Browsing by Author "Watson, Chester C., advisor"
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Item Open Access An urban geomorphic assessment of the Berryessa and Upper Penitencia Creek watersheds in San Jose, California(Colorado State University. Libraries, 2009) Jordan, Brett, author; Watson, Chester C., advisorA paired watershed study for two adjacent urbanized watersheds in San Jose, California was conducted to investigate vastly different channel morphologic response to urbanization and valley subsidence. The urbanized portion of Berryessa Creek (15.5 km2) exhibits system-wide channel instability, meanwhile Upper Penitencia Creek (61.3 km2) has remained stable despite similar urban build out trends. Currently, there is a paucity of field measurements documenting channel response to urbanization and subsidence in the academic literature. Detailed geomorphic field surveys were undertaken to establish 90 permanent cross sections over 9.2 km of urban channel. These surveys were used for sediment transport modeling in this study and will provide a permanent monitoring network. Historic data sources were utilized to establish a baseline context and chronicle change in the watersheds. The historic data sources, field data, and numerical modeling were used to investigate the relative effects of hydrologic alteration, valley subsidence, and river infrastructure on water yield, sediment yield, and channel stability. Drainage area capture by the urban storm sewer network, a component of urbanization that has not previously been addressed in the scientific literature, and engineered river infrastructure elements are the primary causes of system-wide channel instability in the urbanized valley portion of Berryessa Creek. Hydrologic and sediment transport modeling indicates that drainage area capture and urban land use change has increased water yield 48% and sediment yield up to 61% from 76 to 121 tonnes/yr-km2. These hydrologic changes have transformed historically depositional reaches into incised reaches leading to system wide instability. An on-line sedimentation basin and a 1.85 m grade-control structure have reduced downstream sediment yield by 15% from 88 to 76 tonnes/yr-km 2 and increased channel incision rates by capturing coarse bed material in transport. Models indicate that measured valley subsidence of 0.23 m results in upstream incision, however sediment yield is not affected and the morphologic response to subsidence is likely obscured by current instability processes dominant in the system. In the current hydrologic regime of Upper Penitencia Creek, flow diversion and basin reduction by the storm sewer network offset increased runoff produced by the urban landscape and channel stability is not adversely affected by the hydrologic alteration. Water yield is increased by 7%, however sediment yield is reduced by 4% from 41.7 to 39.8 tonnes/yr-km2 at the outlet. River infrastructure in the form of a system of small grade-control structures aids in the stability of the upstream reaches. Valley subsidence of 1.1 m is predicted to cause incision that would progress 1800 m upstream of the zone of maximum subsidence. Modeling results were verified by reach-scale instability observed upstream of the subsidence zone. The reach scale instability is a result of increase stream power resulting from valley subsidence and channel realignment.Item Open Access Numerical analysis of river spanning rock U-weirs: evaluating effects of structure geometry on local hydraulics(Colorado State University. Libraries, 2011) Holmquist-Johnson, Christopher Lee, author; Watson, Chester C., advisor; Abt, Steven R., committee member; Thornton, Christopher I., committee member; Doe, William, committee memberRiver spanning rock weirs are being constructed for water delivery as well as to enable fish passage at barriers and provide or improve the aquatic habitat for endangered fish species. Many design methods are based upon anecdotal information applicable to narrow ranges of channel conditions and rely heavily on field experience and engineering judgment. Without an accurate understanding of physical processes associated with river spanning rock weirs, designers cannot address the failure mechanisms of these structures. This research examined the applicability of a Computational Fluid Dynamics (CFD) model, U2RANS, to simulate the complex flow patterns associated with numerous U-weir configurations. 3D numerical model simulations were used to examine the effects of variations in U-weir geometry on local hydraulics (upstream water surface elevations and downstream velocity and bed shear stress). Variations in structure geometry included: arm angle, arm slope, drop height, and throat width. Various combinations of each of these parameters were modeled at five flow rates: 1/10 bankfull discharge, 1/5 bankfull discharge, 1/3 bankfull discharge, 2/3 bankfull discharge and bankfull discharge. Numerical modeling results duplicated both field observations and laboratory results by quantifying high shear stress magnification near field and lab scour areas and low shear stress magnification near field and lab depositional areas. The results clearly showed that by altering the structure geometry associated with U-weirs, local flow patterns such as upstream flow depth, downstream velocity, and bed shear stress distributions could be altered significantly. With the range of parameters tested, the maximum increase in channel velocity ranged from 1.24 to 4.04 times the reference velocity in the channel with no structure present. Similarly, the maximum increase in bed shear stress caused by altering structure geometry ranged from 1.57 to 7.59 times the critical bed shear stress in the channel for a given bed material size. For the range of structure parameters and channel characteristics modeled, stage-discharge relationships were also developed utilizing output from the numerical model simulations. These relationships are useful in the design process when estimating the backwater effect from a structure for irrigation diversion as well as determining the spacing between structures when multiple structures are used in series. Recommendations were also made, based on the analysis and conclusions gathered from the current study, for further research. The analysis and results of the current study as well as laboratory studies conducted by Colorado State University and field reconnaissance by the Bureau of Reclamation provide a process-based method for understanding how structure geometry affects flow characteristics, scour development, fish passage, water delivery, and overall structure stability. Results of the numerical modeling allow designers to utilize the methods and results of the analysis to determine the appropriate U-weir geometry for generating desirable flow parameters (i.e. upstream flow depth and downstream velocity and bed shear stress magnification) to meet project specific goals. The end product of this research provides tools and guidelines for more robust structure design or retrofits based upon predictable engineering and hydraulic performance criteria.Item Open Access Quasi-equilibrium conditions of urban gravel-bed stream channels in southern Ontario, Canada, and their implications for urban-stream restoration(Colorado State University. Libraries, 2010) Annable, William Kenneth, 1965-, author; Watson, Chester C., advisor; Bledsoe, Brian P., committee member; Fischenich, J. Craig, 1962-, committee member; Julien, Pierre Y., committee memberUrban gravel-bed stream channels in southern Ontario, Canada, identified to be in a state of quasi-equilibrium have been studied over the past 15 years and compared against rural gravel-bed stream channels of the same hydrophysiographic region. Bankfull width and depth versus bankfull discharge were not found to increase as a function of increasing urbanization as has been found in many other studies. The observed annual frequency of bankfull discharge was typically less than a 1-year return period with many sites ranging between two to eighteen bankfull events per year with higher intensity and shorter duration urban flood responses. The cumulative volume of bankfull and larger flood events from the urban-stream channels were very similar to the same annual event volumes in the rural comparison study reaches. Bed-material supply was found to decrease with increasing urbanization and the reduction in bed-material supply appears to be offset by the smaller bankfull channel width, depth, and access to floodplains during large flood events. Field evidence may also suggest an even greater reduction in channel width trajectory, relative to the rural setting, with floodplains to maintain quasi-equilibrium conditions as bed-material supply continues to decrease with increased anthropogenic activity. Compared to surrounding rural watersheds, urban belt widths were found to decrease, while meander wave lengths and radii of curvature were found to increase as a function of bankfull width. The stream-wise elongation of meander wave lengths (and thus increase in radii of curvature) are a result of increased flood flow frequency and volume in the urban environments combined with reductions in bed-material supply. An increased frequency in riffles and pools was also observed along each reach. Additional pools appeared along straight sections between bends, although they were shallower than pools on bends. The changes in bedforms result from brief but frequent discharge events that exceed critical shear values, resulting in sediment pulsing and the frequent placement of keystone clasts that create frequent riffle (and pool) development. Field observations of standing wave patterns in flood flows also support the role of `dune-like' formations as a means of maximizing flow resistance. Several methods of estimating channel-forming discharge were also evaluated to test their applicability in the urban condition. Bankfull stage was identified at a series of locations along each study reach and it was found that the most consistent observations of bankfull discharge occurred during flood conditions where bankfull stage was identified at the top of point bars along the convex arc of bends. The largest errors in estimation occurred at gauge stations where cross-sectional geometry had been altered to conform to bridges or culverts rather than the channel morphology. Independent evaluations of channel-forming discharge were conducted by eleven practitioners ranging from 10 to 43 years of experience with similar findings and errors. Various methods of relating frequency return periods were evaluated using annual peak series discharge observations and continuous 15-minute systematic discharge records using partial duration series analysis. No specific correlations were identified between frequency return periods and land-use change. However, based upon the findings of this study, the applicability of employing annual series peak discharge data to evaluate bankfull frequency return in urban-stream channels is highly discouraged.