Sand spit dynamics in a large tidal-range environment: Insight from multiple LiDAR,UAV and hydrodynamic measurements on multiple spit hook development,breaching, reconstruction,and shoreline changes

Sand spits with distal hooks have been well documented from coasts with low to moderate tidal ranges, unlike high tidal-range environments. Datasets from 15 LiDAR and 3 UAV surveys between 2009 and 2019 on the Agon spit in Normandy (France), a setting with one of the largest tidal ranges in the world (mean spring tidal range: 11 m), combined with in-situ hydrodynamic records between 2013 and 2017, highlight a three-stage pattern of spit hook evolution. Stage 1 (2009–2013) commenced with the onshore migration and attachment of a swash bar, followed by persistent spit accretion updrift of the bar and erosion downdrift because of the slow speed of bar migration in this large tidal-range environment. In stage 2 (2013–2016), three overwash events and a 220 m-wide breach culminating in the total destruction of the spit during winter 2015–2016 involved the landward mobilization of thousands of cubic metres of sand. These events occurred during short durations (a few hours) when spring high tides coincided with relatively energetic waves, underscoring the importance of storms in rapid spit morphological change. Strong spring tidal currents maintained the breach. Stage 3 (2016–2019) has involved new hook construction through welding of a swash bar and spit longshore extension, highlighting the resilience of the spit over the 10-year period, and involving a positive sediment balance of 244 000 m³. The three stages bring out, by virtue of the temporal density of LiDAR and UAV data used, a high detail of spit evolution relative to earlier studies in this macrotidal setting. The large tidal range strongly modulates the role of waves and wave-generated longshore currents, the main process drivers of spit evolution, by favouring long periods of inertia in the course of the spring–neap tidal cycle, but also brief episodes of significant morphological change when storm waves coincide with spring high tides. © 2020 John Wiley & Sons, Ltd.     

Exceptional rates and mechanisms of muddy shoreline retreat following mangrove removal

Probably the largest regular shoreline fluctuations on Earth occur along the 1500 km-long wave-exposed Guianas coast of South America between the mouths of the Amazon and Orinoco Rivers, the world's longest muddy coast. The Guianas coast is influenced by a succession of mud banks migrating northwestward from the Amazon. Migrating mud banks dissipate waves, partially weld onshore, and lead to coastal progradation, aided by large-scale colonization by mangroves, whereas mangrove-colonized areas between banks (inter-bank areas) are exposed to strong wave action and undergo erosion. On large tracts of this coast, urbanization and farming have led to fragmentation and removal of mangroves, resulting in aggravated shoreline retreat. To highlight this situation, we determined, in a setting where mangroves and backshore freshwater marshes have been converted into rice polders in French Guiana, shoreline change over 38 years (1976-2014) from satellite images and aerial orthophotographs. We also conducted four field experiments between October 2013 and October 2014, comprising topographic and hydrodynamic measurements, to determine mechanisms of retreat. The polder showed persistent retreat, at peak rates of up to -200 m/yr, and no recovery over the 38-year period of monitored change. Notwithstanding high erosion rates, mangrove shorelines show strong resilience, with recovery characterized by massive accretion. Retreat of the polder results in a steep wave-reworked shoreface with a lowered capacity for bank welding onshore and mangrove establishment. Persistent polder erosion is accompanied by the formation of a sandy chenier that retreats landwards at rates largely exceeding those in inter-bank situations. These results show that anthropogenic mangrove removal can durably modify the morphodynamics of muddy shorefaces. This limits the capacity for shoreline recovery and mangrove re-establishment even when there is no sustained long-term deficit in mud supply, as in the case of the Amazon-influenced Guianas coast. © 2019 John Wiley & Sons, Ltd.     

Topographic variation in soil erosion and accumulation determined with meteoric 10Be

Understanding natural soil redistribution processes is essential for measuring the anthropogenic impact on landscapes. Although meteoric beryllium-10 (¹⁰Be) has been used to determine erosion processes within the Pleistocene and Holocene, fewer studies have used the isotope to investigate the transport and accumulation of the resulting sediment. Here we use meteoric ¹⁰Be in hilltop and valley site soil profiles to determine sediment erosion and deposition processes in the Christina River Basin (Pennsylvania, USA). The data indicate natural erosion rates of 14 to 21 mm 10⁻³ yr and soil ages of 26 000 to 57 000 years in hilltop sites. Furthermore, valley sites indicate an alteration in sediment supply due to climate change (from the Pleistocene to the Holocene) within the last 60 000 years and sediment deposition of at least 0.5–2 m during the Wisconsinan glaciation. The change in soil erosion rate was most likely induced by changes in geomorphic processes; probably solifluction and slope wash during the cold period, when ice advanced into the mid latitudes of North America. This study shows the value of using meteoric ¹⁰Be to determine sediment accumulation within the Quaternary and quantifies major soil redistribution occurred under natural conditions in this region. © 2018 John Wiley & Sons, Ltd.     

Agricultural Heritage Systems: A Bridge between Urban and Rural Development

In the early 1990s, China stepped into the stage of rapid urbanization with a flourishing economy and new technological development. Surplus labor from rural areas flooded into cities and became the main force of urban development. However some severe threats to human survival emerged from urbanization, such as over- extensive urban development, excessive resource consumption, ecological degradation, food security and safety risks and social crises. It has become an imperative to balance urban and rural development to achieve greater harmony between nature and society. This paper firstly tried to focus on public dietary change, agricultural industry development, institutional guarantee, ecological restoration, as well as cultural tourism in urban and peri-urban agricultural heritage sites. Then, it established a development model, balanced the urbanization and urban-supported agriculture. This paper proposed “Agricultural heritage systems” as an entry point for balancing the development of urban areas and rural areas. Agricultural heritage systems can inherit local traditional culture, keep the green and organic agriculture cultivation systems, exploit the distinct landscape tourism, and the like for diversified development; In addition, agricultural heritage systems can take full advantage of abundant funds, firm the institutional guarantee and advanced technologies from the nearby urban complex for regurgitation-feeding of rural enterprises. With the help of these strategies, we can achieve the harmony of “Ecological Urban” and “Garden Countryside”.     

Effect of Different Sampling Methodologies on Measured Methane Concentrations in Groundwater Samples

Analysis of dissolved light hydrocarbon gas concentrations (primarily methane and ethane) in water supply wells is commonly used to establish conditions before and after drilling in areas of shale gas and oil extraction. Several methods are currently used to collect samples for dissolved gas analysis from water supply wells; however, the reliability of results obtained from these methods has not been quantified. This study compares dissolved methane and ethane concentrations measured in groundwater samples collected using three sampling methods employed in pre‐ and post‐drill sampling programs in the Appalachian Basin. These include an open‐system collection method where 40 mL volatile organic analysis (VOA) vials are filled directly while in contact with the atmosphere (Direct‐Fill VOA) and two alternative methods: (1) a semi‐closed system method whereby 40 mL VOA vials are filled while inverted under a head of water (Inverted VOA) and (2) a relatively new (2013) closed system method in which the sample is collected without direct contact with purge water or the atmosphere (IsoFlask^®). This study reveals that, in the absence of effervescence, the difference in methane concentrations between the three sampling methods was relatively small. However, when methane concentrations equaled or exceeded 20 mg/L (the approximate concentration at which effervescence occurs in the study area), IsoFlask^® (closed system) samples yielded significantly higher methane concentrations than Direct‐Fill VOA (open system) samples, and Inverted VOA (semi‐closed system) samples yielded lower concentrations. These results suggest that open and semi‐closed system sample collection methods are adequate for non‐effervescing samples. However, the use of a closed system collection method provides the most accurate means for the measurement of dissolved hydrocarbon gases under all conditions.     

High-sediment tolerance in the reef coral Turbinaria mesenterina from the inner Great Barrier Reef lagoon (Australia)

Sedimentation is an important stressor on coral reefs subjected to run-off, dredging and resuspension events. Reefs with a history of high-sediment loads tend to be dominated by a few tolerant coral species. A key question is whether such species live close to their tolerance thresholds or near their niche optima. Here, we analyse experimentally the sediment tolerance of a spatially dominant coral, Turbinaria mesenterina (Dendrophylliidae), at nearshore reefs in the central Great Barrier Reef lagoon. Testing was conducted in a 5-week tank experiment under manipulated sediment loading and flow conditions. Physiological stress was assessed based on the behaviour of three key response variables: skeletal growth rate, energy reserves (lipid content) and photosynthetic performance. Because sediment effects are likely to vary between flow regimes, sediment and flow responses were tested using a full factorial design. Sediment loads greater than 110 mg cm⁻² had no effect on any of the physiological variables, regardless of flow (0.7–24 cm s⁻¹). Turbinaria mesenterina is thus tolerant to sediment loads an order of magnitude higher than most severe sediment conditions in situ. Likely mechanisms for such tolerance are that: (1) colonies covered in sediment (60–120 μm) in low-flow were able to clear themselves rapidly (within 4–5 h); and (2) sediment provides a source of food. These results suggest that intensified sediment regimes on coastal reefs may shift coral communities towards dominance by a few well-adapted species.     

Evaluation of tropical cloud regimes in observations and a general circulation model

Tropical cloud regimes defined by cluster analysis of International Satellite Cloud Climatology Project (ISCCP) cloud top pressure (CTP)–optical thickness distributions and ISCCP-like Goddard Institute for Space Studies (GISS) general circulation model (GCM) output are analyzed in this study. The observations are evaluated against radar–lidar cloud-top profiles from the atmospheric radiation measurement (ARM) Program active remote sensing of cloud layers (ARSCL) product at two tropical locations and by placing them in the dynamical context of the Madden–Julian oscillation (MJO). ARSCL highest cloud-top profiles indicate that differences among some of the six ISCCP regimes may not be as prominent as suggested by ISCCP at the ARM tropical sites. An experimental adjustment of the ISCCP CTPs to produce cloud-top height profiles consistent with ARSCL eliminates the independence between those regimes. Despite these ambiguities, the ISCCP regime evolution over different phases of the MJO is consistent with existing MJO mechanisms, but with a greater mix of cloud types in each phase than is usually envisioned. The GISS Model E GCM produces two disturbed and two suppressed regimes when vertical convective condensate transport is included in the model’s cumulus parameterization. The primary model deficiencies are the absence of an isolated cirrus regime, a lack of mid-level cloud relative to ARSCL, and a tendency for occurrences of specific parameterized processes such as deep and shallow convection and stratiform low cloud formation to not be associated preferentially with any single cloud regime.