Shoreface behaviour and equilibrium

Dissertação de Mestrado Integrado em Arquitectura, apresentada ao Departamento de Arquitectura da Faculdade de Ciências e Tecnologia da Universidade de CoimbraMuitas vezes, as periferias das cidades são áreas sem estrutura1 do ponto de vista arquitectónico e urbano. A formação / génese de tais periferias relaciona-se com a grande especulação na densificação de novos territórios, ou, noutros casos, da sua informal ocupação, causando a fragmentação dos tecidos urbanos. No caso de Coimbra, muitos dos planos que foram propostos para a cidade, no século XX, resultaram numa ocupação, densificação do território e realojamento, baseada na formação de bairros periféricos, inicialmente precários. Os realojamentos deram-se em massa, num curto espaço de tempo, adensando as condições para a fragmentação territorial. Inserido no quadro suburbano estão os bairros da Conchada e do Ingote, que, actualmente, sofrem do estigma associado à sua marginalização. Pretende-se, assim, emendar esta realidade, propondo uma estratégia de articulação urbana entre o centro da cidade de Coimbra e os bairros da Conchada e do Ingote, a partir do espaço público. Por um lado, partiu-se de um processo de análise crítico, cuja concretização apoia-se em mapas de análise, para cada um dos bairros, evidenciando a sua inserção urbana, os seus aspectos morfológicos e formais, bem como as suas fragilidades, dando origem a temas / propostas de projecto. Por outro lado e, resultante deste método de análise, surgiu o projecto estratégico. O projecto é pontuado pela conformação de uma nova centralidade nos dois bairros, estimulando a periferia e reforçando o tecido urbano, a partir de uma nova costura urbana.Frequently, the outskirts of cities are unstructured areas2 from the architectural and urban point of view. The creation / genesis of those outskirts relates to the major speculation in the densification of new territories or in other cases from its informal occupation, causing the fragmentation of the urban fabric. In the case of Coimbra, most of the plans which were proposed for the city, in the 20th century resulted in an occupation, densification of the territory and relocation based on creation of outlying neighborhoods initially precarious. The relocations were in mass in a short period of time, densifying the deepening the conditions for the territorial defragmentation. Conchada and Ingote neighborhoods are set in a suburban frame which, currently, suffer from the stigma attached to their marginalization. The objective is to amend this reality, proposing an urban articulation strategy among the center of Coimbra and Conchada e Ingote neighborhoods, from the public space. On the one hand, it was started up a process of critical analysis, whose implementation leans on analysis maps for each of the neighborhoods highlighting its urban integration, its morphological and formal aspects, as well its weaknesses, giving rise to issues / project proposals. On the other hand, and a result from this analysis method, arose the strategic project. The project is punctuated by the formation of a new centrality in two neighborhoods, stimulating the outskirts and reinforcing the urban fabric, through a new urban seam

Puget Sound shoreline inventory and assessment using boat-based lidar

Boat-based lidar of Puget Sound shorelines collected by the Washington State Department of Ecology are developed to provide a comprehensive inventory, classification, and analyses of site conditions and variability. For example, quantitative metrics of shoreline characteristics are derived from DEMs such as bluff crest height, bluff slope, bluff toe elevation, beach slope, and shoreline armoring elevations. These metrics can then be compiled and compared within and among drift cells to determine regional variability such as differences between updrift and downdrift beaches and the effect of fetch, orientation, and other exposure variables. Certain features can also be correlated to characterize how the shoreline landscape may be affecting nearshore ecosystem services. For example, variability and gradients in beach slope and width may be correlated to proximity to feeder bluff activity and the position, length, and elevation of armoring relative to the shoreline and backshore. Upland development and shoreline modification may be correlated to the amount of overhanging vegetation, large woody debris, or beach wrack, and these findings can be compared to conditions at undeveloped shorelines. Details in the lidar point clouds, such as intensity values, can help identify groundwater seepage and potential bluff failure and erosion mechanisms. The complementary photos to the lidar point clouds provide additional documentation of bluff geology, stratigraphy, groundwater flow, and other characteristics to help assess relative bluff stability

High resolution mapping of Puget Sound shorelines

In an effort to collect high-resolution baseline coastal topographic data of beaches and bluffs around the Puget Sound and Strait of Juan de Fuca, the Washington State Department of Ecology Coastal Monitoring & Analysis Program (CMAP) conducted a series of boat-based lidar surveys in October 2013, May through September 2015, and May 2016 at a total of 16 sites spanning 220 km of shoreline and over two dozen drift cells. The drift cells were selected based on a rigorous and systematic geospatial analysis of bluff-backed beaches for their potential for significant bluff sediment supply to intact shorelines identified as having a relative abundance of habitat for forage fish, eelgrass, herring, shellfish, and geoduck, as well as having previous investments in beach restoration projects, and potential for future shoreline armoring and habitat loss based on population growth scenarios. As such, the surveyed drift cells are top candidates for implementing drift cell-scale protection and restoration strategies. The boat-based lidar and GPS topography data were used to produce 0.5-m digital elevation models (DEMs) for the beaches and bluffs at each of the survey sites. These DEMs provide the opportunity to inventory and characterize the shoreline landscape that affects nearshore ecosystem services such as feeder bluff activity, beach slope and width, and the position, length, and elevation of armoring relative to the backshore. Boat-based lidar provides an advantageous point of view of the bluff face, resulting in high resolution data which is needed to gain insight into bluff failure and erosion mechanisms and corresponding sediment transport processes. In addition, it successfully collects data under overhanging vegetation and overwater structures. Repeat surveys in the future would enable change analyses for quantifying bluff sediment supply, changes in marine riparian vegetation, and a better understanding of the linkages between physical and ecological processes

Evaluating methods to obtain high resolution nearshore bathymetry and coastal topography for Puget Sound

The Washington State Department of Ecology Coastal Monitoring & Analysis Program performed a coastal topographic and bathymetric survey of Port Gamble Bay between March 9 and March 28, 2014. Boat-based topographic lidar was collected along the shoreline of the bay and multibeam bathymetric sonar was collected throughout the bay to obtain a seamless topographic-bathymetric surface with complete coverage of Port Gamble Bay. The survey was performed with a R2Sonic 2022 multibeam echosounder, an Optech ILRIS-HD-ER mobile laser scanner, and an Applanix POS MV 320 v5 receiving real-time kinematic positioning corrections. The Joint Airborne Lidar Bathymetry Technical Center of Expertise (JALBTCX) performed a topographic and bathymetric lidar survey of Port Gamble Bay on September 4, 2014. The Coastal Zone Mapping and Imaging Lidar (CZMIL) system was used to obtain seamless coastal topographic-bathymetric coastal intertidal and nearshore coverage of Port Gamble Bay. The bathymetric depth coverage is limited to laser extinction, which is determined by water clarity. The availability of these two datasets provides the unique opportunity to compare data between high-resolution boat-based lidar and multibeam systems and the state-of-the-art airborne topo-bathy lidar system and also assess detection and resolution of specific features throughout a range of water depths across the nearshore important to habitat and restoration efforts. This effort provides a detailed comparison of coverage and resolution of nearshore features and will help clarify differences between these capabilities to aid in planning complementary efforts in coastal zone mapping and monitoring

Behaviour and performance of a dynamic cobble berm revetment during a spring tidal cycle in North Cove, Washington State, USA

In many places, sandy coastlines and their associated assets are at high risk of erosion and flooding, with this risk increasing under climate change and sea level rise. In this context, dynamic cobble berm revetments represent a potentially sustainable protection technique to armour sandy beaches, reduce wave runup and protect the hinterland against wave attack. However, the behaviour and performance of such structures is not well understood. The dynamic cobble berm revetment located in North Cove, WA, USA, was monitored over a spring tidal cycle in January 2019. A representative 60 m alongshore section was monitored over 10 days using 2D laser scanner (lidar) measurements, GPS ground elevation surveys, Radio Frequency Identification of individual cobbles and revetment thickness measurements. These data were used together to assess the dynamic behaviour and functionality of the revetment throughout the experiment. Over the course of the experiment, the surface elevation changed by up to ±0.5 m, and the revetment volume reduced by an average 0.67 m3/m. These changes were found to be caused by relatively large significant wave height and high water levels. The revetment demonstrated a dynamic stability and the capacity to quickly reshape under changing hydrodynamic conditions. The instrumented cobbles were transported along and cross-shore and accumulated at the toe of the revetment, but were never transported seaward of the toe. The revetment also managed to recover some of the lost volume under moderate wave conditions. The revetment behaviour was found to be influenced by variation in the cobble-sand matrix. The underlying sand dynamics – i.e., accumulation or removal of sand within the cobbles – were found to govern the overall volume changes and were important to the overall stability of the revetment. Seven possible transport regimes were identified, and a model of the internal sand dynamics was developed. During the spring tidal cycle measured here, the revetment protected the sand scarp immediately landward and prevented flooding of the hinterland, while armouring the underlying sand. Over time, renourishment will likely be required due to longshore sediment transport, and preliminary guidelines for this and other aspects of design are suggested.</p

Swash-by-swash morphology change on a dynamic cobble berm revetment:High-resolution cross-shore measurements

Dynamic cobble berm revetments are a promising soft engineering technique capable of protecting sandy coastlines by armouring the sand and dissipating wave energy to protect the hinterland against wave attack. They also form composite beaches as they are essentially mimicking natural composite beach structure and behaviour. This type of coastal protections and beaches have recently been investigated, and this led to a better understanding of their overall behaviour under varying water levels and wave conditions. However, the short-term dynamics of the swash zone (where all bed changes occur) has never been studied at high-resolution, and this is needed to fully understand the underlying dynamics of such structures and relate it to observed processes at larger scale. To do so, the revetment at North Cove (WA, USA) was monitored for a nine-day period in January 2019 over a spring tidal cycle and with offshore significant wave height reaching 6 m. A 2-D lidar was used to survey a cross-shore profile of the revetment, and record all surface changes and interaction with swashes at high spatial (0.1 m) and temporal (swash-by-swash) resolution. The revetment was found to rapidly reshape under these energetic conditions, but reached a relatively stable state during the rising tide. The analysis of bed-level changes and net cross-shore mass fluxes over the revetment showed that revetment changes are mainly driven by very small events, with some rare large bed-level changes of a magnitude comparable to the median cobble diameter. The distribution of event mass fluxes nearly balanced out over the duration of a tide, meaning that positive and negative fluxes tended to be symmetrical. Furthermore, measured net fluxes magnitude were 18 times smaller than the absolute fluxes, which demonstrated the dynamic stability of the revetment as substantial movement occur on a wave-by-wave timescale but these balance out over time. The analysis of swash revealed that the revetment section where the swash reaches a maximum depth between 0.15 and 0.45 m undergoes the more extreme fluxes. Swashes deeper than 0.45 m only occurred in zones inundated more than 50% of the time, and smaller extreme fluxes were measured over the revetment section where these deep swashes were recorded. Bed level change oscillations over the revetment were observed, and the cross shore limit of these was correlated with the mean wave period at the toe of the revetment. Overall, the water depth at the toe of the revetment was identified as the key parameter to describe the energy reaching the revetment. This study enables the morphodynamics of dynamic revetment, observed in previous lab and field studies, to be explained at the swash scale, and brought new information on the sediment dynamics of composite beaches and dynamic revetments. These findings allow to suggest some generic guidance for dynamic cobble berm revetment design. Finally, the results are compared to a similar study on sandy beaches.</p

Morphodynamics of prograding beaches: A synthesis of seasonal- to century-scale observations of the Columbia River littoral cell

Findings from nearly two decades of research focused on the Columbia River littoral cell (CRLC), a set of rapidly prograding coastal barriers and strand-plains in the U.S. Pacific Northwest, are synthesized to investigate the morphodynamics associated with prograding beaches. Due to a large sediment supply from the Columbia River, the CRLC is the only extensive stretch of shoreline on the U.S. west coast to have advanced significantly seaward during the late Holocene. Since the last Cascadia Subduction Zone (CSZ) earthquake in 1700, with associated co-seismic subsidence and tsunami, much of the CRLC has prograded hundreds of meters. However, the rates of progradation, and the processes most responsible for sediment accumulation, vary depending on time scale and the morphological unit in question. Remarkably, the 20th and early 21st century shoreline change rates were more than double the late prehistoric rates that include recovery from the last major CSZ event, most likely due to an increase in sediment supply resulting from inlet jetty construction. In some locations detailed beach morphology monitoring reveals that at interannual- to decadal-scale the upper shoreface aggraded about 2 cm/yr, subtidal sandbars migrated offshore and decayed while intertidal bars migrated onshore and welded to the shoreline, the shoreline prograded about 4 m/yr, and 1 to 2 new foredune ridges were generated. A detailed meso-scale sediment budget analysis in one location within the littoral cell shows that approximately 100 m³/m/yr accumulated between − 12 m (seaward limit of data) and + 9 m (crest of landward-most foredune). Gradients in alongshore sediment transport, net onshore-directed cross-shore sediment transport within the surf zone, and cross-shore feeding from a shoreface out of equilibrium with forcing conditions are each partially responsible for the significant rates of sediment supplied to the beaches and dunes of the CRLC during the observational period. Direct observations of beach progradation at seasonal- to decadal-scale are put in context of measured or inferred changes over time scales of decades to centuries.This is the publisher’s final pdf. The published article is copyrighted by Elsevier and can be found at: http://www.journals.elsevier.com/marine-geology/Keywords: Morphodynamics, Columbia, River littoral cell, Foredunes, Sediment budget, Coastal barriers, Progradatio