parking lots. Both the stormwater wetland and wet pond increased water temperatures significantly. Effluent temperatures from the wet pond were significantly warmer than flows from the stormwater wetland from June through September. At both sites, water temperatures were coolest at the bottom depths, and water was cooler than 21°C at the bottom of the stormwater wetland during the early summer and early fall, indicating the thermal benefit of an outlet structure that would draw water from these bottom depths. Water was significantly cooler after conveyance in buried pipes when discharged into the stormwater wetland and wet pond. All of the bioretention areas monitored during the course of this study significantly reduced maximum stormwater temperatures; however, only bioretention areas smaller than 10% of their contributing watershed significantly reduced median stormwater temperatures. The larger bioretention areas provided evidence of substantial reductions in runoff volume, which would reduce effluent thermal loads. Despite temperature reductions, all bioretention areas discharged effluent significantly warmer than 21°C during the summer months. Evaluation of bioretention temperature profiles showed that the coolest effluent temperatures could be obtained from bioretention areas with a soil depth between 90 and 120 cm. Due to its ability to reduce runoff temperatures and flows, bioretention areas are considered to be an effective treatment option for mitigating thermal pollution from urban stormwater runoff. A computer model was developed to simulate the thermal dynamics of a bioretention area. The model used a Green-Ampt based approach to simulate bioretention hydraulics. Pavement and runoff temperature were calculated using a finite difference solution for thermal conduction within the pavement profile in conjunction with a surface heat balance. A number of analytical and empirical methods were used to estimate weather parameters and the antecedent soil temperature profile. Soil and water temperature profiles during infiltration were simulated using a model for conduction and convection in porous media that utilized separate energy equations for the fluid and solid phases. The bioretention thermal model was validated by comparing simulation results with temperature data collected during the course of the monitoring study. The majority of simulated storm events had a root mean squared error less than 2.0°C for bioretention effluent estimates. Predicted effluent temperatures were typically warmer than measured values between 10:00 and 17:00 hours, and cooler for the remainder of the day. A sensitivity analysis showed that effluent temperatures were most affected by input parameters related to the soil and pavement surface heat balances. Simulation results suggested that volume reductions had a larger impact on effluent thermal loads than temperature reductions. Overall, the bioretention thermal model was considered to serve as a valuable tool for predicting bioretention effluent temperatures in trout sensitive regions.
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    Urban Stormwater: First Flush Analysis and Treatment by an Undersized Constructed Wetland
    (2007-10-17) Tucker, Robert Smith; Jean Spooner, Committee Chair; Jean Spooner, Committee Chair; William F. Hunt, Committee Co-Chair; William F. Hunt, Committee Co-Chair; Aziz Amoozegar, Committee Member; Aziz Amoozegar, Committee Member
    Nonpoint stormwater runoff remains a leading threat to surface water quality in the U.S. Increased impervious surfaces, climate change, and increasing water demands put even more pressure on stormwater managers to improve stormwater management practices with regards to cost effectiveness, removal performance, and ecological sustainability. More effective BMPs can be designed by understanding the nature of pollutant runoff loads with respect to the hydrograph. Many studies have been performed on the first flush (FF) phenomenon (the assumption that the initial portion of a rainfall-runoff event is more polluted than the later portions). However, controversy remains on whether or not the first flush truly exists, which environmental factors influence a first flush, and how best to define the first flush phenomenon. The objective of this study as to evaluate the first flush occurrence in two small urban watersheds (differing in extent of impervious area) using multiple analytical methods and definitions previously published in the literature. The first watershed is 4.8 acres with 67% impervious roadway and the second watershed is 5.6 acres with 87% wooded land cover. Statistical tests were performed to analyze for site-specific correlations between first flush strength and rainfall characteristics (e.g. rainfall depth, peak flow rate, runoff volume, peak rainfall intensity, and antecedent dry period) and determine differences in FF strength between different land uses and pollutants. Furthermore, the FF study was utilized to perform an annual treatable load analysis in order to evaluate the effectiveness of two hypothetical BMPs sized to treat stormwater from the highly impervious watershed based on the 1.3 cm, 1.9 cm, 2.6 cm and 3.2 cm of rainfall water quality volumes. A year-long study captured stormwater samples from over 33 storm events and analyzed for TSS, turbidity, nutrients and heavy metals. Samples were collected using flow-based sampling frequencies that yield a more accurate quantification of pollutant mass transport throughout the storm event than time-paced sampling used in most of the previous first flush studies. For each collected storm, normalized cumulative pollutant load (L') and runoff volume curves (V') were generated for each pollutant with a minimum of seven discrete curve points to quantify the first flush effect and evaluate for first flush occurrence based on several published methods and definitions (e.g., max L'>V', max L'-V' > 0.2, and 80% of total load runoff in first 30% of total runoff volume). Linear regression analyses were performed to determine the first flush coefficient (b) for the power function L' = V'b to quantify the FF strength. Relationships between the first flush strength and rainfall characteristics were examined. Analysis of covariance (ANCOVA) was utilized to determine if FF strength (i.e. b-value) significantly differed between wooded and impervious watersheds, and a nonparametric ANOVA (Kruskal-Wallis test) was used to evaluate differences in FF strength among the pollutants within each watershed. As indicated by all the methods utilized in this study, most pollutants exhibit a slight FF effect on average but substantial pollutant loading still occurred in the latter portion of the storm's total runoff volume. Thus, to treat the majority of a storm's total pollutant load requires capturing almost the same fraction of runoff volume. Although the FF phenomenon was not dominant, this study did define a "most efficient" design volume (first 40% of runoff or approximately 1.3 cm of rainfall), which was where the fraction of total pollutant load was greatest compared to the fraction of total runoff volume. Of the rainfall characteristics analyzed, rainfall and runoff volume both inversely affected the FF strength of TSS and heavy metals on the impervious watershed. Although the runoff nature of orthophosphate (O-PO4) at the first portion of the storms did not have a first flush, the relative FF strength for O-PO4 actually increased with increasing rainfall or runoff. Land use did not influence the first flush strength of the pollutants except for Pb, which had significantly stronger FF effect on the more impervious watershed compared to the heavily wooded one. Disregarding the estimated pollutant load from a large 18.2 cm tropical storm, the 1.3 cm and 2.6 cm of rainfall design volumes could potentially capture and treat on average 64% and 82% of the annual runoff volume and 67% and 85% of the annual pollutant load for all the pollutants on the impervious watershed. Although stormwater BMPs designed to capture the first 2.5 cm of rainfall can potentially treat a substantial fraction of yearly pollutant load, this study suggests that in watersheds with limited and expensive land area it is more efficient to use multiple smaller BMPs near the source that capture the smaller, more frequent storms (1.3 cm of rainfall or less). Among all the stormwater BMPs currently utilized to reduce peak runoff volumes and remove contaminants from urban runoff, constructed wetlands have emerged as an optimal choice because of their high performance of water quality improvement and ecological benefits. Due to a stormwater wetland's high land requirement, however, they are often difficult to size properly in urban environments. This study also evaluated the pollutant removal efficiencies of a newly constructed flow-through stormwater wetland that is sized to only capture 20% of runoff generated by the recommended design storm in N.C. (first 2.5 cm of rainfall). The wetland was constructed in December 2006 as a retrofit to a failing level-spreader in order to repair a rapidly eroding head-cut between two stormwater outfall channels and the receiving stream. Following addition of the extended detention function, the undersized wetland has initially removed, on average, 71% of the TSS load, between 39% and 60% of the nutrient load, and 60% of the heavy metal load. For all the pollutants except TP and TKN, lower removal efficiencies were observed for the storms with flow bypass (5 out of 9 events since the extended detention function) on average as compared to the smaller events without bypass. When evaluated for all the sampled storms, event mean concentrations proved to be statistically lower at the outlet for all pollutants except Ti and V. For most pollutants, higher influent EMCs were observed for the storms with higher removal efficiencies. It is important to note, however, that these initial results are skewed by a small number of storms with low rainfall depths (median = 0.46 cm). With that considered, the average removal efficiencies are actually higher than predicted for the 1.3 cm design storm BMP. Undersized wetlands with "flow through" design might provide effective and efficient pollutant removals if designed to safely pass the larger storms. Longer term research will provide a more definitive evaluation.