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The Mississippi River, the longest in North America, is really two rivers geophysically. The volume is less, the slope steeper, the velocity greater, and the channel straighter in its upper portion than in its lower portion. Below the mouth of the Ohio River, the Mississippi meanders through a continental depression that it has slowly filled with sediment over many millennia. Some limnologists and hydrologists consider the transitional middle portion of the Mississippi, where the waters of its two greatest tributaries, the Missouri and Ohio rivers, join it, to comprise a third river, in terms of its behavioral patterns and stream and floodplain ecologies. The Mississippi River humans have known, with its two or three distinct sections, is a relatively recent formation. The lower Mississippi only settled into its current formation following the last ice age and the dissipation of water released by receding glaciers. Much of the current river delta is newer still, having taken shape over the last three to five hundred years. Within the lower section of the Mississippi are two subsections, the meander zone and the delta. Below Cape Girardeau, Missouri, the river passes through Crowley’s Ridge and enters the wide and flat alluvial plain. Here the river meanders in great loops, often doubling back on itself, forming cut offs that, if abandoned by the river, forming lakes. Until modern times, most of the plain, approximately 35,000 square miles, comprised a vast and rich—rich in terms of biomass production—ecological wetland sustained by annual Mississippi River floods that brought not just water, but fertile sediment—topsoil—gathered from across much of the continent. People thrived in the Mississippi River meander zone. Some of the most sophisticated indigenous cultures of North America emerged here. Between Natchez, Mississippi, and Baton Rouge, Louisiana, at Old River Control, the Mississippi begins to fork into distributary channels, the largest of which is the Atchafalaya River. The Mississippi River delta begins here, formed of river sediment accrued upon the continental shelf. In the delta the land is wetter, the ground water table is shallower. Closer to the sea, the water becomes brackish and patterns of river sediment distribution are shaped by ocean tides and waves. The delta is frequently buffeted by hurricanes. Over the last century and a half people have transformed the lower Mississippi River, principally through the construction of levees and drainage canals that have effectively disconnected the river from the floodplain. The intention has been to dry the land adjacent to the river, to make it useful for agriculture and urban development. However, an unintended effect of flood control and wetland drainage has been to interfere with the flood-pulse process that sustained the lower valley ecology, and with the process of sediment distribution that built the delta and much of the Louisiana coastline. The seriousness of the delta’s deterioration has become especially apparent since Hurricane Katrina, and has moved conservation groups to action. They are pushing politicians and engineers to reconsider their approach to Mississippi River management.

Article

Deltas have played a significant role in the growth of human civilization because of their unique economic and ecological importance. However, deltas are becoming increasingly vulnerable because of the impact of intensive human developmental activities, high population and urban growth, subsidence, climate change, and the associated rise in sea level. The trapping of sediments by dams is another major threat to the long-term stability and sustainability of deltas. The emergence and global acceptance of the concept of sustainable development in the 1980s led to the advent of several multidisciplinary and applied fields of research, including environmental science, environmental geology, and sustainability science. Environmental geology focuses on the application of geologic knowledge and principles to broad-ranging environmental and socioeconomic issues, including the specific problems confronting deltas. The key environmental geologic challenges in deltas (especially urban delta areas) are: increasing exposure and vulnerability to geologic hazards (flooding, cyclones, etc.), rise in sea level, decreasing sediment load supply, contamination of soil and water resources, provision of adequate drinking water, and safe waste disposal. The application of geologic knowledge and principles to these challenges requires consideration of the critical geologic controls, such as the geological history, stratigraphy, depositional environment, and the properties of the alluvial sediments. Until recently, most of the traditional engineered solutions in the management of deltas were designed to keep out water (fighting nature), typically without adequate geological/hydrological input, rather than building with nature. Recent innovative approaches to delta management involve a paradigm shift from the traditional approach to a more integrated, holistic, adaptive, and ecologically based philosophy that incorporates some critical geological and hydrological perspectives, for instance, widening and deepening rivers and flood plains as well as constructing secondary channels (i.e., making more room for water). A key challenge, however, is the establishment of a close and functional communication between environmental geologists and all other stakeholders involved in delta management. In addition, there is growing global consensus regarding the need for international cooperation that cuts across disciplines, sectors, and regions in addressing the challenges facing deltas. Integrating good policy and governance is also essential.

Article

Amy W. Ando and Noelwah R. Netusil

Green stormwater infrastructure (GSI), a decentralized approach for managing stormwater that uses natural systems or engineered systems mimicking the natural environment, is being adopted by cities around the world to manage stormwater runoff. The primary benefits of such systems include reduced flooding and improved water quality. GSI projects, such as green roofs, urban tree planting, rain gardens and bioswales, rain barrels, and green streets may also generate cobenefits such as aesthetic improvement, reduced net CO2 emissions, reduced air pollution, and habitat improvement. GSI adoption has been fueled by the promise of environmental benefits along with evidence that GSI is a cost-effective stormwater management strategy, and methods have been developed by economists to quantify those benefits to support GSI planning and policy efforts. A body of multidisciplinary research has quantified significant net benefits from GSI, with particularly robust evidence regarding green roofs, urban trees, and green streets. While many GSI projects generate positive benefits through ecosystem service provision, those benefits can vary with details of the location and the type and scale of GSI installation. Previous work reveals several pitfalls in estimating the benefits of GSI that scientists should avoid, such as double counting values, counting transfer payments as benefits, and using values for benefits like avoided carbon emissions that are biased. Important gaps remain in current knowledge regarding the benefits of GSI, including benefit estimates for some types of GSI elements and outcomes, understanding how GSI benefits last over time, and the distribution of GSI benefits among different groups in urban areas.

Article

Claudia Sadoff, David Grey, and Edoardo Borgomeo

Water security has emerged in the 21st century as a powerful construct to frame the water objectives and goals of human society and to support and guide local to global water policy and management. Water security can be described as the fundamental societal goal of water policy and management. This article reviews the concept of water security, explaining the differences between water security and other approaches used to conceptualize the water-related challenges facing society and ecosystems and describing some of the actions needed to achieve water security. Achieving water security requires addressing two fundamental challenges at all scales: enhancing water’s productive contributions to human and ecosystems’ well-being, livelihoods and development, and minimizing water’s destructive impacts on societies, economies, and ecosystems resulting, for example, from too much (flood), too little (drought) or poor quality (polluted) water.