POSTGLACIAL SEA-LEVEL LOWSTAND ON CUMBERLAND PENINSULA, BAFFIN ISLAND, EASTERN ARCTIC CANADA

This study investigates the postglacial sea-level history of eastern Cumberland Peninsula, a region of Baffin Island, Nunavut where submerged terraces were documented in the 1970s. The gradient in elevation of emerged postglacial marine-limit deltas and fiord-head moraines led Dyke (1979) to propose a conceptual model for continuous postglacial submergence of the eastern peninsula. Multibeam mapping over the past decade has revealed eight unequivocal submerged deltas at 19-45 m below [present] sea level (bsl) and other relict shore-zone landforms (boulder barricade, spits, and sill platform) at 16-51 m bsl. Over a distance of 115 km from Qikiqtarjuaq to Cape Dyer, the submerged coastal features increase in depth toward the east, with a slope (0.36 m/km), somewhat less than that of the marine-limit shoreline previously documented (0.58-0.62 m/km). The submerged ice-proximal deltas, deglacial ice limits, and radiocarbon ages constrain the postglacial lowstand between 9.9 and 1.4 ka cal BP. The glacial-isostatic model ICE-7G_NA (VM7) (Peltier 2020) computes a lowstand relative sea level at 8.0 ka, the depth of which increases eastward at 0.28 m/km. The difference between observed and model-derived lowstand depths ranges from 1 m in the west to 10 m in the east and the predicted tilt is significantly less than observed (p=0.0008). The model results, emerging data on Holocene glacial re-advances on eastern Baffin Island, and evidence for proglacial delta formation point to a Cockburn (9.5-8.2 ka) age for the lowstand, most likely later in this range. This study confirms the 1970s conceptual model of postglacial submergence in outer Cumberland Peninsula and provides field evidence for further refinement of glacial-isostatic adjustment models.

Submarine lava deltas of the 2018 eruption of Kīlauea volcano

Hawaiian and other ocean island lava flows that reach the coastline can deposit significant volumes of lava in submarine deltas. The catastrophic collapse of these deltas represents one of the most significant, but least predictable, volcanic hazards at ocean islands. The volume of lava deposited below sea level in delta-forming eruptions and the mechanisms of delta construction and destruction are rarely documented. Here, we report on bathymetric surveys and ROV observations following the Kīlauea 2018 eruption that, along with a comparison to the deltas formed at Pu‘u ‘Ō‘ō over the past decade, provide new insight into delta formation. Bathymetric differencing reveals that the 2018 deltas contain more than half of the total volume of lava erupted. In addition, we find that the 2018 deltas are comprised largely of coarse-grained volcanic breccias and intact lava flows, which contrast with those at Pu‘u ‘Ō‘ō that contain a large fraction of fine-grained hyaloclastite. We attribute this difference to less efficient fragmentation of the 2018 ‘a‘ā flows leading to fragmentation by collapse rather than hydrovolcanic explosion. We suggest a mechanistic model where the characteristic grain size influences the form and stability of the delta with fine grain size deltas (Pu‘u ‘Ō‘ō) experiencing larger landslides with greater run-out supported by increased pore pressure and with coarse grain size deltas (Kīlauea 2018) experiencing smaller landslides that quickly stop as the pore pressure rapidly dissipates. This difference, if validated for other lava deltas, would provide a means to assess potential delta stability in future eruptions.

Spatial-temporal Variability of the Chlorophyll “a” Concentration in the Krasnodar Reservoir According to the Satellite Images’ Data

The Krasnodar Reservoir has undergone significant transformations during its operation since 1973. As a result, of active delta formation of the Kuban and Belaya rivers, the reservoir was divided into two autonomous reservoirs, its area decreased by 35 %. The study is aimed at determining the significance of the eutrophication factor in the transformation of the Krasnodar Reservoir. Methods. Based on 51 Sentinel-2 satellite images for 2015–2020 the results of calculations of the concentration of chlorophyll «a» as an indicator of phytoplankton biomass in the reservoir are presented. The MSI sensor calculates the concentration of chlorophyll a (chl «a») based on the normalized chlorophyll difference index. Chl «a» estimates were obtained for a total area of 277 km2; the main bowl of the Krasnodar Reservoir and the Tshchik Reservoir, which was cut off from it. We have built maps reflecting the spatial and intra-annual variability of chl «a». Results The highest average concentrations of chl «a» are characteristic of the shallow upper part of the Krasnodar Reservoir, which is influenced by the runoff of the Kuban and Pshish rivers with an increased nutrient content. Locally high average values of chl «a» were recorded in the areas of the runoff current and in the estuaries of the left-bank tributaries. In the Tshchik water body with a low intensity of water exchange, the relationship between the concentration of chl «a» and hydrodynamic processes is not evident. We have revealed features of the annual cycle of phytoplankton biomass. It has been established that the trophic status of the reservoir is subject to seasonal variability: in spring, 2/3 of the water area correspond to eutrophic waters, 1/3 to mesotrophic waters; in autumn, the entire reservoir has a eutrophic status.

On Water Circulation in the Valley Reservoir (Krasnodar Reservoir)

The Krasnodar reservoir has undergone significant transformations during its operation since 1973. As a result of active delta formation of the Kuban and Belaya rivers, the reservoir was divided into two autonomous reservoirs, its area decreased by 35 %. To understand the mechanisms of transformation and the processes of siltation of the reservoir, it is necessary to establish the features of the dynamics of water masses. Based on the results of the ADCP survey carried out in July-August, 2016, the circulation of water masses in the reservoir was analyzed. The distance between survey lines during the survey was 100 m with a total length of 2518 km, the frequency of measurements was 0.28 m-1. The resulting array of data on the velocity vectors (18.6 million values) in combination with the resulting digital model of the reservoir basin was processed in GIS using geostatistical analysis tools. It has been established that the general dynamics of water masses is characterized by cyclonic circulation with a pronounced western runoff current along the right bank of the reservoir. The prevailing velocities of currents are 0.02 ... 0.05 m/s. With steady westerly winds, a shift of the main water jet to the south into the interior of the reservoir was recorded. In the vertical movement of water masses, ubiquitous downward currents were noted in the eastern part of the reservoir, caused by the influx of colder river waters.

Experimental Study at the Reservoir Head of Run-of-River Hydropower Plants in Gravel Bed Rivers. Part II: Effects of Reservoir Flushing on Delta Degradation

Run-of-river hydropower plants (RoR HPPs) are capable of interrupting the sediment connectivity of many alpine rivers. Still, there is a lack of systematical investigations of possible sediment management strategies for small and medium sized RoR HPPs. This study deals with the headwater section of an impoundment and the approach of sediment remobilization during drawdown operations. Therefore, a typical medium sized gravel bed river having a width of 20 m, a mean bed slope of 0.005, a mean flow rate of 22 m3/s, and a 1-year flood flow of 104 m3/s is recreated by a 1:20 scaled physical model. Heterogenous sediment mixtures were used under mobile-bed conditions, representing a range of 14–120 mm in nature. During the experiments, the flow rate was set to be 70% of the 1-year flood (HQ1) regarding on the ability to mobilize all sediment fractions. The possibility to remobilize delta depositions by (partial) drawdown flushing within a reasonable period (≈9 h in 1:1 scale) was shown by the results. The erosion of existing headwater delta deposition was found to be retrogressive and twice as fast as the preceding delta formation process. A spatiotemporal erosion scheme points out these findings. This supports the strategy of a reservoir drawdown at flood events of high reoccurrence rate.

Cordon approach in a delta

Chapter 11 illustrates the consequences of the Cordon approach through the experience of the Bengal Delta, formed by three great river systems—the Ganges, Brahmaputra, and Meghna. The chapter explains the delta formation process in general, noting the stages through which it progresses. It presents the basic facts regarding the Bengal delta, which now spreads across both Bangladesh and India. The chapter reviews the application of the Cordon approach in both parts of the delta. It notes that the approach received wider application in Bangladesh, which contains the larger and active part of the Bengal Delta. In the process, the approach gave rise to different types of cordons, rural and urban; coastal and inland; and partial and full. The chapter shows that while the specificities differ, the Cordon approach in each case led to separation of the land from the nurturing functions of river overflows, emergence of the new danger of catastrophic flooding, and the nagging problem of waterlogging. The Cordon approach also led to the rise of conflicts, pitting people inside the cordons with those remaining outside, who witnessed aggravation of flooding.

Experimental Study at the Reservoir Head of Run-of-River Hydropower Plants in Gravel Bed Rivers. Part I: Delta Formation at Operation Level

This study concerns scaled physical model tests of the delta formation process at the head of a run-of-river hydropower plant (RoR). It forms part of a larger research project to provide a scientific base for RoR sediment management strategies in medium-sized gravel bed rivers. The physical model consisted of an idealized river having a width of 20 m, a mean slope of 0.005, a mean flow rate of 22 m3/s and a 1-year flood flow of 104 m3/s. The model scale was 1:20. For the experiments, five different grain sizes were used, covering a range of 14 to 120 mm at 1:1 scale. Experiments were carried out under mobile-bed conditions at flow rates which correspond to 50%–80% of a 1-year flood HQ1. Even at the head of the reservoir, which is least influenced by the backwater effect of the RoR, sediment transport practically ceases for sediment fractions >14 mm for a flow rate of 0.7 × HQ1. The whole sediment load coming from the undisturbed upstream section accumulates at the head of the reservoir. This delta formation is accompanied by a substantial rise in water levels. A spatio-temporal scheme of the delta formation was derived from the experiments. The study proved that the delta formation increases the flood risk at the head of the reservoir. Conversely, reservoir drawdowns at flood events of high probability may be a promising strategy to enhance sediment connectivity under the specified boundary conditions.

Multiyear changes of the Danube delta marine edge

With the help of field and remote (space) methods, detailed studies of long-term changes of the Danube delta coastline (DCL) were carried out. It was established that the DCL displacements (progradation or retreat) can be an effective indicator of delta formation process as a whole: the dynamics of the delta channel network and the redistribution of water flow and sediment load between the branches, the impact of external factors on the delta (river water flow and sediment load, wind and wave action, rise or drop of the sea level). To determine the causes and characteristics of the Danube DCL changes on seven genetically homogeneous areas the contribution of various factors to the balance of sand sediments forming the DCL was evaluated. Locations of the DCL progradation or retreat were determined. Comparison of the results of a study of the Danube DCL dynamics with the data on changes of morphology and regime of other large deltas may allow one to assess the causes and trends of modern processes in river deltas.