77 research outputs found
Evaluation of an alternative way of river bank protection in the Meghna Estuary
No full textAfter the disastrous floods in Bangladesh in 1987 the Flood Action Plan (FAP) was initiated. The FAP was a co-ordinated action to study the flood problems of Bangladesh. The Meghna Estuary Study (MES) was drawn up as a component of the FAP. The overall goal of MES is to ensure the physical safety and social security of the people living in the coastal areas and on the islands of the Meghna Estuary. This goal is to be realised by retaining and increasing knowledge of the hydraulic and morphological processes and by developing appropriate techniques for efficient land reclamation as well as effective river bank protection. In particular during the monsoon season erosion of the river banks occurs throughout the Meghna Estuary. The erosion causes direct land loss (including loss of livelihood and impoverishment) and, where embankments are eroded, results in flooding of homesteads and agricultural land. Looking at the planform development of the Lower Meghna it is expected that in future years the eastern river bank of the Lower Meghna river starting south of Chandpur up to Haimchar is going to be subject to severe erosion. The river banks are highly erosive due to the non-cohesiveness of the sediments, the small particle size and the high flow velocities. Previous studies carried out have shown that protection of the river bank using conventional river bank protection methods is not generally economically viable at the current level of economic development in Bangladesh. It is expected that at the current level of economic development in Bangladesh alternative Iow cost bank protection measures are the only viable option to protect the river banks against erosion on a large scale. Nevertheless it must be stated that the circumstances are severe and the possibilities and necessary know-how have to be assessed by further research and execution of experiments. Under MES several pilot projects have been executed to study alternative ways of river bank protection. In part I of this study the author has studied the application of flexible bottom screens in combination with permeable spurs. In part II the application of bottom screens to create a roughness field is subject of discussion. Both studies are conducted under the responsibility of the Technical University Delft. One of the pilot projects executed by MES is the Hanar Char erosion control pilot scheme in which flexible bottom screens and permeable spurs are applied as an alternative way of river bank protection. This pilot scheme was executed from March to July 1999 and is part of the proposed Haimchar erosion control pilot project, which covers the complete eastern river bank from Chandpur up to Haimchar. However, up till August 2000 only a part of the Haimchar erosion control pilot project has been executed. The objective of this study is to determine whether flexible bottom screens and permeable spurs are an effective way of river bank protection in the river dominated part of the Meghna Estuary. This is done by studying the available literature and the influence of the structures on the flow pattern and morphology. Flexible bottom screens Flexible bottom screens show great analogy with so called bottom vanes which are placed at a certain angle of attack with the flow. The vanes cause the passing flow to attain a circular motion downstream from it. The sediment-rich bottom flow is directed to the river bank and the erosive upper current is directed towards the centre of the river. In the available literature on bottom vanes the distinction is made between vanes placed at a small angle of attack with the flow and vanes placed at a high angle of attack with the flow.Hydraulic EngineeringCivil Engineering and Geoscience
A study of possible bank protection measures near the Meghna Bridge, Bangladesh
No full textIn 1990, in Bangladesh, the "Meghna roads and highway bridge"was finisthed. This bridge is the first one in a series of two Bridges replacing the ferries over the Upper Meghna River. During construction, and also after the finalization of the Bridge, it was revealed that the designers of this Bridge underestimated the power of the Upper Meghna River here. This revealed itself in various aspects. The most severe ones are the slope failure just upstream from the Bridge which occurred already during the construction, the collapse of (parts of) the slope protection of one of the Bridge's abutments (eight months after the finalization) and the present danger induced by the morphological planform changes of the river just upstream of the Bridge. In the future this could lead to outflanking of the Bridge by the River. This study discusses the various possible measures which can be taken to safeguard the Meghna Bridge on the long term. Firstly, in part I of this report, the present situation at the location of the Meghna Bridge is described and analyzed. Subsequently, in part I I of this Report, a design study is conducted. Designs, solving the present problems, are elaborated onto a detailed design level. These designs are all elaborated with the specific characteristics of Bangladesh being a developing country in mind. Therefore an important aspect during the design study is the use of local resources (both technical as labour) as much as possible. The design study resulted in three final alternatives. These alternatives are evaluated by means of a Multi Criteria Analysis for their non-monetary criteria. Taking also into account the costs estimates for all alternatives resulted in the recommendation of "alternative I I I ". This "alternative III" consists of the following. A series of six groynes is foreseen in the river bend just upstream of the Bridge and a guide bank structure replaces the damaged abutment of the Bridge. A remarkable aspect of this finally recommended design is the fact that all projected structures are to be constructed with a core of sand cement stone and concrete blocks as slope protection. Instead of the (almost classical) method of a soil core with a slope protection of fascine mattresses etc., which is also elaborated being one of the alternatives, it is concluded that the application of sand cement stone as construction material here is the best option. This is explained by a combination of reasons. As the country of Bangladesh is located in one of the largest deltas in the world its soil consists mainly of the fine alluvial deposits as (rather fine) sand. Therefore rock and boulders are rather scarce construction materials here. Another important aspect is the fact that Bangladesh is a developing country. Therefore "high tech" construction techniques as dredging in deep water and the placing of elements under water are locally not (yet) widely known and available. Using sand cement stone and concrete blocks as construction materials requires only simple construction techniques. Another merit is the fact that in the structures rather steep slopes can be applied which results in a (relative) saving of material and therefore results in low costs. These (and others) reasons finally resulted in the recommendation of the mentioned alternative.Hydraulic EngineeringCivil Engineering and Geoscience
Feast at the Fig
No full text56-57The soft golden hue of the early morning
sunshine wafts through the canopy, bathing the evergreen forest in a honeyed
hue. Spider webs gleam their intricate patterns like diamond dust on gossamer,
as the dewdrops on them catch the first shafts of the morning light. The
cackling “kok-koko-koko-kok” of the ubiquitous jungle fowl breaks through the
early stillness, calling the forest to wakefulness
From Fungi to Forests: The Tale of Tropical Tree Diversity
No full textVolume: 75Start Page: 25End Page: 3
Misconstrued dichotomies
No full text[Extract] WE live in an age of trade-offs. In pursuit of material well-being, human societies are transforming earth's land and water in manifold ways, in the process compromising the very natural systems that are essential for our survival. The vast sprawl of homo sapiens across the planet is causing one of the largest extinction crisis – 1000 times higher than ever in earth's history. To burgeoning human numbers, add the societal imperative for economic progress and we certainly face a quandary. How do we ensure development for all people and yet safeguard vital ecological wealth and biodiversity on this finite planet? Equity being an essential prerequisite for sustainability, what measures can ensure resource use in the most equitable manner? In a populous country like India, seeking to maintain its economic growth potential while also preserving its natural wealth, these are questions of paramount importance
Parks protect forest cover in a tropical biodiversity hotspot, but high human population densities can limit success
No full textMaintaining forest cover is important for Biodiversity Hotspots that support many endangered and endemic species but have lost much of their original forest extent. In developing countries, ongoing economic and demographic growth within Hotspots can alter rates and patterns of deforestation, making it a concern to quantify rates of forest loss and assess landscape-scale correlates of deforestation within Hotspots. Such analyses can help set baselines for future monitoring and provide landscape-scale perspectives to design conservation policy. For the Western Ghats Biodiversity Hotspot in India, we examined correlates of forest loss following rapid economic expansion (post-2000 CE). First, we used open-source remote-sensing data to estimate annual trends in recent forest loss (from 2000 to 2016) for the entire Hotspot. Across the entire Western Ghats, we assessed the relative importance of and interactions among demographic, administrative, and biophysical factors that predicted rates of forest loss—measured as the number of 30 × 30-m pixels of forest lost within randomly selected 1 km2 cells. Protected areas reduced forest loss by 30%, especially when forests were closer to roads (33%) and towns (36%). However, the advantage of protection declined by 32% when local population densities increased, implying that the difference in forest loss between protected and non-protected areas disappears at high local population densities. To check scale-dependency of spatial extent, we repeated the modelling process for two landscape subsets within Western Ghats. In contrast with results for the entire Western Ghats, both focal landscapes showed no difference in deforestation with protection status alone or its interactions with village population density and distance to towns. However, deforestation was 88% lower when forests were protected and farther from roads. Overall, our results indicate that protected areas help retain forest cover within a global Biodiversity Hotspot even with rapid development, but high human population densities and road development can reduce the benefits of protection
Post-logging recovery of animal-dispersed trees in a tropical forest site in north-east India
No full textSelective logging is known to alter the structural and community composition of tropical forests and may disrupt plant-frugivore interactions. We hypothesized that even after a sufficient period of recovery, logged areas will not possess as complete a suite of species as an unlogged forest, the differences being more marked for biotically-dispersed species. Species of this functional group are expected to occur at lower densities, have lower species richness and diversity, and be smaller in logged forests. To quantify structural and functional differences in tree communities, we sampled 120 randomly placed plots, 60 each in logged and unlogged forest sites. We found significant differences in species richness and diversity between logged and intact forest. Within biotically-dispersed species, bird dispersed species showed a significant reduction in species richness. Consistent with previous studies, trees in logged forests were smaller, although overall density was not different between the two treatments. We posit that selective logging might have pervasive effects on functional aspects of tropical tree communities, which appear to persist even after two decades of logging cessation
Modelling Centuries of Geo-morphological Development of the Ganges-Brahmaputra-Meghna Delta
No full textThe Ganges-Brahmaputra-Meghna (GBM) Delta is a good example of a large estuarine system with sparse data. This study describes the development and validation of a morphodynamic process-based model (Delft3D) as a tool to predict the dynamic system as a response to climate change, sea-level rise, subsidence and other influences. The modelled sediment transport of the Ganges and Jamuna systems is between 200 and 1100 million ton/year, which is in line with observations. On annual basis sand accounts for less than 20% of the sediment load in the system with the remaining sediment being much finer. Analysis of modelled bed level changes over time reveals that only a few river systems are in an aggrading phase. The 2D model exhibits that about 22% of the supplied sediment deposits in the delta system on floodplains and tidal plains, whereas the remaining 78% of the sediment causes subaquatic delta progradation or is lost in the deep ocean bed. Although the model does not reproduce all-natural phenomena at all spatial scales, it will be a valuable tool to describe and explore the morphodynamic development of the GBM Delta over decadal to centennial timescales for macro-scale understanding, planning, and management.Coastal Engineerin
Dispersal limitation and weaker stabilizing mechanisms mediate loss of diversity with edge effects in forest fragments
No full textEnvironment shapes the spatial organization of tree diversity in fragmented forests across a human‐modified landscape
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