Browsing by Author "Diplas, Panos"

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    Analysis of the vibrations of inflatable dams under overflow conditions
    Wu, Pai-Hung (Virginia Tech, 1995)
    A two-dimensional analysis is applied to the vibrations of inflatable dams under overflow conditions. The static analysis yields the equilibrium state for both the free surface profile and the shape of the dam. The dynamic analysis investigates the small vibrations of the inflatable dam about the equilibrium state. The dam is inextensible, air-inflated, and has two anchored points. The base width, curved perimeter, and internal air pressure are given. The overflow is incompressible, inviscid, and irrotational, and the total head is specified. In the static analysis, the self-weight of the dam is neglected, and the equations of equilibrium from membrane theory are solved by a multiple shooting method. The boundary element method is used to solve Laplace’s equation defined on the overflow domain. An iterative scheme is adopted to obtain the shape of the dam, as well as the location of the free surface. From the equilibrium state, the dynamic analysis is established by a finite difference form of the membrane’s equations of motion and the velocity potential problem is formulated by the boundary element method. After the eigenvalue problem is solved, the eigenvalues and eigenvectors obtained are employed to describe the vibrations of the dam. The effects of the dam‘s density and damping coefficient are illustrated.
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    Evaluation of a Permittivity Sensor for Continuous Monitoring of Suspended Sediment Concentration
    Utley, Barbra Crompton (Virginia Tech, 2009-10-30)
    According to the US Environmental Protection Agency (USEPA) sediment is a leading cause of water quality impairment (US EPA, 2002). The annual costs of sediment pollution in North America alone are estimated to range between $20 and $50 billion (Pimentel et al., 1995; Osterkamp et al, 1998, 2004). Due to the large spatial and temporal variations inherent in sediment transport, suspended sediment measurement is challenging. The overall goal of this research was to develop and test an inexpensive sensor for continuous suspended sediment monitoring in streams. This study was designed to determine if the gain and phase components of permittivity could be used to predict suspended sediment concentrations (SSC). A bench-scale suspension system was designed and tested to guarantee that there were no significant differences in the sediment suspension vertically or horizontally within the system. This study developed prediction models for SSC with input variables of temperature, specific conductivity, and gain and/or phase at multiple frequencies. The permittivity sensor is comprised of an electrode, power source, and a control box or frequency generator. Fixed and mixed effect, multiple, linear regression models were created and compared for target frequencies. However, it was not possible to meet the normality requirements for prediction accuracy. Partial Least Squares (PLS) regression techniques were also applied to gain and phase data for 127 of the 635 frequencies. The three models with the lowest error between predicted and actual values of SSC for validation were further tested with nine levels of independent validation data. The largest model error (error>50%) occurred for the top three models at 0 and 500 mg/L. At the higher concentrations error varied from 1-40%. Once the treatment levels, of the independent validation data set, were near 1000 mg/L the prediction accuracy increased for the top three models. Model 3A, a phase based model, preformed the best. Model 3A was able to predict six of the nine independent validation treatment levels within 300 mg/L. Future research will provide additional laboratory and field testing of the prototype sensor.
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    Influences of Fluctuating Releases on Stream Fishes and Habitat in the Smith River, below Philpott Dam
    Orth, Donald J.; Diplas, Panos; Dolloff, C. Andrew; Newcomb, Tammy J.; Krause, Colin W.; Novinger, Douglas A.; Anderson, Marcy R.; Buhyoff, G. Matthew; Hunter, Anne K.; Shen, Yi (Virginia Tech, 2004-12)
    Operations of Philpott dam for flood control and peak power generation since 1953 have substantially altered downstream ecosystem conditions in the Smith River from the dam to Martinsville. We have described in intensive detail aspects of habitat and fish populations in the tailwater over the past four years (2000 to 2004), analyzed the limited historical information available to develop a better understanding of the mechanisms behind observed changes in the tailwater, and recommend appropriate actions to improve depressed fish populations and environmental conditions. Our research emphasizes that there are no “silver bullet” solutions and the most successful path toward improving the tailwater will reflect numerous tradeoffs to balance environmental, economic, and recreational goals. However, it is clear that enhancing conditions in the Smith River hinge on mitigating the effects of fluctuating releases from Philpott Dam through a combination of flow management (e.g. characteristics of dam operations during baseflow and peak flow periods) and habitat improvement (e.g. channel restoration, temperature management, enhanced biological productivity). In addition, removal or modification of Martinsville Dam to enhance flow, habitat, and fish and sediment passage would benefit fish populations and environmental conditions in the lower tailwater. Current fishery management strategies which are ineffective for enhancing brown trout should be re-evaluated after habitat and flow changes are instituted. Management actions for improving flow and habitat also should be assessed in light of the presence of the Federally Endangered Roanoke logperch Percina rex that also appear to be limited by degraded environmental conditions in the tailwater.