Mashfiqus Salehin

Relative roles of stream flow and sedimentary conditions in controlling hyporheic exchange

Hyporheic exchange is often controlled by subsurface advection driven by the interaction of the stream with sedimentary pore water. The nature and magnitude of the induced exchange flow is dependent on the characteristics of both the stream flow and the sediment bed. Fundamental hydrodynamic theory can be applied to determine general relationships between stream characteristics, sediment characteristics, and hyporheic exchange rates. When the stream bed is fine enough to allow application of Darcys Law, as with sand beds, the induced advective exchange can be calculated from fundamental hydrodynamic principles. Comparison with a wide range of experimental results demonstrates the predictive capability of this theory. Coarser sediments such as gravels are more complex because they admit turbulent interactions between the stream and subsurface flows, which can produce considerable exchange even when the bed surface is flat and no flows are induced by the bed topography. Even for this case, however, scaling arguments can still be used to determine how exchange rates vary with stream and sedimentary conditions. Evaluation of laboratory flume experiments for a wide range of stream conditions, bed sediment types including sand and gravel, and bed geometries demonstrates that exchange scales with the permeability of the bed sediments and the square of the stream velocity. These relationships occur due to fundamental hydrodynamic processes, and were observed to hold over almost five orders of magnitude of exchange flux. Such scaling relationships are very useful in practice because they can be used to extend observed hyporheic exchange rates to different flow conditions and to uniquely identify the role of sedimentary conditions in controlling exchange flux.

Comparison of transient storage in vegetated and unvegetated reaches of a small agricultural stream in Sweden: seasonal variation and anthropogenic manipulation

In this work, we compare solute transport and hyporheic exchange in vegetated and unvegetated reaches of Sava Brook, an agricultural stream in Sweden subject to extreme variations in channel vegetation and morphology due to both natural seasonal effects and anthropogenic manipulation. A solute injection experiment was conducted in September, 2001 (late summer), at which time there was extensive in-channel vegetation in upstream reaches but none in downstream reaches due to channel excavation by farmers. Experimental results are interpreted using both the advective storage path model and the transient storage model. Results from the vegetated and excavated reaches of the stream are compared both with each other and with the results of a previous experiment conducted in April, 1998 (early spring), when the stream was not excavated but there was only minimal vegetation present in the area due to natural seasonal effects. Results from the two injection experiments are compared by using scaled parameters that appropriately include the effects of stream velocity and depth on hyporheic exchange. This analysis indicates that the variation of solute storage time in all non-excavated agricultural reaches is attributable to differences in stream flow depth, velocity, and the hydraulic conductivity of the streambed sediments. Mixing in vegetated reaches is characterized by rapid exchange and considerable lag of the mean solute peak relative to the mean channel velocity. In addition, excavation altered the stream channel geometry so as to increase the storage time of solutes and reduce the effective exchange rate. This work indicates the need to consider the effect of specific processes when analyzing hyporheic exchange using tracer-injection methods, and supports the use of model frameworks with the potential to explicitly include different formulations for various hyporheic and dead zone transport processes.

Flood Risks and Reduction Approaches in Bangladesh

Bangladesh is one of the most flood prone areas of the world because of its unique geographical setting and physiographic features together with a massive and unique hydraulic system. The ever increasing population, ill-planned infrastructural development and massive flood control interventions in a floodplain environment have resulted in flood disasters becoming larger and more frequent in recent times. Socio-economic impact of floods is profound; the flood prone zones represent areas with the highest incidence of the extreme poor, and the number of poor living in high flood risk areas is on the rise. The damage to infrastructure constitutes the major proportion when it comes down to economic damage resulting from floods. Since mid 1960s there has been a steady growth of flood control and drainage projects in Bangladesh through the construction of embankments, drainage channels, and sluices and regulators, with the total coverage area standing at 5.37 million ha. While the projects yielded a number of positive impacts such as increase in agricultural production, increase in economic activities, and reduction of damage to infrastructure inside protected area, the projects in general could not attain the desired objectives because of lack of consideration of interdependence of land, water, ecosystems and socio-economic development. Projects were mostly formulated with single objectives, aiming to solve the immediate problems without giving adequate attention to potential, undesirable long-term consequences. Ill-planned growth of projects have impacted the hydraulic stability of the system, with projects being able to provide protection against normal floods, while largely failing to provide protection during moderate to extreme floods. The cost of environmental degradation has been rather very high. Storage functions of floodplains have been lost, impacting important hydrologic functions of floodplains such as moderation of flood peaks, augmentation of dry season river flows and replenishment of groundwater storage. There has been substantial damage to capture fisheries because of disruption of hydraulic connection between river and floodplain. Disruption of links between river and floodplain by embankments has impacted country boat transports. Livelihood activities of different groups, especially the marginalized, have thus been compromised. In the light of the experiences with the flood control projects, the necessity to harness the beneficial impacts of floods especially in maintaining soil fertility and sustaining fisheries resources has become apparent. All the socio-economic and environmental concerns of earlier practices gave rise to exploration of alternative management strategies which are more resilient and environment friendly. This chapter provides a review of risks associated with different types of floods in Bangladesh. It also gives an account of historic floods and their impacts. Flood risk reduction approaches undertaken in Bangladesh and their impacts and consequences are discussed at length. The chapter also discusses three innovative flood risk reduction approaches that have evolved in Bangladesh, which tend to address the socio-economic and environmental concerns of typical flood control measures.

Integrating Science and Policy Through Stakeholder-Engaged Scenarios

Scenario development for integrated analysis focuses on adopting an interdisciplinary approach covering key elements of the biophysical environment as well as changes in livelihoods, education, economics and governance both locally and internationally. Most importantly, the development of these scenarios generates a dialogue across institutions, stakeholders and sectors, with the use of common data and agreement on shared qualitative and quantitative futures. The scenarios adopted combine three alternative future climates and three socio-economic development pathways. Quantification of these issues included estimation based on published data, expert knowledge and stakeholder engagement, particularly where data are most uncertain or unknown. This chapter demonstrates this approach for coastal Bangladesh.

Mechanisms and Drivers of Soil Salinity in Coastal Bangladesh

Determining soil salinity within the delta is crucial as it is the dominant factor determining crop productivity. There are numerous interacting drivers that influence soil salinity, including climate variability, saline river water inundation, storm surge inundation, depth to groundwater table, groundwater salinity, and shrimp farming (Bagda). For the study area, tidal river salinity appears to influence the soil salinity most, particularly in the south-west of the delta. In northern areas, high groundwater salinity levels, combined with a high groundwater table, are a major contributor to soil salinity. In addition, an increase in salinity of dry season irrigation water is expected to increase salt accumulation in soils, with a possibility of irrigation water salinity exceeding five parts per thousand.

Integrative Analysis Applying the Delta Dynamic Integrated Emulator Model in South-West Coastal Bangladesh

A flexible meta-model, the Delta Dynamic Integrated Emulator Model (ΔDIEM), is developed to capture the socio-biophysical system of coastal Bangladesh as simply and efficiently as possible. Operating at the local scale, calculations occur efficiently using a variety of methods, including linear statistical emulators, which capture the behaviour of more complex models, internal process-based models and statistical associations. All components are tightly coupled, tested and validated, and their behaviour is explored with sensitivity tests. Using input data, the integrated model approximates the spatial and temporal change in ecosystem services and a number of livelihood, well-being, poverty and health indicators of archetypal households. Through the use of climate, socio-economic and governance scenarios plausible trajectories and futures of coastal Bangladesh can be explored.

Avoiding the water-poverty trap: insights from a conceptual human-water dynamical model for coastal Bangladesh

ABSTRACT Water-related risks impact development opportunities and can trap communities in a downward spiral of economic decline. In this article, the dynamic relationship between water-related risks and economic outcomes for an embanked area in coastal Bangladesh is conceptualized. The interaction between flood events, salinity, deteriorating and poorly maintained water infrastructure, agricultural production and income is modelled. The model is used to test the effect of improvements in the reliability, operation and maintenance of the water infrastructure on agricultural incomes and assets. Results indicate that interventions can have non-marginal impacts on indicators of welfare, switching the system dynamic from a poverty trap into one of growth.

Integrative Analysis for the Ganges-Brahmaputra-Meghna Delta, Bangladesh

The Ganges-Brahmaputra-Meghna delta in Bangladesh is one of the world’s most dynamic deltas and supports high population densities based on large provisioning ecosystem services. Analysing the future of these ecosystem services and associated human livelihoods represents a complex multi-scale, multi-disciplinary problem. A conceptual framework aims to identify mechanisms that link physical processes (including climate change) with ecosystem services and social outcomes providing a shared basis for other analysis, including the design of an integrated model. This also allows the characteristics of co-production of ecosystem services at the landscape scale and significant trade-offs between types of ecosystem services to be incorporated. Adopting a systems-based approach of this scale is challenging but essential to support the effective management of coastal Bangladesh.

Defining Social-Ecological Systems in South-West Bangladesh

Seven distinct social-ecological systems are defined for the Bangladesh delta, based on analysis of the ways in which social systems differ according to the ecological system. These systems are rain-fed and irrigated agriculture, brackish and freshwater aquaculture, Charlands, coastal zones, and areas dependent on the Sundarbans mangrove forest. The social systems that inhibit or facilitate access to ecosystem services vary between social-ecological systems. The timing and nature of ecosystem services give rise to different livelihood opportunities, means of access to ecosystem services, and coping mechanisms. Thus, while the common challenge across all social-ecological systems is to design mechanisms by which the poorest populations retain value and benefits, these mechanisms will differ depending on the system.

Ecosystem Services, Well-Being and Deltas: Current Knowledge and Understanding

Deltas are distinct in terms of the concentration of freshwater, nutrients and especially sediment inputs to a small concentrated area of the coastal zone, creating conditions ideal for fertile ecosystems, dense population and high economic activity. Ecosystem services within these areas can provide services significant in the maintenance of well-being for both rural and urban populations. There are significant feedbacks between environmental processes and social dynamics that drive the economic and well-being outcomes for current and future populations. This chapter reviews ecosystem services in deltas and summarises the state of knowledge in this field on how to manage delta ecosystems for the benefit of resident populations and wider society.