Thesis advisor: Noah P. Snyder / Dam-influenced streambank morphology has not been studied extensively in rivers in deglaciated landscapes with high densities of colonial-era milldams. Fluvial restoration in the eastern U.S. often focuses on understanding pre-Colonial floodplain processes. Recent work by Walter and Merritts (2008a) in the Piedmont of the U.S. mid-Atlantic region suggests milldams significantly impact sedimentation by creating surfaces composed of post-dam legacy sediment that are often abandoned by the river and function as fill terraces. I analyze channel morphology and sedimentation patterns upstream of two breached dams on the Sheepscot River in mid-coastal Maine using lidar digital elevation models, historical aerial photographs, radiocarbon dating, and hydraulic modeling. In the past several decades, observable channel morphologic changes occurred at the two study sites: Maxcy's Mills dam (built in 1809, it was 2-m high and breached in the late 1950s), and at Head Tide dam (built in the 1760s, it is 4-m high and was partially breached in 1952). The Sheepscot River has a native population of Atlantic salmon, which is a federally listed endangered species. Understanding the existence and transport of legacy sediment has become an important component of habitat restoration efforts in the region. The goal of this investigation is to determine the extent and morphologic function of legacy sediment in order to better understand how historical dam sites affect channel morphology and sediment transport in a post-glacial, low-gradient river system. Field and remote sensing analyses indicate that surfaces (up to 2.65 m high) composed of mud and sand function as floodplains 1 km upstream from Head Tide dam and surfaces 90 cm high continue 2.5 km upstream from Maxcy's Mills. Analysis of seven radiocarbon dates from pieces of tree bark sampled from the stratigraphy (58-187 cm below the surface) of the two study sites suggest up to 1.8 m of sediment upstream of the two study sites was deposited within the past 300 years and is therefore a legacy of the dams. Quantification of the total volume of stored legacy sediment is on the same scale as volumes observed in the mid-Atlantic Piedmont region, leading to the conclusion that post-glacial rivers in northern New England store milldam sediment in similar fashion to streams analyzed in the Walter and Merritts (2008a) study. / Thesis (MS) — Boston College, 2013. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Earth and Environmental Sciences.

O solos saproliticos sÃo encontrados no fundo das escavaÃÃes de jazidas que sÃo normalmente usadas na construcao de barragens de terra. NÃo sÃo normalmente utilizados pelo desconhecimentos de suas propriedades adequadas para gerar uma estrutura de vedaÃÃo. Em algumas situaÃÃes por inexistÃncias de solos residuais maduros suficientes para construÃÃo da barragem ou por questÃes de distancia e possÃvel construir a barragem com solos pouco desenvolvidos (solos residuais jovens). Este foi a motivaÃao para realizaÃÃo deste pesquisa. Foram realizados todos os ensaios de caracterizaÃÃo e mecÃnicos em amostras solos jovens (saproliticos) e a partir do conhecimento de suas propriedades foram realizadas simulaÃÃes numÃricas para demonstrar ser possÃvel construir pequenas barragens com solo residual jovem com o atendimento das questoes relativas a seguranÃa e a reduÃÃo provavel de custos. Os resultados experimentais e numÃricos comprovaram essa possibilidade / The saprolite soils are found on the bottom of the excavation of deposits that are commonly used in the construction of earth dams. Are not normally used by the unknowns of their suitable properties to generate a sealing structure. In some situations by inexistÃncias sufficient mature residual soils for construction of the dam or distance issues and possible to build the dam with poorly developed soils (young residual soils). This was the motivation for conducting this research. All mechanical and characterization tests on soil samples young (saprolite) and from knowledge of its properties numerical simulations were performed to demonstrate possible to build small dams with young residual soil with the care of the issues related to safety and reduction were performed probable costs. The experimental and numerical results confirmed this possibility

This research investigated the factors influencing the structure of the phytoplanktori community in Chaffey Dam, which is located in sub-tropical Australia. In particular, the research aimed to determine the influence of light at time scales ranging from seconds to seasons, on the growth and photophysiology of the cyanobacterium Anabaena circinalis. On a large scale, field monitoring programs between 1987 and 1997 indicated that the phytoplankton community of Chaffey Dam was dominated by colonial or relatively large phytoplankton that move either with the aid of flagella or can be positively buoyant. Diatoms contributed only a minor component, which may be the result of the reservoir being stratified for much of the year. Several of the dominant taxa bloomed in each of the seasons during the eleven year period, with some blooms lasting >9 months, indicating that environmental variability between seasons can be low. In contrast to other studies, A. circinalis was more likely to grow and bloom during the cooler months (March-October). A two-year intensive monitoring program (1995-1997) identified a seasonal progression that was similar in both years. Chlorophytes occurred in spring, Ceratium in mid summer, a relatively clear period in February, A. circinalis in March and cryptomonads in winter. On a smaller scale, short-term (2-3 day) in-situ and laboratory enclosure experiments found that the light and nutrient requirements of the dominant taxa varied. In comparison to most other phytoplankton, A. circinalis cells disappeared at very rapid rates when supplied irradiances <10 (umol photons m-2 s-1. Over several days of darkness, the filaments broke apart and the cell numbers declined. The experiments also showed that at certain times, field populations of A. circinalis were subject to high losses at all irradiances. Laboratory studies investigating the influence of inter- and intra-daily changes in light availability showed that the growth rate of A. circinalis was not affected by the frequency of daytime light:dark cycles, indicating that the rate of water mixing will not have major influence on its growth if the total daily light dose is maintained. It was also found that A. circinalis cultures did not accumulate large reserves of energy in the form of carbohydrate, other than that required for one night. This strategy may enable the colonies to have a high level of buoyancy each morning so that they float quickly to the surface waters and obtain sufficient light each day to minimise losses. However, this strategy limits the ability of A. circinalis to grow and maintain vital cell processes during extended periods of low irradiances and may be a factor causing them to be susceptible to cell breakdown. Weekly measurements of algal growth rates in Chaffey Dam identified two factors that may have acted singly or simultaneously to influence the development of A. circinalis blooms during 1996 and 1997. The blooms developed during a 4-6 week period when the mean irradiance in the surface mixed layer (SML) was sufficient to prevent high losses. Secondly, the blooms developed when soluble phosphorus in the epilimnion was relatively high but soluble nitrogen was low. This may have favoured A. circinalis, which has the potential to fix atmospheric nitrogen. The decline of A. circinalis blooms was correlated with a deepening of the SML and a reduction of the mean daytime irradiance within the SML. Their decline did not appear to be related to nutrient limitation or to changes in zooplankton concentrations. This research also developed a physiological technique for tracking daily changes in the mean daytime irradiance of A. circinalis and for estimating cell growth rate. This method is based on chlorophyll-a fluorescence quenching analysis of the state transition mechanism, which regulates light availability between the photosystems. The mean daytime irradiance of A. circinalis showed a strong relationship with the degree of non-photochemical quenching (qn), whereas the relative change to the maximum fluorescence showed a strong relationship with cell growth. It is anticipated that this method will provide a useful research tool for determining the relative importance of light and other factors on the net growth of A. circinalis and other cyanobacteria.

When a semi-infinite body of homogeneous fluid initially at rest behind a vertical retaining wall is suddenly released by the removal of the barrier the resulting flow over a horizontal or sloping bed is referred to as a dam-break flow. When resistance to the flow is neglected the exact solution, in the case of a stable horizontal bed with or without `tail water', may be obtained on the basis of shallow-water theory via the method of characteristics and the results are well known. Discrepancies between these shallow-water based solutions and experiments have been partially accounted for by the introduction of flow resistance in the form of basal friction. This added friction significantly modifies the wave speed and flow profile near the head of the wave so that the simple exact solutions no longer apply and various asymptotic or numerical approaches must be implemented to solve these frictionally modified depth-averaged shallow-water equations. When the bed is no longer stable so that solid particles may be exchanged between the bed and the water column the dynamics of the flow becomes highly complex as the buoyancy forces vary in space and time according to the competing rates of erosion and deposition. Furthermore, when the Froude number of the flow is close to unity perturbations in the height and velocity profiles grow into N-waves and the bed below develops ripples which act to sustain the N-waves in the fluid above. It is our intention here to study dam-break flows over erodible sloping beds as agents of sediment transport taking into account basal friction as well as the effects of particle concentrations on flow dynamics including both erosion and deposition. We shall consider shallow flows over initially dry beds and investigate the effects of changes in the depositional and erosional models employed as well as in the nature of the drag acting on the flow and the slope of the bed. These models include effects hitherto neglected in such studies and offer insights into the transport of sediment in the worst case scenario of the complete and instantaneous collapse of a dam. / Mathematics

When a semi-infinite body of homogeneous fluid initially at rest behind a vertical retaining wall is suddenly released by the removal of the barrier the resulting flow over a horizontal or sloping bed is referred to as a dam-break flow. When resistance to the flow is neglected the exact solution, in the case of a stable horizontal bed with or without `tail water', may be obtained on the basis of shallow-water theory via the method of characteristics and the results are well known. Discrepancies between these shallow-water based solutions and experiments have been partially accounted for by the introduction of flow resistance in the form of basal friction. This added friction significantly modifies the wave speed and flow profile near the head of the wave so that the simple exact solutions no longer apply and various asymptotic or numerical approaches must be implemented to solve these frictionally modified depth-averaged shallow-water equations. When the bed is no longer stable so that solid particles may be exchanged between the bed and the water column the dynamics of the flow becomes highly complex as the buoyancy forces vary in space and time according to the competing rates of erosion and deposition. Furthermore, when the Froude number of the flow is close to unity perturbations in the height and velocity profiles grow into N-waves and the bed below develops ripples which act to sustain the N-waves in the fluid above. It is our intention here to study dam-break flows over erodible sloping beds as agents of sediment transport taking into account basal friction as well as the effects of particle concentrations on flow dynamics including both erosion and deposition. We shall consider shallow flows over initially dry beds and investigate the effects of changes in the depositional and erosional models employed as well as in the nature of the drag acting on the flow and the slope of the bed. These models include effects hitherto neglected in such studies and offer insights into the transport of sediment in the worst case scenario of the complete and instantaneous collapse of a dam. / Mathematics

The dangerous hydraulic conditions that can form downstream of a low-head dam were investigated. These dangerous hydraulic conditions have been the cause of hundreds of drowning incidents since the construction of the first low-head dams. Two primary objectives were identified for this study, each of which were primarily performed using the Computational Fluid Dynamics software, Flow-3D®, with physical models used to verify the numerical results. The first objective was the identification of a risk factor made up of easily measured parameters that could accurately predict when the dangerous hydraulic conditions are present at a low-head dam. The risk factor that was found to achieve this objective was calculated as (hu - hd)/P, where hu and hd are the upstream and downstream water depths, respectively, and P is the dam height. For the flat-topped dams tested, the dangerous condition was present within the range of risk factors from 0.343 to 0.708. For the ogee-crested dams tested, the dangerous conditions were present between risk factors of 0.093 and 0.798. The second objective was to identify possible remediation options that would be capable of eliminating the dangerous hydraulic conditions, therefore reducing risk to the public. It was also desired to keep the options easily and inexpensively implemented. Two different remediation options were found to this end, and consisted of either upstream facing ramps spaced along the width of the channel below a low-head dam, or spaced platforms protruding from the downsteam face of the dam slightly below its crest. Three different designs of each configuration were tested, with those for the ramp configuration being identified as R1, R2, and R3. The platform designs were identified as P1, P2, and P3. The options were evaluated based on how long it took for human dummies introduced into the flow to pass through the high risk region of the simulations, with the maximum allowed time being 50 seconds. Any test in which a dummy remained in the danger region for longer than 50 seconds was deemed ineffective. The option found to perform the best was the P2 design, which had an overall performance time of about 17.4 seconds.

Field and modelling investigations of eight failed or removed dams have been undertaken to examine the upstream effects of low head dam decommissioning on channel morphology. Failed or decommissioned sites were selected such that no upstream interventions or channel mitigation had been applied since the time of decommissioning resulting in a physically-based analog consistent with the passive dam removal restoration approach. Field surveys of the sites, which failed between 2 years and 70 years ago, included longitudinal profiles, cross-sections and bed material pavement sampling on each riffle, run, and headcut. Findings demonstrate that vertical disturbances typically in the form of headcuts frequently extend well beyond the backwater limits of most reservoirs. Although in most cases, critical velocity and shear stress thresholds were exceeded, the localized increases in friction slope where headcuts occurred demonstrated that the velocities associated with larger flows exceeded critical thresholds more often than critical shear stress thresholds. Findings show that if the grain size distributions of the underlying alluvial geologic units are close to that of critical velocity thresholds, when headcuts are initiated (with their resulting increase in friction slope), they can result in continued channel degradation upstream of impoundment regions.

Field and modelling investigations of eight failed or removed dams have been undertaken to examine the upstream effects of low head dam decommissioning on channel morphology. Failed or decommissioned sites were selected such that no upstream interventions or channel mitigation had been applied since the time of decommissioning resulting in a physically-based analog consistent with the passive dam removal restoration approach. Field surveys of the sites, which failed between 2 years and 70 years ago, included longitudinal profiles, cross-sections and bed material pavement sampling on each riffle, run, and headcut. Findings demonstrate that vertical disturbances typically in the form of headcuts frequently extend well beyond the backwater limits of most reservoirs. Although in most cases, critical velocity and shear stress thresholds were exceeded, the localized increases in friction slope where headcuts occurred demonstrated that the velocities associated with larger flows exceeded critical thresholds more often than critical shear stress thresholds. Findings show that if the grain size distributions of the underlying alluvial geologic units are close to that of critical velocity thresholds, when headcuts are initiated (with their resulting increase in friction slope), they can result in continued channel degradation upstream of impoundment regions.

Dams have altered flow distributions in rivers everywhere, causing a host of changes in channel morphology and sediment dynamics. Although major changes in flow regime have occurred along Yegua Creek, Texas, since the closure of Somerville Dam in 1967, the issue of sediment transport has not been studied in detail. The extent to which sediment is moving through the system remains unclear. This study addresses the extent to which sediment is moving through and downstream of the dam. Analysis of sediment samples collected at 23 sites in the Yegua Creek channel system showed that coarse sand to silt-sized materials dominate the creek upstream of the dam, whereas finer silt and clay sediments characterize the downstream portions. Calculation of the trapping efficiency of the dam indicates that approximately 99.8% of materials from the upper watershed are trapped behind Somerville Dam. Investigations of sediment mobility further suggest that present flows are capable of mobilizing sediments downstream of the dam. Although a de-coupling between the upper and lower portions of the Yegua Creek watershed has likely occurred due to the high rate of sediment trapping, new sediment sources that include tributaries and alluvial storage likely play a larger role in providing materials for sediment transport downstream. Despite a reuction in peak flows, the channel morphology of Yegua Creek has apparently adjusted over the four decades since construction of Somerville Dam to achieve a new equilibrium characterized by sediment movement. These results are corroborated by analysis of aerial photographs. These findings augment our understanding of the many facets of the response of fluvial systems to the disturbance posed by dam construction. Because Yegua Creek is a major tributary to the Brazos River draining to the Texas coast, increased understanding of sediment dynamics within Yegua Creek provides critical insights into the efficacy of sediment delivery in a regional context, and ultimately to the Texas coastline. The findings of this study also provide useful information for managing stream ecosystems affected by impoundments.

Multi-purpose project requirements lead to construction of large dams. In order to maintain the desired safety level of such dams, comprehensive inspections based on use of a number of precise instruments are needed. The ideal dam instrumentation system should provide time-dependent information about critical parameters so that possible future behavior of the structure can be predicted. New dams are normally equipped with adequate instrumentation systems. Most of the existing dams, however, do not have adequate instruments or current instrumentation systems may not be in good condition. By implementing the modern equipment to existing dams, the uncertainty associated with the impacts of aging or unexpected severe external events will be reduced and possible remedial measures can be taken accordingly. This study summarizes the major causes of dam failures and introduces the instruments to be used to monitor the key parameters of a dam. The concept of the instrument retrofitting to an unmonitored dam is highlighted through a case study. A sample system is proposed for G&ouml / k&ccedil / ekaya Dam, with reference to an investigation of the current condition of the structure. The deficiencies observed during a site visit are listed and the corresponding rehabilitative repair measures are suggested. Finally, different alternatives of a new instrumentation system are introduced and compared in terms of technical and economical aspects.