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Debris flows, which occur in mountain settings worldwide, have been particularly damaging in the glaciated basins flanking the stratovolcanoes in the Cascade Range of the northwestern United States. This ...
Citation Citation
- Title:
- Debris flows in glaciated catchments : a case study on Mount Rainier, Washington
- Author:
- Legg, Nicholas T.
Debris flows, which occur in mountain settings worldwide, have been particularly damaging in the glaciated basins flanking the stratovolcanoes in the Cascade Range of the northwestern United States. This thesis contains two manuscripts that respectively investigate the (1) initiation processes of debris flows in these glaciated catchments, and (2) debris flow occurrence and its effect on valley bottoms over the last thousand years. In a 2006 storm, seven debris flows initiated from proglacial gullies of separate basins on the flanks of Mount Rainier. Gully heads at glacier termini and distributed collapse of gully walls imply that clear water was transformed to debris flow through progressive addition of sediment along gully lengths. In the first study, we analyze gully changes, reconstruct runoff conditions, and assess spatial distributions of debris flows to infer the processes and conditions necessary for debris flow initiation in glaciated catchments. Gully measurements suggest that sediment bulking requires steep gradients, abundant unstable material, and sufficient gully length. Reconstruction of runoff generated during the storm suggests that glaciers are important for generating the runoff necessary for debris flow initiation, particularly because infiltration capacities on glacial till covered surfaces well exceed measured rainfall rates. Runoff generation from glaciers and abundant loose debris at their termini explain why all debris flows in the storm initiated from proglacial areas. Proglacial areas that produced debris flows have steeper drainage networks with significantly higher elevations and lower drainage areas, suggesting that debris flows are associated with high elevation glaciers with relatively steep proglacial areas. This correlation reflects positive slope-elevation trends for the Mount Rainier volcano. An indirect effect of glacier change is thus the change in the distribution of ice-free slopes, which influence a basin’s debris flow potential. These findings have implications for projections of debris flow activity in basins experiencing glacier change. The second study uses a variety of dating techniques to reconstruct a chronology of debris flows in the Kautz Creek valley on the southwest flank of Mount Rainier (Washington). Dendrochronologic dating of growth disturbances combined with lichenometric techniques constrained five debris flow ages from 1712 to 1915 AD. We also estimated ages of three debris flows ranging in age from ca. 970 to 1661. Run-out distances served as a proxy for debris flow magnitude, and indicate that at least 11, 2, and 1 debris flow(s) have traveled at least 1, 3, and 5 km from the valley head, respectively since ca. 1650. Valley form reflects the frequency-magnitude relationship indicated by the chronology. In the upper, relatively steep valley, discrete debris flow snouts and secondary channels are abundant, suggesting a process of debris flow conveyance, channel plugging, and channel avulsion. The lower valley is characterized by relatively smooth surfaces, an absence of bouldery debris flow snouts, few secondary channels, and relatively old surface ages inferred from the presence of tephra layers. We infer that the lower valley is deposited on by relatively infrequent, large magnitude, low-yield strength debris flows like an event in 1947, which deposited wide, tabular lobes of debris outside of the main channel. Debris flows during the Little Ice Age (LIA) predominantly traveled no further than the upper valley. Stratigraphic evidence suggests that the main Kautz Creek channel was filled during the LIA, enhancing debris flow deposition on the valley surface and perhaps reducing run-out lengths. Diminished areas and gradients in front of glaciers during the LIA also likely contributed to decreased run-out lengths. These findings suggest that changes in debris flow source and depositional zones resulting from temperature and glacier cycles influence the magnitude and run-out distances of debris flows, and the dynamics of deposition in valley bottoms.
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292. [Article] Multi-objective Optimization of Reservoir Operation Under Uncertainty with Robust and Flexible Decision Variables
Optimization of reservoir operation is involves various competing objectives for a scarce resource (water). To find the optimal operation of reservoirs, it is essential to consider multiple objectives ...Citation Citation
- Title:
- Multi-objective Optimization of Reservoir Operation Under Uncertainty with Robust and Flexible Decision Variables
- Author:
- Hosseini, Parnian
Optimization of reservoir operation is involves various competing objectives for a scarce resource (water). To find the optimal operation of reservoirs, it is essential to consider multiple objectives simultaneously. There are various sources of uncertainty associated with the reservoir operation problem that should be considered as well. The overarching goal of this research is to develop a framework for finding flexible and reliable solutions to the reservoir operation problem with competing objectives. Because some sources of uncertainty are not well quantified, providing flexible decision variables lets the decision maker choose accordingly from a range of options knowing that all the flexible decision variables are feasible with a specified probability of failure and that are relatively optimal. To accomplish this goal, each flexible decision variable is represented by a random variable within a specific range instead of a single deterministic decision variable. An additional objective is added to the optimization problem, in order to maximize the flexibility of decision variables. The proposed methodology is tested for two mathematical test problems and the operation of the Grand Coulee reservoir, which is located on the Columbia River in the Northwestern United States. The Stochastic Collocation (SC) method is used to sample the random variables and approximate the expected values of the objectives. For the Grand Coulee reservoir, the decision variables are the daily turbine outflows. The first objective of the optimization is to minimize the forebay elevation deviation at the end of the optimization period. The second objective is to maximize the revenue from the hydropower production. The results show that the proposed methodology could find some flexible decision variables with 45% coefficient of variation. The corresponding expected objectives have less than 20% deterioration from the deterministic Pareto solutions. However, the number of function evaluations increases exponentially with the number of decision variables. Therefore, this methodology is suggested for problems with a few decision variables. For finding flexible decision variables in problems with many decision variables, a dimension reduction method called Karhunen Loeve (KL) expansion is implemented in the optimization problem. By extracting useful information from the decision variables, the decision space can be represented with merely a few random variables using a set of deterministic decision variables. The results show that three random variables are sufficient to generate decision variable realizations which have mean and variance less than 1% and 5% different from the original decision variable realizations, respectively. The proposed methodology is capable of efficiently finding flexible decision variables that lead to expected objective values close to the Pareto deterministic solutions. To force the generated decision variable realizations to stay within the feasible bounds and therefore reduce the number of constraints that need to be checked, the data is transformed to be within bounds first, and then the KL-expansion is performed. Using the transformed data decreases the computational time but the decrease in computational time is not significant. The inflow uncertainty is also considered as the only source of input uncertainty. Forecast inflow ensembles can be used as the source of inflow uncertainty. However in this study due to lack of information, historical inflows are used instead. The inflow uncertainties are represented using the KL-expansion. Robust optimization is performed by optimizing the weighted sum of the expectation and standard deviation of the objective due to uncertain inflows. The weights in the robust objective formulation can be changed based on the decision maker’s preference of robustness versus performance. Finally, the combined framework to find robust and flexible decision variables is tested on a reservoir operation problem and the results were compared to the deterministic case.
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Understanding the mechanisms that regulate local species diversity and community structure is a perennial goal of ecology. Local community structure can be viewed as the result of numerous local and regional ...
Citation Citation
- Title:
- Drought, dispersal, and community dynamics in arid-land streams
- Author:
- Bogan, Michael T.
Understanding the mechanisms that regulate local species diversity and community structure is a perennial goal of ecology. Local community structure can be viewed as the result of numerous local and regional processes; these processes act as filters that reduce the regional species pool down to the observed local community. In stream ecosystems, the natural flow regime (including the timing, magnitude, and duration of high and low flow events) is widely recognized as a primary regulator of local diversity and community composition. This is especially true in aridland streams, where low- and zero-flow events can occur frequently and for extended periods of time (months to years). Additionally, wetted habitat patches in arid-land stream networks are often fragmented within and among stream networks. Thus dispersal between isolated aquatic patches may also play a large role in regulating local communities. In my dissertation, I explored the roles that drought, dispersal, and local habitat factors play in structuring arid-land stream communities. I examined the impact of flow permanence and seasonal variation in flow and other abiotic factors on aquatic communities at both fine spatial scales over a long time period (8 years; Chapter 2) and at a broad spatial scale over a shorter time period (1-2 years; Chapter 4). Additionally, I quantified aquatic invertebrate aerial dispersal over moderate spatial scales (≤ 0.5 km) by conducting a colonization experiment using artificial stream pools placed along and inland from two arid-land streams (Chapter 4). Finally, I examined the roles of spatial isolation, microhabitat type, and local abiotic and biotic factors in structuring aquatic communities in freshwater oases scattered across one of the most arid regions of North America, the southern Sonoran Desert (Chapter 5). In Chapter 2, I found that severe drought caused an unprecedented drying event in isolated perennial stream pools, and that several additional drying events occurred over the following four years. This transition to intermittent flow caused the extirpation of several large, long-lived species with low dispersal abilities (including the top predator) and drove the local community into an alternative state. In the colonization experiment described in Chapter 3, I found that several arid-land stream invertebrate taxa disperse widely and frequently. The widespread dispersers identified by this experiment included several of the earliest colonist taxa observed following the severe drought described in Chapter 2. Other taxa, though, only dispersed overland after receiving an environmental cue (rainfall) or preferentially dispersed along stream corridors. In Chapter 4, where I examined invertebrate community structure across a large network of well-connected intermittent and perennial reaches, I found low diversity in intermittent reaches, regardless of their connectivity to diverse upstream perennial reaches. These species-poor, intermittent communities were composed of a unique suite of species with lifehistory adaptations that conferred desiccation resistance, including extended egg and larval diapause stages. The short flow duration of intermittent reaches (<100 days) likely precluded upstream perennial taxa from establishing populations in downstream intermittent reaches before drying occurred, while the relative predictability of flow timing (Dec-Apr) likely allowed for a small number of species to develop appropriate life-history traits (e.g., diapause stage, rapid development time) to exploit these temporally-fleeting habitats. In Chapter 5, I found over 220 species of aquatic animals (including ≥ 5 undescribed species) in the 19 desert oases that were sampled across the southern Sonoran Desert. Local community composition in these oases was strongly driven by microhabitat type. Additionally, native aquatic species richness and abundance in these oases were significantly reduced by the introduction of tilapia, an exotic fish species. The threats to arid-land streams presented by increased drought severity, anthropogenic water withdrawals, and local habitat degradation (e.g., introduced species, unmanaged recreational use) are grave across the southwestern US and northwestern Mexico. I hope that in addition to furthering our understanding of ecological processes in arid-land streams, this dissertation makes a small contribution towards the efforts to preserve these habitats.