Browsing by Author "Daniel H. Willits, 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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    A New Application for Brookfield Viscometers: Viscoelastic Property Determination
    (2006-07-29) Tanjore, Deepti; Daniel H. Willits, Committee Co-Chair; Jason A. Osborne, Committee Member; Christopher R. Daubert, Committee Chair
    Viscoelastic properties are traditionally measured using sophisticated and expensive instruments. Brookfield YR-I is an affordable instrument, primarily developed to measure the yield stress of materials with a vane attachment. In recent years, vane attachments have gained popularity from various advantages, including elimination of wall slip, minimum disturbance to sample, quick single point determination, and easy fabrication and cleaning. Prior research has established that the vane can be used to measure elastic shear moduli of materials. This research project attempts to apply the Brookfield YR-I rheometer to the measurement of viscoelastic properties. Different concentrations of gelatin and polyacrylamide gels served as model systems for viscoelastic and elastic materials, respectively. The concepts developed were applied to the torque-time response obtained for these model systems from the Brookfield YR-I. The data compared favorably with viscoelastic data obtained from oscillatory testing with a stress controlled rheometer. The results helped establish a protocol for the Brookfield YR-I to measure viscoelastic properties. Furthermore, certain commercial products were tested using the protocol, but disagreement between the machines was observed in some materials. The effect of cup size and the position of the assumed Newtonian line are potential reasons for the disparity, and future work should focus on refining these aspects.