Thesis. 1977. Sc.D.--Massachusetts Institute of Technology. Dept. of Civil Engineering. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING. / Vita. / Bibliography : leaves 209-217. / Sc.D.

Nas últimas décadas, a praia de Massaguaçu vem sofrendo processos erosivos intensos, cujos segmentos possuem taxas de recuo acentuadas, denominadas Zonas de Erosão Acentuada (ZEAs). Partindo deste problema este trabalho se propôs a entender a dinâmica sedimentar do arco praial de Massaguaçu e das possíveis causas de formação e manutenção das ZEAs. A coleta de dados se baseou no levantamento batimétrico e do clima de ondas, bem como no levantamento sonográfico de varredura lateral e na coleta de amostras de sedimento de superfície da antepraia de Massaguaçu. Para processar os dados se optou pela aplicação da modelagem numérica utilizando o modelo SWAN (propagação de ondas em águas profundas) e o modelo SMC (módulos OLUCA, COPLA e EROS, que trabalham, respectivamente, com propagação de ondas em águas rasas, simulação de correntes e simulação de transporte sedimentar). Para a formulação da discussão foram selecionados os 6 (seis) casos que apresentaram as condições de ondas mais representativas. Tais casos mostraram que a dinâmica sedimentar e os processos costeiros responsáveis pela formação e manutenção das ZEAs são controlados por dois padrões distintos de ondas incidentes. O primeiro padrão está relacionado às ondas vindas de NE - E, que geram correntes longitudinais para o sul. Suas correntes longitudinais e transversais à costa são de baixa intensidade resultando em um transporte sedimentar e em variações da morfologia praial menos intensos, além de formar células de circulação em vórtices. O segundo padrão, por sua vez, está relacionado às ondas oriundas SSE - SE - ESSE, cujas correntes litorâneas longitudinais rumam ao norte. Tais correntes longitudinais e transversais à costa, apresentam uma intensidade maior, portanto o transporte sedimentar e as variações da morfologia ao longo do arco praial são maiores bem como as células de circulação em vórtices. Partindo desses resultados, percebeu-se que as ZEAs em Massaguaçu estão ligadas a um possível aumento na frequência e na intensidade das tempestades ao longo das últimas décadas e até o presente momento, não há mecanismos naturais que revertam esta retração da linha de costa. / Over the past decades in Massaguaçu beach has happening a strong erosion process with segments that have higher retreat rates, commonly termed as Erosional Hot Spots (EHSs). This research aims the understanding of the sedimentary dynamics along the Massaguaçu embayment and also the possible causes of the formation and maintenance of the EHSs. The methodology was separately on two steps. At first we collected bathymetry, wave and side-scan sonar data, and samples of the surface sediments from the shoreface of Massaguaçu beach. Later, to analyze majority data, we chose to use numerical modeling based on the SWAN model (propagation of waves in deep water) and SMC model (modules OLUCA, COPLA and EROS, that work respectively with the propagation of waves in shallow water, the simulation of currents and finally the simulation of sediment transport). After modeling, we were able to select the 6 (six) most representative wave conditions cases. Those results show that the control of the sediment dynamics and the coastal processes responsible for the formation and the maintenance of ZEA are based on the characteristics of the incident waves. On trying to explain that process, it was possible to recognize two distinct patterns. The first pattern observed is associated with the NE - E waves, generating longshore currents to the south. In this case, both the longshore and the crosshore currents are less intense, forming cells circulation in vortex, resulting in minor variations in sediment transport and beach morphology. The second pattern is related with the SSE - SE - ESE waves, generating longshore currents to the north. In this case both the longshore and the crosshore currents are more intense, although still with occurrence of forming cells circulation in vortex, resulting in a larger sediment transport and morphological changes along the beach. Finally, what we have seem was that the ZEAs in Massaguaçu are possibly linked to a potential increment on storms frequency and intensity over the past decades and unfortunately, at present, there is still no mechanism to recover this natural retreat of the coastline.

Extreme storms are responsible for rapid changes to coastlines worldwide. During the 2013/14 winter, the west coast of Europe experienced a sequence of large, storm-induced wave events, representing the most energetic period of waves in the last 60 years. The southwest coast of England underwent significant geomorphological change during that period, but exhibited a range of spatially variable and complex morphological responses, despite being subjected to the same storm sequence. The 2013/14 storm response along the southwest coast of England was first used as a natural field laboratory to explain the variability in storm response through the introduction and evaluation of a new classification of how sandy and gravel beaches respond to extreme storms. Cluster analysis was conducted using an unique data set of pre- and post-storm airborne Light Detection and Ranging (LiDAR) data from 157 beach sites and the calculation of volumetric beach changes and a novel parameter, the longshore variation index which quantifies the alongshore morphological variability in beach response. The method used can be applied to any sandy and gravel beaches where topographic data with sufficient spatial resolution is available. Four main beach response types were identified that ranged from large and alongshore uniform offshore sediment losses up to 170 m3 m-1 (at exposed, cross-shore dominated sites) to considerable alongshore sediment redistribution but limited net sediment change (at more sheltered sites with oblique waves). The key factors in determining the type of beach response are: exposure to the storm waves, angle of storm wave approach and the degree to which the beach is embayed. These findings provide crucial information for the development of coastal studies at regional scale, especially along coastal areas where abrupt changes in coastline orientation can be observed. A 10-year time series (2007-2017) of supra- and intertidal beach volume from exposed and cross-shore transport-dominated sites was used to examine the extent to which beach behaviour is coherent over a relatively large region (100-km stretch of coast) and predictably coupled to incident wave forcing. Over the study period, 10 beaches, exposed to similar wave/tide conditions, but having different sediment characteristics, beach lengths and degrees of embaymentisation, showed coherent and synchronous variations in sediment volumes, albeit at different magnitudes. This result is crucial for studying coastal changes in remote coastal areas or in areas where only few topographic data are available. The sequence of extreme storms of the 2013/14 winter, which represents the most erosive event over at least a decade along most of the Atlantic coast of Europe, is included in the data set, and three years after this winter, beach recovery is still on-going for some of the 10 beaches. Post-storm beach recovery was shown to be mainly controlled by post-storm winter wave conditions, while summer conditions consistently contributed to modest beach recovery. Skilful hindcasts of regional changes in beach volume were obtained using an equilibrium-type shoreline model, demonstrating that beach changes are coherently linked to changes in the offshore wave climate and are sensitive to the antecedent conditions. Furthermore, a good correlation was found between the beach volume changes and the new climate index WEPA (West Europe Pressure Anomaly), which offers new perspectives for the role and the use of climatic variations proxies to forecast coastline evolution. A process based model, XBeach, was used to model storm response at one macrotidal beach characterized by the largest sediment losses during the 2013/14 sequence of extreme storms. Beach volume changes were modelled over hypothetical scenarios with varying hydrodynamics conditions and beach states to investigate the relative roles of hydrodynamic forcing (i.e., waves and tides), beach antecedent state and beach-dune morphology in beach response to extreme storms. This modelling approach is applicable to any beach system where process based models have been implemented. Beside significant wave height and peak wave period, the beach antecedent state was shown to be the dominant factor in controlling the volumes of sediment erosion and accretion along this cross-shore dominated beach. Modelled volumes of erosion were, on average, up to three times higher along an accreted beach compared to an eroded beach for the same wave conditions. The presence of a dune, being only significantly active during spring tides and storm conditions along this macrotidal beach, was shown to reduce erosion or even cause accretion along the intertidal beach. This work provides a detailed, quantitative insight of the hydrodynamic and morphological processes involved in storm response and beach recovery on a number of spatial and temporal scales. This improved understanding of the potential impact of extreme events will hopefully aid future research efforts and ensure effective management of sedimentary coastlines.

Each year storms impact coastal areas, sometimes causing significant morphologic change. Cold fronts are associated with increased wave energy and frequently occur during the winter months along many coasts, such as the Atlantic and Gulf of Mexico. The higher wave energy can be responsible for a large quantity of the sediment transport resulting in rapid morphologic change. Using streamer traps, the vertical distribution of onshore-directed sediment transport during two different cold fronts on two low-wave energy beaches (i.e., along the northern Yucatan and southeast Florida) were compared with the resulting morphologic change. The objectives of this study are to: 1) analyze the grain size distribution (statistics) of sediment transported during a cold front, 2) compare the vertical sediment distribution throughout the water column, and 3) compare characteristics of bed sediment to the sediment within the water column. Understanding the changing grain size distribution of bottom sediments in comparison to directional transport (throughout the water column) should help determine the sediment fraction(s) being eroded or deposited, which could greatly improve predictions of storm-induced morphology change. / Includes bibliography. / Thesis (M.S.)--Florida Atlantic University, 2017. / FAU Electronic Theses and Dissertations Collection

The aim of this thesis is to understand the role of geomorphological change in the abandonment of Norse harbours in the North Atlantic. Nodes of maritime activities that were established by Norse settlers during the Scandinavian Viking Age often developed into important towns and cities. Some of these, however, disappeared for unknown reasons. Norse harbours in the North Atlantic varied in scale. They ranged from small landing beaches used by small boats for local use through to much larger anchorages handling considerable trade and being important nodes on the transatlantic trading network. Changes in coastal geomorphology necessitated a response from seafarers. In this thesis, a conceptual framework for the formation, recovery and stability of headland-dominated sandy beaches in high-energy environments is established, based on empirical observation and on the use of the MIKE21 numerical sediment transport model. Under persistent calm climatic conditions, nearshore seabed gradient is a weak control on beach formation and persistence in embayments. However, under persistent stormy conditions, nearshore sea bed gradient becomes the prominent control. Embayments with nearshore gradients of > 0.025 m/m inhibit beach recovery on a sub-annual timescale, while gradients < 0.025 m/m promote beach recovery. These ideas are assessed in the Shetland Islands, using numerical modelling, geomorphology and OSL dating on sand blow deposits. In the late Norse era beach landing sites in Unst became prone to depletion and destruction because of increased storminess. Numerical modelling (MIKE21) supports the idea that the recovery time of different sandy beaches on Unst is dependent on average nearshore slope. The beach at Sandwick has shallow nearshore gradients and recovers quickly in the face of storminess, but beach stability at Lunda Wick is more uncertain, and thus Lunda Wick represents a more problematic landing place. The Norse harbour of The Bishop's seat at Garðar in the Eastern Settlement of Greenland is assessed to evaluate the impacts of gradual long term geomorphological change on coastlines that lack soft-sediment. A high resolution, near shore bathymetric survey shows that, due to relative sea level rise of 1 m/500 years, the landing site became more difficult to access during the later period of Norse settlement and key onshore infrastructure was disrupted. The possible role of terrestrial supplies of sediment in changing the viability of landing places is assessed through an evaluation of the Norse trading centre of Gásir in northern Iceland. Geomorphological mapping and analysis of fluvial connectivity indicate that the delta on which Gásir is located is prone to aggradation from large, irregular pulses of sediment derived from landslides in the catchment. Written sources and geomorphological mapping indicate geomorphological changes around the same time that trade was shifting to the use of boats with a deeper draft. Cultural change and environmental changes would have reinforced each other in rendering the harbour site nonviable. Geomorphological forces acting on varying spatial and temporal scales have the potential to disrupt the use of landing sites. Whether environmental changes led to the abandonment of a landing site was strongly influenced by the seafarers' competence and available technology. Higher levels of competence would enable more problematic landing sites to be used, but there are limits to this adaptation. Technological changes, such as the use of larger and deeper draft boats, would have changed the geomorphic requirements for harbour sites, and thus may have led to a passive abandonment of the site over time rather than active abandonment such as that in the face of a catastrophic change of the shoreline. Coastal geomorphology was a critical factor affecting the use of Norse harbours, as it interacted with the wider cultural and economic developments in the North Atlantic realm. This thesis demonstrates that numerical sediment transport analysis is a powerful tool in coastal archaeological research as it can illuminate processes driving observable changes in the empirical record.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1980. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING. / Includes bibliographical references. / by Mohamed Tayeb Benchaita. / Ph.D.

The present study is an integral part of a broader study focused on the design and implementation of self-cleaning culverts, i.e., configurations that prevent the formation of sediment deposits after culvert construction or cleaning. Sediment deposition at culverts is influenced by many factors, including the size and characteristics of material of which the channel is composed, the hydraulic characteristics generated under different hydrologic events, the culvert geometry design, channel transition design, and the vegetation around the channel. The multitude of combinations produced by this set of variables makes the investigation of practical situations challenging. In addition to the above considerations, the field observations, and the laboratory and numerical experiments have revealed additional complexities of the flow and sediment transport through culverts that further increase the dimensions of the investigation. The flow complexities investigated in this study entail: flow non-uniformity in the areas of transition to and from the culvert, flow unsteadiness due to the flood wave propagation, and the complex correlation between the flow and sediment hydrographs produced during storm events. To date, the literature contains no systematic studies on sediment transport through multi-box culverts. Similarly, there is limited knowledge about the non-uniform, unsteady sediment transport in channels of variable geometry. Furthermore, there are few readily useable numerical models that can reliably simulate flow and sediment transport in such complex situations. Given the current state of knowledge, the main goal of the present study is to investigate the above flow complexities in order to provide the needed insights for optimizing the culvert design. The research was phased so that field observations were conducted first to understand the culvert behavior in Iowa landscape. Modeling through complementary hydraulic model and numerical experiments was subsequently carried out to gain the practical knowledge for the development of the self-cleaning culvert designs.

Fluvial sediment transport is a process that has long been important in managing water resources. While we intuitively recognize that increased flow amounts to increased sediment discharge, there is still significant uncertainty in the details. Because sediment transport---and in the context of this dissertation, bed load transport---is a strongly nonlinear process that is usually modeled using empirical or semi-empirical equations, there exists a large amount of uncertainty around model parameters, predictions, and model suitability. The focus of this dissertation is to develop and demonstrate a series of physically- and statistically-based sediment transport models that build on the scientific knowledge of the physics of sediment transport while evaluating the phenomenon in an environment that leads us to robust estimates of parametric, predictive, and model selection uncertainty. The success of these models permits us to put theoretically and procedurally sound uncertainty estimates to a process that is widely acknowledged to be variable and uncertain but has, to date, not developed robust statistical tools to quantify this uncertainty. This dissertation comprises four individual papers that methodically develop and prove the concept of Bayesian statistical sediment transport models. A simple pedagogical model is developed using synthetic and laboratory flume data---this model is then compared to traditional statistical approaches that are more familiar to the discipline. A single-fraction sediment transport model is developed on the Snake River to develop a probabilistic sediment budget whose results are compared to a sediment budget developed through an ad hoc uncertainty analysis. Lastly, a multi-fraction sediment transport model is developed in which multiple fractions of laboratory flume experiments are modeled and the results are compared to the standard theory that has been already published. The results of these models demonstrate that a Bayesian approach to sediment transport has much to offer the discipline as it is able to 1) accurately provide estimates of model parameters, 2) quantify parametric uncertainty of the models, 3) provide a means to evaluate relative model fit between different deterministic equations, 4) provide predictive uncertainty of sediment transport, 5) propagate uncertainty from the root causes into secondary and tertiary dependent functions, and 6) provide a means by which testing of established theory can be performed.

The southern Chandeleur Islands are an ideal setting to study shoal evolution given their history of submergence and re-emergence. Here, numerical models shed light on the attendant processes contributing to shoal recovery/reemergence following a destructive storm event. Simulations of a synthetic winter storm along a cross-shore profile using Xbeach shows that convergence of wave-induced sediment transport associated with repeated passage of cold-fronts initiates aggradation, but does not lead to reemergence. A Delft3d model of the entire island chain shows that as these landforms aggrade alongshore processes driven by incident wave refraction on the shoal platform, backbarrier circulation and resulting transport become increasingly important for continued aggradation and eventual emergence. Aggradation magnitudes are a function of depth ranging from 2 – 10 mm per event (onset to recovery to near mean sea level). In the absence of big storms, this modest aggradation can be more than one meter in a few years.

The Sandy River (OR) is a coastal tributary of the Columbia River and has a steep hydroshed 1316 square kilometers which is located on the western side of Mount Hood (elevation range 3 m to 1800 m). The system exhibits highly variable flow: Its average discharge is ~40 m3/s, and the highest recorded discharge was 1739 m3/s in 1964. In this study I model the geomorphic sensitivity of an 1800m reach located the downstream of the former Marmot Dam, which was removed in 2007. The hydro-geomorphic response to major flood has implications for system management and aquatic life. Studying hydro-geomorphic change requires a systematic approach. Here, I define flows and flood hydrographs for specified return interval based on the observed hydrologic record, and then examine potential hydro-geomorphic changes using a numerical model. A Pearson Type III distribution is used to calculate 100, 75, 50, 25, 10, and 2 year return periods. Extreme event hydrographs are derived by fitting derived and observed flood hydrographs to the gamma distribution curve. Sediment transport and geomorphology are then modeled numerically with Nays2DH, a solver that is part of iRIC software. Because the model is computationally intensive, I model the domain with five different spatial grid resolutions, to find proper grid resolution. The grid resolutions used are 1.5 m, 2 m, 3 m, 4 m, and 5 m. We choose 4 m as optimum grid resolution, based on the convergence of model results. The model is run for extreme event hydrographs with six above return periods. For result visualization and analysis, we focus on flow properties and bed elevation at peak flow and at the end of each event. For both times for each event, important flow and sediment transport parameters are visualized for the entire domain in plane form and eight cross-sections at 200 m intervals. Finally, we divide the geomorphic response into areas of erosion and deposition. Linear regression analyses of mean values of erosion and deposition at peak flow for all extreme events yield R2 of 0.981 for erosion and 0.986 for deposition. The mean erosion and deposition depth at the end of the events is modeled by nonlinear regression with correlation coefficient of 0.965 for erosion and 0.998 for deposition. The regression models provide direct understanding of impacts of different floods on the geomorphic response of the river domain. examination of the model as a whole suggest that the amount of erosion and deposition in the bed and banks is a function of channel geometry, bank and bed geology, riparian area condition and strongly depend on the amount of flow through the channel.