Browsing by Subject "vegetation"
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Item An Analysis of Self-similarity, Momentum Conservation and Energy Transport for an Axisymmetric Turbulent Jet through a Staggered Array of Rigid Emergent Vegetation(2013-05-29) Allen, Jon ScottMarsh vegetation is widely considered to offer protection against coastal storm damage, and vegetated flow has thus become a key area of hydrodynamic research. This study investigates the utility of simulated Spartina alterniora marsh vegetation as storm protection using an ADV measurement technique, and is the first to apply jet self-similarity analysis to characterize the overall mean and turbulent flow properties of a three-dimensional axisymmetric jet through a vegetated array. The mean axial flow of a horizontal axisymmetric turbulent jet is obstructed by three configurations of staggered arrays of vertical rigid plant stems. The entire experiment is repeated over five sufficiently high jet Reynolds number conditions to ensure normalization and subsequent collapse of data by nozzle velocity so that experimental error is obtained. All self-similarity parameters for the unobstructed free jet correspond to typical published values: the axial decay coefficient B is 5:8 +/- 0:2, the Gaussian spreading coefficient c is 85 +/- 5, and the halfwidth spreading rate eta_(1/2) is 0:093 +/- 0:003. Upon the introduction of vegetation, from partially obstructed to fully obstructed, B falls from 5:1+/- 0:2 to 4:2 +/- 0:2 and finally 3:7 +/-0:1 for the fully obstructed case, indicating that vegetation reduces axial jet velocity. Cross-sectionally averaged momentum for the unobstructed free jet is M=M0 = 1:05 +/- 0:07, confirming conservation of momentum. Failure of conservation of momentum is most pronounced in the fully obstructed scenario ? M=M0 = 0:54 +/- 0:05. The introduction of vegetation increases spreading of the impinging jet. The entrainment coefficient alpha for the free jet case is 0.0575; in the fully obstructed case, alpha = 0:0631. Mean advection of mean and turbulent kinetic energy demonstrates an expected reduction in turbulence intensity within the vegetated array. In general, turbulent production decreases as axial depth of vegetation increases, though retains the bimodal profile of the free jet case; the fully vegetated case, however, exhibits clear peaks behind plant stems. Turbulent transport was shown to be unaffected by vegetation and appears to be primarily a function of axial distance from the jet nozzle. An analysis of rate of dissipation revealed that not only does the cumulative effect of upstream wakes overall depress the magnitude of spectral energy density across all wavenumbers but also that plant stems dissipate large anisotropic eddies in centerline streamwise jet flow. This study, thus, indicates that sparse emergent vegetation both reduces axial flow velocity and has a dissipative effect on jet flow. Typically, however, storm surge does not exhibit the lateral spreading demonstrated by an axisymmetric jet; therefore, the results of this study cannot conclusively support the claim that coastal vegetation reduces storm surge axial velocity.Item Application of Remote Sensing Technology and Ecological Modeling of Forest Carbon Stocks in Mt. Apo Natural Park, Philippines(2015-01-23) Leal, Ligaya RubasThis dissertation work explored the application of remote sensing technology for the assessment of forest carbon storage in Mt. Apo Natural Park. Biomass estimation is traditionally conducted using destructive sampling with high levels of uncertainty. A more accurate and non-destructive method for assessment of biomass level is imperative to characterize remaining forest cover. This research study aimed to: 1) examine the vegetation profile and estimate species-specific biomass of Mt. Apo Natural Park, 2) develop an algorithm to assess biomass in plot-level using a terrestrial lidar system (TLS), and 3) generate landscape-level biomass estimates using interferometric synthetic aperture radar (IFSAR). This research endeavors to provide answers to these questions: 1) how the 3 tropical allometries compare in estimating field collected species-level biomass and carbon stocks in three management zones?, 2) what are the significant terrestrial laser scanning-derived metrics to assess plot-level biomass?, and 3) to what degree of uncertainty can IFSAR estimate biomass at the landscape level? Field data was gathered from 1382 trees, covering 52 local species during fieldwork in July and August 2013. Twenty-six plots (30 m x 30 m) were sampled on three management zones: multiple use, strict protection and restoration. Local insurgency problems restricted the research team from sampling additional plots. Destructive sampling was not permitted inside the protected area, thus requiring biomass to be estimated via the use of referenced biomass from 3 allometric equations by relating tree height, diameter-at-breast height, and wood specificity volume. A vegetation profile across the park was generated using a canopy height map (CHM). Results showed that resampled IFSAR products can be used to characterize biomass and carbon storage at the landscape level. This research has demonstrated the adoption of IPCC?s Tier 2, a combination of field and remote sensing data in the assessment of available biomass levels in a tropical forest. The maps created can assist in providing information for biomass and carbon level in MANP for monitoring, reporting and verification in compliance with REDD requirements. Furthermore, this study can provide helpful information regarding policy implications for reforestation and afforestation activities. Results showed that resampled IFSAR products can be used to characterize biomass and carbon storage at the landscape level. This research has demonstrated the adoption of IPCC?s Tier 2, a combination of field and remote sensing data in the assessment of available biomass levels in a tropical forest. The maps created can assist in providing information for biomass and carbon level in MANP for monitoring, reporting and verification in compliance with REDD requirements. Furthermore, this study can provide helpful information regarding policy implications for reforestation and afforestation activities.Item Effects of Hurricane Katrina on the Mammalian and Vegetative Communities of the Barrier Islands of Mississippi(2010-01-14) Scoggin, Annaliese K.The barrier islands of the gulf coast of the U.S. have been shaped and changed by hurricanes for centuries. These storms can alter the vegetation of the barrier islands by redistributing sediments, scouring off vegetation, physical damage to the plants, and by salt stress following the storm. Hurricanes also alter the mammal communities of the barrier islands through direct mortality and by altering vegetative communities. It is important to understand how the vegetation of barrier islands recovers after major hurricanes because the vegetation provides the structure that maintains and builds these islands. Following the landfall of Hurricane Katrina in August of 2005, I studied the changes in the herbaceous ground cover and the density of woody plants in Gulf Islands National Seashore in Mississippi from the winter of 2005 to the summer of 2007. Growth from existing plants and seed banks quickly revegetated the islands after the storm. The amount of live ground cover increased and bare ground decreased on each island and in every vegetation type. Most woody plant species also showed a net increase in density, with the exception of pine (Pinus elliottii) and Florida rosemary (Ceratiola ericoides). The regeneration of woody species and the uniform increase in the live ground cover seemed to indicate that the vegetation of the islands was not irreversibly impacted. I also studied the changes in the composition of mammal populations in Gulf Islands National Seashore from the winter of 2005 to the summer of 2007. Prior to the storm 11 terrestrial mammal species were recorded in studies of the barrier islands. In the 2 years following Hurricane Katrina, I recorded only 1 of the 7 species on Cat Island, 5 of the 9 species on Horn Island and 2 species each on East Ship, West Ship, and Petit Bois Islands (which previously had 4, 4, and 2 each). Populations of mammals that used multiple vegetation types (raccoons [Procyon lotor], nutria [Myocastor coypus], and eastern cottontail [Sylvilagus floridanus]) seemed to show more tolerance to hurricane disturbance than more specialized species (black rat [Rattus rattus], marsh rice rat [Oryzomys palustris]). I also recorded at least one colonization event by river otter (Lutra canadensis), a species not recently recorded on the islands. This research serves as a baseline for future comparison following similar storms.Item Laboratory experiments and numerical modeling of wave attenuation through artificial vegetation(2009-05-15) Augustin, Lauren NicoleIt is commonly known that coastal vegetation dissipates energy and aids in shoreline protection by damping incoming waves and depositing sediment in vegetated regions. However, this critical role of vegetation to dampen wave forces is not fully understood at present. A series of laboratory experiments were conducted in the Haynes Coastal Laboratory and 2-D flume at Texas A&M University to examine different vegetative scenarios and analyze the wave damping effects of incident wave height, stem density, wave period, plant type, and water depth with respect to stem length. In wetland regions vegetation is one of the main factors influencing hydraulic roughness. Traditional open-channel flow equations, including the Manning and Darcy- Weisbach friction factor approach, have been successfully applied to determine bottom friction coefficients for flows in the presence of vegetation. There have been numerous relationships derived relating the friction factor to different flow regime boundary layers to try and derive a wave friction factor for estimating energy dissipation due to bottom bed roughness. The boundary layer problem is fairly complex, and studies relating the wave friction factor to vegetation roughness elements are sparse. In this thesis the friction factor is being applied to estimate the energy dissipation under waves due to artificial vegetation. The friction factor is tuned to the laboratory experiments through the use of the numerical model COULWAVE so that the pipe flow formulation can be reasonably applied to wave problems. A numerical friction factor is found for each case through an iterative process and empirical relationships are derived relating the friction factor for submerged and emergent plant conditions to the Ursell number. These relationships can be used to reasonably estimate a wave friction factor for practical engineering purposes. This thesis quantitatively analyzes wave damping due to the effects of wave period, incident wave height, horizontal stem density, water depth relative to stem length, and plant type for a 6 m plant bed length. A friction factor is then determined numerically for each of the laboratory experiments, and a set of equations is derived for predicting a roughness coefficient for vegetation densities ranging between 97 stems/m2 and 162 stems/m2.Item Study of Kinematics of Extreme Waves Impacting Offshore and Coastal Structures by Non Intrusive Measurement Techniques(2013-11-07) Song, Youn KyungExtreme wave flows associated with a large scale wave breaking during interactions with marine structures or complex coastal geography of is one of the major concerns in a design of coastal and ocean structures. In order to properly understand the impact mechanisms of breaking extreme waves, full field evaluations of impacting multiphase flow velocities should be properly conducted first. In this context, this present dissertation experimentally investigated velocity structures of turbulent, multiphase wave flow velocities during active interactions with various offshore and onshore ocean environments. First, initial inundation flow structures of tsunami-like long waves interacting with complex coastal topography are experimentally investigated. Turbulent wave surface velocities were effectively measured by introducing a non-intrusive video imagery technique, the ?wave front tracing method?. Three distinctive configurations for patch layouts that vary either in characteristic patch diameter (D) or in center-to-center spacing between patches (?S) were employed. That is, patch layouts consisted of six (G1) and twelve (G2), ?small? circular macro roughness patches of D =1.2 m and six, ?large? circular macro roughness patches (G3) of D = 1.7 m were employed, respectively. A patch layout employed for G1 appears to be effective in reducing the u velocities along the centerlines of the reference patch that consistently decreased to 85% of a convergence velocity U = 2m/s and to 45% of U. However, in the channel, u velocities hardly reduced below the convergence velocity. On the other hand, the patch layout G2 is observed as rather effective in uniformly reducing the u velocities alongshore. The hand, the patch layout G3 is observed as effective in suppressing the alongshore variability in flow behind the frontal patches. This may be due to the "holding-up" effects produced by the large patches holding the flow within the patch for a longer duration. Furthermore, such a "holding-up" effect from G3 appears to induce a large inundation depth in the flow along the opening. Next, green water velocities and dynamic impacts of the extreme ocean waves on a fixed offshore deck structure are investigated. The experiments focused on the impacting waves generated in a large-scale, three-dimensional ocean wave basin. Using the BIV technique, overall flow structures and temporal and spatial distributions of the maximum velocities were successfully evaluated. The most significant spatial variability in mean velocities in the propagating direction was found from the protruding wave front near the center of the deck during early stages of the wave run-up. The maximum front speed of 1.4C was first observed in the center of the deck near y = 0 at a midpoint of the deck (x = 0.5L), where C is the wave phase speed. The flow velocities started decreasing below 1C over all fields once the wave frontal flow passed the rear edge and started leaving the deck. Pressure measurements were also conducted at four different vertical positions on vertical measurement planes at three different locations on the horizontal plane. Most of measured pressures showed impulsive impact patterns with sudden rises of pressure peaks. The highest pressure was observed as 1.56pC^(2) at x = L/2. Correlations between wave kinematic energy and dynamic pressure were examined to determine the impact coefficients ci'. ci' varied within relatively narrow ranges 0.29 ? ci' ? 1.56. In the present large scale experiments, the impact pressures on the structures are strongly affected by both variability of flow structures and impulsiveness of impacting waves containing considerable air volumes. Lastly, the study is extended for more violent sloshing wave flows. The study experimentally investigated flow kinematics and impact pressures of a partially filled liquid sloshing flow during the periodic longitudinal motion of a rectangular tank. The horizontal velocities near the free surface reached 1.6C with C being the wave phase speed calculated based on the shallow water assumption. As the tank reached its maximum displacement and about to reverse, the dominant flow changed its direction rapidly to vertical upward after the breaking wave crest impinging on the side wall and forming an up-rushing jet. The vertical velocity of the rising jet reached 3.4C before it impacted the top wall. During the flip-through event as the fast moving wave crest collided with the side wall, the steep wave crest resulted in a focused impact on the side wall at the SWL. The resulting impulsive peak pressure was recorded as about 10gh? immediately followed by the evident pressure oscillation with a frequency approximately 500 Hz. After the wall impact, the multiphase up-rushing jet shot up and impacted the top wall. The magnitude of the pressure was again about 10gh?, similar to that recorded by the breaking wave impact on the side wall. Correlating the dynamic impact pressures with the corresponding local maximum flow velocities in the direction normal to the walls was performed by introducing the impact coefficient ic and the modified impact coefficient c'_(i) , defined as p_(max)=c_(i)pV^(2)= c'_(i)pC^(2) with V_(max) being the magnitude of the maximum local velocities. The average values of the modified impact coefficient c?_(i) between the side wall impacts and the top wall impacts were nearly identical, with the average value of c'_(i)=5.2.