Spatio-temporal patterns of land use/land cover change in the heterogeneous coastal region of Bangladesh between 1990 and 2017

Although a detailed analysis of land use and land cover (LULC) change is essential in providing a greater understanding of increased human-environment interactions across the coastal region of Bangladesh, substantial challenges still exist for accurately classifying coastal LULC. This is due to the existence of high-level landscape heterogeneity and unavailability of good quality remotely sensed data. This study, the first of a kind, implemented a unique methodological approach to this challenge. Using freely available Landsat imagery, eXtreme Gradient Boosting (XGBoost)-based informative feature selection and Random Forest classification is used to elucidate spatio-temporal patterns of LULC across coastal areas over a 28-year period (1990-2017). We show that the XGBoost feature selection approach effectively addresses the issue of high landscape heterogeneity and spectral complexities in the image data, successfully augmenting the RF model performance (providing a mean user's accuracy > 0.82). Multi-temporal LULC maps reveal that Bangladesh's coastal areas experienced a net increase in agricultural land (5.44%), built-up (4.91%) and river (4.52%) areas over the past 28 years. While vegetation cover experienced a net decrease (8.26%), an increasing vegetation trend was observed in the years since 2000, primarily due to the Bangladesh government's afforestation initiatives across the southern coastal belts. These findings provide a comprehensive picture of coastal LULC patterns, which will be useful for policy makers and resource managers to incorporate into coastal land use and environmental management practices. This work also provides useful methodological insights for future research to effectively address the spatial and spectral complexities of remotely sensed data used in classifying the LULC of a heterogeneous landscape

Global variation in anastomosis and end colostomy formation following left-sided colorectal resection

Background End colostomy rates following colorectal resection vary across institutions in high-income settings, being influenced by patient, disease, surgeon and system factors. This study aimed to assess global variation in end colostomy rates after left-sided colorectal resection. Methods This study comprised an analysis of GlobalSurg-1 and -2 international, prospective, observational cohort studies (2014, 2016), including consecutive adult patients undergoing elective or emergency left-sided colorectal resection within discrete 2-week windows. Countries were grouped into high-, middle- and low-income tertiles according to the United Nations Human Development Index (HDI). Factors associated with colostomy formation versus primary anastomosis were explored using a multilevel, multivariable logistic regression model. Results In total, 1635 patients from 242 hospitals in 57 countries undergoing left-sided colorectal resection were included: 113 (6·9 per cent) from low-HDI, 254 (15·5 per cent) from middle-HDI and 1268 (77·6 per cent) from high-HDI countries. There was a higher proportion of patients with perforated disease (57·5, 40·9 and 35·4 per cent; P < 0·001) and subsequent use of end colostomy (52·2, 24·8 and 18·9 per cent; P < 0·001) in low- compared with middle- and high-HDI settings. The association with colostomy use in low-HDI settings persisted (odds ratio (OR) 3·20, 95 per cent c.i. 1·35 to 7·57; P = 0·008) after risk adjustment for malignant disease (OR 2·34, 1·65 to 3·32; P < 0·001), emergency surgery (OR 4·08, 2·73 to 6·10; P < 0·001), time to operation at least 48 h (OR 1·99, 1·28 to 3·09; P = 0·002) and disease perforation (OR 4·00, 2·81 to 5·69; P < 0·001). Conclusion Global differences existed in the proportion of patients receiving end stomas after left-sided colorectal resection based on income, which went beyond case mix alone

Floodplain avulsion channels: understanding their distribution and how they reconnect to the parent channel

© 2018 Dr. Md Abdullah Al BakyThis study is concerned with new river channels that develop on floodplains. These channels can develop gradually, or they can develop more rapidly (avulsions). This study concentrates on the relatively more rapid channel changes known as avulsions. An avulsion specifies the gradual abandonment of an existing river-channel and in response to this, the processes of development of a new channel on a floodplain nearby. The study addresses two specific knowledge gaps: 1) how common are river floodplain avulsions globally, and 2) what are the detailed processes that occur at the up and downstream points where avulsions connect to the main channel? Using random sampling from a global spatial layer I discovered that developing avulsions are extremely common on alluvial floodplains globally, wherever the floodplain is wider than several channel widths. Avulsions are most common on single thread meandering floodplain types, but a review of avulsion literature shows that research is biased to relatively less common floodplain types. Avulsions increase the rate of valley widening, particularly in narrow floodplains. There is a relationship between floodplain width and the number of avulsion channels. The rest of the thesis is focussed in the major process knowledge gap which is how avulsion channels connect into the main channel at the up and downstream ends. The focus of the process component of this study is the broad Murray river floodplain from Yarrawonga to Echuca, SE Australia. I mapped and classified developing channels on the floodplain and found that the avulsion connection point here develops in an unusual way, involving the development and coalescence of low points (depressions) on the levee (this mechanism is very different from normal crevasse splay development). The chain of low points on the alluvial levee coalesce to form a levee channel. Rather than forming by erosion as expected from the literature, form progressively by locally reduced vertical accretion. This identifies a new process by which topography is developed on floodplains. Initially the levee channels are not connected with the Murray main stream and slope away from the river. The connection occurs by lateral migration of the river bank into the levee channel, but also by progressive upslope (river ward) migration of the deepest part of the levee channel towards the river, narrowing the gap between the river and the levee channel. Following connection, the levee channel captures flow from the river, and hydraulic modelling shows that shear stress is sufficient to erode the upstream end of the levee channel. As the channel erodes the shear stress declines, but the proportion of back-flow from the flood recession increases. The result is that the slope of the levee channel reverses to slope towards the river. This is a new mechanism, and it is critical in the sequence of avulsion development. The final stage of the development of avulsion is when a knickzone moves up the levee channel joining another levee channel that is leaving the river upvalley. This is new mechanism of avulsion likely to operate in low energy river systems dominated by fine-grained sediments