Mountain Scholar :: Browsing by Author "Nelson, Peter, committee member"

Browsing by Author "Nelson, Peter, committee member"

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Open Access
Analyzing post-flood recovery after an extreme flood: North St. Vrain Creek, CO
(Colorado State University. Libraries, 2018) Eidmann, Johanna S., author; Rathburn, Sara, advisor; Wohl, Ellen, committee member; Nelson, Peter, committee member
Assessing the ongoing sediment remobilization and deposition following an extreme flood is important for understanding disturbance response and recovery, and for addressing the challenges to water resource management. From September 9-15, 2013, a tropical storm generated over 350 mm of precipitation across the Colorado Front Range. The resulting 200-year flood triggered landslides and extreme channel erosion along North St. Vrain Creek, which feeds Ralph Price Reservoir, water supply for the Cities of Lyons and Longmont, CO. The flood resulted in 10 m of aggradation upstream of the reservoir, transforming the reservoir inlet into an approach channel. 4 years after the flood, downstream transport of flood sediment and deposition in the reservoir continues. This research tracks the fate of flood-derived sediment to understand the evolution of the approach channel and delta to assess post-flood response processes and controls and to quantify sediment remobilization. Photographic analysis and DEM differencing of the approach channel indicates that the majority of channel response to the flood occurred within 1 year following the flood. Evolution of the channel from an initial plane bed occurred through channel incision of up to 2.5 m and widening of up to 10 m, forming a trapezoidal cross section. Channel geometry changes in years 2-5 post-flood are limited in spatial extent, largely dependent on sediment discharge and local variations in channel confinement. Bathymetric DEM differencing from 2014 and 2016 (years 1 and 3 post-flood) indicates a minimum sediment accumulation of 68,000 m3 on the delta plain, and progradation of 170 m of the delta front since the 2013 flood. Between fall 2016 and spring 2017, the reservoir level was dropped approximately 10 m during construction at the spillway, creating a base level drop, delta incision, and causing over 15,000 m3 of sediment to be transported further into the reservoir. Based on bathymetry and reservoir core analyses, a total of 74,000 m3 of sediment was deposited in the delta from 2014 through 2017, producing an estimated loss of 0.4% in reservoir storage capacity. Approximately 184,000 m3 (equivalent to another 1% of reservoir storage capacity) is estimated to remain in storage upstream of the reservoir. Although the approach channel appears to be adjusted to a typical snowmelt runoff, stored sediment remaining upstream of the reservoir indicates that complete recovery of the approach channel may not occur on a management time scale. The remaining large volume of sediment still in storage upstream highlights the potential for future disturbances to trigger additional sediment inputs.
Open Access
Application of large-scale particle image velocimetry at the Hydraulics Laboratory of Colorado State University
(Colorado State University. Libraries, 2018) Chen, Kaiwei, author; Ettema, Robert, advisor; Thornton, Christopher, committee member; Nelson, Peter, committee member; Landers, Stuart, committee member
Large-scale particle image velocimetry (LSPIV) is a nonintrusive technique used to measure free-surface velocities of water flow in a manner that produces a two-dimensional vector field of flow velocity. LSPIV is gradually becoming quite widely used as a technique for measuring flow velocities in a range of flow areas. This study used readily available material and devices, and software, to apply LSPIV to flow fields in two laboratory flumes at the Hydraulics Laboratory of Colorado State University; LSPIV had not been used in this laboratory before this study. The applications used pieces of paper as tracer floats in the flow field, and employed a standard iPhone 6s to record video of the tracers moving in the flow field. The video record of tracer movements was then analyzed using Fudaa LSPIV software and Tecplot 360 software to calculate and present the flow velocity data. The applications demonstrated the utility of the LSPIV technique for determining the free-surface flow patterns, and their variations, in experiments done at the Hydraulics Laboratory. Additionally, this study examined the relationship between the tracer size and LSPIV accuracy with the objective of identifying an optimal width of tracer relative to the width of the flow field and its features. Five sizes of tracer were used in measuring the water-surface flow field through a series of contractions and expansions. It was found that the best tracer size is about from 3.80% to 6.33% of the wide of the channel.
Open Access
Bed sediment transport and channel morphology in a braided channel: insights from a flume experiment
(Colorado State University. Libraries, 2017) Armstrong, Dylan L., author; Ettema, Robert, advisor; Nelson, Peter, committee member; Falkowski, Michael, committee member
This thesis presents the methods and findings from an experiment aimed at relating the rate of bed-sediment transport through a reach of a braided channel to the intensity of the braiding sub-channels (anabranches) along the reach. The experiment was conducted in a large flume located at Colorado State University's Hydraulics Laboratory in Fort Collins, Colorado. No similar flume experiments have been conducted involving braided channels in a wide alluvial plain. Such experiments involve several challenging considerations that greatly complicate such experiments: braided channels are characteristically wide and shallow; have relatively large bed-sediment loads that are difficult to measure, because they move in multiple sub-channels; and the sub-channels (often termed anabranches) are ephemeral. The self-forming nature of the anabranches means that there is little direct control over the exact morphology of the braided channel. The objectives set forth in this experiment overcame the challenges of braided river flume studies, and allowed a comprehensive data set to be obtained of both bed sediment transport data and morphologic braided intensity data. The intensity of braiding was characterized using a braiding index (Flow Width Ratio) developed during this experiment. A relationship was identified and a trend established – as FWR increased, the rate of bed-sediment transport decreased – but the stochastic nature of transport rates and morphology introduced much scatter in the relationship. It was found that local morphologic features have a large impact on the transport of sediment through braided systems, and that the features could help explain some of the scatter in the data.
Open Access
Braided river response to eight decades of human disturbance, Denali National Park and Preserve, AK
(Colorado State University. Libraries, 2016) Richards, Mariah, author; Rathburn, Sara, advisor; Booth, Derek, committee member; Nelson, Peter, committee member; Wohl, Ellen, committee member
The spatial complexity and stochastic nature of braided rivers complicate our ability to quantify natural rates of sediment transport and limit our understanding of braided river response to human disturbance. The Toklat River in Denali National Park and Preserve, a braided tributary of the Kantishna River draining the north-facing slopes of the Alaska Range, exemplifies these challenges. Eight decades of localized channel confinement due to installation of a causeway in the 1930's and over three decades of gravel extraction since the 1980's have occurred on the Toklat River adjacent to the Denali Park Road. A unique, multi-scalar and temporally diverse dataset records the responses of the river over a 10-km reach. I evaluated trends in short-term sediment storage through LiDAR differencing and analyzed long-term planform change using braiding index, braiding beltwidth and topographic ruggedness derived from aerial photographs. Two reference reaches along comparable adjacent braided rivers, with varying levels of confinement and no gravel extraction, illuminate the relative influence of these human disturbances on channel and planform change. Comparisons of 2009 and 2011 LiDAR-derived DEMs showed a statistically significant volumetric loss of -30,300 ± 27,600 m3 over 4 km of active braidplain within the study reach. Braidplain sediment loss adjacent to the channel-confining Denali Park Road bridge crossing was comparable to that removed biennially through gravel extraction downstream (17,100 m3). Upstream of both the gravel extraction site and the bridge crossing, the braiding beltwidth decreased by 400 m and the braiding index lowered from eight to one between 1988 and 2011. The reference reaches did not display such noticeable morphologic adjustments, implying upstream migration of gravel extraction and confinement impacts, which can significantly alter flow character, leading to increased localized stream power, degradation and infrastructure damage. These results are relevant to assessing the variety and spatial extent of human disturbance on braided river systems in general.
Open Access
Controls on post-High Park Fire channel response, South Fork Cache la Poudre Basin, Colorado
(Colorado State University. Libraries, 2015) Shahverdian, Scott M., author; Rathburn, Sara, advisor; Wohl, Ellen, committee member; Nelson, Peter, committee member
Post-fire basin sediment yield is the product of multiple erosional processes operating at multiple spatial scales and in different process domains. Most post-fire erosion response studies have focused on the hillslope scale, yet land management decisions and post-fire treatments are addressed at the watershed scale. The goal of this study was to evaluate how the channel network contributes to the production, transport, and storage of sediment by monitoring post-fire channel response. A better understanding of channel production, transport, and storage of sediment post-fire is required in order to predict basin scale sediment yields and make informed management decisions. Two perennial headwater streams and two ephemeral tributaries of the South Fork Cache la Poudre River were monitored in two severely burned basins in the 2012 High Park Fire burn area of northern Colorado. The basins were either completely or partially mulched with agricultural straw and wood mulch during June 2013. Repeat cross section and longitudinal profile surveys were performed to evaluate event-driven changes. The dominant response in both basins post-fire was net degradation. Steep channel slopes promoted channel incision with no significant overbank deposition, indicating that the channel network was a substantial source of sediment and an efficient transporter of hillslope sediment. In 2013, six storms exceeded the 30 minute maximum intensity 10 mm hr⁻¹ associated with hillslope sediment production while in 2014 two storms exceeded this threshold. Perennial channel response in 2014, measured by mean bed elevation change at cross sections, ranged from -20 to +17 cm, but most cross sections experienced changes between 0-3 cm. Channel response was uncorrelated with channel slope, channel slope*contributing area product, or width to depth ratio. Ephemeral channels showed an alternating cycle of aggradation and degradation on the order of 0-3 cm per event, as well as a scour and fill response during storm events. Scour and fill often resulted in minimal net changes to channel geometry, suggesting that the channel was an important temporary source and sink of sediment and that post-fire peak flow calculations must account for event-based scour. In 2013, suspended sediment concentrations in the South Fork Cache la Poudre exceeded 2500 mg L⁻¹ 12 times, and exhibited a threshold response when MI₃₀ exceeded 10 mm hr⁻¹. In 2014, suspended sediment concentrations exceeded 1500 mg L⁻¹ once, and a MI₃₀ of 40 mm hr⁻¹ was insufficient to cause values to exceed 1500 mg L⁻¹. Post-fire suspended sediment concentrations from the South Fork Cache la Poudre River indicate that hillslopes were the primary source of suspended sediment. Where the straw mulch was retained on hillslopes, it was effective at limiting erosion. The channel network was largely resistant to change during the second year of post-fire monitoring due to the influence of a >200 year storm that occurred in September 2013. Following this storm, the channel network acted primarily to transport sediment rather than produce sediment. Sediment connectivity within the channel network was high in each basin due to steep channel slopes, but the development of an alluvial fan at each basin outlet as well as the morphology of the South Fork Cache la Poudre at each confluence suggest differences in the sediment delivery from each basin to downstream reaches. Sediment connectivity from the hillslopes to the channel network and along the channel network must be addressed in post-fire studies when predicting or interpreting post-fire basin sediment yields. Furthermore, assessing sediment connectivity is a useful tool for land managers making post-fire erosion mitigation decisions.
Open Access
Decreasing stream habitat for Greenback cutthroat trout under future climate projections in headwater streams of the southern Rocky Mountains, Colorado
(Colorado State University. Libraries, 2022) Ma, Chenchen, author; Morrison, Ryan R., advisor; Nelson, Peter, committee member; Kanno, Yoichiro, committee member
Headwaters are vital to the abundance and diversity of biota as they produce various temperatures, light, hydrologic regimes, water chemistry, substrate type, food resources, and species pools. Many studies have shown that headwater streams are especially vulnerable to changing climate, and coldwater fish are especially sensitive to the fluctuations in streamflow and water temperature during summertime low flows. Though previous studies have provided insights on how changes in climate and alterations in stream discharge may affect the habitat requirements for native cutthroat trout species, the suitable physical habitats have not been evaluated under future climate projections for the threatened Greenback Cutthroat Trout (GBCT) occupying the headwater regions in the Southern Rocky Mountains. Thus, this study used field data collected in the summers of 2019 and 2020 from selected headwater streams across the Front Range in the Southern Rocky Mountains to construct one-dimensional hydraulic models (HEC-RAS) to evaluate streamflow and physical habitat under four future climate projections. A principal component analysis (PCA) was then performed to demonstrate the importance of each morphological feature of these streams. Results illustrate high variations in both predicted streamflow reductions and physical habitat for all future climate projections. The projected mean summer streamflow shows much greater decline compared to the projected mean August flow. Moreover, sites located at higher elevations with larger substrate (D50 and D84) and steeper slopes may experience greater reductions in physical habitat under mean summer future climate projections. Future climate change studies on cold-water fisheries need to take multiple influential factors into account instead of heavily focusing on the thermal characteristics. Reintroduction and management efforts for GBCT should be tailored to the individual headwater stream with adequate on-site monitoring that can be applied in a more holistic manner as well.
Open Access
Effects of post-fire mulch applications on hillslope-scale erosion
(Colorado State University. Libraries, 2023) Geller, Jordyn, author; Kampf, Stephanie, advisor; Barnard, David, advisor; Nelson, Peter, committee member
Wildfires are increasing in frequency and intensity, greatly altering the landscape and increasing risk of erosion. Mulching is a common restoration technique used after wildfire to enhance protective ground cover and reduce erosion, yet most studies are conducted at the plot-scale. This study applies an experimental approach to evaluate the impact of mulch treatments at the hillslope-scale using varying mulch levels. Similar adjacent hillslopes were chosen to minimize variability in landscape features. The objectives of this research are to 1) examine the effectiveness of post-fire mulching in reducing erosion at the hillslope-scale, and 2) identify landscape features and precipitation factors contributing to the occurrence and magnitude of sediment yield. Sediment fences were installed in convergent swales and planar hillslopes to quantify sediment yields before and after aerial wood mulch application. Rain gauges were installed to compute rainfall amount (mm), duration (hr), and maximum intensities (mm/hr) by storm event. Field observations, coupled with game camera footage, were utilized to evaluate whether each storm produced sediment in the fences. Surface cover surveys were conducted to assess cover changes over the season. Collectively these data were used to 1) identify rainfall intensity thresholds for erosion, 2) examine controls on sediment generation occurrence with a binomial distribution mixed-effects model, 3) examine controls on the magnitude of sediment yield using a gamma distribution mixed-effect model, and 4) assess relative importance of variables relating to sediment yield using random forest models. Threshold rainfall intensities for generating erosion at the study sites were 32-38 mm/hr for MI5, 11-18 mm/hr for MI15, 7-13 mm/hr for MI30, and 5-8 mm/hr for MI60. Across all models of erosion occurrence and magnitude of sediment yield, maximum rainfall intensity and total precipitation were primary drivers of erosion. There was no evidence of a mulch treatment effect on sediment occurrence or magnitude, likely resulting from insufficient initial mulch cover and a high-intensity storm that removed much of the mulch shortly after it was dropped on the hillslopes. Contributing area, slope mean, and slope length showed no influence on sediment yield, likely due to limited variation in these variables between hillslopes. These results highlight the importance of mulch cover that will stay in place under extreme rainfall. Future hillslope-scale studies should consider dropping mulch during a time period that is unlikely to have high intensity rainfall and explore mulch materials and application methods that will better ensure adequate initial cover for reducing hillslope-scale erosion.
Open Access
Energetics and dynamics of flow through baffle drop shafts using physical and computational model studies
(Colorado State University. Libraries, 2023) Aluthwalage, Kasun Prabodha Sahabandu, author; Venayagamoorthy, Subhas Karan, advisor; Loc, Ho Huu, advisor; Nelson, Peter, committee member; Windom, Bret, committee member
A drop shaft is one of the main hydraulic structures that is used to convey water from higher to lower elevations while dissipating potential energy in storm water management systems, water treatment plants, and hydropower stations. Drop shafts need to be adjusted for higher discharges because of the increased urban flooding due to climate change and rapid urbanization. Traditional baffle drop shafts have limited flow capacity and are unstable due to their asymmetric nature. The novel baffle drop shaft is proposed here for larger range of flow discharges. To the author's knowledge, there are no previous studies that have thoroughly investigated the energy dissipation potential of the novel baffle drop shaft. Hence, there is a need to establish a design relationship between key parameters such as the shaft diameter, baffle spacing, and discharge to inform best design practices. A 1:10 physical model study was carried out to investigate the energy dissipation of a novel baffle drop shaft using different discharges. Pressure and velocity were measured at two locations on the baffles using low range pressure sensors (100 mbar) and an electromagnetic velocity meter. Timed averaged pressure and velocity on the baffles increased with discharge. These averaged quantities were considered to calculate global and local energy dissipation through the shaft. The global energy dissipation efficiency was calculated based on the inlet and outlet channel flow data, and was found to range from 89.6% to 91.9%. The flow regime profiles were quite similar on each baffle section of the shaft; hence, we can consider the energy dissipation in each baffle to be equivalent. Under free-flow conditions, the energy dissipation efficiency decreases as the discharge increases. Physical models are costly and time-consuming for performing parametric studies of flowthrough such structures because each and every geometric configuration needs to be constructed in the lab. Computational Fluid Dynamics (CFD) is a more feasible option to conduct an in-depth investigation of the energetics and dynamics of flow in a baffle drop shaft since it is faster and more cost-effective than a physical model study. The CFD models have been built to simulate the hydraulic behavior of baffle drop shafts using OpenFOAM. This software is adaptable for modeling diverse flow issues due to the variety of models and numerical techniques that it incorporates. A suitable turbulence model that is commonly used in CFD for modeling turbulent flows such as in drop shafts is the RANS-based realizable k- ϵ model. Mesh sensitivity analysis was also performed to establish grid independences of the solution. Benchmark geometry CFD models were calibrated using four locations in the physical model, and velocity and pressure measurements at the edge of the baffle were used for validation with remarkable agreement. A parametric study was conducted using shaft diameters (D) of 0.8 m, 0.9 m, and 1 m, six baffle spacings (h) ranging from 0.23m to 0.48 m, and baffle rotating angles (θ) of 180◦, 250◦, and 270◦. Global energy dissipation efficiency (η) ranged from 92% to 97%. The η value decreased with discharge but was higher under free flow conditions in the baffle drop shaft. The geometric parameters D, h, and θ have little influence on energy dissipation. Considering structural integrity, available space, construction costs, and maintenance costs, the baffle drop shaft needs to be optimized to achieve the desired hydraulic performance. Maximum pressure was observed at the water jet impact location close to the outer shaft wall. Air entrainment is also a significant consideration in designing baffle drop shafts because its impact is critical in applications like hydro power generation. The bulking of the flow due to air entrainment needs to be considered to evaluate the maximum flow carrying capacity of baffle drop shafts. In summary, designing baffle drop shafts requires a multi-criteria approach that is mainly dependent on the design requirements on energy dissipation, structural integrity, construction costs, air entrainment, application, and location.
Open Access
Ensemble-based analyses of liminal extreme rainfall events near Taiwan and northern Colorado
(Colorado State University. Libraries, 2022) Cole, Alexandra S., author; Bell, Michael M, advisor; Rasmussen, Kristen, committee member; Nelson, Peter, committee member
Heavy rainfall is a phenomenon that impacts a variety of climates around the world, from the moisture-rich, tropical northwestern Pacific to the drier Northern Colorado plains. Improvements over decades of numerical weather prediction have allowed for increased accuracy in simulations of heavy rainfall cases, but there are still improvements yet to be made. This thesis, in conjunction with the Prediction of Rainfall Extremes Campaign in the Pacific (PRECIP) field campaign, aims to study the mechanisms behind these heavy rainfall events to increase understanding of their underlying processes and improve the modeling of them. Two weather events are investigated in detail, one in which heavy rainfall was not forecast by global models but greater than 600 mm of rainfall accumulated, and a contrasting case in which heavy rainfall was forecast but little to no rainfall accumulated. On 09 June 2020 near Taiwan, heavy rainfall was produced by quasi-stationary back-building mesoscale convective systems (MCS) associated with a mei-yu front. Peak rainfall amounts totaled over 600 mm with widespread rainfall totals greater than 100 mm. Global model forecast skill was poor in both location and intensity of rainfall. The mesoscale ensemble showed liminal conditions between heavy rainfall or little to no rainfall. The two most accurate and two least accurate ensemble members are selected for analysis via validation against radar-estimated rainfall observations. All members feature moisture-rich environments and moist neutral soundings with low levels of free convection (LFC) and sufficient instability for deep convection, and the synoptic setups do not suggest such different outcomes. Through our analysis, we find that stronger gradients in 100 m virtual potential temperature in the two most accurate members associated with a near-surface frontal boundary provide the primary lifting mechanism for enhanced rainfall. In the two heaviest rain-producing members, air moves north/northeastward and ascends the virtual potential temperature isentropes and rises above the LFC, producing back-building deep, moist convection. The near-surface gradients are weaker and more confined along Taiwan's coast in the two least accurate members, which leads to less rainfall that is misplaced from reality. The analyses suggest that subtle details in the simulation of frontal boundaries and meso-scale flow structures can lead to bifurcations in producing extreme or almost no rainfall. A contrasting event occurred in Northern Colorado on 31 July 2021, where heavy rainfall was forecast and flood warnings were issued, but little to no rainfall and flooding took place in the forecast area. Synoptic and mesoscale conditions were ripe for heavy rainfall, with anomalously high precipitable water values and moderate values of CAPE. Similar to the 09 June 2020 case, the mesoscale ensemble showed a wide spread in rainfall totals, related in part to the variability of surface boundaries and forcing across the ensemble. Weak surface forcing led to very little rainfall in this case despite the high moisture, suggesting similar physical mechanisms and predictability challenges across both the analyzed cases. Implications for improved probabilistic forecasts, increased forecast accuracy, and thus increased public safety for heavy rainfall events are discussed.
Open Access
Estimating changes in streamflow attributable to wildfire in multiple watersheds using a semi-distributed watershed model
(Colorado State University. Libraries, 2023) Wells, Ryan, author; Niemann, Jeffrey D., advisor; Kampf, Stephanie, committee member; Nelson, Peter, committee member
Over half of western U.S. water supply is sourced from forested lands that are increasingly under wildfire risk. Studies have begun to isolate the effects of wildfire on streamflow, but they have used coarse temporal resolutions that cannot account for the numerous, interconnected watershed processes that control the responses to rainfall events. To address these concerns, we developed a method to isolate fine-scale (daily) effects of fire from climate. Wildfire effects were represented by the difference between measured post-fire daily streamflow and simulated unburned post-fire daily streamflow from a hydrologic model calibrated to pre-fire conditions. The method was applied to track hydrologic recovery after wildfires in six burned watersheds across the western U.S.: North Eagle Creek, NM (2012 Little Bear Fire), Lopez Creek, CA (1985 Las Pilitas Fire), and City Creek, Devil Canyon Creek, East Twin Creek, and Plunge Creek, CA (2003 Old Fire). All six watersheds experienced prolonged increases of post-fire streamflow, with the most consistent changes occurring during periods of low streamflow. Following 6 years of increased streamflow, Lopez Creek experienced 6 years of reduced streamflow, before returning to pre-fire streamflow behavior 12 years after the fire. North Eagle Creek and the four watersheds affected by the Old Fire continued to demonstrate elevated streamflow 9 and 18 years post-fire, respectively. This study demonstrates the utility of examining post-fire streamflow at daily resolution over multiple years. In particular, these results captured the variability of change across flow frequencies during recovery periods that would not be quantifiable otherwise.
Open Access
Geophysical constraints on the evolution of an ephemeral channel at the Sand Creek Massacre National Historic Site, Colorado, USA
(Colorado State University. Libraries, 2017) Sparks, Brad, author; Harry, Dennis, advisor; Wohl, Ellen, committee member; Nelson, Peter, committee member
A geophysical survey was conducted on an ephemeral channel, Sand Creek, at Sand Creek Massacre National Historic Site to test three hypothesized migration and depositional models of ephemeral streams. A key motivation for the study is to identify the historical location of Sand Creek, which is critical to establishing the location of the 1864 Sand Creek Massacre. Hammer seismic refraction data were collected on 7 valley-wide lines oriented perpendicular to the channel, and ground penetrating radar data (200 MHz antenna) was collected on a grid overlying the channel and the channel banks. An additional GPR line (100 MHz) was collected on a line spanning the valley bottom. The refraction data show 4 layers: an eolian cap that is 1 - 3 m thick with a velocity of 0.3 km/s; a gradational alluvium layer consisting of ephemeral deposits which is 2 – 3 m thick with velocities ranging from 0.5 - 1 km/s; a gradational alluvium layer consisting of perennial fluvial deposits which is 2 - 7 m thick with velocities ranging from 1.2 - 2.9 km/s; and a homogeneous layer with a velocity of 2.4 km/s which is interpreted to be the Pierre Shale Formation. The radar data located buried channel boundaries and revealed a change in bedforms at 3 - 4 m deep. The change in bedforms is interpreted to indicate a flow regime change from an older perennial to a more recent ephemeral flow. The channel bedforms within the ephemeral flow regime deposits suggest that the channel has not migrated across the modern valley since the ephemeral flow regime was established, but punctuated changes in morphology within the channel have occurred in association with major floods. The results indicate that the channel has not changed position in historical times. This suggests that the modern stream is the proper geographic context for historical accounts that reference the location of Sand Creek when describing events that occurred during the 1864 massacre.
Open Access
Land surface sensitivity of mesoscale convective systems
(Colorado State University. Libraries, 2016) Tournay, Robert C., author; Schumacher, Russ, advisor; Vonder Haar, Thomas, advisor; van den Heever, Susan, committee member; Nelson, Peter, committee member
Mesoscale convective systems (MCSs) are important contributors to the hydrologic cycle in many regions of the world as well as major sources of severe weather. MCSs continue to challenge forecasters and researchers alike, arising from difficulties in understanding system initiation, propagation, and demise. One distinct type of MCS is that formed from individual convective cells initiated primarily by daytime heating over high terrain. This work is aimed at improving our understanding of the land surface sensitivity of this class of MCS in the contiguous United States. First, a climatology of mesoscale convective systems originating in the Rocky Mountains and adjacent high plains from Wyoming southward to New Mexico is developed through a combination of objective and subjective methods. This class of MCS is most important, in terms of total warm season precipitation, in the 500 to 1300m elevations of the Great Plains (GP) to the east in eastern Colorado to central Nebraska and northwest Kansas. Examining MCSs by longevity, short lasting MCSs (<12 hrs), medium (12-15 hrs) and long lasting MCSs (>15 hrs) reveals that longer lasting systems tend to form further south and have a longer track with a more southerly track. The environment into which the MCS is moving showed differences across commonly used variables in convection forecasting, with some variables showing more favorable conditions throughout (convective inhibition, 0-6 km shear and 250 hPa wind speed) ahead of longer lasting MCSs. Other variables, such as convective available potential energy, showed improving conditions through time for longer lasting MCSs. Some variables showed no difference across longevity of MCS (precipitable water and large-scale vertical motion). From subsets of this MCS climatology, three regions of origin were chosen based on the presence of ridgelines extending eastward from the Rocky Mountains known to be foci for convection initiation and subsequent MCS formation: Southern Wyoming (Cheyenne Ridge), Colorado (Palmer divide) and northern New Mexico (Raton Mesa). Composite initial and boundary conditions were developed from reanalysis data, from which control runs of regional MCSs were made as well a series of idealized experiments with imposed large scale soil moisture (SM) anomalies to study to impact to each regional MCS on SM variations in initiation region as well down stream in the GP. Another idealized experiment was made to study the impact of varying the planetary boundary layer (PBL) parameterization in the context of the idealized SM variations. While the distribution of SM has a major impact on CAPE and the location and magnitude of CI, also important is the differences in shear driven by the differences in large scale SM, playing a major, and varying depending on where the regional MCSs interact with the shear anomalies. Utilizing a different PBL parameterization impacts the timing and amount of initial CI, impacting the total precipitation produced by the MCSs, but not nearly the magnitude of alteration to the MCS as varying the SM distribution. A climatology of CI in the Rocky Mountains and adjacent high plains is made using a high resolution observational dataset. From this climatology, the sensitivity of CI to land surface variables, including SM and vegetation is studied. It was found that the timing of CI had a stronger relationship with SM, with earlier CI over wetter than average soils, with the greatest difference in May in the north of the domain, nearly all statistical significant values across regions from north to south in June and July with little difference in August in the northern regions. Outside of May, which showed a strong relationship of earlier CI over less vegetated regions, the relationship was similar, but weaker than, that between SM and CI timing. Examining the CAPE, CIN and PW at CI and null points reveal that the values are generally more conducive to CI over wet soils and anomalously vegetated areas at both CI and null points, with stronger difference in the high plains in the east of regions. Examining the covariance of SM and vegetation at CI points revealed that July and August showed expected covariance relationships with concurrently measured convective variables (i.e., high SM/vegetation associated with high CAPE and vice versa for low SM/vegetation) while May and June higher CAPE and CIN over low vegetation anomalies. A climatology of elevated mixed layers in the central GP was conducted, revealing that the greatest number of EMLS occurred in the northern GP. Back trajectories (BT) were conducted from the radiosonde point of detection for 18 and 36 hours, revealing that the BT point mean for days with severe weather were further west and south from the origin point. The SM and vegetation was sampled at the BT point, revealing a negative, significant correlation with EML depth when pooling the northern stations in 18-hr BTs, and a significant, negative correlation with EVI when pooling the southern sites. A modeling case study was conducted in which an idealized SM anomaly was imposed over the EML origin region. Experiments were also conducted to test the sensitivity of ML formation and EML transport using different PBL parameterizations. While the YSU PBL parameterization produced the deeper PBL over anonymously dry soils in the EML origin region, the EML was not transported to the east as it was in those experiments using the MYNN parameterization, impacting the timing and extent of precipitation in the model runs.
Open Access
Modeling stream evolution and its consequences for watershed scale pollutant loading
(Colorado State University. Libraries, 2018) Lammers, Roderick W., author; Bledsoe, Brian P., advisor; Arabi, Mazdak, committee member; Nelson, Peter, committee member; Rathburn, Sara, committee member
Throughout the world, streams are degraded due to impaired water quality and erosion and sedimentation caused by hydrologic and sediment imbalances. These two issues are linked. Channel erosion not only damages stream habitat but can be a significant source of fine sediment and nutrient pollution in watersheds. Phosphorus in particular is common in streambanks and when these soils are mobilized – for example during amplified high flows in urban streams – they can contribute to eutrophication of downstream water bodies. Understanding these dynamics is important for reversing these impairments and sustainably managing our water resources. In this dissertation, I provide a new tool to quantify the magnitude of channel erosion as a pollutant source. First, I put this issue in context by reviewing recent literature on stream restoration and its ability to either reduce nutrient loading or enhance natural nutrient removal processes. Results suggest that stream restoration can help reduce nutrient pollution, but quantifying these benefits remains challenging. Despite the rapid growth of the stream restoration industry, there is still insufficient monitoring and assessment of project success. Perhaps this is due to a lack of standardized tools and methodologies. The remainder of this dissertation attempts to fill part of this gap --- providing a new tool to predict watershed sediment and phosphorus loading from channel erosion. I develop a new model to simulate stream channel evolution at the watershed scale. This model is built around specific stream power, a variable that is straightforward to calculate using easily quantified parameters: discharge, slope, and width. I first develop new sediment transport equations based on specific stream power. These are used by the model to simulate channel bed aggradation and degradation. I link these processes with a simplified version of a bank erosion model to account for lateral channel adjustment. Model simulations match physical understanding of channel evolution in response to disturbance as well as field datasets of rivers adjusting to both human and natural perturbations. Importantly, the model is structured to quantify uncertainty in model projections. This is essential for understanding both model limitations and more generally for simulating complex systems in a stochastic world. Finally, I apply this new model to estimate sediment and phosphorus loading from bank erosion in two watersheds: Big Dry Creek, Colorado and Lick Creek, North Carolina. Despite their many differences and their unique simulated responses to disturbance, results for both watersheds suggest that channel erosion may be responsible for nearly all of the suspended sediment pollution, but very little of the phosphorus. This new model has a number of applications in both the scientific and management communities --- exploring river behavior in more detail while also answering relevant management questions as we try to more effectively steward our water resources.
Open Access
Monitoring the effectiveness of river realignment on the Upper Colorado River, Rocky Mountain National Park
(Colorado State University. Libraries, 2017) Sparacino, Matthew, author; Rathburn, Sara, advisor; Covino, Tim, committee member; Nelson, Peter, committee member; Ronayne, Mike, committee member
A 2003 debris flow introduced 36,000 m3 of sediment into a high-elevation wetland on the Upper Colorado River in Rocky Mountain National Park. In September 2015, Park staff built an earthen diversion dam and realigned a 190 m reach of the Colorado River into its historic thalweg through the center of Lulu City wetland. Initial dimensions of the constructed channel were 1.6 m wide and 0.4 m deep with an average bed slope of 1.9%. Pre- and post-restoration measurements are compared to assess the hydro-geomorphic response to the channel realignment within the adjacent wetland. The constructed diversion berm redistributed at least 48% of river discharge from a pre-realignment, west-side channel, to a central channel, which decreased surface-water groundwater exchange as well as the size of near-stream hyporheic zones and altered sediment transport capacity. A sodium chloride tracer was injected during base-flow and electrical resistivity was used to monitor changes in near-channel hyporheic exchange across the realigned channel for approximately 24 hours following the injection. Pre-and post-realignment electrical resistivity analyses indicate a loss of hyporheic exchange in the northern wetland, likely a result of decreased river complexity. Tracer mass balances derived from concurrent surface conductivity measurements indicate increases in solute retention throughout Lulu City wetland, possibly due to increased overbank flow. These results imply that solute retention can increase without an equal increase in hyporheic exchange. Furthermore, local incision greater than 0.5 m, widening of 0.2 to 1 m, and upstream knickpoint migration within the realigned channel during 2016 runoff indicate increases in erosion and local sediment transport. The growth of gravel bars upstream of the diversion berm indicate increased sediment deposition at the head of Lulu City wetland. Results from one year of post-realignment monitoring suggest that the channel realignment has had small-scale effects on hyporheic exchange, solute retention, and sediment transport capacity, with potentially negative consequences for the ecosystem services provided by river-wetland systems. Long-term monitoring and increased instrumentation are required to predict how these changes may be amplified in a larger restoration attempt.
Open Access
Nonlinear internal wave - topographic interaction and turbulent mixing using numerical simulations
(Colorado State University. Libraries, 2021) Klema, Matthew Roy, author; Venayagamoorthy, Subhas Karan, advisor; Nelson, Peter, committee member; Rathburn, Sarah, committee member; Thornton, Christopher, committee member
To view the abstract, please see the full text of the document.
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
On the magnitude and frequency of sediment transport in rivers
(Colorado State University. Libraries, 2015) Sholtes, Joel Stephen, author; Bledsoe, Brian, advisor; Wohl, Ellen, committee member; Nelson, Peter, committee member; Arabi, Mazdak, committee member
What flow or range of flows is most responsible for transporting sediment and maintaining sediment continuity in a river over human time scales? This question has inspired scores of studies analyzing the magnitude and frequency of sediment transport (MFA) in rivers and has been a part of the ongoing debate regarding process vs. form-based approaches to stable channel design. MFA in rivers is of general scientific and management interest as it influences channel form, water quality, aquatic habitat, and channel restoration design considerations. The research presented in this dissertation asks the following overarching question: What influences how much and how often sediment is transported in a river? In this dissertation, I consider relationships between the drivers of sediment transport at a point in a river (flow regime, sediment size, and channel form) and metrics describing sediment yield, which integrate the relationship between flow regime and transport over time. To study this question, I use theoretical and empirical approaches in a spectrum of stream types from fine bed streams dominated by suspended load transport to gravel and cobble streams dominated by bed load transport. I find that, for example, the frequency of the most effective discharge decreases and the range of flows most responsible for sediment yield increases with increasing flow regime variability. As river bed material becomes coarser, a more narrow range of less frequent flows becomes most effective in sediment transport. The river management and restoration community has given much effort to predicting the bankfull discharge, Qbf, and associated channel geometry at Qbf for the purposes of channel study, classification, and design. In a study comparing various Qbf predictors in coarse and fine bed rivers, I find that the discharge associated with 50% of cumulative sediment yield based on the flow record—Qs50, the half yield discharge—predicts Qbf better than most other predictors, especially in fine-bed rivers. Other predictors include the most effective discharge, Qeff, and the 1.5-year flood. Using statistical methods to quantify the uncertainty in the sediment load-discharge relationship as well as the empirical flow frequency distribution, I develop methods to propagate uncertainty in estimations of Qeff and Qs50. In an examination of the influence of flow regime non-stationarity on sediment yield metrics, I find that in urbanizing watersheds with increasing trends in flow variance, estimates of Qeff and Qs50 increase dramatically compared to those based on the entire flow period of record. Finally, I estimate Qeff and Qs50 using empirical, sediment load data-driven models and physically-based models driven by one-dimensional flow-depth relationships evaluated at a cross section. Physically-based models that match the slope of the sediment load-discharge relationship performed well. This is the case with total load models for fine bed sites, but generally not the case for bed load models used on coarse bed sites. All daily flow records and sediment load data as well as all Matlab ® and R scripts are contained in the supplementary data.
Open Access
Organic carbon storage in mountain river valley bottoms of the western United States
(Colorado State University. Libraries, 2018) Scott, Daniel N., author; Wohl, Ellen, advisor; Covino, Tim, committee member; Nelson, Peter, committee member; Rathburn, Sara, committee member
Valley bottoms, which include river channels and associated floodplains, are important components of the terrestrial carbon sink. Downed wood and floodplain soil in valley bottoms act as transient pools of organic carbon (OC) that can be stored for up to millennial timescales. This dissertation focuses on quantifying OC storage as downed wood and soil in mountain river valley bottoms in four disparate watersheds that span three mountain ranges across the western United States. Across these four basins, I measured wood load, floodplain OC content, morphologic metrics, and/or vegetation metrics at a total of 178 sites. I find that wood load is a function of metrics that relate to river corridor spatial heterogeneity and wood storage patterns (together determining wood trapping efficiency) at the reach scale and, at a broader spatial scale, wood supply. Wood in an undisturbed basin stores twice as much wood OC as a similar but extensively clearcut basin. In examining floodplain soil OC, I find that much of the variability in OC concentration is due to local factors, such as soil moisture, elevation (a proxy for temperature), and valley bottom geometry. From this, I conclude that local factors likely play a dominant role in regulating OC concentration in valley bottoms, and that inter-basin trends in climate or vegetation characteristics may not translate directly to trends in OC storage. I also use analysis of OC concentration and soil texture by depth to infer that OC is input to floodplain soils mainly by decaying vegetation, not overbank deposition of fine, OC-bearing sediment. Valley bottoms store significant OC stocks in floodplain soil and downed wood (ranging from 0 to 998 Mg C/ha) that vary with valley bottom form and geomorphic processes. Valley bottom morphology, soil retention, and vegetation dynamics determine partitioning of valley bottom OC between soil and wood, implying that modern biogeomorphic process and the legacy of past erosion regulate the modern distribution of OC in river networks. Soil burial is essential to preserving old OC, as measured by an extensive sample of 121 radiocarbon ages of floodplain soil OC. These radiocarbon data indicate a median residence time of floodplain soil OC of 185 yr BP. The age of the floodplain soil OC pool and the distribution of OC between wood and soil imply that OC storage in mountain rivers is sensitive over relatively short timescales to alterations in soil and wood retention, which may have both short- and long-term feedbacks with the distribution of OC between the land and atmosphere. Mountain river valley bottoms act as a high magnitude and moderately long-lasting pool of OC stored on land.
Open Access
Quantifying floodplain health in the contiguous United States using an index of integrity
(Colorado State University. Libraries, 2022) Simonson, Kira, author; Morrison, Ryan, advisor; Nelson, Peter, committee member; Wohl, Ellen, committee member
Despite the numerous hydrological, geological, and ecological benefits produced by floodplain landscapes, floodplains continue to be degraded by human activities at a much higher rate than other landscape types. Although this large-scale landscape modification has been widely observed, a comprehensive, national dataset quantifying the degree to which human activities are responsible for this degradation has not previously been evaluated. Floodplain integrity can be defined as the ability of a floodplain to support essential environmental functions that sustain diversity and ecosystem services through geomorphic, hydrologic, and ecological dynamics. In this research, I seek to analyze floodplain integrity at a national scale for the United States by spatially quantifying the impact of anthropogenic stressors on essential floodplain functions. I assess the prevalence of human modifications through widely available geospatial datasets, which I then use to quantify indicators of floodplain health for five essential floodplain functions. The five essential floodplain functions include flood attenuation, groundwater storage, habitat provision, sediment regulation, and organics and solute regulation. Rather than focusing solely on the ecological health within the floodplain, I develop a more comprehensive integrity evaluation by assessing both the biological and hydrogeomorphic functioning ability of the floodplain. I extend a previously established methodology for quantifying floodplain integrity to better understand the impact that human development has had on floodplain health and critical floodplain functions at the national scale. Additionally, I apply this methodology using land use change data for a 60-year period to analyze how land use has impacted floodplain integrity over time. Quantifying the health of spatially explicit floodplain elements will allow for restoration efforts to be targeted to the areas in most desperate need of preservation.
Open Access
Rain and RELAMPAGO: analysis of the deep convective storms of central Argentina
(Colorado State University. Libraries, 2023) Kelly, Nathan Robert, author; Schumacher, Russ, advisor; Rasmussen, Kristen, committee member; Bell, Michael, committee member; Nelson, Peter, committee member
When, where and how much precipitation falls are fundamental questions to research interests spanning the weather to climate spectrum, yet are difficult to solve. The various methods used to answer "how much" each have sources of error, making it important to obtain knowledge about the characteristics of an individual dataset. This is especially true for rare events such as extreme precipitation. IMERG, TRMM 3B42, MERRA2 and ERA5 precipitation datasets were regridded to the same resolution and compared for 3-hourly heavy rainfall (99th and 99.9th percentile) in subtropical South America, which has some of the strongest convective storms on Earth. Seasonal and dirunal distribution are compared, with similar seasonal distributions between the datasets but at the diurnal scale MERRA2 and ERA5 show more afternoon events than TRMM and IMERG. Thermodynamic environments were compared with MERRA2 events tending to occur in more marginal environments than TRMM 3B42 and ERA5 environments over much of the analyzed region. Overall the satellite datasets showed the highest amounts. Brief case studies are included to illustrate these differences, which reinforce that choice of dataset can be an important factor in precipitation research. How the precipitation falls is also addressed using a case study from the RELAMPAGO field program in Argentina. Many observations are available of this case, which occurred during the mobile operations period of the field program. Mobile surface stations, increased temporal resolution from fixed sounding sites, and six mobile sounding systems provide a high level of detail on the evolution of this storm system. Additionally, a trove of radar data and a GOES mesoscale sector are available. This case is demonstrative of a common occurrence in the region: a strong MCS (Mesoscale Convective System) over the Sierras de Córdoba mountain range. The extent of the backbuilding observed with this MCS was not predicted by the operation convective allowing models used for field program forecasting. To study this event two simulations are presented: one in which backbuilding of the MCS occurs and one where such backbuilding does not occur. The difference between these simulations is the number of vertical levels used in the model which impacts moisture availability upstream of the system via the effect of mountain wave downslope winds.