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Geomorphology and Flooding
Published in Saeid Eslamian, Faezeh Eslamian, Flood Handbook, 2022
Giovanni Barrocu, Saeid Eslamian
River deltas are structures formed by the deposition of sediment at their mouth when the streamflow enters an ocean, sea, estuary, lake, or reservoir. The rate of deposition should be higher than the rate of removal by waves and currents, and it depends on flow velocity and load and the salinity of the stream water and receiving water body. The stream flows into the sea with a turbulent flow into the relatively still body of another fluid, interacting with waves and currents. As river water mixes with saline water, the fine clay parts flocculate into aggregates that are too large to remain long in suspension. Therefore, sedimentation is rapid, but suspended sediments may flow offshore even for several kilometers forming turbidity or density currents.
Fluvial depositional landforms
Published in Richard J. Chorley, Stanley A. Schumm, David E. Sugden, Geomorphology, 2019
Richard J. Chorley, Stanley A. Schumm, David E. Sugden
Deltas are formed where sediment-laden rivers flow into standing bodies of water. The geomorphic characteristics of deltas are determined by a number of groups of factors, including: (1) The relative densities of the river water and the standing water (Bates, 1953).(2) The river hydrology, including the magnitude and variations of the discharge.(3) The amount and calibre of the river load, especially the proportion of bedload to suspended load.(4) The intensity of the coastal processes, particularly of wave action with associated longshore currents and of tidal scour.(5) The geometry of the coast, including its plan view and seaward slope.(6) The tectonic stability of the shoreline, particularly in the vertical sense.(7) The climate, in so far as it exerts additional influences over such matters as the amount and type of vegetational cover and the growth of marine organisms.
California’s Sacramento–San Joaquin Delta
Published in Kathleen A. Miller, Alan F. Hamlet, Douglas S. Kenney, Kelly T. Redmond, Water Policy and Planning, 2017
The Sacramento–San Joaquin Delta has had a key and controversial role in California water politics, policy, and management for decades. The Delta has also been difficult for scientists to understand while also being little known by Californians. Climate change and sea level rise make the Delta more important, more complex and difficult to know, and more controversial too. The natural sciences are not easily integrated, and the difficulties of framing and studying ongoing human-driven change are immense. The scientific difficulties are compounded by the fact that environmental science does not work apart from the existing institutions that structure how science is undertaken; how findings are shared among scientists; and how information is communicated to politicians, policy makers, and the public. The Delta Science Plan and the Delta science conferences are helping to meet the challenge, but institutional changes are also needed to help science address the nature of the Anthropocene.
Strategies for climate change adaptation: lessons learnt from long-term planning in the Netherlands and Bangladesh
Published in Water International, 2021
Jos van Alphen, Jaap de Heer, Ellen Minkman
Deltas are geomorphological areas situated in the transitional zone between the marine and riverine environments and largely defined by their flat low-lying surface form. Low-lying delta areas derive their special and dynamic character by the ongoing interaction between the supply of fresh water, sediment and nutrients by the rivers and tidal dynamics and salt intrusion from the sea. Deltas have always been attractive places to live, resulting in a concentration of people and business centres in delta cities. Nowadays, deltas face multiple and complex challenges, including rapid population growth, loss of ecosystems, land subsidence and salt intrusion, which are aggravated by climate change. When insufficiently addressed, the risks of property damage, economic loss and human casualties increase (Ahmed & Suphachalasai, 2014). Existing planning practices are focused on the short to medium terms, while addressing present and future challenges of urban deltas requires a perspective of multiple decades to accommodate for climate change and tailor future developments to changing circumstances. However, these long timescales also entail uncertainty about future conditions, design of interventions and development pathways, posing challenges to policymakers and planners. Moreover, adequate planning also requires solid institutional arrangements, continued funding and stakeholder commitment. Long-term planning in urban deltas thereby entails dealing with the uncertainty related to climate change induced threats.
Governing resilient landscapes across the source-to-sea continuum
Published in Water International, 2021
Rebecca Welling, Paulina Filz, James Dalton, Douglas Mark Smith, Janaka de Silva, Peter Manyara
The source-to-sea continuum presents a number of intricately linked physical, ecological and socioeconomic processes, providing humans with a variety of ecosystems services (Thom et al., 2019 [2020]). People benefit from coastal habitats that are at the same time sensitive to human activities upstream along the river course that provide food, water and energy; and in a negative way through water retention for irrigation, pollution from toxins, pesticides and sewage discharge. The cycle of water determines freshwater quality and quantity, and ultimately seawater quality, affecting coastal ecosystems with the associated human activities such as fishing, aquaculture, tourism, recreation and urbanization opportunities. Similarly, sediment transport along the river channels interacts with coastal zone dynamics, having an impact on nutrient cycles to the sea, beach nourishment and biodiversity (Monteiro & Marchand, 2009; Silvestri & Kershaw, 2010). Sediment supply is necessary for delta formation and prevents erosion in habitats important for humans (mangrove forests, coral reefs, seagrass beds, beaches) (Monteiro & Marchand, 2009).
Linear stability analysis of sediment-laden planar turbulent jets in shallow waters
Published in Journal of Hydraulic Research, 2022
Dongdong Shao, Adriano C. de Lima
Currently, numerous deltas throughout the world are experiencing land loss due to anthropogenic and natural stressors such as reduced sediment supply, land subsidence and sea level rise, and restoration projects such as artificial diversions that deliver sediment to build deltaic land in eroding areas are increasingly practised (Canestrelli et al., 2014; Fagherazzi et al., 2015; Falcini & Jerolmack, 2010). In terms of the implications of our analysis for artificial diversion, the designed levee breach should maximize the value of jet stability parameter to produce stable jets and thus the formation of a nearshore mouth bar to maximize the land building. This can be achieved by increasing the breach width, reducing the water depth, or increasing bottom friction in the receiving basin by adding artificial roughness as discussed in Fagherazzi et al. (2015). As far as the carried sediment is concerned, with everything else being equal, coarser sediments in low and moderate concentration tend to produce a stable jet and the desirable mouth bar deposition, whereas the presence of excessive fine (cohesive) sediments would lead to the opposite. It is worth pointing out that the above discussions are restricted to idealized conditions, and more realistic considerations of other confounding factors such as wind waves (Nardin & Fagherazzi, 2012; Nardin et al., 2013), tides (Leonardi et al., 2013) and basin slope (Jiménez-Robles et al., 2016) etc. can be found in Fagherazzi et al. (2015), which reviewed the effects of shallow jet stability on sediment transport and the associated consequences for depositional patterns at river mouths.