VTechWorks Repository :: Browsing by Author "Moglen, Glenn E."

Browsing by Author "Moglen, Glenn E."

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Bioretention Hydrologic Performance in an Urban Stormwater Network
James, Matthew Bruce (Virginia Tech, 2010-04-30)
While many studies have evaluated the hydrologic effects of bioretention at the site level, few have investigated the role bioretention plays when distributed throughout a watershed. This study aims to assess bioretention's effects on an urbanized watershed using two modeled scenarios: one where runoff from many land uses was routed through the practice, and another in which only runoff from large impervious areas was routed. Peak flows, volumes, and lag times from these models were compared to the watershed's current and predeveloped conditions. Both scenarios provided reductions in peak flows with respect to existing conditions for modeled storm events, sometimes to levels below the predeveloped condition. Neither case was able to reduce volumes to predevelopment levels; the option to treat impervious areas had a negligible effect on runoff volume. Both cases were able to extend lag times from the existing development condition. Based on these results, bioretention appears to have the capability to improve watershed hydrologic characteristics. Furthermore, only treating impervious areas could be a viable alternative when funds or space are limiting factors.
Effect of Urbanization on the Hyporheic Zone: Lessons from the Virginia Piedmont
Cranmer, Elizabeth Nadine (Virginia Tech, 2011-06-28)
As the world's population shifts toward living in cities, urbanization and its deleterious effects on the environment are a cause of increasing concern. The hyporheic zone is an important part of stream ecosystems, and here we focus on the effect of urbanization on the hyporheic zone from ten first-to-second-order streams within the Virginia Piedmont. We use sediment hydraulic conductivity and stream geomorphic complexity (vertical undulation of thalweg, channel sinuosity) as metrics of the potential for hyporheic exchange (hyporheic potential). Our results include bivariate plots that relate urbanization (e.g., total percent impervious) with hyporheic potential at several spatial scales. For example, at the watershed level, we observed a decrease in horizontal hydraulic conductivity with urbanization and an increase in vertical hydraulic conductivity, which ultimately results in a negligible trend from conflicting processes. Vertical geomorphic complexity increased with total percent impervious cover. This trend was somewhat unexpected and may be due to erosion of legacy sediment in stream banks. At the reach level, hydraulic conductivity increased and sinuosity decreased as the riparian buffer width increased; these trends are weak and are essentially negligible. The hydraulic conductivity results conform to expected trends and are a product of aforementioned concomitant processes. Our results emphasize the complexity of hydrologic and geomorphic processes occurring in urban stream systems at multiple scales. Overall, the watershed level effects enhancing hyporheic exchange, which is contrary to expectations. Given the importance of hyporheic exchange to stream function, further study is warranted to better understand the effects of urbanization.
A Generalized Log-Law Formulation For a Wide Range of Boundary Roughness Conditions Encountered in Streams
Plott, James Read (Virginia Tech, 2011-08-30)
It is demonstrated that the method for locating a velocity profile origin, or plane of zero velocity, by fitting log profiles to streamwise velocity measurements is applicable to a larger range of roughness scales than previously expected. Five different sets of detailed, experimental velocity measurements were analyzed encompassing sediment-scale roughness elements, roughness caused by rigid vegetation, and large-scale roughness elements comprised of mobile bedforms. The method resulted in similar values of normalized zero-plane displacement for all roughness types considered. The ratios of zero-plane displacement, dh, to roughness height, ks, were 0.20 and 0.26 for the sediment- and vegetation-scale experiments, respectively. The results for the two experiments with bedform dominated roughness were 0.34 and 0.41. An estimate of dh/ks ranging from 0.2 to 0.4 is therefore recommended for a range of roughness types with the higher end of the range being more appropriate for the larger, bedform-scale roughness elements, and the lower end for the sediment-scale roughness elements. In addition, it is demonstrated that the location of the plane of zero velocity is temporally constant even when the bed height is not. The effects of roughness element packing density were also examined with the identification of a possible threshold at 4%, above which zero-plane displacement is independent of packing density. The findings can be applied to field velocity measurements under mobile bed conditions, facilitating the calculation of turbulence parameters such as shear velocity, by using point measurements and providing guidelines for the estimation of an appropriate value for zero-plane displacement.
Hydrologic Modeling of a Probable Maximum Precipitation Event Using HEC-HMS and GIS Models - A Case Study of Two Watersheds in Southern Virginia-
Kingston, William John III (Virginia Tech, 2012-06-11)
Presented in this thesis is a case study of two study watersheds located in south central Virginia. For each, a HEC-HMS event-based hydrologic model was constructed to simulate the rainfall-runoff response from the Probable Maximum Storm (PMS), the theoretical worst-case meteorological event that is capable of occurring over a particular region. The primary goal of these simulations was to obtain discharge hydrographs associated with the Probable Maximum Flood (PMF) at key locations in each of the watersheds. These hydrographs were subsequently used to develop flood inundation maps of the study areas and to characterize sediment transport phenomena in the study reaches under severe flooding conditions. To build the hydrologic basin models, ArcHydro, HEC-GeoHMS and ArcGIS were employed to assimilate the substantial amount of input data and to extract the pertinent modeling parameters required for the selected simulation methods. In this, the SCS Loss and Transform Methods, along with the Muskingum Routing Method, were adopted for the HEC-HMS simulations. Once completed, the basin models were calibrated through a comparison of simulated design storm flows to frequency discharge estimates obtained with regional regression techniques and a flood frequency analysis. The models were then used to simulate their respective PMS events, which were developed following recommendations from the Hydrometeorological Branch of the National Weather Service and the U.S. Army Corps of Engineers. Descriptions of each of the study sites, explanations of the modeling theory and development methodologies, and discussions of the modeling results are all detailed within.
Low Flow Variations in Source Water Supply for the Occoquan Reservoir System Based on a 100-Year Climate Forecast
Maldonado, Philip Pasqual (Virginia Tech, 2011-09-14)
The reliability of future water supplies comes into question with the onset of global climate change and the variations in local weather patterns that it brings. Changes in temperature, precipitation, soil moisture, and sea level can all have an impact on drinking water storage and supply. As these impacts are realized, it is increasingly important to use forward projecting estimates of future supply through the use of general circulation models (GCMs). GCMs can be used to predict changes in local weather over the next century. Using GCM data as input to a hydrologic model of local water supplies, water supply managers can assess and be better prepared for the impact of these possible changes. Land use/demand in particular has an impact on runoff characteristics within a watershed. By incorporating changes in land use/demand into hydrologic model simulations, a more complete picture can be generated of the possible runoff characteristics, and thereby source water supply. The four land use scenarios used in this study are: 1) present day land use/demand; 2) projected land use/demand to 2040; 3) projected land use/demand to 2070; and 4) projected land use/demand to 2100. This study uses established techniques to incorporate both climate and land use/demand change into a hydrologic model of the Occoquan watershed, which encompasses an area of approximately 1,550 square kilometers in Northern Virginia, U.S.A., and is part of the drinking water supply to approximately 1.7 million residents.
Optimization of Multi-Reservoir Management Rules Subject to Climate and Demand Change in the Potomac River Basin
Stagge, James Howard (Virginia Tech, 2012-07-16)
Water management in the Washington Metropolitan Area (WMA) is challenging because the system relies on flow in the Potomac river, which is largely uncontrolled and augmented by the Jennings-Randolph reservoir, located 9-10 days travel time upstream. Given this lag, release decisions must be made collectively by federal, state and local stakeholders amid significant uncertainty, well in advance of accurate weather forecasts with no ability to recapture excess releases. Adding to this uncertainty are predictions of more severe and sporadic rainfall over the next century, caused by anthropogenic climate change. This study aims to evaluate the potential impacts of demand and climate change on the WMA water supply system, identifying changes in system vulnerability over the next century and developing adaptation strategies designed to maximize efficiency in a nonstationary system. A daily stochastic streamflow generation model is presented, which succesfully replicates statistics of the historical streamflow record and can produce climate-adjusted daily time-series. Using these time series, a multi-objective evolutionary algorithm is used to optimize the system's operating rules given current and future conditions, considering several competing objectives.
Performance Assessment of a Water Supply System under the Impact of Climate Change and Droughts: Case Study of the Washington Metropolitan Area
Bhatkoti, Roma; Triantis, Konstantinos P.; Moglen, Glenn E.; Sabounchi, Nasim S. (2018-09)
Fresh water demand is rising due to factors such as population growth, economic development, and land use changes. At the same time, climate change is rendering the water supply even more uncertain for the future. Due to recurring water restrictions and increasing water-related fees triggered by droughts and water shortages, there is a widespread, growing discomfort with respect to future water availability. Among key stakeholders and local policy makers, this has led to an increased interest in modeling the availability of water resources, with the aim of developing and implementing the appropriate water resource infrastructure and management strategies. This paper examines the Washington metropolitan area (WMA) water supply system and uses a system dynamics approach as a planning tool to make an exploratory assessment of the adequacy of the study area's water supply system to meet future water demand under the influence of substantial droughts and climate change. This assessment finds that the study area is self-sufficient under normal climate conditions during the entire planning horizon but that it will be strained under moderately severe droughts. On the basis of the temperature, streamflow and precipitation projections made by climate change models specific to the WMA region, climate change is expected to improve the water supply reliability. However, climate change has uncertainty associated with it. One of the four climate models for the Potomac River basin projects a decrease in the precipitation and streamflow, which may result in a reduction in the water supply and the system's reliability. Regulating the price and the system losses are valuable tools that can be leveraged. But these policy interventions require stakeholder participation (price regulation) and capital investments (reduction of distribution losses). Finally, system reliability can also be improved by increasing water supplies.
Quantifying Hydrological Impacts of Climate Change Uncertainties on a Watershed in Northern Virginia
Baran, Ayden A.; Moglen, Glenn E.; Godrej, Adil N. (2019-12)
Forecasted changes to climate were used to model variations in the streamflow characteristics of a northern Virginia catchment. Two emission scenarios were applied from international climate projections along with four general circulation models (GCMs) by using two statistical downscaling methods to drive the hydrological simulations in two future time periods (2046-2065 and 2081-2100). Incorporation of these factors yielded 32 runoff simulation models for a 130-km(2) watershed located in northern Virginia. These models were compared with historical streamflow data from the late 20th century. Changes in streamflow were compared using median, low, and high flows. Results showed a general increase in median flows in both the mid- and late 21st century. Low flows were projected to decrease, whereas high flows were projected to increase, creating a larger range between low flows and high flows. In addition, statistical tests were conducted to identify the main factors that affected variations in future climate projections. The choice of the downscaling method emerged as the main source of uncertainty. This research quantifies the impacts of climate change as well as uncertainties within climate change projections for regional water resources. Considering the essential role of this watershed for water supply in northern Virginia, the findings of this study illustrate likely impacts of climate change on water supply reliability, supporting climate resiliency in the study area. (C) 2019 American Society of Civil Engineers.
Relations between Landscape Structure and a Watershed's Capacity to Regulate River Flooding
Mogollon Gomez, Beatriz (Virginia Tech, 2014-11-03)
Climate and human activities impact the timing and quantity of streamflow and floods in different ways, with important implications for people and aquatic environments. Impacts of landscape changes on streamflow and floods are known, but few studies have explored the magnitude, duration and count of floods the landscape can influence. Understanding how floods are influenced by landscape structure provides insight into how, why and where floods have changed over time, and facilitates mapping the capacity of watersheds to regulate floods. In this study, I (1) compared nine flood-return periods of 31 watersheds across North Carolina and Virginia using long-term hydrologic records, (2) examined temporal trends in precipitation, stream flashiness, and the count, magnitude and duration of small and large floods for the same watersheds, and (3) developed a methodology to map the biophysical and technological capacity of eight urban watersheds to regulate floods. I found (1) floods with return periods ≤ 10 years can be managed by manipulating landscape structure, (2) precipitation and floods have decreased in the study watersheds while stream flashiness has increased between 1991 and 2013, (3) mapping both the biophysical and technological features of the landscape improved previous efforts of representing an urban landscape's capacity to regulate floods. My results can inform researchers and managers on the effect of anthropogenic change and management responses on floods, the efficacy of current strategies and policies to manage water resources, and the spatial distribution of a watershed's capacity to regulate flooding at a high spatial resolution.
Spatial distribution of imperviousness and the space-time variability of rainfall, runoff generation, and routing
Mejia, Alfonso I.; Moglen, Glenn E. (American Geophysical Union, 2010-07-01)
We study the relationship between the spatial distribution of imperviousness and the space-time variability of rainfall, runoff generation, and hydrologic response. For this study we follow an analytical framework that is able to represent space-time variability and use it to determine relationships for quantities commonly used in hydrology, for example, the amount of rainfall excess, the total runoff from a storm, the runoff ratio of developed land use to undeveloped land use, and the mean time and variance of the runoff time. The relationships are derived such that the space-time variability of rainfall, runoff, and the hydrologic response, and their relative importance, can be identified and compared. In addition, the method allows the separation of pervious and impervious contributions to runoff and the estimation of their relative influence on the hydrologic response. We illustrate the estimation of the relationships from available data and apply them to two cases. In the first case, the space-time variability of rainfall and its interaction with impervious cover is investigated. In the second case, we examine the impacts of the imperviousness pattern on runoff relationships. We find that the imperviousness and rainfall pattern can interact to either increase or decrease the average amount of rainfall excess. We also find that the influence of pervious and impervious contributions on the response can depend on the form of the overall imperviousness pattern. The proposed framework can be a useful tool for identifying the importance of different space-time hydrologic components in mixed pervious-impervious landscapes.
Using SLEUTH Land Cover Predictions to Estimate Changes in Runoff Quality and Quantity in the Delmarva Peninsula
Ciavola, Suzanne J. (Virginia Tech, 2011-04-18)
Anticipating future trends in land development and climate change is a constant challenge for engineers and planners who wish to effectively compensate for the resulting changes in stormwater runoff that will inevitably follow. This study is a regional attempt at predicting how predicted changes in land cover will affect runoff characteristics in a number of watersheds throughout the Delmarva Peninsula when compared to the current state. To predict changes in land cover and the associated land use, the SLEUTH model coupled with PED utilized a number of different inputs including population growth trends, existing geography, current land planning policies as well as different growth factors to predict where urban growth is most likely to occur. The model creates maps which show the approximate location of predicted growth for the year 2030. Using SLEUTH output, the magnitude of changes that can occur in runoff quality and quantity due to land cover changes were estimated in each of the seventeen representative watersheds that were chosen within the Delmarva Peninsula. Changes in water quality were calculated based on nutrient loading rates for sediment, phosphorus, and nitrogen. These nutrient loading rates correspond to different land uses within different county segments in the peninsula. The expected changes in water quantity were quantified using the United States Department of Agriculture's Natural Resources Conservation Services' TR-20 which estimated the peak flows for each watershed based on watershed's size, land cover, soils, and slope. Evaluating the magnitude of these potential changes in the Delmarva Peninsula provides an important look into the effects of increased urban development on the predominantly agrarian land mass, the majority of which drains to the Chesapeake Bay.
Vegetated Swales in Urban Stormwater Modeling and Management
White, Kyle Wallace (Virginia Tech, 2012-04-30)
Despite the runoff reduction efficiencies recommended by various regulatory agencies, minimal research exists regarding the ability of vegetated swales to simultaneously convey and reduce runoff. This study assessed the effect water quality swales distributed among upstream sub-watersheds had on watershed hydrology. The study was also posed to determine how certain design parameters can be dimensioned to increase runoff reduction according to the following modeling scenarios: base, base check dam height, minimum check dam height, maximum check dam height, minimum infiltration rate, maximum infiltration rate, minimum Manning's n, maximum Manning's n, minimum longitudinal slope, and maximum longitudinal slope. Peak flow rate, volume, and time to peak for each scenario were compared to the watershed's existing and predevelopment conditions. With respect to the existing condition, peak flow rate and volume decreased for all scenarios, and the time to peak decreased for most scenarios; the counterintuitive nature of this result was attributed to software error. Overall, the sensitivity analysis produced results contrary to the hypotheses in most cases. The cause of this result can likely be attributed to the vegetated swale design and modeling approaches producing an over designed, under constrained, and/or over discretized stormwater management practice.
Water Resources Adaptation to Climate and Demand Change in the Potomac River
Stagge, James H.; Moglen, Glenn E. (2017-11)
The effects of climate change are increasingly considered in conjunction with changes in water demand and reservoir sedimentation in forecasts of water supply vulnerability. Here, the relative effects of these factors are evaluated for the Washington, DC metropolitan area water supply for the near (2010-2039), intermediate (2040-2069), and distant (2070-2099) future by repeated water resources model simulations. This system poses water management challenges because of long water-delivery travel times that increase uncertainty, multiple water jurisdictions that constrain potential decisions, and future scenarios that simultaneously increase demand and decrease water supply during the critical summer period. Adaptation strategies were developed for the system using a multiobjective evolutionary algorithm. Optimized reservoir management policies were compared using six distinct objectives ranging from reservoir storage to environmental and recreational benefits. Simulations of future conditions show water stress increasing with time. Reservoir sedimentation is projected to more than double (114% increase) the severity of reservoir storage failures by 2040. Increases in water demand and climate change are projected to further stress the system, causing longer periods of low flow and a loss of recreational reservoir storage. The adoption of optimized rules mitigates some of these effects, most notably returning simulations of 2070-2099 climate to near historical levels. Modifying the balance between upstream and downstream reservoirs improved storage penalties by 20.7% and flowby penalties by 50%. Changing triggers for shifting load to off-line reservoirs improved flowby (8.3%) and environmental (4.1%) penalties slightly, whereas changing demand restriction triggers provided only moderate improvements, but with few adverse effects. (C) 2017 American Society of Civil Engineers.

Browsing by Author "Moglen, Glenn E."

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