Browsing by Author "Julien, Pierre, committee member"

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Open Access
Design of baffled hydraulic jump stilling basins for dams
(Colorado State University. Libraries, 2025) Moses, Dana Wesley, author; Thornton, Christopher, advisor; Crookston, Brian, committee member; Ettema, Robert, committee member; Julien, Pierre, committee member; Rathburn, Sara, committee member
The hydraulic jump has been studied and used as a primary means of energy dissipation for hydraulic structures for well over a century. By the 1920s and 1930s, baffled hydraulic jump stilling basins were in widespread use as energy dissipators for large dams. These hydraulic jump stilling basins often consisted of toe blocks, a negative step and/or toe curve; one or more rows of baffle blocks; and a solid or dentated end sill. By the 1950's standard design guidance was developed by multiple agencies and universities. A comparison of the standard baffled hydraulic jump guidance illustrates a drastic difference in recommended stilling basin geometry with identical incoming flow conditions. For example, given the same incoming flow conditions, the height of baffle block determined for a U.S. Bureau of Reclamation standard design can be more than twice the block size for a standard outlet works stilling basin determined from US Army Corps of Engineers guidance. The use and/or geometry associated with chute blocks, number of rows of baffle blocks, length of basin, distance to baffle blocks, and end sill geometry have similar discrepancies. The current research includes a systematic physical model evaluation, performing over 400 individual experiments of the most often utilized baffled stilling basin design configurations. These experiments include 15 stilling basin configurations, each being evaluated for six discharge conditions and six tailwater scenarios. In addition, the USACE standard stilling basin configuration was evaluated for the general scour tendencies for discharges below and above the design discharge by means of mobile bed physical modeling. A numbered list of significant findings associated with the current research are provided below. Detailed descriptions of these significant findings and other conclusions and recommendations associated with the research objectives are provided subsequently. 1. The USACE (1992) stilling basin configuration is recommended for incoming Froude Numbers less than 4.5. 2. The Modified Type III stilling basin configuration is recommended for incoming Froude Numbers in the range of 4.5 to 8. 3. The stilling basin length and minimum required tailwater for the USACE (1992) and USBR (1984) can be expressed by unified equations. 4. A toe curve is not recommended for the hydraulic jump basin due to increase in length required and the decrease in jump stability. 5. Intermittent ramps associated with the tapered baffle block configuration are not recommended due to increase in downstream scour potential, decreased tailwater resilience, cost, general lack of observed cavitation damage, and unproven effectiveness in reducing cavitation damage. 6. Macro-scale turbulent structures exiting the stilling basin are the primary phenomenon controlling downstream scour potential. A maximum downstream attack angle of 15-degrees from horizontal was determined for the USACE (1992) stilling basin configuration for incoming Froude Numbers in the range of 3 to 5.
Open Access
Mixing of scalars in turbulent flows using direct numerical simulations
(Colorado State University. Libraries, 2015) Nithianantham, Ajithshanthar, author; Venayagamoorthy, S. Karan, advisor; Julien, Pierre, committee member; Sakurai, Hiroshi, committee member
The research presented in this thesis focuses on scalar mixing in unstratified (neutral) flows and stably stratified flows using Direct Numerical Simulations (DNS). Such flows are ubiquitous in natural flows such as rivers, estuaries, oceans and the atmosphere. First, a detailed study was performed to investigate the effect of varying Schmidt numbers (Sc) on turbulent mixing of a passive scalar in a stationary homogeneous unstratified flow using forced DNS. A total of 6 simulations were performed for 0.1 ≤ Sc < 3. Qualitative and quantitative results of the flow field and the passive scalar fields are presented and discussed. The effect of the Schmidt number on the turbulent mixing was found to be negligible and becomes important (as it should) only when mixing occurs under laminar flow conditions. Using a model proposed by Venayagamoorthy and Stretch in 2006 for the turbulent diascalar diffusivity as a basis, a practical (and new) model for quantifying the turbulent diascalar diffusivity is proposed asKS = 1.1 γ' LT k1/2, where LT is defined as the Thorpe length scale, k is the turbulent kinetic energy and γ' is one-half of the mechanical to scalar time scale ratio, which was shown by previous researchers to be approximately 0.7. The novelty of the proposed model lies in the use of LT, which is a widely used length scale in stably stratified flows (almost exclusively used in oceanography), for quantifying turbulent mixing in unstratified flows. LT can be readily obtained in the field using a Conductivity, Temperature and Depth (CTD) profiler or obtained from density fields in a numerical model. The turbulent kinetic energy is mostly contained in the large scales of the flow field and hence can be measured in the field using devices such as an Acoustic Doppler Current Profiler (ADCP) or modeled in numerical simulations. Comparisons using DNS data show remarkably good agreement between the predicted and exact diffusivities. Finally, the suitability of the proposed model for stably stratified flows was explored for varying degrees of stratification ranging from mildly stable flow conditions to strongly stable conditions. In stably stratified flows, density variations of the fluid dynamically affect the flow field and hence the density acts as what is widely known as an active scalar. Under strongly stable conditions, the DNS results indicate an inverse relationship between the Thorpe scale LT and kinetic energy length scale Lkε, which is different to the direct (almost one to one correspondence) relationship that was found for unstratified flows. Hence, in order to account for this difference, a modified turbulent diascalar diffusivity model was proposed as Kd = 13 γ' LT3 k1/2. It must be noted that this modified model while dimensionally inconsistent (due to the inverse relationship between the length scales), provides reasonable quantitative estimates of the diffusivity under stably stratified flow conditions. The models proposed in this study require further (extensive) testing under higher Reynolds number flow conditions. If shown to be valid, they would be widely useful for quantifying turbulent mixing using field measurements of large scale quantities (i.e. LT and k) as well as a simple and improved turbulence closure scheme.
Open Access
Observations from a series of flume experiments on contraction scour along a rectangular channel
(Colorado State University. Libraries, 2020) Nowroozpour, Alireza, author; Ettema, Robert, advisor; Julien, Pierre, committee member; Nelson, Peter, committee member; Zevenbergen, Lyle, committee member; Gallen, Sean, committee member
To view the abstract, please see the full text of the document.
Open Access
Quantitative assessment of floodplain functionality in Colorado using an index of integrity
(Colorado State University. Libraries, 2019) Karpack, Marissa Nicole, author; Morrison, Ryan, advisor; Julien, Pierre, committee member; Wohl, Ellen, committee member
Floodplain integrity can be defined as the ability of a floodplain to support essential geomorphic, hydrologic, and ecological functions that maintain biodiversity and ecosystem services. Humans alter floodplain functionality by changing the physical landscape of the floodplain or by altering river flow regimes and subsequent floodplain inundation dynamics. This research evaluates floodplain integrity by assessing the prevalence of anthropogenic modifications to hydrology and landscape. Specifically, the objectives of this research are to: 1) develop a methodology to assess floodplain integrity using geospatial datasets available for large spatial scales; and 2) use the methodology to evaluate spatial patterns of floodplain integrity in the state of Colorado. To accomplish these objectives, I evaluated the critical floodplain functions of attenuating floods, storing groundwater, regulating sediment, providing habitat, and regulating organics and solutes. At present, this work is the first to quantify the integrity of specific floodplain functions instead of measuring floodplain health solely by ecological integrity. I applied the index of floodplain integrity methodology in the state of Colorado to analyze the integrity of each of the five floodplain functions and the aggregated overall integrity. In Colorado, overall floodplain integrity decreased as stream order increased above third order streams. Floodplain integrity was also lower in floodplains that intersected urban areas than those that did not, which indicates the index of floodplain integrity captured the adverse relationship between development and floodplain health established in literature. By quantifying anthropogenic reductions to floodplain functionality at broad spatial scales, the index of floodplain integrity can help target restoration efforts towards the most affected functions and areas.
Open Access
Spatial simulation of snow and frozen ground using a modified temperature-based model
(Colorado State University. Libraries, 2018) Follum, Michael Lee, author; Niemann, Jeffrey, advisor; Fassnacht, Steven, committee member; Julien, Pierre, committee member; Kampf, Stephanie, committee member
Volume and timing estimates of snowpack and subsequent streamflow are vital for water management and flood forecasting in snow-dominated regions. Numerical models are often employed to estimate the depth of snowpack and presence of frozen ground for assessment of the resulting streamflow. Air temperature based models, such as temperature-index (TI) snow models and degree-day (DD) frozen ground models, are commonly used due to their simplicity and low data requirements. However, because air temperature (a surrogate for available energy) is the main forcing variable, the snowpack and frozen ground in TI and DD models vary spatially based only on elevation. The overall objective of this research is to improve the representation of spatial variations in snowpack and frozen ground within watersheds in order to improve streamflow simulations. To accomplish this goal, this study replaces air temperature in a TI snow model and a DD frozen ground model with a proxy temperature for available energy. The proxy temperature is calculated using a simplified radiation energy balance (requiring precipitation, air temperature, and cloud cover data) that accounts for spatial heterogeneity in both shortwave and longwave radiation due to topography and vegetation. The modified-TI model, referred to as the Radiation-derived Temperature-Index (RTI) snow model, is tested at Senator Beck basin (SBB) in Colorado and at Sleepers River Experimental Watershed (SREW) in Vermont. The RTI model outperforms a pre-existing TI model in simulation of snow water equivalent (SWE) and improves simulation of snow covered area (SCA) at both SBB and SREW. The improvements in snow simulation using the RTI model also improve the streamflow simulation at SBB. The modifications to the DD model, referred to as the modified Continuous Frozen Ground Index (modCFGI) model, also account for insulation of soil by ground cover and simulate frost depth. When tested at SREW, the modCFGI model more accurately captures the variations in frozen ground between the sites, inter-annual variations in frozen ground depths at a given site, and the occurrence of frozen ground than the pre-existing Continuous Frozen Ground Index model. Overall, the modifications made to the snow and frozen ground methods increase the spatial accuracy without requiring much additional data. The RTI and modCFGI methods are also readily transferrable to other hydrologic models.
Open Access
The development of a decision support system for concurrently evaluating changes in instream and floodplain habitats caused by flow modifications
(Colorado State University. Libraries, 2020) Passero, Elaina, author; Morrison, Ryan, advisor; Ross, Matthew, committee member; Julien, Pierre, committee member
Assessments of changes to riverine ecosystems due to flow modifications have historically focused on instream habitat. Thus, considerations of floodplain habitat have often been neglected in assessment tools, creating difficulties for understanding the comprehensive impacts of flow changes to both instream and floodplain environments. To support improved habitat management and protection of naturally variable flows, I developed a decision support system that evaluates both fish and vegetation habitat availability in alternative flow scenarios. This system uses the results of high resolution 2D hydrodynamic models to quantify and map suitable habitat for fish and floodplain vegetation at a range of discharges in a river reach. Depth, velocity, and substrate habitat preference information was used to determine available fish habitat at each modeled discharge. Vegetation habitat was quantified from logistic regression equations relating long-term habitat inundation patterns to probability of occurrence of vegetation. I demonstrate the use of this tool on the Verde River in Arizona, USA. Habitat was evaluated for the historic flow record and two alternative flow scenarios: reduced high flows and reduced baseflows. The two scenarios were compared by evaluating changes in monthly and overall habitat availability, the balance of native and non-native fish habitat, and potential for vegetation movement. Reducing high flows created more habitat for fish with non-natives having the largest increases and led to vegetation encroachment. Reducing baseflows did not affect vegetation habitat, but native and non-native fish habitat was greatly reduced.
Open Access
Untangling the effects of seasonality and post-fire stream channel erosion on the hydrologic response of a burned mountain catchment
(Colorado State University. Libraries, 2022) Gieschen, Michael, author; Nelson, Peter, advisor; Covino, Tim, committee member; Julien, Pierre, committee member
Stream channel incision and deposition are common after wildfire, and these geomorphic changes may impact runoff mechanisms and the composition of pre-event and event water in runoff. To investigate this, we monitored discharge and electrical conductivity at 6 nested sites within a 15.5 km2 watershed in the northern Colorado Front Range that had recently burned, experienced large flooding, and well-documented and significant channel erosion and deposition. Over the study period, the watershed experienced seven precipitation events. For each hydrograph, we separate baseflow from runoff using a new method to characterize and account for the strong diurnal signal in the baseflow. Electrical conductivity is used as a tracer in a two-component end-member mixing analysis to separate the event hydrographs into event and pre-event water. Correlation coefficients were computed between key variables of the hydrologic response (such as runoff ratio, volumes of event and pre-event water) to storm and basin characteristics (including stream channel erosion/deposition, fraction of high/moderate burn severity, precipitation intensity, and antecedent precipitation). The strength and significance of correlations was found to vary seasonally. In the early season, event and pre-event volumes did not vary significantly with basin or storm characteristics. In the late season, antecedent precipitation correlated with a decrease in event runoff (R2 = 0.34) and total runoff (R2 = 0.40), increased precipitation intensity correlated with an increase in event runoff (R2 = 0.48), and local erosion correlated with an increase in pre-event runoff (R2 = 0.60) and total runoff (R2 = 0.53). These findings indicate that seasonality and post-fire stream channel erosion influence the makeup of runoff response, most likely through their impact on the gradient of the near-stream groundwater table.