Browsing by Author "Rathburn, Sara L., committee member"

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Open Access
A framework for the analysis of coastal infrastructure vulnerability under global sea level rise
(Colorado State University. Libraries, 2017) O'Brien, Patrick S., author; Julien, Pierre Y., advisor; Watson, Chester C., committee member; Ettema, Robert, committee member; Rathburn, Sara L., committee member
The assumption of hydrologic stationarity has formed the basis of coastal design to date. At the beginning of the 21st century, the impact of climate variability and future climate change on coastal water levels has become apparent through long term tide gauge records, and anecdotal evidence of increased nuisance tidal flooding in coastal areas. Recorded impacts of global sea rise on coastal water levels have been documented over the past 100 to 150 years, and future water levels will continue to change at increasing, unknown rates, resulting in the need to consider the impacts of these changes on past coastal design assumptions. New coastal infrastructure plans, and designs should recognize the paradigm shift in assumptions from hydrologic stationarity to non-stationarity in coastal water levels. As we transition into the new paradigm, there is a significant knowledge gap which must address built coastal infrastructure vulnerability based on the realization that the underlying design assumptions may be invalid. A framework for the evaluation of existing coastal infrastructure is proposed to effectively assess vulnerability. The framework, called the Climate Preparedness and Resilience Register (CPRR) provides the technical basis for assessing existing and future performance. The CPRR framework consists of four major elements: (1) datum adjustment, (2) coastal water levels, (3) scenario projections and (4) performance thresholds. The CPRR framework defines methodologies which: (1) adjust for non-stationarity in coastal water levels and correctly make projections under multiple scenarios; (2) account for past and future tidal to geodetic datum adjustments; and (3) evaluate past and future design performance by applying performance models to determine the performance thresholds. The framework results are reproducible and applicable to a wide range of coastal infrastructure types in diverse geographic areas. The framework was applied in two case studies of coastal infrastructure on the east and west coasts of the United States. The east coast case study on the Stamford Hurricane Barrier (SHB) at Stamford CT, investigated the navigation gate closures of the SHB project. The framework was successfully applied using two performance models based on function and reliability to determine the future time frame at which relative sea level rise (RSLR) would cause Navigation Gate closures to occur once per week on average or 52 per year. The closure time analysis also showed the impact of closing the gate earlier to manage internal drainage to the Harbor area behind the Stamford Hurricane Barrier. These analyses were made for three future sea level change (SLC) scenarios. The west coast case study evaluated four infrastructure elements at the San Francisco Waterfront, one building and three transportation elements. The CPRR framework applied two performance models based on elevation and reliability to assess the vulnerability to flooding under four SLC scenarios. An elevation-based performance model determined a time horizon for flood impacts for king tides, 10 and 100-year annual exceedance events. The reliability-based performance model provided a refinement of results obtained in the elevation-based model due to the addition of uncertainty to the four infrastructure elements. The CPRR framework and associated methodologies were successfully applied to assess the vulnerability of two coastal infrastructure types and functions in geographically diverse areas on the east and west coasts of the United States.
Open Access
Full spectrum analytical channel design with the capacity/supply ratio (CSR)
(Colorado State University. Libraries, 2017) Stroth, Travis R., author; Bledsoe, Brian P., advisor; Nelson, Peter A., committee member; Rathburn, Sara L., committee member
Analytical channel design tools have not advanced appreciably in the last decades, and continue to produce designs based upon a single representative discharge that may not lead to sediment continuity. It is beneficial for designers to know when a simplified design may be problematic and to efficiently produce alternative designs that approximate sediment balance over the entire flow regime. The Capacity/Supply Ratio (CSR) approach, an extension of the Copeland method of analytical channel design for sand channels, balances the sediment transport capacity of a design reach with the sediment supply of a stable upstream reach over the entire flow duration curve (FDC) rather than just a single discharge. Although CSR has a stronger physical basis than previous analytical channel design approaches, it has not been adopted in practice because it can be a cumbersome and time-consuming iterative analysis without the use of software. I present a novel design tool that was developed using the Visual Basic for Applications (VBA) programming language in Excel® and produces stable channel slope/width combinations based on the CSR methodology for both sand- and gravel-bed streams. The CSR Stable Channel Design Tool's (CSR Tool) code structure was based on Copeland's method in SAM and HEC-RAS (Hydrologic Engineering Center – River Analysis System) and was tested with a single discharge to verify outputs. Eighteen sand-bed rivers were investigated with the tool in a comparison of designs based on the CSR approach and five single-discharge metrics: the effective discharge (Qeff) or discharge that transports the most sediment over time, the 1.5-year recurrence interval discharge (Q1.5), bankfull discharge (Qbf), and the discharges associated with 50th (Qs50) and 75th (Qs75) percentiles of the cumulative sediment yield curve. The Qs50 and Qs75 single-discharge designs match the CSR output most closely followed by the Qbf and Qeff. The Qeff proved to be the most inconsistent design metric because it can be highly dependent on the binning procedure used in the effectiveness analysis. Furthermore, I found that the more rigorous physical basis of the CSR analysis is potentially most important in designing "labile' channels with highly erodible substrate, high perennial flow "flashiness', low width-to-depth ratio, and high incoming sediment load. The CSR Tool provides a resource for river-restoration practitioners to utilize process-based design techniques that can promote more reliable and sustainable designs for dynamic fluvial systems.
Open Access
Island dynamics and their role in regulating sediment flux in the Middle Snake River, Idaho
(Colorado State University. Libraries, 2014) Thomas, Dai B., author; Wohl, Ellen E., advisor; Anthony, Deborah J., committee member; Rathburn, Sara L., committee member; Bledsoe, Brian P., committee member
This study was conducted to provide an improved understanding of the dynamics of river islands and to investigate the role of islands in regulating sediment flux within the fluvial system. The study showed that the islands in entrenched geomorphic subreaches of the MSR form, erode, and reform in locations controlled by lateral constrictions. The geometry of the islands adjust on a decadal or even longer time scale in response to a disturbance or changes in water and sediment supply, and thus, the islands form part of a temporal and spatial continuum of bedforms. The formation of the islands regulates sediment flux through the reach. The study reach of the Middle Snake River (MSR) in Idaho contains over 300 islands within approximately 200 km between Swan Falls Dam and Brownlie Reservoir. The hydrology of the study reach has been significantly altered by upstream dams on the mainstem and dams on tributaries within the study reach. Data used in the study include: (1) historical aerial photos (1938/1939) and topographic maps (c1894-1906), (2) topographic and bathymetric survey data collected in 1997 through 1999, (3) flow measurements from 1911 to present, (4) bed material samples, (5) morphostratigraphic mapping of 194 islands and (6) stratigraphic soil profile data collected on 95 islands. The soil profile data included soil stratigraphy, soil samples (used for sediment gradations and pollen analyses), pedological descriptions, historical artifacts and charcoal fragments (used for carbon dating). A previously developed 1-dimensional hydraulic model of the study reach was used to evaluate the hydraulic conditions along MSR and to calculate the overtopping discharges of the islands. Comparison of historical aerial and topographic data with 2012 aerial photography showed evidence of the growth and erosion of islands and reworking of island chains to form new configurations, illustrating the dynamic nature of the islands over the last approximately 100 years. The historical document review also showed that the location of almost all larger islands and island groups are controlled by lateral constrictions such as tributary fans. Soil profile data, pollen analyses, historical artifacts and radiocarbon dating of soil charcoal were used to determine the approximate age of islands and to evaluate the erosional and depositional activity of the islands. The soil profile data showed an extreme range in age at some islands where the gravel platform of the islands is old (circa 7,000 years), but the overlying sediments are young (on the order of hundreds of years). Two-dimensional sediment-transport models were developed to evaluate the baseline conditions and simulate island development. Baseline conditions modeling showed the gravel- to cobble-sized material forming the core of the islands is not mobilized under the current hydrology. The islands formed in response to more recent floods from silt-sand sized sediment supply, which explains the relatively young soils overlying older gravel cores. Modeling results showed that: (1) the island geometry adjusts to a disturbance or a change in sediment supply, (2) the formation of islands regulates sediment flux, and (3) the islands form, erode, and reform in the same general locations, which supports the study hypotheses that islands form part of a temporal and spatial continuum of bedforms.
Open Access
Modeling hydrologic processes associated with soil saturation and debris flow initiation during the September 2013 storm, Colorado Front Range
(Colorado State University. Libraries, 2019) Timilsina, Sujana, author; Niemann, Jeffrey D., advisor; Rathburn, Sara L., committee member; Nelson, Peter A., committee member
Seven days of extreme rainfall during September 2013 produced more than 1,100 debris flows in the Colorado Front Range, about 78% of which occurred on south-facing slopes (SFS). A few previously-published soil moisture observations suggest that SFS were wetter than north-facing slopes (NFS) during the event, which would contrast with soil moisture patterns observed during dry conditions. Various causes have been hypothesized for the preferential saturation of SFS, but those hypotheses remain largely untested. The objectives of this study are to analyze the soil moisture patterns using additional soil moisture observations, determine the hydrologic processes behind the preferential saturation of SFS, and to evaluate the importance of soil moisture in determining the debris flow initiation sites. Soil moisture patterns are simulated using the Equilibrium Moisture from Topography, Vegetation, and Soil (EMT+VS) model for a study region that includes 63% of the observed debris flow locations. Five hypotheses are implemented in the model including: (1) higher rainfall rates, (2) lower interception rates, (3) lower porosity, (4) thinner soils, and (5) reduced deep drainage on SFS. The EMT+VS model is also coupled with an infinite slope stability model to produce factor of safety maps. The hypotheses are tested by comparing the modeled soil moisture patterns to soil moisture observations and the debris flow initiation sites. The results suggest that differences in interception and deep drainage between the slopes were primarily responsible for producing wetter SFS, but the soil moisture pattern likely played a smaller role than vegetation and slope in determining the debris initiation sites.
Open Access
Morphodynamic modeling of flow and sediment transport over low-head, Run-of-River dams
(Colorado State University. Libraries, 2018) Queen, Robert William, author; Neslon, Peter A., advisor; Rathburn, Sara L., committee member; Morrison, Ryan R., committee member
Low-head or Run-of-River (RoR) dams exist on all types of rivers throughout the United States, yet the exact mechanisms of how sediment moves around the structures have not been well researched. Due to the increasing use of RoR dams in small hydroelectric projects, there is a need to better understand the controlling factors of how sediment passes over these dams. A one-dimensional morphodynamic model was developed to investigate the effects of RoR dams on channel morphology over long time scales. The model solves the gradually varied flow equations to compute the flow field in the vicinity of the dam, computes grain-size-specific sediment transport rates, and uses sediment mass conservation and vertical storage bookkeeping to calculate the evolution of bed elevation, the bed surface grain-size distribution, and the vertical pattern of stratigraphy. The model's hydraulic calculations were calibrated using data collected from a series of flume experiments performed with a model RoR dam to better capture the non-hydrostatic flow over a dam. Numerical experiments designed to investigate how the grain-size distribution of the sediment supply rate, flow rate (steady and unsteady), and dam height act as controls on sediment passage over RoR dams were conducted using parameters reported in the literature for a RoR dam in northern Delaware.These one-dimensional simulations were complemented with a few simulations using, a two-dimensional morphodynamic model, Nays2DH. The 1D simulation results show that the stored sediment upstream of RoR dams does depend on the sediment supply, dam height, grain-size and flow discharge. Once sedimentation in the reservoir has reached equilibrium, high flow events will reduce or scour the sediment while lower flows will typically increase the amount of sediment behind the dam. Finally, a dam that stores more sediment will have greater downstream effects in terms of changes to grain-sizes and bed elevation due to the increased time it takes to pass sediment over the dam and reach an equilibrium condition on the upstream side of the dam.
Open Access
Optimization of Sangju weir operations to mitigate sedimentation problems
(Colorado State University. Libraries, 2016) Kim, Hwa Young, author; Julien, Pierre Y., advisor; Fontane, Darrell G., committee member; Thornton, Christopher I., committee member; Rathburn, Sara L., committee member
To view the abstract, please see the full text of the document.
Open Access
Seasonal precipitation and soil moisture dynamics of a hyperarid wash in the Sonoran Desert, U.S.A.
(Colorado State University. Libraries, 2013) Howe, Susan R., author; Wohl, Ellen E., advisor; Laituri, Melinda J., committee member; Kelly, Eugene F., committee member; Rathburn, Sara L., committee member
Precipitation and runoff in arid and hyperarid landscapes is infrequent and both spatially and temporally variable, and the relationship between these hydrologic components and vegetation, soils, and geomorphology in these environments is complex and not well understood. In this study, precipitation and soil moisture were monitored beneath three cover types in three locations across two geomorphic surfaces in the Yuma Wash watershed, located in the Lower Colorado River Valley of the Sonoran Desert, on the US Army Yuma Proving Grounds in Yuma, Arizona. Monitoring, sampling, and characterization occurred from July 2006 to February 2010. Six tipping bucket rain gages and sixty time domain reflectometry soil moisture sensors recorded moisture inputs and storage on a middle to late Pleistocene age alluvial terrace, and a younger, Holocene age alluvial wash. Sensors were spatially distributed in the lower, middle and upper locations of the watershed, beneath bare ground at 2.5, 25, 50, and 100 cm, and beneath the dripline radius of Olneya tesota and Parkinsonia microphylla, at 25, 50, and 100 cm depths. These data suggest that precipitation is highly variable in space and time, and is generally greater than the surrounding valley bottoms of Yuma Proving Grounds. Findings also suggest that soils beneath the dripline radius of these plant species on terraces are wetted more frequently and to greater depths in response to smaller magnitude and lower intensity storm events relative to soils beneath the same species on washes, and relative to bare ground soils. Threshold precipitation conditions necessary to generate changes in soil moisture were compared across surfaces, and illustrate that the vesicular structure in the A (Av) horizons beneath desert pavement plays a key role in redistribution of moisture as runon to O. tesota and P. microphylla on terraces, and that soils beneath the dripline radius of both species on washes receive moisture only during rainfall events exceeding 30 mm. There is also some evidence to suggest precipitation and near surface soil moisture may be greater in the upper basin relative to the mid- and lower basin on both surfaces, but at depths of 25-100 cm, soil moisture responses were difficult to interpret due to local soil properties not quantified in this study. The influence of soil temperature on the imaginary permittivity component of soil moisture readings due to high soluble salt content, the presence of enriched clay layers, soil compaction and induration is discussed. Findings highlight the need to quantify these age-dependent soil pedogenic and hydrologic properties when assessing soil moisture response to spatially variable precipitation in these water-limited environments. Implications for management of military lands are discussed.
Open Access
Stratigraphic feedbacks on alternate bar morphology
(Colorado State University. Libraries, 2017) Brown, Ryan A., author; Nelson, Peter A., advisor; Morrison, Ryan R., committee member; Rathburn, Sara L., committee member
As gravel bed rivers aggrade, they can develop subsurface stratigraphy consisting of heterogeneous grain-size distributions in the downstream, cross-stream, and vertical directions. During subsequent periods of degradation, this heterogeneous stratigraphy can be exhumed and may feedback on the processes that drive morphodynamic evolution. However, these surface-stratigraphy feedbacks are poorly understood and difficult to predict. These feedbacks are investigated by implementing the ability to store, track, and access bed stratigraphy in the 2-dimensional mixed-grain-size morphodynamic model FaSTMECH. The stratigraphy framework consists of a 3-dimensional grid of subsurface layers containing grain size fractions. The active layer (surface) is then allowed to exchange sediment with bedload as well as the stratigraphy layers. During aggradation, size fractions of sediment in the active layer and bedload are mixed with the highest stratigraphy layer size fractions. During degradation, the active layer takes on the sediment properties stored in the stratigraphy. This model is used to investigate stratigraphic feedbacks on the coevolution of surface patchiness and alternate bar morphology. When alternate bars are forced by an obstruction, differences between model simulations with and without stratigraphy enabled are minimal because bars quickly stabilize and become fixed. With no obstruction, however, migrating alternate bars formed with stratigraphy enabled are wider and display stronger sorting patterns than those formed without accounting for stratigraphy. The repeated aggradation and degradation associated with bar migration results in frequent interaction between subsurface and surface material. The repeated access of material finer than the initial bulk material during degradation allows for greater degrees of surface sorting where coarse bar tops become coarser and fine pools become finer. Changes in sorting patterns are shown to increase bar width and increase bar celerity by 1 cm/min. This suggests that surface/subsurface interactions play an important role in setting bed morphology, sorting patterns, and bedform dynamics in gravel bed rivers.
Open Access
The sediment yield of South Korean rivers
(Colorado State University. Libraries, 2019) Yang, Chun-Yao, author; Julien, Pierre Y., advisor; Ettema, Robert, committee member; Nelson, Peter, committee member; Rathburn, Sara L., committee member
South Korea is experiencing increasing river sedimentation problems, which requires a reliable method to predict the sediment yield. With the recent field measurements at 35 gaging stations in South Korea provided by K-water, we quantified the sediment yield by using the flow duration curve and sediment rating curve. The current sediment yield models have large discrepancies between the predictions and measurements. The goal of this dissertation is to provide better understanding to the following questions: (1) How much of the total sediment load can be measured by the depth-integrated samplers? (2) Can we predict the sediment yield based only on watershed area? (3) Is there a parametric approach to estimate the mean annual sediment yield based on the flow duration curve and sediment rating curve? With 1,962 sediment discharge measurements from the US D-74 sampler, the total sediment discharge is calculated by both the Modified Einstein Procedure (MEP) and the Series Expansion of the Modified Einstein Procedure (SEMEP). It is concluded that the SEMEP is more accurate because MEP occasionally computes suspended loads larger than total loads. In addition, SEMEP was able to calculate all samples while MEP could only compute 1,808 samples. According to SEMEP, the ratio Qm/Qt of measured sediment discharge Qm to total sediment discharge Qt is a function of the Rouse number Ro, flow depth h, and the median grain size of the bed material d50. In Korean sand and gravel bed rivers, the materials in suspension are fine (silt or clay) and Ro ≈ 0. The ratio Qm/Qt reduces to a function of flow depth h, and at least 90% of the total sediment load is measured when h > 1 m. More than 80% of the sediment load is measured when the discharge Q is larger than four times mean annual discharge ¯Q(Q/¯Q > 4). The ratio Qs/Qt of suspended sediment discharge Qs to total sediment discharge can be also analyzed with SEMEP and the result shows that Qs/Qt is a function of h/d50 and Ro. When Ro ≈ 0, the ratio Qs/Qt increases with h/d50. The suspended load is more than 80% of the total sediment load when h/d50 > 18. The relationship between specific sediment yield, SSY, and watershed area, A, is SSY = 300A-0.24 with an average error of 75%. Besides the specific sediment yield, the mean annual discharge, the normalized flow duration curve, the sediment rating curve, the normalized cumulative distribution curve, and the half yield discharge vary with watershed area. From the normalized flow duration curve at an exceedance probability of 0.1%, small watersheds (A<500 km2) have 42 5000 km2) which have 14 < Q/¯Q < 33. In terms of sediment rating curves, at a given discharge, the sediment load of small watersheds is one order of magnitude higher than for large watersheds. From the normalized cumulative distribution curves, the half yield (50% of the sediment transported) occurs when the discharge is at least 15 times the mean discharge. In comparison, the half yield for large watersheds corresponds to Q/¯Q < 15. The flow duration curve can be parameterized with â and ˆb by using a double logarithmic fit to the flow duration curve. This parametric approach is tested with 35 Korean watersheds and 716 US watersheds. The value of â generally increases with watershed area. The values of ˆb are consistently between 0.5 and 2.5 east of the Mississippi River and the Pacific Northwest. Large variability in ˆb is found in the High Plains and in Southern California, which is attributed to the high flashiness index in these regions. A four-parameter model is defined when combining with the sediment rating curve. The four parameters are: â and ˆb for the flow duration curve, and ā and ¯b for the sediment rating curve. The mean annual discharge ¯Qs is calculated by ¯Qs = āâ¯bΓ(1+ ˆb¯b). The model results are compared to the flow-duration/sediment-rating curve method. The average error of this four-parameter model is only 8.6%. The parameters can also be used to calculate the cumulative distribution curves for discharge and sediment load.