Organic carbon burial by river meandering partially offsets bank-erosion carbon fluxes in a discontinuous permafrost floodplain

Arctic river systems erode permafrost in their banks and mobilize particulate organic carbon (OC). Meandering rivers can entrain particulate OC from permafrost many meters below the depth of annual thaw, potentially enabling OC oxidation and the production of greenhouse gases. However, the amount and fate of permafrost OC that is mobilized by river erosion is uncertain. To constrain OC fluxes due to riverbank erosion and deposition, we collected riverbank and floodplain sediment samples along the Koyukuk River, which meanders through discontinuous permafrost in central Alaska. We measured sediment total OC (TOC), radiocarbon content, water content, bulk density, grain size, and floodplain stratigraphy. Radiocarbon abundance and TOC were higher in samples dominated by silt as compared to sand, which we used to map OC content onto floodplain stratigraphy and estimate carbon fluxes due to river meandering. Results showed that sediment being eroded from cutbanks and deposited as point bars had similar OC stocks (mean ± 1SD of 125.3 ± 13.1 kgOC m−2 in cutbanks versus 114.0 ± 15.7 kgOC m−2 in point bars) whether or not the banks contained permafrost. We also observed radiocarbon-depleted biospheric OC in both cutbanks and permafrost-free point bars. These results indicate that a significant fraction of aged biospheric OC that is liberated from floodplains by bank erosion is subsequently re-deposited in point bars, rather than being oxidized. The process of aging, erosion, and re-deposition of floodplain organic material may be intrinsic to river-floodplain dynamics, regardless of permafrost content.

Challenges Related to Pipeline Free Spans in Watercourse Crossings

Free span assessment in watercourse crossings for the on-shore pipeline industry has become a more and more important part of pipeline integrity practice. One reason is that it has become increasingly well known that the dominant cause of pipeline failures in watercourse crossings is fatigue failure due to vortex induced vibrations at pipeline free spans. Recognition of this is now being identified in industry recommended practices and owners are incorporating this type of assessment into their pipeline integrity management practice. On shore pipelines are not designed with an allowable free span as is the practice with off-shore pipelines, but are buried. Design codes specify minimum depths of cover when constructed and indicate that pipelines should be maintained so that no excessive loads occur. In the past the no excessive loads requirement has been interpreted that the pipeline must remained buried. As experience from the off-shore environment and increasingly from experience on-shore has shown that most exposed and/or free spans do not fail. Due to various river erosion mechanisms; scour, bank erosion or avulsion, previously buried pipelines do develop free spans. Some of the free spans fail and release products directly into the watercourse. Failures, particularly for liquid products, are very expensive due to the economic loss, repair costs and environment clean-up of the watercourse and its banks. Similarly, costs associated with pipeline replacement for free spanning pipelines or repair of pipelines that might develop free spans are relatively high. It is important to develop an understanding of the probability of the pipeline failing due to a free span, or put another way, determine which free span is a threat to integrity. This paper discusses some of the challenges with assessing free spans in watercourse crossings as part of integrity programs and highlights experiences in assessing this threat to integrity. The objective of this paper is to discuss some of the key uncertainties related to the management of the threat due to free spans. These uncertainties are due to the reliability of information about the free span, water velocity and condition of the pipelines.

Fracture variability in basalts and its effect on river erosion: a case study in the Paraná Volcanic Province

The variation in the structural characteristics (cooling joints and tectonic fractures) of basaltic flows implies potential variability in the intensity of erosion by plucking. The erosive behavior of the rivers that sculpt these areas depends on their interaction with the diverse fracture systems. In view of this, we analyzed the effect of fracture variability in basalts on erosion in a bedrock river reach located in the Continental Volcanic Province of the Paraná Basin, southern Brazil. The 120-m-long reach is influenced somewhat by a possible fault that crosses it near one end. The fracture density and fracture direction were evaluated through field photogrammetry in seven sample areas distributed along the reach. The fracture direction and main erosion axes were also surveyed by remote piloted aircraft (RPA) aerial imaging. Tectonic fractures were identified in the field; they do not always appear in the survey of the sample areas but are evident in the RPA survey. The main erosion axes coincide with the principal fracture directions (tectonic fractures), which are disposed obliquely to the channel flow direction, making an average angle of 50°. The more abundant and multidirectional cooling joints act to control the plucking process and not to determine the erosion direction. The fracture density decreases with increasing distance from the fault crossing zone (from 9.62 to 3.73 m/m²), although the lower value is influenced by the presence of an amygdaloidal basalt zone. The higher fracture density favors more intense plucking.

Impact of Climate Change and Ongoing Adaptation Measures in the Bangladesh Sundarbans

The climate of Bangladesh has changed drastically which may put considerable adverse impacts on mangrove fishers but very few studies focused on this professional group. An attempt was made to perceive the impact and adaptation measures of the Sundarbans mangrove resource users, employing interviews and focus group discussions. A total of 150 respondents were randomly selected from the Sundarbans west under Shyamnagar Upazila of Satkhira District. It was revealed that the abundance of fishes, fuel woods, honey, golpata (Nypa fruticans), and shrimp post-larvae (PL) was reduced considerably. The resource users have adapted themselves by changing their occupation and becoming jobless and depending on the other family members. PL collection, honey collection, shrimp culture, and wood collection were found professional adapting strategies to adopt cyclone, flood, salinity intrusion, river erosion, and drought. Several recommendations are elicited, the implementation of which is important to ensure livelihood sustainability of the mangrove communities.

Mechanism of the Rapid Shrinkage of Limboto Lake in Gorontalo, Indonesia

This study explores the mechanisms behind the rapid shrinkage of Limboto Lake, Gorontalo, Indonesia, using remotely sensed imagery and river outcrop investigation data. The results show that more than 70% of the sedimentation resulting in shrinkage is contributed by riverbank erosion causing rivers to drain into the lake during the period 2003–2017. From geological investigation, it is found that the lowland area to the west of Limboto Lake comprises 1 m of flood sediments, followed by at least 5 m of fine-grained inner bay sediments. Severe riverbank erosion is also observed at many points. Hence, it is concluded that the shrinkage of Limboto Lake resulted from rapid-induced rapid erosion of inner bay sediments formed during plate collision which readily flowed into and were deposited in Limboto Lake; this created a delta, especially on the west side of the lake. Accelerated sedimentation caused by river erosion has led to rapid lake shrinkage. This phenomenon could be typical of the transformation of enclosed seas into lakes by the rapid uplifting movement of land in collision zones.