Slack Water Research Articles

Hydrodynamic modelling and model sensitivities to bed roughness and bathymetry offset in a micro-tidal estuary

AbstractTidal estuaries support everyday functions for over 80% of Australia's population living within 50 km of the coastline and thus come under immense pressure of physicochemical changes. Most studies in estuarine applications have used the bed roughness as the single calibration parameter to calibrate hydrodynamic modelling, yet errors in bathymetric data can significantly impose uncertainties into the model outputs. In this study, we evaluated the sensitivity of a hydrodynamic model of a micro-tidal estuary to both the bed roughness and bathymetry offset through comparing observed and modelled water level and velocity. Treating both bathymetry offset and bed roughness as calibration parameters, three calibration scenarios were tested to examine the impact of these parameters. To validate the model, Lagrangian drifter data as a new dataset in shallow estuaries were used. The analysis shows that model outputs are more sensitive to the variation of bathymetry offset than bed roughness. Results show that calibrating the bathymetry offset alone can significantly improve model performance. Simultaneous calibration of both parameters can provide further improvement, particularly for capturing the water level. Drifter and modelled velocities are highly correlated during flood tides, whereas the correlation is low for slack water because of wind-induced current on drifters.

Remote video methods for studying juvenile fish populations in challenging environments

Understanding the dynamics of juvenile fish populations is a key component to understanding variation in adult populations, however, juvenile fish are frequently found in a variety of structurally complex habitats which are often not easily sampled. The use of baited (BRUV) and unbaited (RUV) remote underwater stereo-video to study juvenile fish populations is uncommon and thus far, no study has directly compared the efficacy of these two methods solely for juvenile fish assemblages.The macro-tidal Kimberley region in the north-west of Australia is challenging to sample using diver-based methods due to the strong currents and prevalence of saltwater crocodiles (Crocodylus porosus). We sampled 60 Stereo-RUVs and 60 Stereo-BRUVs across four shallow-water (1-6 m) benthic habitats: coral, mangrove, macroalgae, and seagrass, to contrast the effect of the presence or absence of bait, deployment period, in-water visibility and tidally driven water speed on estimates of relative total abundance, species richness, and composition of the juvenile fish assemblage.No difference was detected in the ability of stereo-BRUV or stereo-RUV to quantify the relative total abundance, species richness, or assemblage composition of juvenile fish. We also found little effect of in-water visibility or tidally driven water speed, likely as a result of stratifying our sampling to slack water periods.We found that a deployment period of 10 min for Stereo-BRUVs and 15 min for Stereo-RUVs was optimum for sampling the juvenile fish assemblage across all four contrasting habitats. Since no statistical significance was observed between 10 and 15 min, we recommend that Stereo-RUVs deployed for 15 min during tidal slack water conditions are an optimum way to provide consistent results for comparisons of juvenile fish assemblage metrics across the benthic habitats studied within this region.The controlled video methodologies described here provided an opportunity to access un-sampled juvenile fish habitats and assemblages where traditional diver-based methodologies are impossible. Applying these methods broadly can help to improve our understanding of the status of juvenile fish and infer the reproductive success and viability of their populations across a range of habitat and environments.

Tidally Forced Lee Waves Drive Turbulent Mixing Along the Arctic Ocean Margins

AbstractIn the Arctic Ocean, limited measurements indicate that the strongest mixing below the atmospherically forced surface mixed layer occurs where tidal currents are strong. However, mechanisms of energy conversion from tides to turbulence and the overall contribution of tidally driven mixing to Arctic Ocean state are poorly understood. We present measurements from the shelf north of Svalbard that show abrupt isopycnal vertical displacements of 10–50 m and intense dissipation associated with cross‐isobath diurnal tidal currents of ∼0.15 m s−1. Energy from the barotropic tide accumulated in a trapped baroclinic lee wave during maximum downslope flow and was released around slack water. During a 6‐hr turbulent event, high‐frequency internal waves were present, the full 300‐m depth water column became turbulent, dissipation rates increased by a factor of 100, and turbulent heat flux averaged 15 W m−2 compared with the background rate of 1 W m−2.

Linking fish assemblages to hydro‐morphological units in a large regulated river

AbstractFlow‐related changes of physical habitat represent a potentially significant environmental filter determining the presence and composition of fish assemblages in rivers. The mesoscale (100–103 m) of river habitat has been identified as an appropriate resolution to model linkages between fish and their abiotic environment that are relevant yet logistically feasible for management of large rivers with complex habitats and diverse fish assemblages. This study identified well‐defined mesohabitat types (i.e. hydro‐morphological units) that influence the fish community of the lower Saint John River, New Brunswick, downstream of a large hydropower generating station (the Mactaquac Generating Station). Four hydro‐morphological units or habitats (i.e. pool, riffle, run and slack water) were identified and linked to three distinct fish assemblages. Eurytopic species represent habitat generalists that were common among all habitats throughout the study area. Rheophilic species preferred fast‐flowing run habitat, whereas limnophilic species were mainly associated with slack water habitat. Riffle habitats that frequently run dry during low flows were mostly vacant of fish species, suggesting that fish assemblages that would naturally occur in these environments could be affected by fluctuations in flow (i.e. hydropeaking) due to dam operation. Our improved understanding of the relationship between fish assemblages and hydro‐morphological units is a fundamental first step to develop meaningful habitat models that can facilitate the effective evaluation of flow management options regarding hydropower and other flow manipulation activities in large rivers with diverse fish fauna.

Linking land subsidence to soil types within Hue city in Central Vietnam

Coastal areas of Southeast Asia are progressively threatened by flooding as a consequence of more frequent precipitation extremes and rising sea levels. Especially urban areas are affected by flood risk which is additionally increased by surface subsidence related to building activities and groundwater extraction. However, the severity of subsidence as well as its triggers and environmental interrelations are only little understood. This study measures surface subsidence for Hue city by using persistent scatterer radar interferometry (PS-InSAR). A series of 53 images acquired by the Sentinel-1 radar satellite between 2018 and 2019 was analyzed to reliably retrieve surface changes at the millimeter scale. The overall displacement ranges between -25 and +10 millimeters per year. Its spatial distribution was then compared to the extent of different soil types in the study area to conduct an analysis of variance (ANOVA). The results confirmed a significant difference between the soil types with Plinthic Acrisols as the soil type having the largest negative average surface velocity. Possible triggers are the intrusion of slack water from the surrounding rice cultivation areas and construction activities which lead to increasing weight and soil compaction. The findings shall raise awareness for the topic and underline the demand for further research.
 Mưa lớn và nước biển dâng là những nguyên nhân gây lũ lụt ngày càng nghiêm trọng ở các khu vực ven biển Đông Nam Á. Đặc biệt việc gia tăng công trình xây dựng và khai thác nước ngầm gây sụt lún bề mặt dẫn đến ngập lụt ở các vùng đô thị. Tuy nhiên, các nghiên cứu về mối tương quan giữa sụt lún bề mặt với các hiện tượng môi trường chưa được chú trọng nhiều. Trong nghiên cứu này, độ lún bề mặt của thành phố Hue được đo bằng phương pháp giao thoa radar tán xạ liên tục (PS-InSAR). Phân tích 53 ảnh vệ tinh Sentinel-1 từ năm 2018-2019 cho thấy sự thay đổi tổng thể bề mặt dao động từ -25mm đến 10mm mỗi năm. Phân tích phương sai (ANOVA) cho thấy sự thay đổi bề mặt khác nhau tùy từng loại đất, trong đó đất đỏ vàng (Plinthic Acrisols) có tốc độ sụt lún trung bình cao nhất. Các tác nhân có thể là do sự xâm nhập của nước từ các vùng trồng lúa xung quanh và các hoạt động xây dựng dẫn đến tăng trọng lượng và nén đất. Những phát hiện này là cơ hội nâng cao nhận thức về sự sụt lún bề mặt và cần được nghiên cứu thêm.

Using clast geometries to establish paleoriver discharges: Testing records for aggradation and incision from the upper Indus River, Ladakh Himalaya

River systems in the Himalaya have been studied extensively to understand the processes driving periodic river valley aggradation and incision resulting due to rainfall, high relief, and ongoing deformation. When climatic forces are considered, the debate continues on whether wetter climatic intervals with localized glacier melting promote river aggradation through increased discharge and enhanced sediment load, or instead, it is during drier conditions that aggradation occurs through increased sediment to water ratio. This study, for the first time, attempts to constrain discharge during periods of established river aggradation and incision over late Quaternary. Here, geometric data from imbricated gravels of channel fills are used to calculate paleodischarges during net river aggradation at 47–23 ka, and preserved slack water deposits (SWDs) at 14–10 ka are used to constrain paleodischarges that occurred during net river incision. Catchment area-normalized discharge derived from these valley fill sequences ranges from 0.78 to 3.26 (×10−7 m s−1). Syn-incision discharge estimates yielded normalized discharge values of 6.17 (×10−7 m s−1) are considerably higher than discharges estimated from periods of aggradation. Morphometric analysis, chi slope, and Ksn, along the Indus River, observed no relation with the paleodischarge. This case study implies that net river incision in the upper Indus River occurs predominantly during relatively wetter climatic conditions than the climate during aggradation.

Numerical Modeling for Hydrodynamics and Near-Surface Flow Patterns of a Tidal Confluence

Dai, W.; Bilal, A.; Xie, Q.; Ahmad, I., and Joshi, I., 2020. Numerical modeling for hydrodynamics and near-surface flow patterns of a tidal confluence. Journal of Coastal Research, 36(2), 295–312. Coconut Creek (Florida), ISSN 0749-0208.Because of the flow influx of tributaries, a confluence forms a unique environment carrying interesting hydrodynamic features and other attributes. The understanding of flow behavior here is important, particularly if it is on a tidally influenced channel in a harbor metropolitan. Because of communal requirements, there is a possibility of building wading structures, which may interplay with the flow in this zone. The knowledge of unidirectional river or flume confluences so far is not readily applicable for similar features in channels near coastal areas that have tidal flow in addition to river runoff. In this study, a tidal confluence that has a highly dynamic bidirectional flow is investigated. Near-surface flow patterns in a tidal cycle are simulated by using a numerical model. A field survey provides the bathymetry, time-series boundary conditions, and corresponding verification data. Good agreement is reached between calculated and measured results. Based on the condition of tidal current, four scenarios are selected for which confluence flow patterns are observed, both spatially and temporally. The results indicate that at least one recirculation is always in the tidal confluence for all flow conditions, which rotates counterclockwise for the ebb flow and clockwise for the flood flow. In addition, there is no absolute slack water condition at the tidal junction in the study area. The study also finds that the flows of all three connected channels at the confluence change in a looped pattern with respect to one another. Furthermore, the study reports unique relationships among the ratios of different flows.

The impact of sea level rise on maritime navigation within a large, channelized estuary

ABSTRACT Vessel handling guidelines in Tampa Bay have been developed and refined over many years to prevent vessel groundings, collisions, and other maritime incidents. These guidelines restrict navigation based upon vessel characteristics such as vessel length, draft, displacement, and destination, often in conjunction with tidal levels and current speeds. A numerical circulation model of the Tampa Bay estuary was used to examine the occurrence frequency of the most common limiting current speeds within the main shipping channel under four sea level rise (SLR) scenarios ranging from 0.25 to 2 m. Near the bay mouth, the occurrence of limiting speeds generally decreased with SLR, but farther up the channel it increased, mostly due to changes in the M2 tide, with contributions from other primary tides. The relative occurrence of slack water, the lowest of the limiting speeds, was most sensitive to SLR, with absolute changes in occurrence ~50%. Higher speed limits had smaller relative changes. A methodology is developed using data from the Automatic Identification System that identifies vessel traffic potentially impacted by SLR and changing tidal currents. Some implications of reduced tidal currents for port and vessel traffic management are discussed.

Holocene extreme paleofloods and their climatological context, Upper Colorado River Basin, USA

Given its singular importance for water resources in the southwestern USA, the Upper Colorado River Basin (UCRB) is remarkable for the paucity of its conventional hydrological record of extreme flooding. Short-term record-based flood frequency analyses lead to very great aleatory uncertainties about infrequent extreme flood events and their climate-driven causal associations. This study uses paleoflood hydrology to examine a small portion of the underutilized, but very extensive natural record of Holocene extreme floods in the UCRB. We perform a meta-analysis of 82 extreme paleofloods from 18 slack water deposit sites in the UCRB to show linkages between Holocene climate patterns and extreme floods. The analysis demonstrates several clusters of extreme flood activity: 8040–7960, 4400–4300, 3600–3460, 2900–2740, 2390–1980, 1810–720, and 600–0 years BP. The extreme paleofloods were found to occur during both dry and wet periods in the paleoclimate record. When compared with independent paleoclimatic records across the Rocky Mountains and the southwestern USA, the observed temporal clustering pattern of UCRB extreme paleofloods shows associations with periods of abruptly intensified North Pacific-derived storms connected with enhanced variability of El Niño. This approach demonstrates the value of creating paleohydrological databases and comparing them with hydro-climatic proxies in order to identify natural patterns and to discover possible linkages to fundamental processes such as changes in climate.

The links between entrance geometry, hypsometry and hydrodynamics in shallow tidally dominated basins

AbstractGeomorphological characteristics of tidal basins control hydrodynamics and sediment transport potential within such basins, for example, by adjusting the balance in tidal asymmetry. In this study we examine the effects of entrance geometry on tidal velocity asymmetry, slack water asymmetry, bed shear stress patterns and hypsometric profile shapes by comparison of six shallow meso‐tidal basins of Tauranga Harbour, New Zealand. Numerical model results show how tidal distortion increases with distance from a basin entrance. A simple ratio between basin width and entrance width defines levels of basin dilation. Sub‐basins with a constricted geometry and deep entrance channels are associated with small bed shear stress values and high rates of flood‐directed tidal velocity asymmetry in the sheltered basin centres, indicating a large potential for sediment deposition of larger particles. Moreover, slack water asymmetry within these basins is weakly ebb‐directed, indicating a small potential for transport of fine sediments out of the basins. The constricted depositional basins are characterized by convex hypsometric profiles with elevated intertidal regions. Unconstricted geometries are associated with larger bed shear stress values and more ebb‐directed tidal velocity asymmetry within basin centres, suggesting limited potential for overall sediment deposition. The slack tide duration asymmetry is weakly flood‐dominant indicating that limited input of fine sediment into the basins is possible. The comparatively high‐energy conditions within these exposed basins are associated with a less convex hypsometric intertidal profile. The ability to estimate tidal asymmetries is advantageous when developing management strategies related to ecosystem functioning, navigability or coastal protection in specific geomorphic settings. © 2019 John Wiley & Sons, Ltd.

Reconstructing human–landscape interactions in the context of ancient iron smelting on Elba Island, Italy, using sedimentological evidence

AbstractWe reconstruct human–landscape interactions in the context of ancient bloomery smelting on Elba Island, Tyrrhenian Sea, based on sedimentological evidence. Elba was, together with Populonia, the center of iron production in antiquity (mid‐6th century B.C.E. to 1st century C.E.). The sediment sequences obtained by vibracoring in the Campo plain reveal that the Holocene transgression triggered aggradation during the Early–Middle Holocene (>5.4 ka cal BP). Nevertheless, wetland conditions prevailed in low‐lying areas during Roman times. Correspondingly, ancient smelting sites are found only on elevated areas along the edge of the plain. The palaeolandscape thus plays a key role in understanding location factors for smelting sites in antiquity. Our sediment sequences indicate that during Etrusco–Roman times (2.4–2.0 ka cal BP) morphodynamics on the island increased, as is evident from the alteration of slope deposits and slack water deposits in upstream positions, and the change from lacustrine to alluvial deposition in downstream areas. The charcoal record shows that fire dynamics increased simultaneously. The sediments that are synsedimentary to the ancient smelting period show significantly increased arsenic contents and high sediment magnetic susceptibility, whereas overall concentrations of heavy metals remain in the range of the geogenic background signal.

Estimating spatial pattern of hyporheic water exchange in slack water pool

Hyporheic zone (HZ) influences hydraulic and biogeochemical processes in and alongside streams, therefore, investigating the controlling geographic factors is beneficial for understanding the hydrological processes in HZ. Slack water pool (SWP) is an essential micro-topographic structure that has an impact on surface water and groundwater interactions in the HZ during and after high flows. However, only a few studies investigate HZ surface water and groundwater exchange in the SWP. This study used the thermal method to estimate the HZ water exchange in the SWP in a segment of the Weihe River in China during the winter season. The findings show that on the flow-direction parallel to the stream, river recharge dominates the HZ water exchange, while on the opposing flow-direction bank groundwater discharge dominates the water exchange. The water exchange in the opposing flow-direction bank is about 1.6 times of that in the flow-direction bank. The HZ water exchange is not only controlled by flow velocity but also the location and shape of the SWP. Great water exchange amount corresponds to the shape with more deformation. The maximum water exchange within the SWP is close to the river bank where the edge is relatively high. This study provides some guidelines for water resources management during flooding events.

Tidal asymmetry and residual sediment transport in a short tidal basin under sea level rise

Abstract Tidal asymmetry in estuaries and lagoons (tidal basins) controls residual sediment transport, and quantifying tidal asymmetry is important for understanding the factors contributing to long-term morphological changes. Asymmetry in peak flood and ebb currents (Peak Current Asymmetry – PCA) controls residual transport of coarse sediment, and asymmetry in slack water duration preceding flood and ebb currents (Slack Water Asymmetry – SWA) controls residual transport of fine sediment. PCA and SWA are analyzed herein for Newport Bay, a tidal embayment in southern California, based on the skewness of tidal currents predicted for several stations by a hydrodynamic model. Use of skewness for tidal asymmetry is relatively new and offers several advantages over a traditional harmonic method including the ability to resolve variability over a wide range of time scales. Newport Bay is externally forced by mixed oceanic tides that are shown to be ebb dominant because of shorter falling tide than rising tide. Both PCA and SWA indicate ebb dominance that favors export of coarse and fine sediments, respectively, to the coastal ocean. However, we show that the ebb dominance of SWA is derived from the basin geometry and not the external forcing, while ebb dominance of PCA is linked to the external forcing and the basin geometry. We also show that tidal flats in Newport Bay play an important role in maintaining ebb dominated transport of both coarse and fine sediments. Loss of tidal flats could weaken PCA and reverse SWA to become flood dominant. Specifically, we show that sea level rise > 0.8 m that inundates tidal flats will begin to weaken ebb dominant PCA and SWA and that sea level rise > 1.0 m will reverse SWA to become flood dominant. This feedback mechanism is likely to be important for predicting long-term evolution of tidal basins under accelerating sea level rise.

A laboratory study on the generation of estuarine negatively, neutrally and positively buoyant bio-mediated flocs

In estuaries and coastal areas, the mixing of inorganic and organic sediments results in the production of bio-mediated flocs. Their densities depend on the ratio between the light organic fraction and the mineral ballast. Transparent exopolymer particles (TEP) are ubiquitous in estuarine and marine environments. Because of their positive buoyancy, they form biofilms at the water surface. In order to investigate how the bio-mediated flocculation involving TEP impacts the sedimentation processes in the near surface layer of estuaries, we determined at the laboratory, the ascending/descending velocity of the mineral particles and their flocs size distributions (FSDs) of sediments sampled from the Cam River estuary (Vietnam), for concentrations ranging from 5 to 50 mg L−1, for two levels of turbulence representative to the slack water and mid ebb conditions of this estuary and for various TEP concentration and stickiness. The obtained results showed that the sediment velocity varied over two orders of magnitude depending of the conditions of the experiment. Ascending velocities were mostly measured for mid ebb conditions. The generated positively buoyant bio-mediated flocs were formed by microflocs coated by a TEP matrix. For the slack water conditions, FSDs were highly dependent of the TEP concentration and stickiness. Positively buoyant bio-mediated flocs were generated for specific concentrations and stickiness of TEP only. The corresponding FSDs were consistent with positively buoyant macroflocs, with TEP located mostly inside the flocs.

Determining coral reef calcification and primary production using automated alkalinity, pH and pCO2 measurements at high temporal resolution

We investigated coral reef carbonate chemistry dynamics and metabolic rates using an automated system that measured total alkalinity (TA, 30 min intervals), pH on the total scale (pHT, 10 min intervals) and the partial pressure of carbon dioxide (pCO2, 1 min intervals) over 2 weeks at Heron Island (Great Barrier Reef, Australia). The calculation of pHT (using the pCO2 and TA pair) and pCO2 (using the pH and TA pair) had similar values to the measured pHT and pCO2 values. In contrast, calculated TA from the pCO2-pH pair showed a large discrepancy with measured TA (average difference between measured and calculated TA = 52 μmol kg−1). High frequency sampling allowed for detailed analysis of the observations and an assessment of optimum sampling intervals required to characterise the net ecosystem calcification (NEC) and production (NEP) using a slack water approach. Depending on the sampling interval (30 min–2 h time steps) used for calculations, the estimated daily NEC and NEP could differ by 12% and 30%, respectively. Abrupt changes in both NEC and NEP were observed at dawn and dusk, with positive NEC during these periods despite negative NEP. Integrating NEC and NEP over a full diel cycle using 1 or 2 h integration time steps resulted in small differences of 2–7% for NEC and 1–3% for NEP. A diel hysteresis pattern rather than a simple linear relationship was observed between the aragonite saturation state (Ωar) and NEC. The observed hysteresis supports recent studies suggesting that short-term observations of seawater Ωar may not be a good predictor of long-term changes in NEC due to ocean acidification. The slope of the DIC to TA relationship was slightly higher (0.33) in 2014 than in an earlier study in 2012 (0.30). The automated, high frequency sampling approach employed here can deliver high precision data and can be used at other coral reef research stations to reveal long-term changes in NEC and NEP potentially driven by ocean acidification, eutrophication or other local changes.

Integrative geoarchaeological research on settlement patterns in the dynamic landscape of the northwestern Nile delta

Settlement activity in the Nile delta is characterized by its profound connection to the branches of the river Nile. Major ancient settlements were founded next to waterways. The constant shifting of these Nile branches – coming either too close or moving too far away – was a fundamental challenge for settlements. This research focuses on the region around Buto (Tell el-Fara'in) in the northwestern Nile delta. The massive increase in settlements in the Ptolemaic, Roman and in particular late Roman periods (4th c BC – 7th c AD) needs to be connected with a fundamental change in the landscape and the way people interacted with the landscape. A particular challenge to reconstructing the ancient land and waterscape was posed by the regional lack of an indicative modern surface relief. A linear settlement pattern of ancient sites was interpreted as showing the distribution along a defunct river branch. A combination of remote sensing data, in particular a new high resolution DEM based on Tandem-X data, and a program of over thirty cores on the ground, have clarified the landscape, especially its fluvial pattern, and the placements of associated settlements. In the north of the study region, the DEM shows elevated levees of former palaeorivers belonging to a finely ramified subdelta, with all settlements placed on alluvial levees. The corings uncovered different artificial channels and identified ancient natural riverbeds at a deep level but similar depth, suggesting that the streams were active during the occupation of these sites and the ancient settlements were either in direct vicinity of the natural rivers or connected via artificial channels. These artificial channels found in corings next to the settlements show characteristics of slack water regimes. In essence, the massive increase of settlements spread over the northwestern delta in Classical Antiquity was spurred by multiple branches that provided routes of transportation, fresh water for irrigation and good conditions for agriculture on their elevated and fertile levees.

Water velocity modeling can delineate nearshore and main channel plankton environments in a large river

Methods to distinguish nearshore and main channel ecosystems within large rivers are essential for observing physical, chemical, and biological features that influence overall river ecosystem function. Water velocity fields based on hydrodynamic modeling of water flow trajectory were used to evaluate water history (i.e., water moving slowly as in a slack water region, or more rapidly, as characteristic of the main channel) prior to passing a given point in the Upper St. Lawrence River. Using this method to differentiate zones in the river, observations of biotic and abiotic variables in nearshore and main channel zones during late spring and summer (June–August) were compared to assess the difference in these water column river ecosystems. Differences in plankton community composition between nearshore and main channel waters along the Upper St. Lawrence River were investigated to test the hypothesis that nearshore and main channel environments in large river systems have different biotic (e.g., phytoplankton, crustacean zooplankton, and rotifer concentrations) and abiotic [e.g., water temperature, specific conductivity, silicate, colored dissolved organic matter (CDOM), and total phosphorus] characteristics. Nearshore water had significantly higher concentrations of CDOM and chlorophyll-a than main channel waters. With distance downstream, crustacean zooplankton and rotifers decreased in abundance in both nearshore and main channel regions. This study describes an effective method for stratified sampling design that differentiates nearshore and main channel ecosystems in the water column of large rivers.

Harbour seals (Phoca vitulina) around an operational tidal turbine in Strangford Narrows: No barrier effect but small changes in transit behaviour

Abstract Data were obtained from 32 electronic tags that were glued to the fur of harbour seals (Phoca vitulina) in and around Strangford Lough, Northern Ireland, during the environmental monitoring of the SeaGen tidal turbine. This study provides the first detailed information on the behaviour of marine mammals close to a commercial‐scale tidal energy device. The turbine did not prevent transit of the animals through the channel and therefore did not result in a ‘barrier’ effect. However, the animals' behaviour did change when the turbine was operating, demonstrating the importance of allowing for behavioural responses when estimating collision risks associated with tidal turbines. Tagged animals passed the location of the device more frequently during slack water than when the current was running. In 2010 the frequency of transits by tagged seals reduced by 20% (95% CI: 10–50%) when the turbine was on, relative to when it was off. This effect was stronger when considering daylight hours only with a reduction of transit rate of 57% (95% CI: 25–64%). Seals tagged during the operational period transited approximately 250 m either side of the turbine suggesting some degree of local avoidance compared with the pre‐installation results. The results presented here have implications for monitoring and managing the potential interactions between tidal turbines and marine wildlife. Principally that the design of telemetry studies for measuring change in response to developments should seek to understand and take into account variability in seal behaviour. This study only looked at the effects of a single turbine rather than an array, and mitigation limited the ability to determine close range interactions. However, the study indicates that the effect of the turbine on Strangford Lough harbour seals was minor and that collision risk was reduced by the behaviour of the seals.

Indikator Penciri Penanggalan Hijriah pada Pergerakan Pasang Surut

Rhythmic movement of the tides follow the movement of the moon,as the dominant component of which has a rhythmic, should the movement of tidal inform month trip in the form of style variations of tide generating force (GPP) which indicates the time in the Hijra calendar. This article aims to determine the indicator of the tidal movement indicating the time of the Hijri calendar. The timing of slack water (t sw) the movement receded into pairs (tsw s) in peak II relatively consistent for the three months of data were analyzed tidal movement. Consistency is shown on the results of the statistical analysis by comparing the Probability (Pr) to the Wilks’ Lambda ( ) is very small(<0.0001) which means that the time of slack water (t sw) can be a primary identifier in determining the Hijra calendar. Further research is needed to determine the time point t SWS belt of a general nature in order to facilitate the determination of the Hijra calendar through the movement of the tides.DOI: 10.15408/ajis.v17i2.5686

Use of time-depth recorders to determine fishing depth of commercial setnets and inform management in Cook Inlet, Alaska

BackgroundAccess to harvestable surpluses of sockeye salmon is often limited by incidental harvest of Chinook salmon in the mixed-species gillnet fishery of Cook Inlet, Alaska, particularly in years of low Chinook abundance. This is restricting economic opportunity and creating allocation conflict between user groups. Prior work quantified differences in migration depth of Chinook and sockeye salmon and suggested that the use of shallower surface-hung fixed gillnets (“setnets”) could potentially reduce Chinook interception rates while leaving sockeye harvest rates relatively stable. However, lack of knowledge concerning the actual fishing depth of setnets was identified as an important uncertainty in implementing regulatory change. Cook Inlet is subject to strong tidal currents and high suspended sediment loads which make the water virtually opaque, preventing simple visual assessment of the depth of fishing nets.ResultsWe used time-depth recorders attached to setnets within Cook Inlet to record the leadline depths of 29, 36, and 45 mesh setnets during all fishing openings in 2015. Recorded depths were far shallower than the physical construction of the nets would suggest for most of the tidal cycle (as shallow as 0.6 m), and only near slack water did they extend down to near their maximum potential depth.ConclusionsIn this study, we demonstrate how time-depth recorders can be used to document the behavior of fishing gear and provide information useful for fine-tuning changes to regulations governing fishing net construction. Most Chinook harvest presumably occurs around slack water because it was only during these narrow time windows that the nets extended into the depths where Chinook primarily migrate. Switching to reduced-depth nets would avoid much of the Chinook distribution. In contrast, sockeye are potentially exposed to nets over much of the tidal cycle, although data on the efficiency of nets at various stages of the tide are lacking. Reduced sockeye harvest rates might be compensated for by some increase in allowable fishing time.