VTechWorks Repository :: Browsing by Author "Kibler, David F."

Browsing by Author "Kibler, David F."

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The Accuracy of River Bed Sediment Samples
Petrie, John Eric (Virginia Tech, 1998-12-17)
One of the most important factors that influences a stream's hydraulic and ecological health is the streambed's sediment size distribution. This distribution affects streambed stability, sediment transport rates, and flood levels by defining the roughness of the stream channel. Adverse effects on water quality and wildlife can be expected when excessive fine sediments enter a stream. Many chemicals and toxic materials are transported through streams by binding to fine sediments. Increases in fine sediments also seriously impact the survival of fish species present in the stream. Fine sediments fill tiny spaces between larger particles thereby denying fish embryos the necessary fresh water to survive. Reforestation, constructed wetlands, and slope stabilization are a few management practices typically utilized to reduce the amount of sediment entering a stream. To effectively gauge the success of these techniques, the sediment size distribution of the stream must be monitored. Gravel bed streams are typically stratified vertically, in terms of particle size, in three layers, with each layer having its own distinct grain size distribution. The top two layers of the stream bed, the pavement and subpavement, are the most significant in determining the characteristics of the stream. These top two layers are only as thick as the largest particle size contained within each layer. This vertical stratification by particle size makes it difficult to characterize the grain size distribution of the surface layer. The traditional bulk or volume sampling procedure removes a specified volume of material from the stream bed. However, if the bed exhibits vertical stratification, the volume sample will mix different populations, resulting in inaccurate sample results. To obtain accurate results for the pavement size distribution, a surface oriented sampling technique must be employed. The most common types of surface oriented sampling are grid and areal sampling. Due to limitations in the sampling techniques, grid samples typically truncate the sample at the finer grain sizes, while areal samples typically truncate the sample at the coarser grain sizes. When combined with an analysis technique, either frequency-by-number or frequency-by-weight, the sample results can be represented in terms of a cumulative grain size distribution. However, the results of different sampling and analysis procedures can lead to biased results, which are not equivalent to traditional volume sampling results. Different conversions, dependent on both the sampling and analysis technique, are employed to remove the bias from surface sample results. The topic of the present study is to determine the accuracy of sediment samples obtained by the different sampling techniques. Knowing the accuracy of a sample is imperative if the sample results are to be meaningful. Different methods are discussed for placing confidence intervals on grid sample results based on statistical distributions. The binomial distribution and its approximation with the normal distribution have been suggested for these confidence intervals in previous studies. In this study, the use of the multinomial distribution for these confidence intervals is also explored. The multinomial distribution seems to best represent the grid sampling process. Based on analyses of the different distributions, recommendations are made. Additionally, figures are given to estimate the grid sample size necessary to achieve a required accuracy for each distribution. This type of sample size determination figure is extremely useful when preparing for grid sampling in the field. Accuracy and sample size determination for areal and volume samples present difficulties not encountered with grid sampling. The variability in number of particles contained in the sample coupled with the wide range of particle sizes present make direct statistical analysis impossible. Limited studies have been reported on the necessary volume to sample for gravel deposits. The majority of these studies make recommendations based on empirical results that may not be applicable to different size distributions. Even fewer studies have been published that address the issue of areal sample size. However, using grid sample results as a basis, a technique is presented to estimate the necessary sizes for areal and volume samples. These areal and volume sample sizes are designed to match the accuracy of the original grid sample for a specified grain size percentile of interest. Obtaining grid and areal results with the same accuracy can be useful when considering hybrid samples. A hybrid sample represents a combination of grid and areal sample results that give a final grain size distribution curve that is not truncated. Laboratory experiments were performed on synthetic stream beds to test these theories. The synthetic stream beds were created using both glass beads and natural sediments. Reducing sampling errors and obtaining accurate samples in the field are also briefly discussed. Additionally, recommendations are also made for using the most efficient sampling technique to achieve the required accuracy.
Analysis of Roanoke Region Weather Patterns Under Global Teleconnections
LaRocque, Eric John (Virginia Tech, 2006-02-07)
This work attempts to relate global teleconnections, through physical phenomena such as the El Nino-Southern Oscillation (ENSO), Artic Oscillation (AO), North Atlantic Oscillation (NAO), and the Pacific North American (PNA) pattern to synoptic-scale weather patterns and precipitation in the Roanoke, Virginia region. The first chapter describes the behavior of the El Nino-Southern Oscillation (ENSO) by implementing non-homogeneous and homogeneous Markov Chain models on a monthly time series of the Troup Southern Oscillation Index (SOI), a sea level pressure based index. Meanwhile, in the second chapter the author has related or an attempt has been made to relate global teleconnections (through ENSO and AO) to a synoptic scale, station-centered set of weather types in order to assess trends in precipitation. The final portion of this work describes spatial variability of seasonal precipitation in southwestern Virginia in a context that incorporates global teleconnections (through AO, PNA, NAO, and ENSO) and frontogenesis. It was found that the Markov property can be used to describe and predict the monthly evolution of ENSO. Also evident is an increased probability of a wetter spring in the Roanoke region when El Nino combines with the negative phase of the AO during the previous winter. Meanwhile, Roanoke winters subsequent to a fall season described by this same El Nino-AO condition are predicted to receive more precipitation than average. This work additionally showed possible trends between frontal-precipitation events in the Roanoke region and global teleconnections.
Application of the Analytic Hierarchy Process Optimization Algorithm in Best Management Practice Selection
Young, Kevin D. (Virginia Tech, 2006-05-30)
The efficiency of a best management practice (BMP) is defined simply as a measure of how well the practice or series of practices removes targeted pollutants. While this concept is relatively simple, mathematical attempts to quantify BMP efficiency are numerous and complex. Intuitively, the pollutant removal capability of a BMP should be fundamental to the BMP selection process. However, as evidenced by the absence of removal efficiency as an influential criterion in many BMP selection procedures, it is typically not at the forefront of the BMP selection and design process. Additionally, of particular interest to any developer or municipal agency is the financial impact of implementing a BMP. Not only does the implementation cost exist, but there are long-term maintenance costs associated with almost any BMP. Much like pollutant removal efficiency, implementation and maintenance costs seem as though they should be integral considerations in the BMP selection process. However, selection flow charts and matrices employed by many localities neglect these considerations. Among the categories of criteria to consider in selecting a BMP for a particular site or objective are site-specific characteristics; local, state, and federal ordinances; and implementation and long-term maintenance costs. A consideration such as long-term maintenance cost may manifest itself in a very subjective fashion during the selection process. For example, a BMPs cost may be of very limited interest to the reviewing locality, whereas cost may be the dominant selection criterion in the eyes of a developer. By contrast, the pollutant removal efficiency of a BMP may be necessarily prioritized in the selection process because of the required adherence to governing legislation. These are merely two possible criteria influencing selection. As more and more selection criteria are considered, the task of objectively and optimally selecting a BMP becomes increasingly complex. One mathematical approach for optimization in the face of multiple influential criteria is the Analytic Hierarchy Process. "The analytic hierarchy process (AHP) provides the objective mathematics to process the inescapably subjective and personal preferences of an individual or a group in making a decision" (Schmoldt, 2001, pg. 15). This paper details the development of two categories of comprehensive BMP selection matrices expressing long-term pollutant removal performance and annual maintenance and operations cost respectively. Additionally, the AHP is applied in multiple scenarios to demonstrate the optimized selection of a single BMP among multiple competing BMP alternatives. Pairwise rankings of competing BMP alternatives are founded on a detailed literature review of the most popular BMPs presently implemented throughout the United States.
Assessing Drought Flows For Yield Estimation
Gillespie, Jason Carter (Virginia Tech, 2002-12-12)
Determining safe yield of an existing water supply is a basic aspect of water supply planning. Where water is withdrawn from a river directly without any storage, the withdrawal is constrained by the worst drought flow in the river. There is no flexibility for operational adjustments other than implementing conservation measures. Where there is a storage reservoir, yields higher than the flow in the source stream can be maintained for a period of time by releasing the water in storage. The determination of safe yield in this situation requires elaborate computation. This thesis presents a synthesis of methods of drought flow analysis and yield estimation. The yield depends on both the magnitude of the deficit and its temporal distribution. A new Markov chain analysis for assessing frequencies of annual flows is proposed. The Markov chain results compare very well with the empirical data analysis. Another advantage of the Markov chain analysis is that both high and low flows are considered simultaneously; no separate analyses for the lower and upper tails of the distribution are necessary. The temporal distribution of drought flows is considered with the aid of the generalized bootstrap method, time series analysis, and cluster sequencing of worsening droughts called Waitt's procedure. The methods are applied to drought inflows for three different water supply reservoirs in Spotsylvania County, Virginia, and different yield estimates are obtained.
Assessing the Nonpoint Source Pollutant Removal Efficiencies of a Two-Basin Stormwater Management System in an Urbanizing Watershed
Lovern, Sharla Benjamin (Virginia Tech, 2000-05-15)
Monitoring of a regional stormwater management facility, located on the Virginia Tech campus in Blacksburg VA, was conducted in order to assess its efficacy in reducing nonpoint source pollutant losses downstream. The facility design includes both an upper water quality (wet) pond and a lower 100-yr-event quantity (dry) pond. These on-stream ponds capture both baseflow and storm runoff from the southern portion of the Virginia Tech campus and surrounding lands, and release the water back to the unnamed stream shortly above its conjunction with Stroubles Creek, a tributary of the New River. Monitoring sites for flow measurement, water quality sampling, and biotic assessments (habitat evaluation and rapid bioassessment of benthic macroinvertebrates) were located above and below each of the ponds. Both grab samples and automated samples were collected at these stations. Between 1997 and 1999, water quality grab samples included 35 baseflow samples and 22 stormflow samples. The grab samples were analyzed for concentrations of total suspended solids (TSS), metals, bacteria, and nutrients as well as temperature, pH, dissolved oxygen, conductivity, total organic carbon (TOC), and chemical oxygen demand (COD). Automated flow-weighted sampling was initiated in February of 1999 and results are reported through the end of October 1999. Thirty-three storms in 1999 were monitored for flow and various water quality parameters (TSS, TOC, COD, and nutrients). Pollutant loads and pollutant removal estimates were calculated with regard to the wet pond, dry pond, and the combined facility. Two types of pollutant removal efficiencies were calculated: (1) the EMC efficiency, based on pollutant concentrations from individual storms; and (2) the SOL efficiency, based on pollutant loads, to estimate long-term performance over the study period. Benthic macroinvertebrate sampling and habitat assessment were performed in both 1997 and 1999. In addition, a preliminary investigation of pond characteristics was conducted, including measurements of water quality and composition, sediment deposition and composition, and residence time. As a system, the stormwater management facility appears to have minimum impact on improving the downstream water quality. Pollutant concentrations and loads both appear to increase downstream of the facility as compared to upstream, during both storm event and baseflow periods. Monitoring results of the benthic assemblages showed evidence of moderate to high impairment at all sampling locations, and habitat assessments showed evidence of high sedimentation levels within the stream, even after installation of the stormwater management facility. Total suspended solids (TSS) concentration removal efficiency was 10% for the combined wet pond and dry pond system, much lower than the 80 to 90% TSS removal expected for properly functioning stormwater management facilities (Hartigan, 1989). There is some evidence of sedimentation within the ponds because of a slight reduction in sediment-bound constituent export, but the dissolved nutrient constituents had either very low and most often negative (indicating pollutant export) removal efficiencies. Concentrations of metals measured in the stream often exceeded their respective acute and chronic water quality criteria at all sampling locations. Pollutant removal efficiencies measured in the wet pond are atypical of those reported in the literature (Schueler, 1993). Insufficient residence time (two days compared to the optimal two weeks), and wet pond embankment failure are likely the principal causes of the wet pond's inadequate performance and thus, the inadequate performance of the overall facility. TSS removal efficiencies were low in the wet pond (19% for concentrations and 33% for loads) compared to the 80 to 90% expected for similar ponds. Nevertheless, the wet pond reduced the concentrations of several pollutants typically associated with TSS and not likely to be associated with the fill material for the wet pond embankment. Zinc concentrations in sediment cores were highest near the pond inlet, where the majority of sedimentation occurs. During storm events, the following results were noted. Copper and zinc concentrations in 1998 were lower at the pond outlet as compared to the pond inlet, and TOC concentrations and loads were also reduced by the wet pond (13% for concentrations and 12% for loads). However, sedimentation is also expected to remove phosphorusl, and wet pond phosphorus loads were only reduced by 10% and 3% for orthophosphorus and total phosphorus, respectively. Because the wet pond is undersized with respect to the watershed it serves (surface area less than 1% of the watershed area (0.87 ha), as compared to the 3% ratio often recommended for optimal pollutant removal (Athanas, 1988)), higher removal efficiencies were found during baseflow periods. The greatest reductions in baseflow concentrations were for ammonia (67%), nitrate (57%), total nitrogen (54%), and COD (45%). However, the residence time of two days appears to be insufficient to reduce fecal coliform concentrations in the stream, and over 40% of the fecal coliform samples collected exceeded the water quality standard for contact recreation (DEQ-WQS, 1997). Furthermore, the wet pond did not appear to reduce TSS or TOC during baseflow periods. Export of TSS (-29% EMC efficiency) and TOC (-44% EMC efficiency) from the wet pond during baseflow periods is likely due to the wet pond embankment failure as well as pond eutrophication. Eutrophication processes are favored by the water temperature increase as flow passes through the shallow wet pond. The wet pond increased downstream temperatures by approximately 8°C above inflow temperatures during the summer, and to levels above 21°C which cannot be tolerated by sensitive coldwater species (Schueler, 1987). The dry pond did not remove dissolved nutrient constituents or other pollutants during baseflow periods, but there is some evidence of sedimentation within the dry pond during storm events. During storm events, the dry pond was effective in removing TSS, with a concentration removal efficiency of 69% (EMC efficiency) and loading removal efficiency of 43% (SOL Efficiency). Removal of TKN and total phosphorus (36% and 37% respectively for concentrations) within the dry pond is further evidence of sedimentation within the dry pond. The wet pond embankment was built in 1997, and monitoring occurred during a potential stabilization period when evidence of water quality benefits are slow to appear, especially with respect to downstream habitat and aquatic communities. Some benefits which could have been observed more immediately may have been negated or masked by the progressive erosion of the wet pond embankment as a result of a design flaw. Further complicating the results is the appearance; based on observations of extended drawdown time and results from a water budget analysis in the wet pond (where inflow substantially exceeds inflow); that groundwater interacts with the pond in a complicated fashion, possibly including both recharge and discharge. To fully understand the impact of the stormwater management facility on the water quantity and quality within this tributary of Stroubles Creek, monitoring efforts should continue after the wet pond embankment is repaired and is fully operational. If biotic community improvement is desired, the stabilization period could be defined by the time necessary to flush out accumulated sediment within the channel. Monitoring efforts should also expand to include the investigation of the groundwater regime and water level fluctuations within the wet pond. Further measurements of pollutant removal processes and influences upon those processes within the wet pond should also be considered. Last, the influence of the stormwater management facility on downstream flow regimes should be investigated to assess the adequacy of its performance with regard to flow control and prevention of stream channel degradation.
Assessing Urban Non-Point Source pollutants at the Virginia Tech Extended Dry Detention Pond
Hodges, Kimberly Jean (Virginia Tech, 1997-05-23)
With a growing concern for the environment and increasing urbanization of rural areas, understanding the characteristics of urban non-point source pollution has become a focus for water quality investigators. Once thought to be a small contributor to the pollution problem, urban non-point sources are now responsible for transporting over 50% of all pollutants into natural waterways. Assessing non-point source pollution is the key to future water quality improvements in natural receiving waters. The purpose of this research was to investigate the water quality of an urbanized watershed, analyze current prediction methods and to investigate the effectiveness of an extended dry detention basin as a pollutant removal management practice on a 21.68-acre urban watershed on the Virginia Tech Campus. This research included extensive stormwater monitoring and sampling to characterize the runoff and water quality from an urban watershed. The resulting analysis included comparing well-known desktop prediction methods with pollutant removal rates using an extended dry detention basin and comparison with different literature values. Finally, the study team calibrated the PSRM-QUAL model for watershed prediction of non-point source runoff and pollution. The results of the stormwater monitoring process show that water quality prediction methods are not very successful on a storm by storm basis, but can be fairly accurate over longer periods of time with little or no storm water quality sampling. The extended dry detention basin is a simple yet effective management practice for the removal of sediments and sediment bound pollutants.
Calibration of Snowmaking Equipment for Efficient Use on Virginia's Smart Road
Shea, Edward (Virginia Tech, 1999-08-04)
Virginia's Smart Road, to be completed by early 2000, is a test bed for numerous research activities including snow and ice control, remote sensor testing, snow removal management, safety and human factors, and vehicle dynamics. An all-weather testing system will feature 75 automated snowmaking towers. In order to provide timely and repeatable weather scenarios, equipment operators will need to understand fully the limitations and capabilities of the snowmaking system. The research presented herein addresses the hydraulic and hydrologic variables and design methodology to implement efficient snowmaking at a transportation research facility. Design variables include nozzle configuration, water pressure and flowrate, compressed air pressure and flowrate, tower orientation, snow inducer concentration, water and compressed air temperature, and ambient weather conditions. Testing and data collection was performed at the Snow Economics, Inc. research and development site at Seven Springs Mountain Resort in Champion, PA. The results of this work will be used to guide the operators of the Smart Road on the most efficient use of the snowmaking equipment.
Characterization of palmer drought index as a precursor for drought mitigation
Lohani, Vinod K. (Virginia Tech, 1995-08-15)
Coping with droughts involves two phases. In the first phase drought susceptibility of a region should be assessed for developing proper additional sources of supply which will be exploited during the course of a drought. The second phase focuses on the issuance of drought warnings and exercising mitigation measures during a drought . These kinds of information are extremely valuable to decision making authorities. In this dissertation three broad schemes i) time series modeling, ii) Markov chain analysis, and iii) dynamical systems approach are put forward for computing the drought parameters necessary for understanding the scope of the drought. These parameters include drought occurrence probabilities, duration of various drought severity classes which describe a region's drought susceptibility, and first times of arrival for non drought classes which signify times of relief for a drought-affected region. These schemes also predict drought based on given current conditions. In the time series analysis two classes of models; the fixed parameter and the time varying models are formulated. To overcome the bimodal behavior of the Pallner Drought Severity Index (PDSI), primarily due to the backtracking scheme to reset the temporary index values as the PDSI values, the models are fitted to the Z index in addition to the PDSI for the forecasting of the PDSI.
A Comparative Study of Stream-Gaging Methods Employed in Nonpoint Source Pollution Studies in Small Streams
Mitchem, Charles E. Jr. (Virginia Tech, 1998-04-09)
The U.S. Geological Survey started measuring stream flow in 1888 as part of a public land irrigation study. The demand for accurate stream flow measurement has increased with the rising concern about nonpoint source (NPS) pollution. NPS pollution studies, such as TMDL development, often involve quantification of flow in small first and second order streams. This application of technology intended for use in larger streams presents special problems that must be addressed by the user. The goal of this study was to conduct a comparative analysis of the current technologies used to measure flow in small streams with respect to accuracy and cost. The analyses involved field investigations, laboratory experiments, and a cost analysis. The specific study objectives were: 1) Compare the accuracy of various methods for estimating stream discharge in small first and second order streams, 2) Compare the accuracy of various methods for estimating stream discharge in a controlled laboratory environment, and 3) Evaluate the costs associated with installation, operation, and maintenance of each of the systems investigated. Ten stream-gaging methods were evaluated for their field performance, laboratory performance, and costs. Analysis of the field investigation data indicated that the Marsh McBirney current meter and the One-orange method were the most accurate among the methods studied. The results of the laboratory experiments imply that the Starflow acoustic Doppler and Valeport BFM001 current meter performed best among the ten methods. The Starflow acoustic Doppler device also proved to be the most cost-effective method. Overall, the Marsh McBirney and Valeport BFM001 current meters exhibited the best performance for both field and laboratory situations among the methods evaluated.
A Comprehensive Decision Support System(CDSS) for Optimal Pipe Renewal using Trenchless Technologies
Khambhammettu, Prashanth (Virginia Tech, 2001-07-20)
Water distribution system pipes span thousands of miles and form a significant part of the total infrastructure of the country. Rehabilitation of this underground infrastructure is one of the biggest challenges currently facing the water industry. Water main deterioration is twofold: the main itself loses strength over time and breaks; also, there is degradation of water quality and hydraulic capacity due to build of material within a main. The increasing repair and damage costs and degrading services demand that a deteriorating water main be replaced at an optimal time instead of continuing to repair it. In addition, expanding business districts, indirect costs, and interruptions including protected areas, waterways and roadways require examination of trenchless technologies for pipe installation. In this thesis a new threshold break rate criterion for the optimal replacement of pipes is provided. As opposed to the traditional present worth cost (PWC) criterion, the derived method uses the equivalent uniform annualized cost (EUAC). It is shown the EUAC based threshold break rate subsumes the PWC based threshold break rate. In addition, practicing engineers need a user-friendly decision support system to aid in the optimal pipeline replacement process. They also need a task-by-task cost evaluation in a project. As a part of this thesis a comprehensive decision support system that includes both technology selection knowledge base and cost evaluation spreadsheet program within a graphical user interface framework is developed. Numerical examples illustrating the theoretical derivations are also included.
Determination of Optimal Stable Channel Profiles
Vigilar, Gregorio G. Jr. (Virginia Tech, 1997-01-28)
A numerical model which determines the geometry of a threshold channel was recently developed. Such a model is an important tool for designing unlined irrigation canals and channelization schemes, and is useful when considering flow regulation. However, its applicability is limited in that its continuously curving boundary does not allow for sediment transport, which is an essential feature of natural rivers and streams. That model has thus been modified to predict the shape and stress distribution of an optimal stable channel; a channel with a flat-bed region over which bedload transport occurs, and curving bank regions composed of particles that are all in a state of incipient motion. It is the combination of this channel geometry and the phenomenon of momentum-diffusion, that allows the present model to simulate the "stable bank, mobile bed" condition observed in rivers. The coupled equations of momentum-diffusion and force-balance are solved over the bank region to determine the shape of the channel banks (the bank solution). The width of the channel1s flat-bed region is determined by solving the momentum-diffusion equation over the flat-bed region (the bed solution), using conditions at the junction of the flat-bed and bank regions that ensure matching of the bed and bank solutions. The model was tested against available experimental and field data, and was found to adequately predict the bank shape and significant dimensions of stable channels. To make the model results more amenable to the practic ing engineer, design equations and plots were developed. These can be used as an alternative solution for stable channel design; relieving the practitioner of the need to run the numerical program. The case of a stable channel that transports both bedload and suspended sediment is briefly discussed. Governing equations and a possible solution scheme for this type of channel are suggested; laying the groundwork for the development of an appropriate numerical model.
Determining the Sustainability of Coal Mine Cavity Discharge as a Drinking Water Source
Anderson, Eric T. (Virginia Tech, 1999-03-30)
In southwestern Virginia, adequate sources of public water for small isolated communities are difficult to find. While many alternatives exist, one of the largest sources of water in this region is flooded abandoned coal mines. One such coal mine aquifer was chosen for a sustainability study in Dickenson County, Virginia. A flowrate monitoring system was installed at the point of discharge from the mine, and the flow records from three months of data collection were analyzed. The recording period included one of the driest periods in recent years, and the flowrate data recorded provided useful information regarding the sustainability of the system. After a study of the geology and groundwater flow patterns in the region, it was determined that a coal mine aquifer is very similar to the extremely heterogeneous system seen in karst landscapes. Thus, techniques common to karst phenomenon were used to analyze the spring hydrograph. A spring recession analysis was performed upon five storm recessions, and the coefficients for each recession compared and discussed in light of known geologic information. It was discovered that the recession coefficients described the flow from the mine very adequately and that the mine response to a rainfall pulse was very similar to the response of certain types of karst aquifers. This information was used to predict a sustainable flow from the mine. A cross-correlation analysis was performed in an attempt to fit a "black box" model to the flow data, as well as to verify the results of the spring recession analysis. The correlation analysis proved that one rainfall event produced many separate reactions in the flowrate at the mine discharge point. This strengthened results concluded by the recession analysis. It was found that the flow record was not long enough to adequately create a statistical model, but a procedure was described that could be used to model flows once a larger flow record was available.
Development and Evaluation of a Gis-Based Spatially Distributed Unit Hydrograph Model
Kilgore, Jennifer Leigh (Virginia Tech, 1997-12-10)
Synthetic unit hydrographs, which assume uniform rainfall excess distribution and static watershed conditions, are frequently used to estimate hydrograph characteristics when observed data are unavailable. The objective of this research was to develop a spatially distributed unit hydrograph (SDUH) model that directly reflects spatial variation in the watershed in generating runoff hydrographs. The SDUH model is a time-area unit hydrograph technique that uses a geographic information system (GIS) to develop a cumulative travel time map of the watershed based on cell by cell estimates of overland and channel flow velocities. The model considers slope, land use, watershed position, channel characteristics, and rainfall excess intensity in determining flow velocities. The cumulative travel time map is divided into isochrones which are used to generate a time-area curve and the resulting unit hydrograph. Predictions of the SDUH model along with the Snyder, SCS, and Clark synthetic unit hydrographs were compared with forty observed storm events from an 1153-ha Virginia Piedmont watershed. The SDUH model predictions were comparable or slightly better than those from the other models, with the lowest relative error in the peak flow rate prediction for 12 of the 40 storms, and a model efficiency of at least 0.90 for 21 of the storms. Despite the good predictions of the hydrograph peak flow rate and shape, the time to peak was underpredicted for 34 of the 40 storms. Runoff from the 40 storms was also generated for two subwatersheds (C: 462 ha; D: 328 ha) in Owl Run to assess the effect of scale on the SDUH model. Peak flow rate predictions were more accurate for the entire watershed than for either subwatershed. The time to peak prediction and model efficiency statistics were comparable for the entire watershed and subwatershed D. Subwatershed C had poorer predictions, which were attributed to a large pond in the main channel, rather than to scale effects. The SDUH model provides a framework for predicting runoff hydrographs for ungauged watersheds that can reflect the spatially distributed nature of the rainfall-runoff process. Predictions were comparable to the other synthetic unit hydrograph techniques. Because the time to peak and model efficiency statistics were similar for the 1153-ha watershed and a 328-ha subwatershed, scale does not have a major impact on the accuracy of the SDUH model.
Development and Evaluation of the Profile Synthesis Method for Approximate Floodplain Redelineation
Dickerson, Thomas Ashby (Virginia Tech, 2007-11-15)
In the United States, the floodplain maps used in the administration of the National Flood Insurance Program are created and maintained by the Federal Emergency Management Agency. Currently, a nationwide map modernization program is underway to convert the existing paper floodplain maps into a digital format, while continuing to improve the maps and expand the scope of the studies. The flood zones depicted on these maps are developed through engineering studies, using a variety of accepted methods to model and predict flood-prone areas. These methods are classified as detailed, limited detailed, or approximate, corresponding to varying levels of expense and accuracy. Current flood map revision activities across the nation typically consist of developing new hydraulic models, or reusing existing hydraulic model results in conjunction with new, more detailed LiDAR terrain models. This research develops a profile synthesis method for redelineation of approximate flood boundaries, and evaluates the method's performance and usability. The profile synthesis method is shown to perform reliably on simple floodplain geometry, recreating a water surface profile based only on its floodplain boundaries. When applied to a real-world floodplain studied in a previous flood insurance study, the profile synthesis method is shown to perform adequately, with results comparable to an approximate hydraulic model developed in HEC-RAS. Methods similar to this profile synthesis method for reuse of existing approximate zone boundaries have not been widely documented or evaluated; nevertheless, methods such as this are believed to be common in the revision of approximate zone flood boundaries. As such, this work explores concepts which will be of interest to individuals actively involved in flood map revision and modernization.
Development of a Computerized Version of the Universal Soil Loss Equation and the USGS Pollutant Loading Functions
Espiritu, Kenneth (Virginia Tech, 1997-06-11)
A computerized program of soil loss and pollutant loading equations was developed in a Windows PC environment. The program implements the Universal Soil Loss Equation (USLE), the Modified Universal Soil Loss Equation (MUSLE), Roehl's sediment delivery ratio equation, and a sediment delivery ratio equation based on both the USLE and the MUSLE. Also implemented into the program were ten pollutant loading equations based on the USGS Nationwide Regression Equations (NRE) for predicting water quality in urban runoff. The programs developed here will become a part of the Virginia Tech/Penn State Urban Hydrology Model (VT-PSUHM).
Distributed Hydrologic Modeling of the Upper Roanoke River Watershed using GIS and NEXRAD
McCormick, Brian Christopher (Virginia Tech, 2003-03-27)
Precipitation and surface runoff producing mechanisms are inherently spatially variable. Many hydrologic runoff models do not account for this spatial variability and instead use "lumped" or spatially averaged parameters. Lumped model parameters often must be developed empirically or through optimization rather than be calculated from field measurements or existing data. Recent advances in geographic information systems (GIS) remote sensing (RS), radar measurement of precipitation, and desktop computing have made it easier for the hydrologist to account for the spatial variability of the hydrologic cycle using distributed models, theoretically improving hydrologic model accuracy. Grid based distributed models assume homogeneity of model parameters within each grid cell, raising the question of optimum grid scale to adequately and efficiently model the process in question. For a grid or raster based hydrologic model, as grid cell size decreases, modeling accuracy typically increases, but data and computational requirements increase as well. There is great interest in determining the optimal grid resolution for hydrologic models as well as the sensitivity of hydrologic model outputs to grid resolution. This research involves the application of a grid based hydrologic runoff model to the Upper Roanoke River watershed (1480km2) to investigate the effects of precipitation resolution and grid cell size on modeled peak flow, time to peak and runoff volume. The gridded NRCS curve number (CN) rainfall excess determination and ModClark runoff transformation of HEC-HMS is used in this modeling study. Model results are evaluated against observed streamflow at seven USGS stream gage locations throughout the watershed. Runoff model inputs and parameters are developed from public domain digital datasets using commonly available GIS tools and public domain modeling software. Watersheds and stream networks are delineated from a USGS DEM using GIS tools. Topographic parameters describing these watersheds and stream channel networks are also derived from the GIS. A gridded representation of the NRCS CN is calculated from the soil survey geographic database of the NRCS and national land cover dataset of the USGS. Spatially distributed precipitation depths derived from WSR-88D next generation radar (NEXRAD) products are used as precipitation inputs. Archives of NEXRAD Stage III data are decoded, spatially and temporally registered, and verified against archived IFLOWS rain gage data. Stage III data are systematically degraded to coarser resolutions to examine model sensitivity to gridded rainfall resolution. The effects of precipitation resolution and grid cell size on model outputs are examined. The performance of the grid based distributed model is compared to a similarly specified and parameterized lumped watershed model. The applicability of public domain digital datasets to hydrologic modeling is also investigated. The HEC-HMS gridded SCS CN rainfall excess calculation and ModClark runoff transformation, as applied to the Upper Roanoke watershed and for the storm events chosen in this study, does not exhibit significant sensitivity to precipitation resolution, grid scale, or spatial distribution of parameters and inputs. Expected trends in peak flow, time to peak and overall runoff volume are observed with changes in precipitation resolution, however the changes in these outputs are small compared with their magnitudes and compared to the discrepancies between modeled and observed values. Significant sensitivity of runoff volume and consequently peak flow, to CN choices and antecedent moisture condition (AMC) was observed. The changes in model outputs between the distributed and lumped versions of the model were also small compared to the magnitudes of model outputs.
Effect of Spatial Scale on Hydrologic Modeling in a Headwater Catchment
Fedak, Ryan Michael (Virginia Tech, 1999-01-29)
In this study, two hydrologic models were applied to the mountainous Back Creek catchment, located in the headwaters of the Roanoke River in Southwest Virginia. The two models employed were HEC-1, an event based lumped model, and TOPMODEL, a continuous semi-distributed model. These models were used to investigate (a) the issue of spatial scale in hydrologic modeling, and (b) two approaches to modeling, continuous versus event based. Two HEC-1 models were developed with a different number of subareas in each. The hydrographs generated by each HEC-1 model for a number of large rainfall events were analyzed visually and statistically. No observable improvement resulted from increasing the number of subareas in the HEC-1 models from 20 to 81. TOPMODEL was applied to the same watershed using a series of different size grid cells. The first step in applying TOPMODEL to a watershed involves GIS analysis which results in a raster grid of elevations used for the calculation of the topographic index, ln(a/tan b). The hydrographs generated by TOPMODEL with each grid cell size were compared in order to assess the sensitivity of TOPMODEL hydrographs to grid cell size. An increase in grid cell size from 15 to 120 meters resulted in increased values of the watershed mean of the topographic index. However, hydrographs generated by TOPMODEL were completely unaffected by this increase in the topographic index. Analyses were also performed to determine the sensitivity of TOPMODEL hydrographs to several model parameters. It was determined that the parameters that had the greatest effect on hydrographs generated by TOPMODEL were the m and ln(To) parameters. The modeling performances of the event based HEC-1 and the continuous TOPMODEL were analyzed and compared visually and statistically for a number of large storms. The limited number of storms used to compare HEC-1 and TOPMODEL makes it difficult to determine definitively which model simulates large storms better. It does appear that perhaps HEC-1 is slightly superior in that regard. TOPMODEL was also executed as an event based model for two single events and the resulting hydrographs were compared to the HEC-1 and continuous TOPMODEL results. Both HEC-1 and TOPMODEL (when used as a continuous model) simulate large storms better than TOPMODEL (when used as an event based model).
Evaluating the Effectiveness of the Skimmer Versus the Perforated Riser in Sedimentation Basins
Hoechst, Lisa Marie (Virginia Tech, 1997-12-04)
Erosion, transportation, and deposition of sediment into receiving waters can have substantial environmental and economic impacts. Sedimentation basins are a remediation technique used to limit sediment transport from earth disturbance activities. Retention efficiency is used as a measure of a sedimentation basin's effectiveness. Several factors influence retention efficiency including the type of principal spillway used. The most common spillway is the perforated riser which dewaters the basin throughout its entire vertical profile. However, a relatively new outlet device, the skimmer, has been developed, which dewaters the basin from the water surface. A laboratory study was conducted to compare the skimmer with the perforated riser for three different soil types and determine if there were any significant differences in the trapping efficiencies of the two outlets. The test basin dewatered over a three hour period. The parameters observed were dewatering rate, effluent sediment concentration, sediment loss rate, and retention efficiency. The skimmer treatments consistently had higher values of sediment retention efficiencies. A statistical analysis performed on the retention efficiency data showed that retention efficiency was not influenced by any combination of outlet and soil type and that outlet was significant at the 5% level. Overall, the skimmer outperformed the perforated riser for all soil types tested. Additionally, retention efficiencies were predicted for shorter dewatering times. The results indicated shorter dewatering times may have smaller impacts on the retention efficiency of basins where the skimmer is utilized rather than the perforated riser.
From Landscapes to Waterscapes: A PSE for Landuse Change Analysis
Rubin, E.J.; Diet, R.; Chanat, J.; Speir, C.; Dymond, Randel L.; Lohani, Vinod K.; Kibler, David F.; Bosch, D. D.; Shaffer, Clifford A.; Ramakrishnan, Naren; Watson, Layne T. (Department of Computer Science, Virginia Polytechnic Institute & State University, 2000)
We describe the design and implementation of L2W — a problem solving environment (PSE) for landuse change analysis. L2W organizes and unifies the diverse collection of software typically associated with ecosystem models (hydrological, economic, and biological). It provides a web-based interface for potential watershed managers and other users to explore meaningful alternative land development and management scenarios and view their hydrological, ecological, and economic impacts. A prototype implementation for the Upper Roanoke River Watershed in Southwest Virginia, USA is described.
GIS based optimal design of sewer networks and pump stations
Agbenowosi, Newland Komla (Virginia Tech, 1995-11-08)
In the planning and design of sewer networks, most of the decisions are spatially dependent because of the right of way considerations and the desire to have flow by gravity. This research addresses the application of combined optimization-geographic information system (GIS) technology in the design process. The program developed for the design uses selected manhole locations to generate the candidate potential sewer networks. The design area is delineated into subwatersheds for determining the locations for lift stations when gravity flow is not possible. Flows from upstream subwatersheds are transported to the downstream subwatersheds via a force main. The path and destination of each force main in the system is determined by applying the Dijkstra's shortest path algorithm to select the least cost path from a set of potential paths. This method seeks to minimize the total dynamic head. A modified length is used to represent the length of each link or force main segment. The modified length is the physical length of the link (representing the friction loss) plus an equivalent length (representing the static head). The least cost path for the force main is the path with the least total modified length. The design approach is applied to two areas in the town of Blacksburg, Virginia. The resulting network and the force main paths are discussed.

Browsing by Author "Kibler, David F."

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