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Sellanes, L.; Alonso, R., and Solari, S., 2024. Performance analysis of groyne fields on the Uruguayan coast. In: Phillips, M.R.; Al-Naemi, S., and Duarte, C.M. (eds.), Coastlines under Global Change: Proceedings from the International Coastal Symposium (ICS) 2024 (Doha, Qatar). Journal of Coastal Research, Special Issue No. 113, pp. 814-818. Charlotte (North Carolina), ISSN 0749-0208. Groyne fields are one of the most common coastal structures on the Uruguayan coast of the Rio de la Plata estuary. However, the performance of these structures has not been systematically evaluated since the late 1970s. Knowing and understanding the past performance of these structures is key to predict their future performance, and to plan the adaptation of these beaches to climate change. A background review of the performance of the groynes prior to the 1980s was carried out, as well as the analysis of aerial photos and historical plans prior to 1985. Then, satellite images from 1985-present, processed with CoastSat, were analysed to quantify the evolution of the coastline, with particular attention to the areas where new groynes were built, or existing ones were enlarged in the early 1980s. At the same time, the hydrodynamics of the groyne fields were analysed using XBeach to relate the observed behaviour to different geometric variables (e.g. spacing and length of the groynes). The results indicate that the groyne fields located on the Uruguayan coast have been successful in terms of their functional performance, managing to advance and maintain the position of the coastline, and that in general their hydrodynamic behaviour is that of short groynes. However, several groynes have presented poor structural performance, in some cases of such importance that their functional performance has been compromised. In addition, the rate of sediment accumulation in some groyne fields has allowed gaining insight into the sediment budget in the littoral cells in which they are installed.

The influence of groynes in large rivers on caddisflies has been poorly studied in the literature. Therefore, we carried out an investigation on the 420-km stretch of the River Oder equipped with groynes. At 29 stations, we caught caddisflies in four habitats: current sites, groyne fields, riverine control sites without groynes and in the river’s oxbows. We found that groyne construction increased species richness, diversity, evenness, and altered the structure of functional groups into more diversified and sustainable ones compared to the control sites. The groyne field fauna is similar to that of natural lentic habitats, but its composition is largely governed by the presence of potential colonists in the nearby oxbows. We distinguished three of the river’s caddisfly assemblages. The distribution of Trichoptera was governed inter alia by the plant cover and the amount of detritus, and consequently, the food resources. Oxygen, nitrates, phosphates and electrolytic conductivity were important as well. Groynes have had positive effects for caddisflies—not only those in the river itself, but also those in its valley. They can therefore be of significance in river restoration (although originally they served other purposes), especially with respect to the radically transformed ecosystems of large rivers.

Chennai Port (13.099872° N, 80.297407° E), located along the southeast coast of India, has been a hub for maritime trade since the fifteenth century. An artificial harbour was initially constructed in 1881 which underwent numerous expansions in the following years. The breakwaters of the harbour intercepted the heavy sediment-laden littoral drift along the coast, resulting in the formation of the world’s second-longest urban beach south of the port, i.e. on its up-drift side. Meanwhile, the coast north of the port, i.e. the down-drift side, experienced intense erosion due to a lack of sediment supply and forces induced on the coast due to waves and currents. The shoreline change study in this paper investigates a shoreline stretch of about 6 km (protected by transitional groynes), north of Chennai Port by dividing it into three segments. The rate of shoreline changes over a period of about 12 years across three different segments was assessed using statistical parameters by employing remote sensing techniques complemented with geographical information system (GIS) and digital shoreline analysis system (DSAS) tool. It is inferred that the coast has witnessed accretion and sizable growth in beach width has been observed post the construction of groyne field.

In this paper we provide a description of the three-dimensional flow induced by a sequence of lateral obstacles in a straight shallow open-channel flow with flat bathymetry. The obstacles are modelled as rectangular blocks and are located at one channel wall, perpendicular to the main stream direction. Two aspect ratios of the resulting dead zones are analysed. The flow structure is experimentally characterised by particle image velocimetry measurements in a laboratory flume and simulated using three-dimensional Large Eddy Simulations. Good agreement between experimental measurements and numerical results is obtained. The results show that the effect of the obstacles in the main channel is observed up to one obstacle length in the spanwise direction. The spacing between obstacles does not seem to have a large influence in the outer flow. The mean flow within the dead zone is characterised by a large recirculation region and several additional vortex systems. They are discussed in the paper, as well as the mean and root-mean-square wall shear-stresses.

Design guidelines were developed for a number of in-stream structures; however, the knowledge about their morphological and hydraulic function is still incomplete. A variant is submerged groynes, which aim to be applicable for bank protection especially in areas with restricted flood water levels due to their shallow height. Laboratory experiments were conducted to investigate the backwater effect and the flow resistance of submerged groyne fields with varying and constant field length and groyne distance. The effect of the shape of a groyne model was investigated using two types of groynes. The validity of different flow types, from “isolated roughness” to “quasi smooth”, was analyzed in relation to the roughness density of the groyne fields. The results show a higher backwater effect for simplified groynes made of multiplex plates, compared to groynes made of gravel. The relative increase of the upstream water level was lower at high initial water levels, for short length of the groyne field, and for larger distance between the single groynes. The highest roughness of the groyne fields was found at roughness densities, which indicated wake interference flow. Considering a mobile bed, the flow resistance was reduced significantly.

Costa da Caparica beach, in Portugal, has suffered from chronic erosion for the last 50 years, a phenomenon that has been countered by various management interventions. This study aims at comparing sixteen possible interventions, thus identifying the most effective one(s) in terms of reducing beach erosion or even promoting beach accretion. This exercise is achieved using a one-line shoreline evolution model, calibrated with in situ field data, forced by local wave conditions. The target management period is 25 years. In the calibration phase, it is found that the annual mean alongshore net sediment transport along the 24 km sandy coast is variable in direction and magnitude, but it is mostly smaller than ±50 × 103 m3/year. This net transport results from the imbalance of northward/southward-directed bulk transports of circa tenfold-larger magnitudes. This affects the overall sediment balance at the urban beaches, as well as the effectiveness of the intervention strategies. The results show that the present management strategy is effective in holding the shoreline position, although deploying the same nourishment volume but over a shorter area could lead to better results. The best solutions, which are capable of promoting beach accretion, implicate the lengthening of the terminal groin at the northern extremity of the beach. The results from this study can support decision makers in identifying the most appropriate management action, not just locally but also at other coastal regions where similar problems persist and the same methodology could be applied.

Alonso, R.; Solari, S., and Teixeira, L., 2018. Erosion Problem on a Fluvial Beach. The Case Study of “La Concordia” in the Uruguay River. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 131–135. Coconut Creek (Florida), ISSN 0749-0208. A case study of beach erosion at “La Concordia” is presented. It is a fluvial beach located in the most downstream stretch of the Uruguay River. Despite it is a fluvial environment, the 10 km width that the river has in this zone allows for the generation of wind-waves that play an important role in the morphological processes that shape its sandy margins. This was initially proven by assessing the evolution of the coastline from satellite images and aerial photos, since the main change observed in the coastline is the expected one when groins are built on a sea coast. After that, the predominant influence of waves was confirmed by: 1) a good agreement between the geometry of the system (i.e. plan-shape and profiles) with the one predicted by formulations conceived from wave dominated marine environments, 2) a cross–section river flow velocity distribution, calculated for an historical maximum discharge, that shows that outside of the river channels velocities are lower than the critical expected for bottom sediment transport, 3) an estimation of the littoral transport in agreement with the historical coastline evolution. This verification supports the use of models and methods commonly employed in coastal engineering for the study of this type of fluvial beaches. In this case, the numerical model XBeach was used to simulate the response of the system to extreme events, in order to provide an input to a beach nourishment design.

Alluvial rivers at lowland are very complex in nature. Severe bank erosion at high flow and undue sedimentation at low flow are very common there. Groins are not functioning successfully with their present arrangements. This study investigates various orientations of groins to identify the optimum one for the effective functioning at high flow and low flow both. A 2D numerical model, RIC-Nays is utilized upon confirmation through detailed experimental data. Two types of groins: non-permeable and permeable, and four orientations: 100°, 90°, 80° and 70° to the bank line downstream are considered. Computation reveals that smaller angled groins function better through deepening the main channel and minimizing the local scour, except deposition near bank reduces.

The role of man-made “flow-reduced zones” as a sink or a source of water content is unknown for the River Elbe, Germany. We measured and compared: a) the concentration of suspended matter at the inflow and the outflow of one special groyne field (“UFZ groyne field”) weekly for a period of two years; and b) the intensity of oxygen metabolism within several groyne fields and the main stream during a Lagrangian survey. Under discharge conditions near or below mean water, we found a significant reduction of suspended particulate matter and particle bound nutrients in the “UFZ groyne field”. In contrast, concentrations of most soluble water contents and chlorophyll a did not significantly change between in- and outflow. During the Lagrangian survey, pelagic production and respiration rates developed nearly identically in both the main stream and the adjacent groyne fields but oxygen time curves showed higher amplitudes for the groyne fields compared to the river. This higher net-oxygen production in the groyne fields is due to reduced water depth and reduced stream velocity. It enhances the concentration of oxygen in the main stream. The contrary results show the coexistence of both sink and source functions of “flow-reduced zones” in rivers.

Despite improvements in water quality over the last 15 yr, priority pollutant contamination of Elbe River sediments has not reached acceptable levels. Analysis of sediment core samples shows elevated nutrient and pollutant loads deposited on the floodplains and in the river course and demonstrates the specific sink function of both sites. Sediments in the river course may be remobilized, indicating that the long-term sink function can temporarily become a significant source character. Implications for risk assessment, water quality management, and dredged sediment management are examined.