Mountain Scholar :: Browsing by Author "Wohl, Ellen, committee member"

Browsing by Author "Wohl, Ellen, committee member"

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Open Access
A channel stability assessment and logistic regression model for a reach of muddy creek below Wolford Mountain Reservoir, in north-central Colorado
(Colorado State University. Libraries, 2014) Williams, Cory A., author; Rathburn, Sara, advisor; Wohl, Ellen, committee member; Bledsoe, Brian, committee member
Water resource managers face increasing pressure to meet community water needs while responsibly managing resource infrastructure and preserving aquatic and riparian ecosystems. Management of many western rivers involves multiple uses for multiple stakeholders, especially rivers downstream from dams impounding water-supply reservoirs. These resource management issues arise in a reach of Muddy Creek near Kremmling, Colorado, which serves multiple functions including: (1) wetland mitigation, (2) private, recreational fishing, (3) and agriculture. Alteration of streamflow from reservoir operations and loss of upstream sediment supply, in conjunction with legacy management effects, have resulted in channel instability and increased streambank erosion in Muddy Creek below Wolford Mountain Reservoir. A typical response to channel instability on managed rivers includes installation of erosion-control structures. However, installation of these structures to protect property and infrastructure is expensive and can have unintended consequences at adjacent locations, highlighting the need for resource managers to better understand the underlying geomorphic processes controlling channel adjustment within the reach. To address these issues, field reconnaissance and channel surveying completed during base-flow conditions were used to (1) determine the dominant erosive and resistive processes within the reach that contribute to channel stability and response, and (2) assess the validity of using logistic regression techniques as an analytical framework and to estimate the probability or risk of localized streambank erosion. These findings can be used in conjunction with local management objectives to evaluate or gage acceptable risk to current infrastructure and to target and prioritize where monitoring or remediation should be conducted. Understanding the geomorphic processes and reach characteristics driving streambank erosion can be used to guide management and operational decisions within the reach to minimize impacts. A map of probability of erosion, for each streambank, is presented which shows risk (as a probability of streambank erosion, ranging from < 3 to 80 percent) based on significant explanatory variables from the logistic regression model. The study found that stream-induced scour and undercutting have differing effects within the reach due to changes in the erosive power of the stream and relative difference in streambank and riparian characteristics. Areas most susceptible to streambank erosion occurred in wider cross sections where fluvial energy was oriented into the streambanks, not necessarily in areas with the greatest fluvial energy and potential erosive power (i.e., areas with the steepest bed slope). This suggests additional, localized conditions within the reach need to be considered. Differences in streambank and riparian characteristics were shown to have varying levels of resistance to streambank erosion within the study reach. Larger streambank heights increased the probability of streambank erosion when these streambanks were not supported by bedrock outcrops of Pierre Shale or alluvial fans and talus slopes. Erosion-control structures decreased the probability of streambank erosion where structures retained original positions relative to flow. Where changes to flow orientation occurred, the probability of streambank erosion around these structures increased substantially. Riparian vegetation type also influenced streambank erosion as well as channel top-width. Streambanks covered in willows were found to decrease the risk of streambank erosion, whereas areas dominated by grasses increased streambank erosion potential as well as increased channel top-widths relative to areas dominated by willows. Additional effects from reach-scale characteristics were evaluated. Areas of greater sinuosity and wider valley widths show increased probability of streambank erosion, as well as in areas located downstream of an irrigation diversion structure. This may be due to a combination of effects from further confinement of the reach as valley widths decrease, increased streamflow from tributaries, and/or the occurrence of increased seepage along areas near an irrigation ditch. Streambanks with observed saturated soils were also found to have between 20-44 percent increased odds of streambank erosion. A linkage between proximity of streambank seeps and unlined irrigation ditches and irrigation turn-outs was highly significant, with potential effects extending to distances up to 250 m from the irrigation water sources.
Open Access
Air concentration and bulked flow along a curved, converging stepped chute
(Colorado State University. Libraries, 2020) Biethman, Blake W., author; Ettema, Robert, advisor; Thornton, Christopher, advisor; Wohl, Ellen, committee member
This thesis focuses on the air-entrainment performance of a stepped spillway of unique form. The performance was determined using a hydraulic model constructed at a length scale (prototype length/model length) of 24. The new stepped spillway is part of the Gross Reservoir Expansion (GRE) project, which by 2025 is expected to raise the existing Gross Dam about a third of its current height. The stepped spillway will be the tallest stepped spillway in the United States. The model spillway consisted of a chute whose step dimensions, vertical to horizontal, were 0.051 m by 0.025 m, resulting in a chute slope (V:H) of 2.0 and a chute angle of 63.4°. Additionally, the chute conformed, in planform, to the curved planform of raised Gross Dam. At the spillway's crest, that radius of curvature, at model scale, was 22.2 m. The chute width converged by about 20% from the top of the chute to the stilling basin at the base of the chute. The chute's steepness, height, curvature and convergence made the chute's geometry unique among existing stepped spillways. The evaluation involved measurements of air entrainment and flow velocity along the stepped chute, for which the skimming flow regime prevailed for discharge larger than about 9% of the spillway's design discharge. To date, the effect on water flow and air entrainment of chute curvature in stepped spillways had not been investigated. The investigation was facilitated from measurements obtained using a dual-tip conductivity probe, which detected the instantaneous void fraction of the air-water mixture. The probe also enabled measurement of the velocity of the bulked flow along the chute. The study showed that, when the chute conveyed the design discharge (at model scale, 0.347 m3/s), streamwise values of air concentration and flow depth (bulked with entrained air) were basically constant near the bottom of the chute. Additionally, the chute's planform curvature resulted in non-uniform flow across the chute. At the design discharge, and near the bottom of the chute, the flow depth along the chute's centerline was nominally about 30% greater than the flow depth at the sidewall. When the chute's curvature was accounted for, the water surface along the centerline of the chute was approximately level with the water surface near the sidewall. Further, the depth-averaged concentration of entrained air near the bottom of the chute decreased with increasing water discharge. The chute's converging sidewalls mildly affected the flow near the sidewalls, causing slight increases in flow depth and reductions in flow velocity. These changes, though noticeable, were negligible in terms of spillway performance because of their magnitude.
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 member
Gravel-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.
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
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 member
In 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.
Open Access
Assessing the swimming and jumping performance of Wyoming fishes with implications for fishway design
(Colorado State University. Libraries, 2024) Garvey, Chase, author; Myrick, Christopher A., advisor; Kanno, Yoichiro, committee member; Wohl, Ellen, committee member
To successfully begin to solve the complex problems facing native fish conservation efforts, we need to seek a deeper understanding of the fish that inhabit the communities that we hope to conserve. With regards to the issue facing the fragmentation of our lotic ecosystems, and the effects that dams and similar structures can have on fish communities, fishways are a common tool used to restore the connectivity of streams by allowing the uninhibited passage of fish. In these experiments we studied the swimming and jumping abilities of Hornyhead Chub (Nocomis biguttatus), Bigmouth Shiner (Notropis dorsalis), Iowa Darter (Etheostoma exile), and Brook Stickleback (Culaea inconstans) in order to provide fisheries managers with criteria for fishway design that will allow the passage of desirable target species, and potentially block the spread of invasive Brook Stickleback. We did this by testing the jumping abilities of each species at various temperatures using artificial waterfalls. After testing groups of fish at various waterfall heights, we used logistic regression to predict the probability of individual fish passage under various conditions. To test the swimming ability of each species, we used swim tunnels to determine the maximum swimming velocity of each species, and estimate their endurance at various swimming velocities. Our results show that a vertical barrier greater than 15 cm will block the upstream movement of nearly all individual Hornyhead Chub. Heights greater than 8.4 cm will block the upstream passage of Bigmouth Shiner, and barriers taller than 6.0 cm will block the upstream passage of Iowa Darter and Brook Stickleback. Given this information, if vertical drops and pool designs were to be incorporated into fishways designed for these species, drops between fishway pools should remain within these thresholds if successful passage is to be achieved. The results of the swimming performance experiments show that each species' swimming abilities are unique relative to their raw maximum swimming abilities and overall endurance. The problem facing engineers and biologists is that many different species that make up these communities are unique, and have characteristics that pose specific challenges or advantages to assisting their movements. Additionally, fisheries scientists must remain cognizant that fishways that allow native fish movement will likely allow the movement of non-target species (including potential invasives) present in the same systems. Managers must balance the trade-offs between restoring native ecosystems and protecting areas that have not been invaded.
Open Access
Baffle-post structures for flow control in open channels
(Colorado State University. Libraries, 2015) Ubing, Caroline, author; Thornton, Christopher, advisor; Ettema, Robert, advisor; Bledsoe, Brian, committee member; Wohl, Ellen, committee member
This thesis presents theory and laboratory findings regarding the hydraulic performance of baffle-post structures used as a means for controlling flow in open channels. Such structures comprise one to two parallel rows of posts that extend slightly higher than the anticipated depth of flow, and offer a useful means for retarding flow in various channel situations where there is a need to reduce flow energy, possibly to reduce flow capacity to transport bed sediment and manage channel morphology. Observations and data regarding headloss and discharge coefficients and backwater flow profiles associated with varying structure geometry were obtained so as to determine the extent to which a baffle-post structure will retard an approach flow and reduces its capacity to convey bed sediment. The creation of a M₁ gradually varied flow profile in the upstream reach complicates the use of headloss to characterize hydraulic performance of the baffle-post structures. Instead, the parameter, y₁/y₀, offers a practical means for describing such performance; y₁= flow depth at the upstream face of the structure, and y₀= the depth of uniform flow prior to use of a structure. The most influential geometric variable was influencing structure performance was the lateral spacing between posts, s; it is expressed non-dimensionally as s/D, where D = post diameter. Qualitative results regarding sediment transport confirm a reduction in bed-sediment transport rate upstream of the structure. However, the turbulent flow structures at the baffle-post structures promote local scour at the base of such structures. Due to the flow acceleration between posts, baffle-posts structures could potentially obstruct fish and other aquatic life passage along the channel.
Open Access
Biogeochemical implications of beaver-mediated fluvial complexity in river-floodplain meadows
(Colorado State University. Libraries, 2018) Weiss, Tristan N. M., author; Covino, Tim, advisor; Wohl, Ellen, committee member; Rhoades, Charles, committee member; Falkowski, Michael, committee member
Mountain river networks alternate between narrow, transport dominated segments and low gradient wide valley segments that can be important locations for the retention and processing of carbon and nutrients. In North America, beaver (Castor canadensis) engage in dam building that enables the establishment of complex river-floodplain meadows (hereafter "meadows"), characterized by wide riparian corridors, multi-thread channels, and high levels of river-floodplain connectivity. However, in many river-floodplain systems, human land-use and the removal of beaver has led to fluvial simplification characterized by reductions in riparian vegetation and channel incision. We examined differences in hydrology and biogeochemistry among four meadows of varying beaver activity and associated fluvial complexity within Rocky Mountain National Park, USA. We quantified water and dissolved organic carbon (DOC) flux, measured fluorescent dissolved organic matter (DOM) character, and monitored ecosystem metabolism. At complex meadow segments, we observed increased stability across space and through time in patterns of water flux and DOC concentration, export, and character. While DOC dynamics were stable at complex meadow segments, in simplified meadow segments we observed increases in DOC concentration and export, and shifts toward more terrestrially sourced, aromatic, and humic DOM. These results suggest that complex river-floodplain systems facilitate stability in stream flows and maintain water quality with respect to DOC concentration, flux, and form. Conversely, the loss of fluvial complexity in simplified meadow segments can lead to lost hydrologic and biogeochemical stability and make the valley segments more sensitive to future perturbations. Because wide-complex meadow segments can act as key locations of hydrologic retention in mountain stream networks, understanding how fluvial simplification alters water and DOC dynamics may be important for developing targeted restoration strategies for altered mountain headwater systems.
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
Dammed ponds! A study of post-fire sediment and carbon dynamics in beaver ponds and their contributions to watershed resilience
(Colorado State University. Libraries, 2023) Dunn, Sarah B., author; Rathburn, Sara, advisor; Wohl, Ellen, committee member; Morrison, Ryan, committee member
Excess sediment generated by wildfires threatens stream water quality, riparian habitat, and infrastructure. Beavers construct dams that pool water and capture sediment. Beaver ponds may bolster watershed resilience by providing sediment and carbon storage following wildfire. I tested the hypotheses that (1) burned ponds store greater relative volumes of sediment compared to unburned ponds, (2) post-fire sedimentation rates exceed pre-fire and unburned rates, and (3) post-fire sediment stored in beaver ponds is coarser and has a higher abundance of organic carbon relative to pre-fire sediment. I surveyed 48 beaver ponds in the Colorado Rocky Mountains. Approximately half of the ponds are in areas that burned in 2020 wildfires, whereas the other half remain unburned. Sites also spanned a range of geomorphic, vegetation, and individual pond characteristics. I conducted sediment probe surveys and collected sediment cores to quantify pond sediment storage and characterize sediment composition. Stratigraphic units present in sediment cores were analyzed for grain size and total organic carbon (TOC). Results indicate that beaver ponds in the Rocky Mountains store high volumes of sediment (mean = 796 m3). Burned ponds contain statistically significantly more relative sediment storage and have higher sedimentation rates than unburned ponds. Beaver ponds recorded high post-fire sedimentation rates (median = 19.8 cm/yr). Moreover, post-fire sedimentation rates are an order of magnitude higher than pre-fire rates in ponds with both pre- and post-fire sediments. Total sediment volume, sedimentation rates, grain size, and TOC content did not vary significantly between burned and unburned ponds. Geomorphology, vegetation, and pond characteristics exert additional influences on pond sediment dynamics. Pond characteristics determine the sediment trapping efficiency of ponds. Larger ponds store greater volumes of sediment, as do off-channel and older ponds. Ponds abandoned by beaver store greater volumes of sediment than actively maintained or human- constructed dams. Beaver activity and dam maintenance is critical for maintaining storage availability in ponds. Additionally, sedimentation rates are higher in ponds that are on-channel and recently constructed compared to off-channel and older ponds. These findings indicate that beaver-based restoration can be implemented prior to fire to provide critical post-fire sediment storage, thus enhancing watershed resilience and recovery.
Open Access
Developing rock ramp fishway criteria for fishes of regional conservation concern
(Colorado State University. Libraries, 2024) Paik, Kira, author; Myrick, Christopher A., advisor; Kanno, Yoichiro, committee member; Wohl, Ellen, committee member
Rivers and streams in the United States have been greatly fragmented by the construction of instream structures such as dams, diversions, and culverts to meet the growing needs of human populations. Many of these structures inhibit upstream movement by fish species, negatively affecting abundance as well as overall survival. Conservation efforts are looking at restoring connectivity through the installation of fish passage structures or fishways. To improve effectiveness and functionality of these fish passage structures, the swimming abilities of the target species should be considered when creating the design. Rock ramp fishways are becoming increasingly utilized because they can allow passage of a large assortment of species with variable swimming abilities and are highly customizable. Creating cast concrete fishways in this style can also help to reduce the cost of construction of passage structures. We evaluated the passage success of five fish species of conservation concern using an experimental rock ramp fishway at slopes of 2-10%, in 2% increments. This study focused on species of national or regional conservation concern including Topeka Shiner Notropis topeka, Suckermouth Minnow Phenacobius mirabilis, Rio Grande Chub Gila pandora, Rio Grande Sucker Catostomus plebeius, and Mottled Sculpin Cottus bairdii. Our results showed that decreased slope and distances would lead to higher passage success for the five species. For the entire length of the fishway (6.1 m), all species had very high passage probabilities (> 0.9) at the lowest slopes 2 and 4%, and for all species except the Topeka Shiner, the 6% slope also had high passage probabilities (> 0.8). At 8% and 10% slopes, passage success for these species decreased drastically (< 0.31) and would not be recommended for longer fishways (> 2.03 m between resting areas). Based on these results, managers designing fishways for these species should be able to pick ideal combinations of slope and length to successfully allow passage of an acceptable proportion of the fish to attain management goals.
Open Access
Effectiveness of light traps for detecting Razorback Sucker larvae
(Colorado State University. Libraries, 2019) de Vlaming, Catherine M., author; Bestgen, Kevin, advisor; Bailey, Larissa, committee member; Wohl, Ellen, committee member
Current management strategies for improving the status of wild and endangered Razorback Sucker Xyrauchen texanus rely on adequate larval sampling of wetland and riverine backwater habitats to evaluate post-reproductive survival, spatial and temporal patterns of distribution and abundance, and entrainment into wetlands. One strategy uses the detection of Razorback Sucker larvae to prompt flow releases to inundate Green River floodplain wetlands, habitat which may increase survival of those early life stages. Light traps, a passive sampling gear which exploits the innate attraction of fish early life stages to light, are thought an effective gear type for sampling, but little is known of their efficacy to capture or retain larvae. Therefore, we assessed usefulness of light traps for sampling or retaining Razorback Sucker larvae under a variety of environmental conditions using laboratory and field experiments. In the laboratory, we investigated effects of light trap set time, release distance from trap, light presence, turbidity, light source, cover, and trap aperture on capture and retention probabilities of five early life stages of Razorback Sucker. Mean capture probability of protolarvae prior to the development of a swim bladder (7-9 mm total length [TL]) was 40% (28-55%) over the various treatments, but rose to 76% (73-80%) after protolarvae formed a swim bladder (9-10 mm TL). Mesolarvae (11-17 mm TL), the most commonly captured life stage in field sampling, had similar mean capture probabilities as later protolarvae at 86% (82-90%). Capture probability of metalarval (mean = 42%, range 21-63%; 15-24 mm TL) and juvenile (mean = 24%, range 20-28%; 22-37 mm TL) life stages were lower. Retention probabilities of larvae placed in traps were generally >75% and increased to 97% for juveniles, but some fish nearly always escaped. The relationship between set time and release distances of 1, 3, and 5 m on capture indicated longer set times positively influenced capture probabilities while distance had little effect. Light presence in traps greatly increased capture and retention of larvae compared to unlit traps, and indicated traps lit with light-emitting diodes (LED) increased capture of Razorback Sucker larvae due to increased light intensity when compared to chemical-light-stick-lit traps. Light trap aperture widths of 4 or 6 mm did not influence capture or retention. Overall, laboratory experiments provided valuable information on how specific variables affect capture and retention of Razorback Sucker larvae in light traps and provide a framework for interpreting and designing field studies, which we were able to subsequently carry out. Field experiments consisted of experimental releases of unmarked, single, and double-marked (immersion in oxytetracycline hydrochloride [OTC]) Razorback Sucker larvae over three nights in a managed wetland of the Green River, Utah at the Ouray National Wildlife Refuge. Batches of released larvae were paired with 1 of 12 light traps each night in various densities (10, 50, 250, 1,000 fish per trap), 3 and 10-m release distances from light traps, LED and chemical-light-stick light sources, and two release times to evaluate effects on larvae capture probabilities. In addition, batches of 25 single-marked larvae were placed in light traps and set on a fourth night in various environmental light conditions (night, sunrise, and sunlight) to evaluate effects on larvae retention. Light traps recaptured larvae each night, even with low density releases in the 53.5 ha wetland, and recapture probabilities ranged from 0 to 0.68. The LED trap capture probabilities were up to 2.5X greater than for chemical light stick traps, but capture probabilities were not influenced by release distance or larvae density. Inexplicably, retention was very low, a result inconsistent with the previous laboratory tests. Both laboratory and field experiments indicated light traps are a useful gear to monitor abundance of larvae, evaluate reproductive success of adults, and detect even low densities of larvae in large and open habitats. Additionally, light traps are suitable to detect presence of Razorback Sucker larvae in riverine backwaters each spring, the timing of which is used to begin high flow releases from Flaming Gorge Dam to inundate Green River, Utah, floodplain wetlands. Expanded ecological understanding of early life stages of Razorback Sucker will contribute to their conservation in the Colorado River basin.
Open Access
Evaluating channel morphologic changes and bed-material transport using airborne lidar, upper Colorado River, Rocky Mountain National Park, Colorado
(Colorado State University. Libraries, 2014) Mangano, Joseph F., author; Rathburn, Sara, advisor; Wohl, Ellen, committee member; Bledsoe, Brian, committee member
A debris flow associated with the 2003 breach of Grand Ditch in Rocky Mountain National Park, Colorado provided an opportunity to determine controls on channel geomorphic responses following a large sedimentation event. Due to the remote site location and high spatial and temporal variability of processes controlling channel response, repeat airborne lidar surveys in 2004 and 2012 were used to capture conditions along the upper Colorado River and tributary Lulu Creek i) one year following the initial debris flow, and ii) following two bankfull flows (2009 and 2010) and a record-breaking long duration, high intensity snowmelt runoff season (2011). Locations and volumes of aggradation and degradation were determined using lidar differencing. Channel and valley metrics measured from the lidar surveys included water surface slope, valley slope, changes in bankfull width, sinuosity, braiding index, channel migration, valley confinement, height above the water surface along the floodplain, and longitudinal profiles. Reaches of aggradation and degradation along the upper Colorado River are influenced by valley confinement and local controls. Aggradational reaches occurred predominantly in locations where the valley was unconfined and valley slope remained constant through the length of the reach. Channel avulsions, migration, and changes in sinuosity were common in all unconfined reaches, whether aggradational or degradational. Bankfull width in both aggradational and degradational reaches showed greater changes closer to the sediment source, with the magnitude of change decreasing downstream. Local variations in channel morphology, site specific channel conditions, and the distance from the sediment source influence the balance of transport supply and capacity and, therefore, locations of aggradation, degradation, and associated morphologic changes. Additionally, a complex response initially seen in repeat cross-sections is broadly supported by lidar differencing, although the differencing captures only the net change over eight years and not annual changes. Lidar differencing shows great promise because it reveals vertical and horizontal trends in morphologic changes at a high resolution over a large area. Repeat lidar surveys were also used to create a sediment budget along the upper Colorado River by means of the morphologic inverse method. In addition to the geomorphic changes detected by lidar, several levels of attrition of the weak clasts within debris flow sediment were applied to the sediment budget to reduce gaps in expected inputs and outputs. Bed-material estimates using the morphologic inverse method were greater than field-measured transport estimates, but the two were within an order of magnitude. Field measurements and observations are critical for robust interpretation of the lidar-based analyses because applying lidar differencing without field control may not identify local controls on valley and channel geometry and sediment characteristics. The final sediment budget helps define variability in bed-material transport and constrain transport rates through the site, which will be beneficial for restoration planning. The morphologic inverse method approach using repeat lidar surveys appears promising, especially if lidar resolution is similar between sequential surveys.
Open Access
Evaluating surface water–groundwater interactions in floodplains using SWAT+ and gwflow
(Colorado State University. Libraries, 2023) Schulz, Evan, author; Morrison, Ryan R., advisor; Bailey, Ryan T., advisor; Wohl, Ellen, committee member
Floodplains are essential ecosystems that provide a variety of economic, hydrologic, and ecologic services. Within floodplains, surface water-groundwater exchange plays an important role in facilitating biogeochemical processing and can have a strong influence on hydrology through infiltration or discharge of water. These functions can be difficult to assess due to the heterogeneity of floodplains and monitoring constraints, so numerical models are useful tools to estimate fluxes, especially at a large scale. In this study, the gwflow module of the SWAT+ (Soil and Water Assessment Tool) ecohydrological model quantified magnitudes and spatiotemporal patterns of floodplain surface water-groundwater exchange in a mountainous watershed using an updated version of the module that directly calculated floodplain-aquifer interactions during periods of floodplain inundation. The gwflow module is a spatially distributed groundwater modeling subroutine within the SWAT+ code that uses a gridded network and physically based equations to calculate groundwater storage, groundwater head, and groundwater fluxes. I used SWAT+ to model an area of 7,516 km2 in the Colorado Headwaters HUC8 watershed (14010001) and used streamflow data from USGS gages in the watershed for calibration and testing. I evaluated model performance for scenarios with and without simulated floodplain-groundwater exchange and for three gwflow grid cell sizes. Models that included floodplain-groundwater interactions outperformed those without such interactions and provided valuable information about floodplain inundation and exchange rates. Furthermore, I found that smaller gwflow cell sizes showed similar or better performance than larger cell sizes and simulated additional information about local variations in groundwater fluxes, especially within floodplains. Finally, my analyses on the location of floodplain fluxes in the watershed showed that wider areas of floodplains, "beads," exchanged a higher net and per area volume of water, as well as higher rates of exchange, than narrower areas, "strings." These outputs remained consistent across all studied cell sizes, with smaller cells simulating greater differences between bead and string floodplain regions. Study results show that floodplain surface water-groundwater exchange is a valuable process to include in hydrologic models, and model outputs could inform land conservation practices by indicating priority locations where substantial hydrologic exchange occurs.
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 member
Meandering 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.
Open Access
From the Colorado Front Range to global topography: evaluating the roles of tectonics and climate on long term landscape evolution
(Colorado State University. Libraries, 2022) Marder, Eyal, author; Gallen, Sean, advisor; Pazzaglia, Frank, committee member; Wohl, Ellen, committee member; Schutt, Derek, committee member; Kampf, Stephanie, committee member
Landscapes are primarily shaped by the interactions between tectonics and climate, and their interplay and relative roles in landscape evolution over thousands to millions of years have a significant impact on global erosion and nutrient and sediment productions. Thus, understanding and quantifying the impact of tectonics and climate on short- to long-term landscape evolution has large implications on natural global cycles (e.g., climate change, atmospheric and terrestrial carbon circulations), biodiversity and ecological sustainability, hazard management (e.g., earthquakes, landslides), infrastructure planning, and decision making. In the last decades, significant progress has been made in the field of tectonic geomorphology to try and resolve the relative roles of tectonics and climate in landscape evolution. Yet, many questions remained unresolved, for instance: - What drives landscape evolution in post-orogenic settings? - What is the relative role of climate in landscape evolution at the global scale? In my PhD, I address these questions by investigating the impact of tectonics and climate on fluvial topography and geomorphology at different spatiotemporal scales. In my first chapter, I present a local study in the southern Colorado Front Range to explore the relative roles of tectonics and climate on observed landscape unsteadiness that affected the area during the late Cenozoic. In the second chapter, I extend this study and address this question to the scale of the entire Colorado Front Range. In my third chapter, I explore the impact of climate on fluvial topography at the global scale. For all these studies, I integrate field data, digital topographic analysis, geochronology, and modeling to compare new and existing predictions for the roles of tectonics and climate at the local (chapter I), regional (chapter II), and global (chapter III) scales to empirical observations. Results from these studies shed light on some ongoing controversies (e.g., what drives topographic rejuvenation in the Colorado Front Range) and resolve misunderstood concepts (e.g., how climate is recorded in fluvially-dominated landscapes). The first and third chapters in this dissertation were submitted to peer-reviewed journals and are under review, while the second chapter is in its final stage as a third manuscript for a peer-reviewed journal. FIRST CHAPTER: LATE CENOZOIC DEFORMATION IN THE SOUTHERN COLORADO FRONT RANGE REVEALED BY RIVER PROFILE ANALYSIS AND FLUVIAL TERRACES Post-orogenic landscapes are important sources of sediment and nutrients relevant to many natural global cycles and ecological sustainability. Many of these settings exhibit evidence of recent landscape unsteadiness, but their driving mechanisms are poorly understood. The Colorado Front Range (FR), a post-orogenic setting that maintains high relief, elevated topography, and evidence of ongoing unsteadiness, is a good example of this enigma. Two prevailing hypotheses have been proposed to explain the geologically-recent landscape unsteadiness in the FR: (1) mantle dynamics and active tectonics during the late Cenozoic; (2) enhanced erosional efficiency associated with a Quaternary climate change. Here we evaluate these end-member hypotheses through a case study of tectonic geomorphology of the Upper Arkansas River basin in southern Colorado. We perform river profile analysis on bedrock channels in the eastern Rockies and map and analyze fluvial terraces in the western High Plains. We find that knickpoints in the eastern Rockies record a one- to two-stage increase in base level fall rate downstream of the FR mountain front and an eastward increase in the magnitude of incision. Similarly, terraces in the western High Plains record an eastward increase in the magnitude of incision. Collectively, and supported by flexural and supplemental geomorphic analyses, these results suggest a previously undetected regional-scale, west-directed back tilting signal associated with differential rock uplift. Based on existing geodynamic models, we interpret these deformation patterns and related landscape response as a result of a migrating dynamic topography that swept the southern FR from west to east during the late Cenozoic. SECOND CHAPTER: TECTONIC AND GEODYNAMIC CONTROL ON REJUVENATION IN THE COLORADO ROCKY MOUNTAINS The Colorado Rocky Mountains (CRM) ancient foreland basin, currently known as the High Plains, shows a steeper long-wavelength tilt away from its hinterland relative to other active mountain range foreland basins worldwide. Further, studies showed that the High Plains experienced a transition from a system of net deposition to one characterized by net erosion at ~5 Ma. However, the mechanisms proposed to explain these observations are the center of ongoing debate. Some argue that the tilting and the transition from deposition to erosion were facilitated by tectonically- or geodynamically-driven changes in rock uplift rate, while others argue that these records are simply the result of an increase in erosional efficiency driving river incision and relaxation with some amount of isostatic rebound. One of the main reasons this controversy continues is that empirical studies trying to address this question were conducted mostly in the High Plains, where landscape geomorphic signatures used to distinguish between these two hypotheses are ambiguous. Here, we conduct a geomorphic analysis of the Colorado Rockies, which lies upstream of the High Plains province and is characterized by a harder crystalline basement, where bedrock rivers might still achieve a record of the transient landscape of the CRM and help clarify potential drivers. We combine river profile analysis with a compilation of new and existing basin average erosion rates from cosmogenic 10Be and channel incision rates from luminescence dating on fluvial terraces to differentiate two geomorphic zones in the Colorado Rockies: 1. an upper, relict topography upstream of convex upward knickpoints that is consistent with lower long-term background erosion rates of ~0.03 mm/yr and lower channel steepness of ~80-100 m0.9; 2. a transient landscape downstream of these knickpoints that is consistent with higher channel incision rates of ~0.3 mm/yr and higher channel steepness that increases systematically from ~150 m0.9 in the northern CRM to 300 m0.9 in the southern CRM. These results and their spatial patterns across the CRM are inconsistent with existing predictions from a climate-induced increased erosional efficacy during the last Cenozoic. Rather, they imply a long-wavelength deformation and a sustained tectonic uplift rate associated with active tectonics and geodynamics that impacted the CRM in the last 5 Ma. THIRD CHAPTER: CLIMATE CONTROLS ON FLUVIAL TOPOGRAPHY Conceptual and theoretical models for landscape evolution suggest that fluvial topography is sensitive to climate. However, it has remained challenging to demonstrate a compelling link between fluvial topography and climate state in natural landscapes. One possible reason is that many studies compare erosion rates to climate data, although theoretical studies note that, at steady-state, climate is encoded in topography rather than in erosion rates. Here, we use an existing global compilation of 10Be basin average erosion rates to isolate the climate signal in topography for fluvially-dominated catchments underlain by crystalline bedrock that appear to be in morphological steady state. Our results show that the nonlinearity between erosion rates and the normalized river channel steepness index, which is a proxy for fluvial relief, systematically increases with increasing mean annual precipitation and decreasing aridity. When interpreted in the context of detachment-limited bedrock incision models that account for incision thresholds and stochastic distribution of floods, this systematic pattern can be explained by a decrease in discharge variability in landscapes that are wetter and less arid, assuming incision thresholds are important. Our results imply a climate control on topography at a global scale and highlight new research directions that can improve understanding of climate’s impact on landscape evolution.
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
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 member
Emergent 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.
Open Access
Hydrologic and hydraulic response to wildfires in the upper Cache la Poudre watershed using a SWAT and HEC-RAS model cascade
(Colorado State University. Libraries, 2015) Havel, Aaron, author; Arabi, Mazdak, advisor; Baker, Daniel, committee member; Wohl, Ellen, committee member
The enhanced possibility of catastrophic wildfires in the western USA and other regions around the world has increased the need to evaluate the effects of wildfire on the hydrology of watersheds and the hydraulic behavior of rivers. Understanding the effects of wildfires is vital in water-resources management and for public safety especially in regions where communities depend on surface water supply. Similarly, areas adjacent to river systems may be at risk of increased flooding due to wildfires in their upstream watersheds. Effects of wildfires on hydrologic fluxes in watersheds and rivers have been extensively studied; but, characterization of responses to wildfires is difficult due to the spatial variability of post-wildfire conditions. At the watershed scale, hydrologic responses comprise a network of complex nonlinear interactions. Hence, comprehensive watershed models serve as a useful tool to understand these relationships. Watershed models commonly lack the ability to represent channel geometry and channel process with sufficient spatial frequency. Thus, a hydrologic and hydraulic model cascade provides a bridge between the nonlinear interactions of the uplands and the river responses at the channel scale. The overall goal of this study is to examine the spatial variability of the effects of the 2012 High Park and Hewlett wildfires that occurred within the headwaters of the Cache la Poudre River located in northern Colorado, USA. Two commonly used models were calibrated and used in combination. First, the Soil and Water Assessment Tool (SWAT) was used to evaluate the hydrologic responses of the upper Cache la Poudre watershed to the wildfire events. Subsequently, the results from the SWAT model were used as inputs for the hydraulic model Hydrologic Engineering Center River Analysis System (HEC-RAS) to simulate channel hydraulics along 42.5 km of the upper Cache la Poudre River. The baseline SWAT model was established to simulate the hydrology of the study area between the years 2000 and 2014. This model accounts for wildfires by modifying land use/land cover inputs and corresponding parameters during simulations. Daily streamflow data were used for model calibration and testing. Using the calibrated baseline model, no-wildfire and wildfire scenarios were created. The two scenarios were then compared for changes in average annual total runoff volume, water budgets, and full streamflow statistics at the watershed and sub-basin scales. Then a HEC-RAS model was developed to simulate the hydraulic responses of the stream network using streamflows for various floods extracted from the two SWAT scenarios. High resolution DEM data and surveyed water surface elevations are used for model calibration and testing, respectively. Channel hydraulic behavior including flood inundation area, streamflow velocities, and channel shear stress were compared for the two scenarios at the channel scale. At the watershed scale, wildfire conditions have little effect on the hydrologic responses, but at the sub-basin scale a total runoff increase up to 75 percent between scenarios was found. Generally, wildfire affected water budgets showed more surface runoff versus subsurface runoff, suggesting a decrease in infiltration rates under post-wildfire conditions. Flow-duration curves developed using full streamflow statistics for burned sub-basins show that less frequent streamflows become greater in magnitude leading to ecosurplus values up to 0.279. Also, simulations revealed that there is a strong and significant (R2 > 0.8 and p < 0.001) positive correlation between runoff increase and percentage of burned area upstream. Streamflow increases were between 2 and 14 percent depending on the reach’s proximity to the wildfire and the flood. Lastly, along the main stem only slight increases in flood area, average cross section velocity, and shear stress as a result of wildfire were observed in the simulations. The results have important implications on improving post-wildfire water resources management.