Browsing by Author "Nelson, Peter, advisor"
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Item Open Access Alternate bar dynamics in response to increases and decreases of sediment supply(Colorado State University. Libraries, 2016) Bankert, Andrew, author; Nelson, Peter, advisor; Bledsoe, Brian, committee member; Wohl, Ellen, committee memberGravel-bed rivers can accommodate changes in sediment supply by adjusting their bed topography and grain size in both the downstream and cross-stream directions. Under high-supply aggradational conditions, this can result in spatially non-uniform stratigraphic patterns, and the morphodynamic influence of heterogeneous stratigraphy during subsequent degradational periods is poorly understood. We conducted an experiment in an 18.3 m long, 1.2 m wide straight rectangular channel where we developed alternate bars in a gravel-sand mixture under constant discharge and sediment supply then developed stratigraphy over existing bars through aggradation with two supply increases. The supply was then reduced back to the initial supply rate, causing degradation through that self-formed stratigraphy. We collected stratigraphic samples and made frequent measurements of the bed topography and flow depth, which were used with a two-dimensional hydrodynamic model to characterize flow conditions throughout the experiment. Migrating alternate bars stabilized during the first equilibrium phase creating bed surface sorting patterns of coarse bar tops and fine pools. During the first supply increase the bars remained stable as the pools aggraded. During the second supply increase the pools aggraded further, causing the boundary shear stress over the bar tops to increase until the bars gained the capacity to migrate and eventually stabilize in new locations. As aggradation occurred, the original sediment sorting patterns were preserved in the subsurface. During the degradational phase, the pools experienced incision and the bars eroded laterally, but this lateral erosion ceased when coarse sediment previously deposited during the bar-building phase became exposed. Our results suggest that if a sediment supply increase is capable of filling the pools it can cause stable bars to migrate and the bed to be reworked. Our findings also show that heterogeneous stratigraphy can play an important role in determining whether bars persist or disappear after a sediment supply reduction.Item Open Access Assessing and managing urban riverscapes: integrating physical processes and social-ecological values(Colorado State University. Libraries, 2022) Murphy, Brian Michael, author; Nelson, Peter, advisor; Wohl, Ellen, committee member; Grigg, Neil, committee member; Morrison, Ryan, committee memberIn the age of the Anthropocene, human influence has spread far and wide across our planet affecting the physical, chemical, and biological condition of the rivers, streams, and floodplains in the urban environment, our "urban riverscapes." The human connection to urban riverscapes includes both the built environment created and accessed by people and the intangible community values that humans place upon flowing water. The value of these benefits encourages stewardship of our waterways by integrating experiential, aesthetic, and cultural attributes that foster appreciation for streams as natural systems in the built environment. However, when poorly managed, human activities adversely impact our natural ecosystems, resulting in less resilient stream systems, poor aesthetics, and unsafe conditions. The research presented in this dissertation asks the following overarching research question: How can managers and practitioners apply multi-scale social-ecological, hydrologic, geomorphologic, and riparian ecological remote sensing and field data to advance urban riverscape management? Four chapters follow from this hypothesis: urban riverscape problems lie on a spectrum of complexity where solutions are often conceivable but difficult to implement. Integrating diverse perspectives and knowledge extends the scope of stakeholder perspectives so that social-ecological context is considered alongside the physical processes that typically characterize riverscapes. This approach entails leveraging existing and new methods to create frameworks that integrate the multi-scale assessment of physical conditions and social-ecological qualities underlying applied riverscape management. I explore the integration of diverse knowledge to enhance management outcomes through the concept of "wicked problems." I analyze the connections between diverse types of knowledge and practices through numerous case studies. My analysis shows how systematically characterizing project attributes, such as the prominence of local government and technical knowledge or the weakness of academia and indigenous knowledge, requires an approach that builds capacity and collaboration within transdisciplinary stakeholder groups. I find that the importance of integrating communities, including under-represented knowledge bases, into urban riverscape management can generate equitable and incremental solutions. To evaluate connections between social values, ecological conditions, and hydrogeomorphic processes, I outline a framework for urban riverscape assessment that advances the practice of managing urban riverscapes facing complex problems. The framework is based upon evaluation across four foundational categories, or facets, critical to the management of urban riverscapes: (1) human connections and values, (2) hydrologic processes and hydraulic characteristics, (3) geomorphic forms and processes, and (4) ecological structure and processes. I structure the framework around three tiers of actionable steps, which tackle the questions: Why are we assessing this riverscape (Tier 1)? What do we need to understand in and along this riverscape (Tier 2)? How will we assess the riverscape to develop that understanding (Tier 3)? I find that the answer to the first question is context-based and dependent upon integrating diverse types of knowledge, while the response to the second question involves examining the functions and values of urban riverscapes through the lens of the four facets and their inter-related processes. Answering the third question requires developing and testing a novel assessment method – the "Urban Riverscape conditions-Based Assessment for management Needs" (URBAN). I base URBAN on riverscape context and on integrating the assessment of facets at multiple scales. I apply the method to a test data set of publicly available and site-specific data across a study area in the Denver metropolitan region to illustrate its overall performance, including its ability to evaluate specific riverscape physical conditions and social-ecological qualities. I find reach typologies combined with urban riverscape characteristics provide tangible management strategies that managers can use to inform planning and decision making. The overarching conclusion of this dissertation is that managing urban riverscapes requires assessment methods that consider scale (spatial, temporal, and topical) and context (both physical and social characteristics), and the use of indicators and metrics that directly support decision-making among interdisciplinary stakeholders. It is possible to move toward this vision by using remote-sensed and field data that provides both social and physical information, to assess the relationship between physical condition and social-ecological values, and to use that information to determine where and how to prioritize management strategies for urban riverscapes.Item Open Access Evaluating post-fire geomorphic change on paired mulched and unmulched catchments using repeat drone surveys(Colorado State University. Libraries, 2023) Hayter, Lindsey, author; Nelson, Peter, advisor; Kampf, Stephanie, committee member; Morrison, Ryan, committee memberSediment redistribution after wildfire can dramatically alter a catchment and pose risks to local infrastructure and water quality. Mulch application is increasingly being used to mitigate post-fire hillslope runoff and erosion, although relatively little is known about its effects at the catchment scale. In this study we used repeat drone surveys to measure erosion and deposition across 6 small (0.5-1.5 km2) catchments, 3 mulched and 3 unmulched, in the 2020 Colorado Cameron Peak Fire burn scar. The objectives were to (1) quantify sediment volumes and spatial patterns of erosion and deposition on a catchment and channel scale, (2) compare geomorphic change to mulch coverage, vegetation cover, precipitation intensity, burn severity, and morphologic metrics, and (3) identify conditions in which mulch may be most appropriate based on findings. Initial drone surveys were gathered in the spring of 2022 shortly after mulching and were differenced to surveys collected in fall of 2022, capturing the erosional effects of a Colorado monsoon season within a 6.4 cm horizontal resolution DEM of Difference (DoD). Structure from motion (SfM) errors were thresholded out of the DoD to yield maximum and mean levels of detection at 14 cm and 5 cm respectively. Vegetation was filtered from the DoD by supervised classification of vegetation in the drone imagery. We found hillslope erosion dominated the sediment budget, with the mulched catchments eroding 141% more per area than the unmulched. A regression model suggested erosion to be most influenced by vegetation, hillslope length, and maximum 60-minute rainfall intensity. Channels were overall net depositional, and patterns of erosion and deposition in channels were controlled by changes in slope and stream power as well as local morphologic metrics. Our analysis does not find a significant impact of mulch at the catchment scale especially when coverage is low (~22%) and highlights the importance of understanding catchment attributes and processes when making post-fire treatment decisions.Item Open Access Experimental flume and numerical studies into the influence of floodplain vegetation on river-corridor hydrodynamic processes(Colorado State University. Libraries, 2023) White, Daniel C., author; Morrison, Ryan, advisor; Nelson, Peter, advisor; Thornton, Chris, committee member; Rathburn, Sara, committee memberThe active channel has historically been the primary focus of river hydrodynamic process studies and river engineering. However, increased global flood risk and awareness of ecosystem services provided by floodplains has encouraged managers to broaden their perspective beyond the banks. As water exits and reenters the channel during floods, water, nutrients, and sediment are exchanged with the floodplain. This flux is heavily influenced by both channel-floodplain hydrologic connectivity, or the ability of water to access the floodplain, and by floodplain land cover types. River and hydrologic modifications that result in disconnected floodplains include channel planform and cross-section geometry alterations, diversions and dams, levees, land cover change, and river sediment mining. As river managers, land-use managers, and landowners acknowledge the benefits of functional, laterally connected river corridors, more river restoration projects are undertaken with a primary goal of reconnecting a river channel to the adjacent floodplain. A major component of large river restoration and river engineering projects includes designing for and predicting future flow scenarios using hydraulic models and other analytical methods. Developing a hydraulic model for river restoration design relies on the theory and science of fluvial morphodynamic processes as well as the parameterization of hydraulic roughness coefficients. Because of the historical emphasis on in-channel processes, the scientific literature related to channel-floodplain hydrodynamics and floodplain roughness parameterization is sparse. Specifically, there are limited studies investigating the influence of vegetation on channel-floodplain exchange flow, lateral connectivity, and resulting channel topography. To address this knowledge gap, I conducted a series of physical and numerical modeling experiments where floodplain vegetation and flow parameters were varied. In Chapters 2 and 3, I present the results of flume experiments where I measured bedform topography and the flow field under varied floodplain vegetation conditions at two overbank flow depths. The experiments were conducted in a 1-m wide meandering compound channel inset in a 15.4-m long, 4.9-m wide basin. The channel bed was a mobile sand-and-gravel mixture with a median sediment size of 3.3 mm, and sediment transport occurred only within the channel. I tested bare and vegetated floodplain conditions with 2.7-cm diameter rigid emergent vegetation elements at spacings of 3.0 units m-2 and 12.1 units m-2. My observations of the flow field indicate that high density vegetation enhances secondary circular flow through the meander bend and reduces momentum exchange at the channel-floodplain interface. At a low relative depth, flow through high density vegetation was deflected away from the down-valley direction and forced to reenter the channel at a steep angle with respect to the channel centerline. However, at a high relative depth, dense vegetation steered in-channel surface flows more closely following the channel centerline. These observations shed light on the hydrodynamic processes leading to flood wave attenuation, enhanced nutrient cycling, and channel altering stresses, and these results may inform river restoration riparian management best practices. To investigate bedform response, I performed a moving-window analysis of topographic surface metrics including skewness, coefficient of variation, and standard deviation, as well as topographic patch analysis of area and contagion to measure changes in bedform heterogeneity as flow depth and vegetation density were varied. My results show that both greater density vegetation and larger flows can increase bedform topographic heterogeneity. These findings suggest that floodplain vegetation and natural hydrologic regimes that include overbank flows can enhance stream habitat complexity. Designing for the effects of established vegetation conditions and prioritizing floodplain vegetation planting may be useful for river managers striving to achieve successful biomic river restoration. Expanding on the observations made in the flume, I explored the ability of a 2D hydraulic model to predict the effects of vegetation on meandering channel flow dynamics. I used the TreeLS point cloud processing tool to automatically extract woody floodplain vegetation characteristics and estimate Manning's roughness coefficients for vegetation from aerial LiDAR. I investigated the influence of varied vegetation densities on channel-floodplain exchange flows in HEC-RAS 2D. I developed hydraulic models for three reaches along the Butokamabetsu River in the Hokkaido University Uryu Experimental Forest in Northern Japan where each reach had distinct biogeomorphic characteristics including channel width, slope, sinuosity, and floodplain vegetation density. I found that in the lower gradient, higher sinuosity reaches, floodplain vegetation density had more influence on channel-floodplain exchange flow attenuation. These results highlight the importance of planning for the presence and density of vegetation in river restoration projects particularly in lower gradient, more sinuous stretches of river. The results and analysis presented in this dissertation suggest that biological drivers such as rigid emergent floodplain vegetation play an important role in river form and function particularly in conjunction with floods that occasionally access the floodplain. These detailed observations of flow, sediment, and resulting bed morphology as well as analysis of innovative remote sensing techniques provide a basis for an improved understanding of morphodynamic processes in meandering rivers.Item Open Access Flume study of mechanisms responsible for particle sorting in gravel-bed meandering channels(Colorado State University. Libraries, 2019) White, Daniel, author; Nelson, Peter, advisor; Morrison, Ryan, committee member; Wohl, Ellen, committee memberMeandering gravel-bed rivers tend to exhibit bed surface sorting patterns with coarse particles located in pools and fine particles on bar tops. The mechanism by which these patterns emerge has been explored in sand-bed reaches; however, for gravel-bed meandering channels it remains poorly understood. Here we present results from a flume experiment in which bed morphology, velocity, sediment sorting patterns, and bed load transport were intensively documented in a single-bend meandering channel. The experimental channel is 1.35 m wide, 15.2 m long, and its centerline follows a sine-generated curve with a crossing angle of 20 degrees. Water and sediment input were held constant throughout the experiment at 104.8 L/s and 230 kg/h, respectively, and measurements were collected under quasi-equilibrium conditions once the sediment input and output were approximately equal and the bed was essentially unchanging. Measurements of the three-dimensional velocity field indicate the development of a helical flow where near-bed velocity is directed toward the inner bank and flow at the surface is directed toward the outer bend. Calculated cross-stream bed load transport rates show that the trajectories of fine and coarse particles cross downstream of the bend apex, with fine sediment directed inward toward the point bar and coarse sediment directed toward the outer pool. Boundary shear stress, calculated from near-bed velocity measurements, indicates that in a channel with mild sinuosity, deposition of fine particles on bars is a result of divergent shear stress at the inside bend of the channel just downstream of the apex. The strong inward secondary currents that developed near the outside bend of the channel have little impact on the fine sediment deposition occurring on the bar under the conditions of this study. Boundary shear stress at equilibrium in the upstream half of the pool was below the critical value for coarse particles (>8 mm), which were only found in the pool. Selective transport toward the sloped region connecting the pool and bar top was responsible for winnowing of fine particles in the pool. Similarly, boundary shear stress near the bar front at equilibrium was below the critical value for particles near the D₅₀ of the bulk sediment feed (≤4 mm). Here, only fine particles were mobilized and transported downstream to the bar top. Fine and coarse sediment followed essentially identical trajectories through the meander bend, which contrasts earlier studies of sand-bedded meanders where fine and coarse particles cross paths. This suggests a different sorting mechanism for gravel bends. This experiment shows that a complex interaction of quasi-equilibrium bed topography, selective sediment transport, and currents that develop as a result of curved channel geometry are responsible for the sorting patterns seen in gravel bed, meandering channels.Item Open Access Hydrodynamics in meandering compound channels with varied emergent floodplain vegetation densities: a 3D numerical modeling study(Colorado State University. Libraries, 2021) Brouillard, Nicolas P., author; Morrison, Ryan, advisor; Nelson, Peter, advisor; Wohl, Ellen, committee memberEmergent floodplain vegetation can influence the hydrodynamic interactions between floodplain and main channel flows during floods in meandering compound channels. These interactions impact the flow and boundary shear stress fields in the main channel, which govern sediment transport, channel morphodynamics, and the capacity to convey flood flows. These processes are important to sustaining aquatic habitats, understanding geomorphic change, and predicting flood severity. However, the effects of emergent floodplain vegetation density on flow phenomena in meandering compound channels are poorly understood. Therefore, this study had three objectives: 1) accurately numerically model three-dimensional (3D) flows at different relative depths (ratio of floodplain to main channel flow depths) in a meandering compound channel with a fixed rectangular main channel cross section and a smooth floodplain using data from published physical experiments, 2) use the numerical model to simulate varied emergent floodplain vegetation density conditions, and 3) analyze the effects of different emergent floodplain vegetation densities on the main channel and floodplain hydrodynamics. Specifically, the effects of floodplain vegetation conditions on primary flows, secondary flows, and boundary shear stresses in the main channel were explored. This study also looked at how floodplain vegetation density affected total discharge capacity as well as inbank and overbank layer-averaged flow patterns. Smooth floodplain, low floodplain vegetation density, and high floodplain vegetation density scenarios were modeled with uniform arrays of emergent cylinders with non-dimensional vegetation densities (portion of the control volume occupied by vegetation) of 0, 0.00946, and 0.0368, respectively, based on natural floodplain forests. These scenarios were modeled for eleven relative depths ranging from 0 to 0.80. Previous research in meandering compound channels with smooth and roughened floodplains has shown that minimum average streamwise velocities and boundary shear stresses in the main channel occur at a given threshold value of overbank relative depth. Therefore, a major focus of this research was to examine the relationships between vegetation densities, overbank relative depths, and minima in average main channel streamwise velocities and boundary shear stresses. The 3D numerical model accurately replicated the results of previously published physical experiments (objective 1) based on calibrated error metrics comparing free surface elevations and main channel streamwise velocities. Results from the calibrated numerical model show that as floodplain vegetation density increased, the initial minimum values of average main channel streamwise velocities and boundary shear stresses were lower in magnitude and occurred at greater relative depths and discharges (objectives 2 and 3). Unlike in the smooth and low vegetation density floodplain scenarios, these average main channel values generally did not increase with relative depth and discharge above the initial minimum case for the high vegetation density scenario. Furthermore, the main channel boundary shear stress field had strong gradients and had greater variations in magnitude in the vegetated floodplain scenarios compared with the smooth floodplain scenario. Additionally, increasing floodplain vegetation density greatly reduced the discharge capacity as well as the average main channel streamwise velocities and boundary shear stresses above the lowest relative depths. Finally, the character of the main channel primary and secondary flow structures as well as the inbank and overbank layer-averaged flows were also affected by floodplain vegetation density. As vegetation density increased, floodplain flows deviated further from the valley-wise direction and plunged more steeply into the main channel below the bankfull level, thus increasing interactions between inbank and overbank flow layers. The strength of separation between inbank and overbank flow layers at an imaginary bankfull level horizontal plane is believed to influence energy losses in the flow, which helps to explain trends in the flow velocity and boundary shear stress fields. In conclusion, this study illustrates why river scientists and engineers should consider the effects of floodplain vegetation density on main channel hydrodynamic processes in similar meandering compound channel systems.Item Open Access Large-scale remote sensing of geomorphic change in mulched and unmulched watersheds burned in the 2020 East Troublesome Fire, Colorado(Colorado State University. Libraries, 2023) Murray, John Thomas, author; Nelson, Peter, advisor; Kampf, Stephanie, committee member; Morrison, Ryan, committee memberElevated levels of sediment transport in post-wildfire landscapes can degrade the hydrologic and geomorphic processes of a river system, damage aquatic habitat, and pose a threat to downstream infrastructure. Hillslope mulching applications have proven to be effective at mitigating runoff and erosion at plot and hillslope scales but the impacts of mulching at the watershed scale remain generally unknown. We conducted repeat aerial surveys of one unmulched and five partially mulched watershed outlets (0.61-1.44 km2) to quantify erosion and deposition in the East Troublesome Fire burn scar. The objectives of the study were (1) to quantify volumes of erosion and deposition for hillslopes and channels for a variety of sites at a range of elevations (2) to identify and quantify the drivers of erosion and deposition and their relative contributions within and across watersheds (3) to determine the impact of slope, width, and vegetation cover on sediment storage and transport within watersheds; and (4) to assess the impacts of a large-scale aerial mulching operation at scales from hillslopes to watersheds. Multiple drone flights were conducted for each study site between July and October 2022. The earliest and latest surveys were differenced to produce DEM of Difference (DoD), with spatial resolutions ranging from 3.8 to 4.4 cm. Vertical uncertainties calculated from measurement uncertainty and Structure from Motion (SfM) errors were filtered out of the DoD at a 95% confidence interval (CI), resulting in maximum and mean detection thresholds of 11 and 4 cm, respectively. A supervised classification algorithm was used to filter out changes due to vegetation growth and decay, which varied in effectiveness across the six study sites. Hillslope erosion and deposition volumes were at least three times higher than near-channel volumes, with most sites being an order of magnitude higher. However, near-channel erosion and deposition magnitudes normalized by area were higher than normalized hillslope magnitudes at all sites. A bootstrap forest regression model was used to determine relationships between various site-specific parameters and erosion and deposition for each watershed individually, and for all six sites combined. The model indicated mean slope, absence of vegetation, mean differenced normalized burn ration (dNBR), and hillslope length to be strong drivers of erosion and deposition for the individual models. Total precipitation accumulation and maximum 60-minute rainfall intensity were stronger contributors in the combined models. Near-channel storage and transport was influenced by local relationships between width, stream power, and absence of vegetation. Mulch coverage area was found to be weakly correlated with erosion and deposition at the watershed scale, with contributions possibly being dependent on coverage rate. These findings emphasize the importance of applying mulch in areas where it is both necessary and can have a measurable impact on reducing erosion rates.Item Open Access Modeling risk of landslide initiation and runout in the Colorado Front Range under current and future climates(Colorado State University. Libraries, 2021) Byron, Elizabeth, author; Nelson, Peter, advisor; Niemann, Jeffrey, advisor; Gallen, Sean, committee memberPrecipitation-induced landslides pose risks to humans through property damage, disruption of infrastructure, injury, and loss of life. Due to the spatial and temporal heterogeneity of soil moisture and landscape characteristics that impact slope stability and potential impacts of climate change on landslide location, quantifying landslide risk to humans is difficult as uncertainties are not represented in available datasets. Recent developments have improved our ability to probabilistically model landslide initiation, thus allowing for the incorporation of spatial and temporal uncertainty in the prediction of the onset of hillslope failures. The ability to incorporate uncertainty in landslide models is particularly valuable for considering how climate change, which could impact vegetation cover and associated root cohesion, might alter the vulnerability of people and infrastructure to landslides. The aim of this analysis is to probabilistically forecast landslide susceptibility under climate change by incorporating changes in the type and distribution of vegetation while accounting for uncertainties in key properties. Using Landlab, a Python-based toolkit for landscape modeling, we perform Monte Carlo simulations with an infinite slope stability model to make spatially explicit calculations of the probability of landslide initiation. The soil moisture input to the landslide model is from the Equilibrium Moisture from Topography, Vegetation, and Soil (EMT+VS) model, which downscales coarse-resolution soil moisture by incorporating the dependence of soil moisture on topographic, vegetative, and soil characteristics. We evaluate model sensitivity and identify that vegetation, which impacts cohesion and soil depth, has a large impact on the model. We evaluate model performance by simulating landslide susceptibility over a 1333 km2 area of the Colorado Front Range as there is a large inventory of more than 1300 landslides from an extreme precipitation event in 2013. One anticipated effect of climate change in the Colorado Front Range is a reduction in the survivability of trees, which we incorporate through applying reductions to vegetative cohesion and vegetation cover. For the 2013 event, the model predicts 79.6% of the mapped landslides and 5.8% of the rest of the study area as being unstable. A deterministic model using mean values from the probability model and assuming FS ≤ 1 is unstable captures only 42% of observed landslides, supporting the use of the probabilistic model. The probabilities are low (P(F) < 0.2) for the majority of predicted failures with a concentration at higher (P(F) > 0.8) values, with the latter having higher slopes and lower vegetation. 66% of nodes with P(F) > 0 occur on south facing slopes where trees are less abundant. After incorporating climate change, we see an increase in the areas susceptible to landslides and a shift to more instability on north-facing slopes. Our study suggests that vegetation changes due to climate change could result in major shifts in the people and infrastructure susceptible to landslides in the Colorado Front Range. In conjunction with landslide initiation, determining landslide runout is important to fully analyze landslide risk. Landslide runout modeling for large areas is difficult due to limited information and the complexity of landslides. The difficulties of physically modeling landslides on large spatial scales have led to the development of empirical methods based on topographic attributes. While empirical models are limited in that they require calibration in new areas and thus can only be applied to areas with landslide inventories, they provide a way to model landslide runout at large spatial scales and identify areas for further, potentially more physically-based, analyses. We investigate whether topographic controls can be used to predict landslide termination. We develop a landslide runout model and apply it to a 10-m elevation grid. Our model routes landslides downslope with d8 flow direction method and uses a critical slope, defined as a minimum slope a landslide must encounter to end, and slope persistence, defined as the distance the landslide must travel under the critical slope, to represent landslide stopping locations. We apply our model to see if it can replicate landslide runout in the Colorado Front Range due to a large landslide inventory from a 2013 precipitation event that induced approximately 1300 mapped landslides. The calibrated model has a critical slope of 3° and a slope persistence of 20 m and predicts landslide distance in both the calibration and evaluation areas with a Nash-Sutcliffe (NS) value of 0.69 and 0.58, respectively. We compare our calibrated model to an angle of reach approach, an approach that has been applied previously for landslide runout mapping which determines the slope between the start and end of a landslide, and determine that the best NS value of 0.14 occurs at an angel of 20°. Our results show that within our study area, topographic controls provide plausible initial estimates of runout endpoints and an improvement over similarly simplistic methods such as the angle of reach. The potential of using critical slope combined with slope persistence to capture topographic controls to predict runout endpoints is a promising opportunity for landslide hazard mapping at large spatial extents.Item Open Access Multi-scalar response of an experimental fixed-wall meandering channel to a sediment supply increase(Colorado State University. Libraries, 2020) Cortese, David, author; Nelson, Peter, advisor; Morrison, Ryan, committee member; Wohl, Ellen, committee memberMeandering river planforms are prevalent and well-studied features in the natural landscape. These rivers commonly exhibit a characteristic morphology of fine-grained point bars along the inner banks of meander bends with coarser pools along the outer banks. If subjected to a change in sediment supply, these rivers are likely to respond at various spatial and temporal scales through adjustments to sorting patterns, cross-sectional shape, and reach-scale morphology. In this study, a flume experiment was conducted to document the temporal progression of responses across scales of a fixed-wall meandering channel to a sediment supply increase. The 0.344 m wide experimental channel consisted of four meander bends following a sine-generated trace with a 20-degree crossing angle, meander wavelength of 2.75 m, and a unimodal sediment mixture with median grain size of 0.62 mm. The channel was provided constant flow and sediment supply until an initial equilibrium was established, after which the sediment supply was doubled until a new equilibrium state was reached. The experimental channel developed characteristic bar-pool morphologies and sorting patterns with superimposed, mobile, scaled gravel-dune bed forms during both phases of the experiment. After the sediment supply increase, dynamic adjustments occurring from smaller to larger scales took place. Initially, the dunes essentially disappeared, after which the relief of the bars decreased. Both of these sub-reach-scale responses were temporary, however, and ultimately the dunes and bar-pool morphology returned to their conditions at the beginning of the sediment supply increase. The long-term and largest-scale response to the supply increase was a 44% increase in bed slope. To explain these observations, we propose a conceptual model wherein the channel undergoes a temporal progression of responses from smaller to larger spatial scales, with the total response potential at each scale related to the conditions and constraints at that scale. This conceptual understanding allows us to reconcile seemingly divergent outcomes from previous research on how meandering rivers adjust to sediment supply changes.Item 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 memberStream 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.